PC Power Supply Hacking - 50V Output
thedatastream, Tue Jan 09 2007, 04:33PMEDIT: change thread title to reflect performance achieved.
I want to bodge an AT PC power supply to have a variable output voltage up to 30V (don't really care where the bottom end is). This is for powering my Titch SSTC.
Assuming that the AT has some similarities with this ATX supply
and this AT supply 
in terms of the output and control stage then I should be able to find my way around it. Obvious differences are that there is no 5V standby and no 3V3 supply on the AT.I intend to
- Disconnect all rails except the 12V from the transformer
- Uprate the capacitor and diode on the output
- Disconnect overvoltage circuit (if any)
- Locate and alter the feedback components to get maximum duty cycle on the transformer input (with suitable load on output) to figure out how many turns I need to add to the transformer to get my maximum output voltage
- Add appropriate number of turns
- Add variable resistor in feedback circuit to adjust output
Things to be concerned about
- Stability could be affected
- Saturation of transformer needs to be avoided
- Fitting more turns ont already crowded transformer could be difficult - perhaps remove existing secondary if possible?
Any suggestions or tips most welcome, or if there is anything obvious that I have overlooked then please let me know.
James
Re: PC Power Supply Hacking - 50V Output
Steve Conner, Tue Jan 09 2007, 05:12PM
Hi James
The 12V rail on these things comes from a centre tapped winding rectified by a two-diode full wave rectifier. (Usually in a TO220 package on the output side heatsink: the TO247 is the 5V rectifier.) If you added another two diodes to make a bridge rectifier, got rid of all the 5V rectifier bits, and disconnected the centre tap then the same winding would want to output 24V.
**edit** sorry, that is the standard circuit for an ATX, I can't remember how the AT ones worked. There were all different circuits.
There's a thread on the old board that documents how I "overclocked"
a 5V SMPS to 13.8V 
with some help from Richie Burnett. This might give you some pointers on how to get started.
Steve Conner, Tue Jan 09 2007, 05:12PM
Hi James
The 12V rail on these things comes from a centre tapped winding rectified by a two-diode full wave rectifier. (Usually in a TO220 package on the output side heatsink: the TO247 is the 5V rectifier.) If you added another two diodes to make a bridge rectifier, got rid of all the 5V rectifier bits, and disconnected the centre tap then the same winding would want to output 24V.
**edit** sorry, that is the standard circuit for an ATX, I can't remember how the AT ones worked. There were all different circuits.
There's a thread on the old board that documents how I "overclocked"
a 5V SMPS to 13.8V with some help from Richie Burnett. This might give you some pointers on how to get started.Re: PC Power Supply Hacking - 50V Output
Marko, Tue Jan 09 2007, 05:13PM
Wee. Tradition continues...
Old threads are about ATX modding, though; AT's are almost extinct today.. I don't remember those electronics anymore
- woops, steve already got it.
I may be wrong, but I think that highest voltage that can be 'hacked' from single 12V rail was some 25V with ATX. I don't know if AT will get any better.
-5V could probably be used but windings are almost surely too tiny..
About the transformers, I found it to be a very frustrating thing, unless you are very lucky.
Cores are stuck to their forms with some hardcore epoxy and is practically impossible to get apart in one piece.
Mess isn't closely worth even buying a new transformer.
Best thing I can think off is using a real fullwave rectifier t double the output voltage at start.
Marko, Tue Jan 09 2007, 05:13PM
Wee. Tradition continues...
Old threads are about ATX modding, though; AT's are almost extinct today.. I don't remember those electronics anymore
- woops, steve already got it.I may be wrong, but I think that highest voltage that can be 'hacked' from single 12V rail was some 25V with ATX. I don't know if AT will get any better.
-5V could probably be used but windings are almost surely too tiny..
About the transformers, I found it to be a very frustrating thing, unless you are very lucky.
Cores are stuck to their forms with some hardcore epoxy and is practically impossible to get apart in one piece.
Mess isn't closely worth even buying a new transformer.
Best thing I can think off is using a real fullwave rectifier t double the output voltage at start.
Re: PC Power Supply Hacking - 50V Output
Dr. Dark Current, Tue Jan 09 2007, 06:18PM
If I'm not mistaken, the 12V output from the xfmr also supplies power to the pwm chip (494), if you up this voltage to 30V, isn't it going to x-plod in your face?
Dr. Dark Current, Tue Jan 09 2007, 06:18PM
If I'm not mistaken, the 12V output from the xfmr also supplies power to the pwm chip (494), if you up this voltage to 30V, isn't it going to x-plod in your face?
Re: PC Power Supply Hacking - 50V Output
Marko, Tue Jan 09 2007, 07:22PM
494 works well up to 40V, LM393 and 339 up to 36 volts, and judging atx already acheiving to close voltages I don't think that would be a problem.
I'm although, confused with one thing; how TL hets it's startup power?
It looks like transformer T2 does it, but in a rather wierd way (in series with main transformer?) Is thew supply ''kick-start'', or is that transformer actually same core as gate drive transformer? Steve?
Marko, Tue Jan 09 2007, 07:22PM
jmartis wrote ...
If I'm not mistaken, the 12V output from the xfmr also supplies power to the pwm chip (494), if you up this voltage to 30V, isn't it going to x-plod in your face?
If I'm not mistaken, the 12V output from the xfmr also supplies power to the pwm chip (494), if you up this voltage to 30V, isn't it going to x-plod in your face?
494 works well up to 40V, LM393 and 339 up to 36 volts, and judging atx already acheiving to close voltages I don't think that would be a problem.
I'm although, confused with one thing; how TL hets it's startup power?
It looks like transformer T2 does it, but in a rather wierd way (in series with main transformer?) Is thew supply ''kick-start'', or is that transformer actually same core as gate drive transformer? Steve?
Re: PC Power Supply Hacking - 50V Output
Steve Conner, Tue Jan 09 2007, 10:11PM
Hi Firkragg
In the ATX power supply schem posted by thedatastream, the 5V standby supply is a separate converter that runs all the time the PC is plugged in. It supplies just enough power to the TL494 to get it started. Once the main power supply is turned on, the +12V output powers the TL494 and driver transistors, through the diode marked D in the schem.
I have no idea how the AT schematic he posted works. I bet it has something to do with the mysterious point A. T2 in that circuit is a current transformer to operate the current limiter.
Steve Conner, Tue Jan 09 2007, 10:11PM
Hi Firkragg
In the ATX power supply schem posted by thedatastream, the 5V standby supply is a separate converter that runs all the time the PC is plugged in. It supplies just enough power to the TL494 to get it started. Once the main power supply is turned on, the +12V output powers the TL494 and driver transistors, through the diode marked D in the schem.
I have no idea how the AT schematic he posted works. I bet it has something to do with the mysterious point A. T2 in that circuit is a current transformer to operate the current limiter.
Re: PC Power Supply Hacking - 50V Output
Marko, Tue Jan 09 2007, 10:53PM
AT thingie was what I asked about.. never mind.
Datastream: ATX'-es are dirt cheap today, and if this thing starts sucking, it may be just easier to buy a new 350W supply for $10 and mod it
You can also series them for more power, but low voltage grounds of all except one supply must be floated.
Marko, Tue Jan 09 2007, 10:53PM
I have no idea how the AT schematic he posted works. I bet it has something to do with the mysterious point A. T2 in that circuit is a current transformer to operate the current limiter.
AT thingie was what I asked about.. never mind.
Datastream: ATX'-es are dirt cheap today, and if this thing starts sucking, it may be just easier to buy a new 350W supply for $10 and mod it
You can also series them for more power, but low voltage grounds of all except one supply must be floated.
Re: PC Power Supply Hacking - 50V Output
..., Tue Jan 09 2007, 11:13PM
I would recomend buying an ATX power supply, and modding that do do whatever to 18v, then series that with the 12v on the existing power supply ;)
..., Tue Jan 09 2007, 11:13PM
I would recomend buying an ATX power supply, and modding that do do whatever to 18v, then series that with the 12v on the existing power supply ;)
Re: PC Power Supply Hacking - 50V Output
thedatastream, Wed Jan 10 2007, 08:37AM
Hi folks, thanks for all the responses
I wonder if the AT engages in a bit of self oscillation to get enough energy through the winding to start up the TL494 (although a quick glance at the circuit doesn't reveal an obvious method).
Reasons for sticking with using an AT supply rather than buying an ATX are:
Also, I'd prefer not to series the supplies, as I want a single ATX shaped box as the base of the coil - two boxes would be out of proportion and would mess up me feng shui like.
Putting the cores in a pan of boiling water, or perhas in the vacuum oven at work should allow me to separate them OK - should I need to that is. If the secondary is on the outside, I may be able to get away with just unwrapping the layers without dissasembling the core.
UVLO on the TL494 is 7V max, so if I make the bottom end of the adjust range = 8V and stick a heatsunk 7812 regulator between my adjustable output and the TL494 supply then that should be good enough. Too much voltage will affect the current being driven into the main bipolar transistor bases - I don't want to fry the gate drive circuit components.
I'll open up the power supply this lunchtime, have a look inside and let you know how I get on.
Rgds,
James
thedatastream, Wed Jan 10 2007, 08:37AM
Hi folks, thanks for all the responses
I wonder if the AT engages in a bit of self oscillation to get enough energy through the winding to start up the TL494 (although a quick glance at the circuit doesn't reveal an obvious method).
Reasons for sticking with using an AT supply rather than buying an ATX are:
- I've got two that were donated for free
- They are a bit less complicated inside so I've got more chance of finding the correct components easily
Also, I'd prefer not to series the supplies, as I want a single ATX shaped box as the base of the coil - two boxes would be out of proportion and would mess up me feng shui like.
Putting the cores in a pan of boiling water, or perhas in the vacuum oven at work should allow me to separate them OK - should I need to that is. If the secondary is on the outside, I may be able to get away with just unwrapping the layers without dissasembling the core.
UVLO on the TL494 is 7V max, so if I make the bottom end of the adjust range = 8V and stick a heatsunk 7812 regulator between my adjustable output and the TL494 supply then that should be good enough. Too much voltage will affect the current being driven into the main bipolar transistor bases - I don't want to fry the gate drive circuit components.
I'll open up the power supply this lunchtime, have a look inside and let you know how I get on.
Rgds,
James
Re: PC Power Supply Hacking - 50V Output
Marko, Wed Jan 10 2007, 12:56PM
There is apsolutely no chance you are going to boil the cores off.
The temperature needed for epoxy to melt was some 400C or so, with hot air gun.
I completely charred the plastic form, and core ended broken in several places due to massive overheating.
To get 30V, as already suggested, you don't need to do anything with the transformer;
using a full 4-diode bridge will give you 24V output wich I'm pretty sure for can be boosted to 30V:
You can then use one branch to provide power to supply's electronics without frying it.
This way you could get 50V out of most ATX supplies; you are probably the first one to do the same with AT.
From the moddng side, AT's arent any different than ATX, complexity isn't a factor there.
ATX mods are already done and verified many times.
Marko, Wed Jan 10 2007, 12:56PM
Putting the cores in a pan of boiling water, or perhas in the vacuum oven at work should allow me to separate them OK - should I need to that is. If the secondary is on the outside, I may be able to get away with just unwrapping the layers without dissasembling the core.
There is apsolutely no chance you are going to boil the cores off.
The temperature needed for epoxy to melt was some 400C or so, with hot air gun.
I completely charred the plastic form, and core ended broken in several places due to massive overheating.
To get 30V, as already suggested, you don't need to do anything with the transformer;
using a full 4-diode bridge will give you 24V output wich I'm pretty sure for can be boosted to 30V:
You can then use one branch to provide power to supply's electronics without frying it.
This way you could get 50V out of most ATX supplies; you are probably the first one to do the same with AT.
From the moddng side, AT's arent any different than ATX, complexity isn't a factor there.
ATX mods are already done and verified many times.
Re: PC Power Supply Hacking - 50V Output
thedatastream, Wed Jan 10 2007, 01:55PM
Dur... should read more carefully - a full bridge rectifier is ideal. I shall implement forthwith (obviously not using 1N4148s
waits for bang)
I hear what you are saying about the epoxy, sounds like I won't have to go down that route now.
Progress so far has been good, I've butchered the PSU and removed almost everything on the secondary power side except the coupled choke and the 12V inductor. It's nice to see that the circuit is almost identical to the AT schematic I linked to in my first post. This means it has been fairly easy to track down the relevant components.
PSU now works with 12V in isolation and no-preload necessary. I've put a 7805 from the 12V to the 5V rail as it powers some of the ancilliary circuitry.
BEFORE
AFTER
REMOVED PARTS
To do
Thanks again for the help so far
James
thedatastream, Wed Jan 10 2007, 01:55PM
Dur... should read more carefully - a full bridge rectifier is ideal. I shall implement forthwith (obviously not using 1N4148s
waits for bang)
I hear what you are saying about the epoxy, sounds like I won't have to go down that route now.
Progress so far has been good, I've butchered the PSU and removed almost everything on the secondary power side except the coupled choke and the 12V inductor. It's nice to see that the circuit is almost identical to the AT schematic I linked to in my first post. This means it has been fairly easy to track down the relevant components.
PSU now works with 12V in isolation and no-preload necessary. I've put a 7805 from the 12V to the 5V rail as it powers some of the ancilliary circuitry.
BEFORE
AFTER
REMOVED PARTS
To do
- Upgrade output diodes to withstand correct voltage and current in fullbridge configuration
- Remove feedback resistor from 5V line
- Insert variable resistor into 12V feedback line
- Test
Thanks again for the help so far
James
Re: PC Power Supply Hacking - 50V Output
Avalanche, Wed Jan 10 2007, 03:58PM
From my experience separating cores, I've found that there are two different kinds of resin used in ferrite transformers - the white stuff (not a chance, too high a melting point) and then some transformers use a kind of opaque brown resin, that is very brittle and chips off. That kind of resin melts easily into a liquid if you drop it into a pan of boiling water, I separated a transformer from an old monitor like this. Dropped it into a pan and was surprised when 2 minutes later the transformer just fell apart!
Avalanche, Wed Jan 10 2007, 03:58PM
From my experience separating cores, I've found that there are two different kinds of resin used in ferrite transformers - the white stuff (not a chance, too high a melting point) and then some transformers use a kind of opaque brown resin, that is very brittle and chips off. That kind of resin melts easily into a liquid if you drop it into a pan of boiling water, I separated a transformer from an old monitor like this. Dropped it into a pan and was surprised when 2 minutes later the transformer just fell apart!
Re: PC Power Supply Hacking - 50V Output
thedatastream, Fri Jan 12 2007, 09:55AM
The gain over the conventional half wave rectified output appears to be high at light loads, factor of x2.8 at 10W, although I would expect this to come down as the load increases.
More hacking away this lunchtime, but it's looking pretty good
Rgds all,
James
thedatastream, Fri Jan 12 2007, 09:55AM
Steve Conner wrote ...
If you added another two diodes to make a bridge rectifier, got rid of all the 5V rectifier bits, and disconnected the centre tap then the same winding would want to output 24V.
The key to this sentence is "disconnect the centre tap", otherwise the windings get shorted out to ground when the bottom winding goes negative - see attached image.If you added another two diodes to make a bridge rectifier, got rid of all the 5V rectifier bits, and disconnected the centre tap then the same winding would want to output 24V.
The gain over the conventional half wave rectified output appears to be high at light loads, factor of x2.8 at 10W, although I would expect this to come down as the load increases.
More hacking away this lunchtime, but it's looking pretty good
Rgds all,
James
Re: PC Power Supply Hacking - 50V Output
Steve Ward, Sat Jan 13 2007, 02:08AM
Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.
I just recently built another switch mode supply using the old TL494. Its a constant current source for charging NiMH batteries (our robot uses a moster 50x sub-C cell pack). I was happy to see it pumping 38V at 7A with ease. It was actually a rather easy project, the only set back was when i used too small of wire on my ferrite core, and it got rather toasty! The 10awg litz fixed that up
.
Steve Ward, Sat Jan 13 2007, 02:08AM
The gain over the conventional half wave rectified output appears to be high at light loads, factor of x2.8 at 10W, although I would expect this to come down as the load increases.
Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.
I just recently built another switch mode supply using the old TL494. Its a constant current source for charging NiMH batteries (our robot uses a moster 50x sub-C cell pack). I was happy to see it pumping 38V at 7A with ease. It was actually a rather easy project, the only set back was when i used too small of wire on my ferrite core, and it got rather toasty! The 10awg litz fixed that up
.Re: PC Power Supply Hacking - 50V Output
thedatastream, Sat Jan 13 2007, 10:49AM
Success! Yesterday evening yielded 1.5A @ 48V = 72W into an electronic load - the maximum voltage output. I would hope to be able to get 3A out at this voltage = 144W with the fan cooling running in the chasis (originally a 200W supply). A 100k pot in series with a 1k resistor in the feedback gives me the full adjustment range from 3V to 48V.
However, I still need a regulated 12V supply for the SSTC control electronics, so I've had to install a NatSemi simple switcher to get a 12V 3A rail from the variable output. As its a buck converter, it will only regulate when the input is above 12V. Since the main output can vary lower than this, some kind of "12V good" signal will be required - a zener and bipolar transistor should be good enough.
The intention now is to mount the control cct on the inside of the lid and mount the H-bridge and coil on the top of the case.
Thanks once again for all the assistance in helping me achieve this goal, I will do a full write up on my website before long.
James
thedatastream, Sat Jan 13 2007, 10:49AM
Steve Ward wrote ...
Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.
Yes, there is feedback from the output rail. The power supply was operating in burst mode when I made this measurement (buzzing away merrily) so when it is operating properly, this ratio should come down a bit.Ahem... have you "closed the loop"? Cant expect even a forward converter to regulate properly without negative feedback from the output.
Success!
Yesterday evening yielded 1.5A @ 48V = 72W into an electronic load - the maximum voltage output. I would hope to be able to get 3A out at this voltage = 144W with the fan cooling running in the chasis (originally a 200W supply). A 100k pot in series with a 1k resistor in the feedback gives me the full adjustment range from 3V to 48V.However, I still need a regulated 12V supply for the SSTC control electronics, so I've had to install a NatSemi simple switcher to get a 12V 3A rail from the variable output. As its a buck converter, it will only regulate when the input is above 12V. Since the main output can vary lower than this, some kind of "12V good" signal will be required - a zener and bipolar transistor should be good enough.
The intention now is to mount the control cct on the inside of the lid and mount the H-bridge and coil on the top of the case.
Thanks once again for all the assistance in helping me achieve this goal, I will do a full write up on my website before long.
James
Re: PC Power Supply Hacking - 50V Output
Marko, Sat Jan 13 2007, 10:54AM
You can use a buck-boost converter on -5V rail so it will correct it to positive, any voltage you want. I used buck boost in power supply for my little class E coil with good results.
Marko, Sat Jan 13 2007, 10:54AM
However, I still need a regulated 12V supply for the SSTC control electronics, so I've had to install a NatSemi simple switcher to get a 12V 3A rail from the variable output. As its a buck converter, it will only regulate when the input is above 12V. Since the main output can vary lower than this, some kind of "12V good" signal will be required - a zener and bipolar transistor should be good enough.
You can use a buck-boost converter on -5V rail so it will correct it to positive, any voltage you want. I used buck boost in power supply for my little class E coil with good results.
Re: PC Power Supply Hacking - 50V Output
thedatastream, Sat Jan 13 2007, 06:01PM
A buck boost, specifically a SEPIC regulator is not a bad idea. The main reason I've gone for a buck is that I already had the circuit built up.
What controller did you use in your application? Obviously something that would run at 5V... I can't use a UC3843 as it has a max turn on voltage of 9V.
James
thedatastream, Sat Jan 13 2007, 06:01PM
A buck boost, specifically a SEPIC regulator is not a bad idea. The main reason I've gone for a buck is that I already had the circuit built up.
What controller did you use in your application? Obviously something that would run at 5V... I can't use a UC3843 as it has a max turn on voltage of 9V.
James
Re: PC Power Supply Hacking - 50V Output
Marko, Sat Jan 13 2007, 08:14PM
I used LM2575 since I didn't need anything under 7V, but there are tons of usable regulators around:
I often dig my regulators out of old motherboards and computer trash, and they can be sampled too.
Good luck...
Marko, Sat Jan 13 2007, 08:14PM
I used LM2575 since I didn't need anything under 7V, but there are tons of usable regulators around:
I often dig my regulators out of old motherboards and computer trash, and they can be sampled too.
Good luck...
Re: PC Power Supply Hacking - 50V Output
thedatastream, Sun Jan 14 2007, 10:29AM
Incidentally, I've come across some more AT PSU schematics on this page - click on the power supply link.
James
thedatastream, Sun Jan 14 2007, 10:29AM
Firkragg wrote ...
I used LM2575 since I didn't need anything under 7V, but there are tons of usable regulators around:
I often dig my regulators out of old motherboards and computer trash, and they can be sampled too.
Cool thanks, I shall investigate my options.I used LM2575 since I didn't need anything under 7V, but there are tons of usable regulators around:
I often dig my regulators out of old motherboards and computer trash, and they can be sampled too.
Incidentally, I've come across some more AT PSU schematics on this page
- click on the power supply link.James
Re: PC Power Supply Hacking - 50V Output
thedatastream, Sat Jan 20 2007, 02:21PM
Webpage detailing the modifcation is now up
I'm looking at using a NatSemi LM3578 in a SEPIC configuration to provide my 12V rail.
James
thedatastream, Sat Jan 20 2007, 02:21PM
Webpage detailing the modifcation is now up
I'm looking at using a NatSemi LM3578
in a SEPIC configuration to provide my 12V rail.James
Re: PC Power Supply Hacking - 50V Output
uzzors2k, Sat Jan 20 2007, 06:08PM
Good job, and well documented! I'm tempted to give this a try again, seeing you were able to keep the short circuit protection intact. Too bad you didn't have an ATX, as modding the seperate 5v standby to 12v would be no problem.
uzzors2k, Sat Jan 20 2007, 06:08PM
Good job, and well documented! I'm tempted to give this a try again, seeing you were able to keep the short circuit protection intact. Too bad you didn't have an ATX, as modding the seperate 5v standby to 12v would be no problem.
Re: PC Power Supply Hacking - 50V Output
Dr. Shark, Wed Jan 24 2007, 12:03PM
Nice work! I have also spend considerable time hacking AT supplys, sorry I did not jump in any earlier. Well, as you have realised AT and ATX are almost identical other than the "extra stuff" in ATX.
I've still got one thing to add though: From my experience it is possible and feasible to rewind the power transformer. Breaking the core is a non-issue since most people have about 10 dead PSUs to pick replacements from, and once the core is open, rewinding is straighforward. Annoingly the primary is the outer layer, so it needs to be removed first, but it is also fairly short. After removing the 5V winding, there is a lot of space on the core for longer, thicker wire, so it worth doing.
Dr. Shark, Wed Jan 24 2007, 12:03PM
Nice work! I have also spend considerable time hacking AT supplys, sorry I did not jump in any earlier. Well, as you have realised AT and ATX are almost identical other than the "extra stuff" in ATX.
I've still got one thing to add though: From my experience it is possible and feasible to rewind the power transformer. Breaking the core is a non-issue since most people have about 10 dead PSUs to pick replacements from, and once the core is open, rewinding is straighforward. Annoingly the primary is the outer layer, so it needs to be removed first, but it is also fairly short. After removing the 5V winding, there is a lot of space on the core for longer, thicker wire, so it worth doing.
Re: PC Power Supply Hacking - 50V Output
Dr. Shark, Sun Jan 28 2007, 06:09PM
Whoops, I did it again!
Not much of a Hack really since all I did was take out all parts of the secondary side except for the 12V rail and rewind the power transformer. After a few seconds in boiling water it comes apart nicely. I removed the 5V windings and doubled the length of the 12V wire. I did not mess with the rectifiers and the fan is running off the full 28.7V - this should provide plenty of cooling
Dr. Shark, Sun Jan 28 2007, 06:09PM
Whoops, I did it again!
Not much of a Hack really since all I did was take out all parts of the secondary side except for the 12V rail and rewind the power transformer. After a few seconds in boiling water it comes apart nicely. I removed the 5V windings and doubled the length of the 12V wire. I did not mess with the rectifiers and the fan is running off the full 28.7V - this should provide plenty of cooling
Re: PC Power Supply Hacking - 50V Output
thedatastream, Mon Jan 29 2007, 08:00AM
Nicely done joe, you could always use a 7812 regulator to drive the fan...
BTW, I'm leaving the LM3578 based SEPIC 12V power supply for a later date, I've blown two chips due to overvoltage on the switch. I'm now going to have to re-think my psu design which will be detailed in a different thread.
Rgds
James
thedatastream, Mon Jan 29 2007, 08:00AM
Nicely done joe, you could always use a 7812 regulator to drive the fan...
BTW, I'm leaving the LM3578 based SEPIC 12V power supply for a later date, I've blown two chips due to overvoltage on the switch. I'm now going to have to re-think my psu design which will be detailed in a different thread.
Rgds
James
Re: PC Power Supply Hacking - 50V Output
Marko, Sat Feb 03 2007, 09:43PM
Hi joe and datastream
Using a 40V converter obivously isn't going to work well on former 12V rail you boosted to 50V.
My idea, from upper post, was to use a 5V rail, or even -5V as simpler buck-boost converter. From your site, I figured you removed all dides and stuff from all outputs.
Could you revive the -5V rail to run a buck-boost from it?
In worst case, you can always use a small transformer to power your TL494, fan and coil electronics.
It's not elite like switching converter but is very sturdy and robust.
I boiled my transformers for about half an hour but none of them wanted to budge.
Hot air gun is only method I found to work, but it torches a hell out of ferrite and it's no wonder that it just falls apart after... simply not worth effort.
If I needed to wind a transformer, I would just get a large enough flyback core, or, if I need specific cores, order some from cws.
Marko, Sat Feb 03 2007, 09:43PM
Hi joe and datastream
BTW, I'm leaving the LM3578 based SEPIC 12V power supply for a later date, I've blown two chips due to overvoltage on the switch. I'm now going to have to re-think my psu design which will be detailed in a different thread.
Using a 40V converter obivously isn't going to work well on former 12V rail you boosted to 50V.
My idea, from upper post, was to use a 5V rail, or even -5V as simpler buck-boost converter. From your site, I figured you removed all dides and stuff from all outputs.
Could you revive the -5V rail to run a buck-boost from it?
In worst case, you can always use a small transformer to power your TL494, fan and coil electronics.
It's not elite like switching converter but is very sturdy and robust.
I've still got one thing to add though: From my experience it is possible and feasible to rewind the power transformer. Breaking the core is a non-issue since most people have about 10 dead PSUs to pick replacements from, and once the core is open, rewinding is straighforward. Annoingly the primary is the outer layer, so it needs to be removed first, but it is also fairly short. After removing the 5V winding, there is a lot of space on the core for longer, thicker wire, so it worth doing.
I boiled my transformers for about half an hour but none of them wanted to budge.
Hot air gun is only method I found to work, but it torches a hell out of ferrite and it's no wonder that it just falls apart after... simply not worth effort.
If I needed to wind a transformer, I would just get a large enough flyback core, or, if I need specific cores, order some from cws.
Re: PC Power Supply Hacking - 50V Output
thedatastream, Mon Feb 26 2007, 05:03PM
I've changed the output rectification scheme as I found that two diodes can be used instead of four.
This grounding of one end of the transformer gets round the problem of trying to run the KA7500 control voltage from one of the transformer taps and having the ground return currents messed up causing the power supply to trip out at higher voltages.
It also gives me a 1/2 Vcc output fromt the old grounded centre tap of the transformer to run my SEPIC off for the control circuitry for my Titch tesla coil.
updates soon
James
thedatastream, Mon Feb 26 2007, 05:03PM
I've changed the output rectification scheme as I found that two diodes can be used instead of four.
This grounding of one end of the transformer gets round the problem of trying to run the KA7500 control voltage from one of the transformer taps and having the ground return currents messed up causing the power supply to trip out at higher voltages.
It also gives me a 1/2 Vcc output fromt the old grounded centre tap of the transformer to run my SEPIC off for the control circuitry for my Titch tesla coil.
updates soon
James
Re: PC Power Supply Hacking - 50V Output
thedatastream, Thu Mar 01 2007, 10:46PM
OK, the half wave rectification seems to have resolved a lot of the stability problems I had, but I now have a problem using this modified power supply to power my Titch SSTC.
I've tested Titch on a bench dc power supply (30V, 2A linear) with no issues but when using the modified PC supply, ramping the power up to more than what I estimate to be a maybe 10 watts (coil running interrupted at 50Hz, about 20% duty) the power supply cuts out in a similar fashion to when the output is shorted out unintentionally.
I've measured the output voltage - With 4 x 820uF, low impedance capacitors on the output, there is about 0.5V to 1V of ripple when the coil turns on. I haven't measured the ripple current, although I suspect that the pulsed currents are enough to trip whatever over current protection exists in the supply. I've tried decoupling the output with a 3300uF additional low ESR cap but to no effect.
To try:
* Increase decoupling and filtering using the above large capacitor and a big series inductor
* Find and either slug off the response or disable the over current protection circuit
Does anyone have any other suggestions of things to try?
Thanks
James
thedatastream, Thu Mar 01 2007, 10:46PM
OK, the half wave rectification seems to have resolved a lot of the stability problems I had, but I now have a problem using this modified power supply to power my Titch SSTC.
I've tested Titch on a bench dc power supply (30V, 2A linear) with no issues but when using the modified PC supply, ramping the power up to more than what I estimate to be a maybe 10 watts (coil running interrupted at 50Hz, about 20% duty) the power supply cuts out in a similar fashion to when the output is shorted out unintentionally.
I've measured the output voltage - With 4 x 820uF, low impedance capacitors on the output, there is about 0.5V to 1V of ripple when the coil turns on. I haven't measured the ripple current, although I suspect that the pulsed currents are enough to trip whatever over current protection exists in the supply. I've tried decoupling the output with a 3300uF additional low ESR cap but to no effect.
To try:
* Increase decoupling and filtering using the above large capacitor and a big series inductor
* Find and either slug off the response or disable the over current protection circuit
Does anyone have any other suggestions of things to try?
Thanks
James
Re: PC Power Supply Hacking - 50V Output
Steve Conner, Fri Mar 02 2007, 10:37AM
Yes, I'm not convinced the halfwave rectification is a good idea. It will put a high average DC current through the transformer and saturate it, which is probably what's tripping your overcurrent protection on the primary side.
The other hookup with the 4 diode full bridge is what higher voltage commercial SMPSs use. I just bought a 400W SMPS for an audio amp that delivers +/-80V, and it has the 4 diode setup. The centre tap of the transformer is grounded, and there are two output filter inductors, one on each output of the bridge.
If you use the 4 diodes and apply a filter inductor to the centre tap (ie replace D5 in your before schematic with an inductor) it should give you half the main supply voltage. You also need a filter inductor on the main +50v rail, but I assumed you left the original one in. Hooking the bridge output straight to a capacitor will cause high peak currents and trouble with the regulation.
Steve Conner, Fri Mar 02 2007, 10:37AM
Yes, I'm not convinced the halfwave rectification is a good idea. It will put a high average DC current through the transformer and saturate it, which is probably what's tripping your overcurrent protection on the primary side.
The other hookup with the 4 diode full bridge is what higher voltage commercial SMPSs use. I just bought a 400W SMPS for an audio amp that delivers +/-80V, and it has the 4 diode setup. The centre tap of the transformer is grounded, and there are two output filter inductors, one on each output of the bridge.
If you use the 4 diodes and apply a filter inductor to the centre tap (ie replace D5 in your before schematic with an inductor) it should give you half the main supply voltage. You also need a filter inductor on the main +50v rail, but I assumed you left the original one in. Hooking the bridge output straight to a capacitor will cause high peak currents and trouble with the regulation.
Re: PC Power Supply Hacking - 50V Output
Marko, Fri Mar 02 2007, 12:40PM
Lots of forward converters use halfwave rectification (with a free wheeling diode) and seem to work fine.
If only a single switch is used then there must be a tetriary winding with a diode wich rectifies the stored energy from the core back to supply rails and prevents saturation.
Computer PSU is also basically a forward converter except it uses a halfbridge, and emmiter-collector diodes are supposed to rectify the energy from 'dead' cycle and prevent saturation much in same way.
It may be misuse of halfbridge and it's qualities, max possible output power is reduced but it looks like it should work. Steve?
PS. just one more question, for Steve: in all time, only thing I haven't completely figured out yet is that yellow-toroidal output inductor thingie. It couples all rails strongly together but itself is very lossy, so it could have no real benefit in forward operation (real forward inductors are wound on ferrite rods and are in series with it).
?
Marko, Fri Mar 02 2007, 12:40PM
Yes, I'm not convinced the halfwave rectification is a good idea. It will put a high average DC current through the transformer and saturate it, which is probably what's tripping your overcurrent protection on the primary side.
Lots of forward converters use halfwave rectification (with a free wheeling diode) and seem to work fine.
If only a single switch is used then there must be a tetriary winding with a diode wich rectifies the stored energy from the core back to supply rails and prevents saturation.
Computer PSU is also basically a forward converter except it uses a halfbridge, and emmiter-collector diodes are supposed to rectify the energy from 'dead' cycle and prevent saturation much in same way.
It may be misuse of halfbridge and it's qualities, max possible output power is reduced but it looks like it should work. Steve?
PS. just one more question, for Steve: in all time, only thing I haven't completely figured out yet is that yellow-toroidal output inductor thingie. It couples all rails strongly together but itself is very lossy, so it could have no real benefit in forward operation (real forward inductors are wound on ferrite rods and are in series with it).
?
Re: PC Power Supply Hacking - 50V Output
Steve Conner, Fri Mar 02 2007, 01:17PM
Firkragg, you could be right. I'm confused now
The Astec SMPS I modified to power my ham rig was exactly that kind, it used a diagonal half-bridge (a la coilgun) feeding a transformer that fed a half-wave rectifier with freewheel diode identical to James's. I somehow assumed that its transformer would need an airgap, but when I think about it more, I guess it doesn't.
*edit* As far as I know, the yellow toroid thingie is a proper filter choke and would (does, even) function as a forward inductor. The multiple windings thing is just a cheap money saving trick. Because all the rails come off the same transformer, the voltage waveforms on them are identical in shape, so the same inductor can (sort of!) smooth them all, if you make the turns ratio on the inductor windings the same as on the transformer windings.
Steve Conner, Fri Mar 02 2007, 01:17PM
Firkragg, you could be right. I'm confused now
The Astec SMPS I modified to power my ham rig was exactly that kind, it used a diagonal half-bridge (a la coilgun) feeding a transformer that fed a half-wave rectifier with freewheel diode identical to James's. I somehow assumed that its transformer would need an airgap, but when I think about it more, I guess it doesn't.*edit* As far as I know, the yellow toroid thingie is a proper filter choke and would (does, even) function as a forward inductor. The multiple windings thing is just a cheap money saving trick. Because all the rails come off the same transformer, the voltage waveforms on them are identical in shape, so the same inductor can (sort of!) smooth them all, if you make the turns ratio on the inductor windings the same as on the transformer windings.
Re: PC Power Supply Hacking - 50V Output
Marko, Sat Mar 03 2007, 12:44AM
Em, hi guys...
Well, best I can think of that it is a simple overload occuring during the conduction cycle since only one cycle is used instead of two. If you're worried about that you can scope the halfbridge, current transformer and the output and see how it looks.
That's what I guessed at first, but existence of ''normal'' ferrite inductors in series with it is controversal for the low-cost theory. Nobody would put two different types of inductors if he wants to save money..? (as in schematic )
Yellow inductor is also quite lossy at these frequencies, but this probably only comes out at very high output currents. So meh..
Marko, Sat Mar 03 2007, 12:44AM
Em, hi guys...
Well, best I can think of that it is a simple overload occuring during the conduction cycle since only one cycle is used instead of two. If you're worried about that you can scope the halfbridge, current transformer and the output and see how it looks.
*edit* As far as I know, the yellow toroid thingie is a proper filter choke and would (does, even) function as a forward inductor. The multiple windings thing is just a cheap money saving trick. Because all the rails come off the same transformer, the voltage waveforms on them are identical in shape, so the same inductor can (sort of!) smooth them all, if you make the turns ratio on the inductor windings the same as on the transformer windings.
That's what I guessed at first, but existence of ''normal'' ferrite inductors in series with it is controversal for the low-cost theory. Nobody would put two different types of inductors if he wants to save money..? (as in schematic
)Yellow inductor is also quite lossy at these frequencies, but this probably only comes out at very high output currents. So meh..
Re: PC Power Supply Hacking - 50V Output
Steve Conner, Sat Mar 03 2007, 11:08AM
I'd guess that the two series inductors should have a capacitor to ground between them, which would make it a LCL filter. The schematic guy probably forgot it.
Steve Conner, Sat Mar 03 2007, 11:08AM
I'd guess that the two series inductors should have a capacitor to ground between them, which would make it a LCL filter. The schematic guy probably forgot it.
Re: PC Power Supply Hacking - 50V Output
Marko, Sat Mar 03 2007, 01:25PM
He didn't; what you say is true only for 5V rail, and schematic seems right compared to other schematics I have and real PCB. It's probably some kind of factory thinkerness I don't want to be bothered with, so thanks anyway.
Marko, Sat Mar 03 2007, 01:25PM
I'd guess that the two series inductors should have a capacitor to ground between them, which would make it a LCL filter. The schematic guy probably forgot it.
He didn't; what you say is true only for 5V rail, and schematic seems right compared to other schematics I have and real PCB. It's probably some kind of factory thinkerness I don't want to be bothered with, so thanks anyway.
Re: PC Power Supply Hacking - 50V Output
thedatastream, Sun Mar 04 2007, 10:13AM
The yellow coil is a powdered iron toroid which is used as a coupled choke. This aids cross regulation between the outputs when one output is loaded and the other isn't. Basically it is a current sharing choke.
From what I've seen in PC power supplies, they don't use an LCL, they use: transformer > diodes > coupled choke > single choke > large capacitor > maybe another capacitor or a preload resistor
I'm not sure that the output rectification makes a difference but I'll keep it up my sleeve if nothing else seems to do the job. Will have a tinker when I get back to work and see what we can do!
Thanks
James
thedatastream, Sun Mar 04 2007, 10:13AM
The yellow coil is a powdered iron toroid which is used as a coupled choke. This aids cross regulation between the outputs when one output is loaded and the other isn't. Basically it is a current sharing choke.
From what I've seen in PC power supplies, they don't use an LCL, they use: transformer > diodes > coupled choke > single choke > large capacitor > maybe another capacitor or a preload resistor
I'm not sure that the output rectification makes a difference but I'll keep it up my sleeve if nothing else seems to do the job. Will have a tinker when I get back to work and see what we can do!
Thanks
James
Re: PC Power Supply Hacking - 50V Output
Ken M., Sun Apr 08 2007, 03:10AM
Wow this info helps alot! Thanks guys! Well It worked great until I discovered my PSU HAD a pot that let me adjust 12v+ between 11-13v I replaced it with a bigger pot, found the short circuit protect transistor, PARTIALY disconnected it and could get 11-18v+ and -12v for 11-30v..at least till the golden rule of electronics was broken (I can't remember the name of the rule) "ALL Electronics run off smoke! IF you let the smoke out the device will no longer work!" So as a result the excesive +V split the transistor in 1/2.
Ken M., Sun Apr 08 2007, 03:10AM
Wow this info helps alot! Thanks guys! Well It worked great until I discovered my PSU HAD a pot that let me adjust 12v+ between 11-13v I replaced it with a bigger pot, found the short circuit protect transistor, PARTIALY disconnected it and could get 11-18v+ and -12v for 11-30v..at least till the golden rule of electronics was broken (I can't remember the name of the rule) "ALL Electronics run off smoke! IF you let the smoke out the device will no longer work!"
So as a result the excesive +V split the transistor in 1/2.Re: PC Power Supply Hacking - 50V Output
ragnar, Sun Apr 08 2007, 04:50AM
I hacked my first TL494 supply last week... it was too easy -- I just put a fixed resistor in series with the existing feedback on pin1, then replaced the smoothing capacitor on the output with one with a more suitable voltag rating. Voila, 18V at 5A, and they only cost me $20. Now I can run my IXDD414s from it.
Though in reality, I'd really much prefer to have 24V, the transformer starts squealing at 19V, so I think a rewind is in order.
^_^
ragnar, Sun Apr 08 2007, 04:50AM
I hacked my first TL494 supply last week... it was too easy -- I just put a fixed resistor in series with the existing feedback on pin1, then replaced the smoothing capacitor on the output with one with a more suitable voltag rating. Voila, 18V at 5A, and they only cost me $20. Now I can run my IXDD414s from it.
Though in reality, I'd really much prefer to have 24V, the transformer starts squealing at 19V, so I think a rewind is in order.
^_^
Re: PC Power Supply Hacking - 50V Output
uzzors2k, Sun Apr 08 2007, 09:43AM
Yeah, PSU modding is surprisingly simple. However to get any lifetime out of it, a rewind and lots of component replacement is necessary, which makes building your own smps from scratch worth the trouble.
uzzors2k, Sun Apr 08 2007, 09:43AM
Yeah, PSU modding is surprisingly simple. However to get any lifetime out of it, a rewind and lots of component replacement is necessary, which makes building your own smps from scratch worth the trouble.
Re: PC Power Supply Hacking - 50V Output
thedatastream, Mon Apr 09 2007, 01:22AM
My modification was performed with no rewinding of the transformer - only the choke had turns removed and the output rectification was changed.
Glad to see that this mod is coming in useful
thedatastream, Mon Apr 09 2007, 01:22AM
My modification was performed with no rewinding of the transformer - only the choke had turns removed and the output rectification was changed.
Glad to see that this mod is coming in useful
Re: PC Power Supply Hacking - 50V Output
ragnar, Mon Apr 09 2007, 05:45AM
I hope I don't sound too thick when I ask: why did you take turns off the choke and alter the rectification? Did you put a voltage doubler on the rectifier?
ragnar, Mon Apr 09 2007, 05:45AM
I hope I don't sound too thick when I ask: why did you take turns off the choke and alter the rectification? Did you put a voltage doubler on the rectifier?
Re: PC Power Supply Hacking - 50V Output
thedatastream, Tue Apr 10 2007, 01:57PM
If you look at you can see that the rectification has changed so that it uses each end of the transformer outputs instead of the centre tap. I didn't use a voltage doubler
I didn't remove turns from the choke, I removed windings - specifically the 5V, -5V and -12V ones. I used the 12V winding unmodified as the output choke.
Webpage
thedatastream, Tue Apr 10 2007, 01:57PM
wrote ...
I hope I don't sound too thick when I ask: why did you take turns off the choke and alter the rectification? Did you put a voltage doubler on the rectifier?
I hope I don't sound too thick when I ask: why did you take turns off the choke and alter the rectification? Did you put a voltage doubler on the rectifier?
If you look at
you can see that the rectification has changed so that it uses each end of the transformer outputs instead of the centre tap. I didn't use a voltage doublerI didn't remove turns from the choke, I removed windings - specifically the 5V, -5V and -12V ones. I used the 12V winding unmodified as the output choke.
Webpage
Re: PC Power Supply Hacking - 50V Output
Steve Conner, Tue Apr 10 2007, 03:00PM
Hi James, all,
Richie Burnett sent me the following in relation to Firkragg's question about using inductors in series.
Steve Conner, Tue Apr 10 2007, 03:00PM
Hi James, all,
Richie Burnett sent me the following in relation to Firkragg's question about using inductors in series.
richie burnett wrote ...
Those yellow (type-26) iron powder cores in PC PSUs are the main "buck
choke" and do indeed set the ripple current for the supply. The reason for
the little ferrite chokes that follow is that the main iron-powder core is
usually wound with multiple layers and multiple windings. (Sharing one core
improves cross-regulation of outputs because feedback is only taken from the
highest power rated output.) Interwinding capacitance in this "monster
choke" couples the fast switching edges through to the output capacitor,
particularly at maximum line voltage. Therefore it is followed by a single
layer ferrite "stick" inductor to remove high frequency spikes that would
drop voltage across the ESR of the output electrolytic. They could have
made a T-filter by placing a capacitor to ground between the two inductors,
but it's effectiveness is limited because of the self capacitance of the
buck-choke and any ESR of the capacitor chosen for this position. It also
introduces another pole in the transfer function which makes closing the
voltage loop very difficult.
This "two inductors in series" technique is quite common in active PFC
applications too. Most App Notes and text books say it's essential to wind
boost PFC chokes with a single layer to minimise end-to-end capacitance -
otherwise you get a lot of HF hash fed straight back from the switching node
into the supply. This is true, but it is certainly more compact and
cost-effective to make more use of the winding window and reduce HF hash
with a ferrite choke in series. (Or use ripple steering at high powers.)
Those yellow (type-26) iron powder cores in PC PSUs are the main "buck
choke" and do indeed set the ripple current for the supply. The reason for
the little ferrite chokes that follow is that the main iron-powder core is
usually wound with multiple layers and multiple windings. (Sharing one core
improves cross-regulation of outputs because feedback is only taken from the
highest power rated output.) Interwinding capacitance in this "monster
choke" couples the fast switching edges through to the output capacitor,
particularly at maximum line voltage. Therefore it is followed by a single
layer ferrite "stick" inductor to remove high frequency spikes that would
drop voltage across the ESR of the output electrolytic. They could have
made a T-filter by placing a capacitor to ground between the two inductors,
but it's effectiveness is limited because of the self capacitance of the
buck-choke and any ESR of the capacitor chosen for this position. It also
introduces another pole in the transfer function which makes closing the
voltage loop very difficult.
This "two inductors in series" technique is quite common in active PFC
applications too. Most App Notes and text books say it's essential to wind
boost PFC chokes with a single layer to minimise end-to-end capacitance -
otherwise you get a lot of HF hash fed straight back from the switching node
into the supply. This is true, but it is certainly more compact and
cost-effective to make more use of the winding window and reduce HF hash
with a ferrite choke in series. (Or use ripple steering at high powers.)
Re: PC Power Supply Hacking - 50V Output
Marko, Tue Apr 10 2007, 10:41PM
Hi steve
Well this explains a lot (thanks to richie)...
I didn't quite understand
this part but I guess it's not important.
Marko, Tue Apr 10 2007, 10:41PM
Hi steve
Well this explains a lot (thanks to richie)...
I didn't quite understand
It also
introduces another pole in the transfer function which makes closing the
voltage loop very difficult.
this part but I guess it's not important.
Re: PC Power Supply Hacking - 50V Output
thedatastream, Tue Apr 17 2007, 06:47AM
I received this email from Richie the other day
It sounds like adjusting (or as suggested above, delaying) the current limit would allow this hacked supply to work with a pulsed SSTC application. However, since I've got bigger TCs to fry I'm not going to pursue it further at this time for the purposes of powering Titch.
Thanks Richie :)
James
thedatastream, Tue Apr 17 2007, 06:47AM
I received this email from Richie the other day
wrote ...
Hi James,
I just noticed this post from you on 4HV, and have some input if you are
interested...
> I've tested Titch on a bench dc power supply (30V, 2A linear) with no
issues but when using the modified PC supply, ramping the power up to more
than what I estimate to be a maybe 10 watts (coil running interrupted at
50Hz, about 20% duty) the power supply cuts out in a similar fashion to when
the output is shorted out unintentionally.
This is common behaviour when you try to use a voltage-mode controlled SMPSU
to charge a capacitor in pulsed-power applications. I get exactly the same
behaviour if I try to use that Xantrex 600V-1kW supply to charge a cap bank
for a pulsed SSTC. It trips out on over-current when the SSTC is fired up -
even at low power.
> I've measured the output voltage - With 4 x 820uF, low impedance
capacitors on the output, there is about 0.5V to 1V of ripple when the coil
turns on.
That is the cause of the problem. Once the SSTC is fired and it discharges
the capacitors by a volt or so, the output of the PC power supply *DOES*
essentially see a short circuit. The supply is connected to a bunch of big
capacitors that are pulling its output down to a potential lower than its
output set-point. This is what trips the over current protection and causes
the supply to shut down.
> I haven't measured the ripple current, although I suspect that the pulsed
currents are enough to trip whatever over current protection exists in the
supply.
I would bet that the current out of the PSU skyrockets and hits the
over-current trip point when the capacitors are discharged.
In order to smoothly charge capacitors in a pulsed application you really
need to use a power supply with a constant-current output. Or at least use
a power supply that limits to a constant-current line when overloaded
instead of folding back or shutting down. A constant voltage power supply
that uses current mode control for the inner-loop and an outer voltage loop
can be made to work in the way that you want. This is because peak current
is still bounded even if the output voltage goes out of spec and the voltage
error amplifier is saturated. Just watch out for the voltage overshooting
when it charges the capacitors back up to the voltage set-point.
Unfortunately most cheap shitty PC power supplies use archaic topologies,
ie voltage-mode control with peak-current limiting via hiccup mode. In a PC
power supply s/c protection is important to prevent destruction when someone
accidentally shorts the output. Constant current limiting is not important,
and is probably undesirable because it would burn up PCB traces on
motherboards with shorted peripherals etc. Unfortunately there isn't much
you can do to alter the way the current limiting works. If you delay the
onset of current limiting the supply will most likely blow-up under a
short-circuit test, so this isn't a wise move. The best solution is to
build a current-mode buck or forward converter with average current mode
control or even a small flyback converter if the power level is below 100W.
I hope this helps,
-Richie
Hi James,
I just noticed this post from you on 4HV, and have some input if you are
interested...
> I've tested Titch on a bench dc power supply (30V, 2A linear) with no
issues but when using the modified PC supply, ramping the power up to more
than what I estimate to be a maybe 10 watts (coil running interrupted at
50Hz, about 20% duty) the power supply cuts out in a similar fashion to when
the output is shorted out unintentionally.
This is common behaviour when you try to use a voltage-mode controlled SMPSU
to charge a capacitor in pulsed-power applications. I get exactly the same
behaviour if I try to use that Xantrex 600V-1kW supply to charge a cap bank
for a pulsed SSTC. It trips out on over-current when the SSTC is fired up -
even at low power.
> I've measured the output voltage - With 4 x 820uF, low impedance
capacitors on the output, there is about 0.5V to 1V of ripple when the coil
turns on.
That is the cause of the problem. Once the SSTC is fired and it discharges
the capacitors by a volt or so, the output of the PC power supply *DOES*
essentially see a short circuit. The supply is connected to a bunch of big
capacitors that are pulling its output down to a potential lower than its
output set-point. This is what trips the over current protection and causes
the supply to shut down.
> I haven't measured the ripple current, although I suspect that the pulsed
currents are enough to trip whatever over current protection exists in the
supply.
I would bet that the current out of the PSU skyrockets and hits the
over-current trip point when the capacitors are discharged.
In order to smoothly charge capacitors in a pulsed application you really
need to use a power supply with a constant-current output. Or at least use
a power supply that limits to a constant-current line when overloaded
instead of folding back or shutting down. A constant voltage power supply
that uses current mode control for the inner-loop and an outer voltage loop
can be made to work in the way that you want. This is because peak current
is still bounded even if the output voltage goes out of spec and the voltage
error amplifier is saturated. Just watch out for the voltage overshooting
when it charges the capacitors back up to the voltage set-point.
Unfortunately most cheap shitty PC power supplies use archaic topologies,
ie voltage-mode control with peak-current limiting via hiccup mode. In a PC
power supply s/c protection is important to prevent destruction when someone
accidentally shorts the output. Constant current limiting is not important,
and is probably undesirable because it would burn up PCB traces on
motherboards with shorted peripherals etc. Unfortunately there isn't much
you can do to alter the way the current limiting works. If you delay the
onset of current limiting the supply will most likely blow-up under a
short-circuit test, so this isn't a wise move. The best solution is to
build a current-mode buck or forward converter with average current mode
control or even a small flyback converter if the power level is below 100W.
I hope this helps,
-Richie
It sounds like adjusting (or as suggested above, delaying) the current limit would allow this hacked supply to work with a pulsed SSTC application. However, since I've got bigger TCs to fry I'm not going to pursue it further at this time for the purposes of powering Titch.
Thanks Richie :)
James
Re: PC Power Supply Hacking - 50V Output
Steve Conner, Tue Apr 17 2007, 10:11AM
I have modified ordinary SMPS to have a constant-current output before. It's not perfect, but it might work if you're interested.
I just hooked up a sense amplifier, current set point control, and error amp/integrator, and hooked the resulting mess to the feedback pin of the TL494 via a diode. If the current goes higher than the setpoint, the integrator swings over, the diode conducts, and the new loop takes over from the TL494's internal error amps.
The resulting gadget has voltage and current knobs and meters on the front, and behaviour similar to a linear bench power supply, but not nearly as good. To get stability, I had to make the integrator very slow, so when the output is shorted it barfs its full 8 amps for a fraction of a second and I hear a burst of the fizzing sound that the SMPS's original protection circuit makes. This is nothing like the "proper" way to make a constant-current supply, and I expect Richie to throw rocks at me soon
Nevertheless, it turned out very useful as a general purpose 30V, 7A power supply and earned a permanent place on my bench. It's great for charging batteries as it's more efficient than a linear unit, and ripple has never been a problem. Although I did start with a good SMPS that originally powered a broadcast TV camera.
Steve Conner, Tue Apr 17 2007, 10:11AM
I have modified ordinary SMPS to have a constant-current output before. It's not perfect, but it might work if you're interested.
I just hooked up a sense amplifier, current set point control, and error amp/integrator, and hooked the resulting mess to the feedback pin of the TL494 via a diode. If the current goes higher than the setpoint, the integrator swings over, the diode conducts, and the new loop takes over from the TL494's internal error amps.
The resulting gadget has voltage and current knobs and meters on the front, and behaviour similar to a linear bench power supply, but not nearly as good. To get stability, I had to make the integrator very slow, so when the output is shorted it barfs its full 8 amps for a fraction of a second and I hear a burst of the fizzing sound that the SMPS's original protection circuit makes. This is nothing like the "proper" way to make a constant-current supply, and I expect Richie to throw rocks at me soon
Nevertheless, it turned out very useful as a general purpose 30V, 7A power supply and earned a permanent place on my bench. It's great for charging batteries as it's more efficient than a linear unit, and ripple has never been a problem. Although I did start with a good SMPS that originally powered a broadcast TV camera.
Re: PC Power Supply Hacking - 50V Output
Dago, Thu Aug 16 2007, 06:36PM
I tried modding an ATX-psu for higher voltages but I cant really get it to work.
I dremeled off all the connections to the main transformer secondaries (including center tap ground) and put up a full-wave (made of schottky diodes) rectifier + smoothing cap on the 12V winding. The feedback is taken from the cap to the TL494 pin 1 (which has a trimmer pot + resistor in parallel going to the ground) via a pot + 1k resistor. I also disconnected two diodes coming of the feedback line to... somewhere (not sure what was their purpose) but I presume atleast the other one was the "standby" powersupply override diode.
The PSU has some circuitry I'm not 100% clear on what its doing there, especially a LS168 BCD bidirectional counter chip next to the TL494. The power good signal wire goes there so I guess it has something to do with it. Two of its flip-flop outputs are connected to the TL494 pins 15, 14, 13. Any idea what its doing?
Well the thing didnt work at all with the full-wave rectifier (not 100% sure why... but possibly an already blown diode), but with a single rectifier diode it worked. But the thing is that the PSU oscillates like mad and makes a fairly loud screeching/sizzling noise and the TL494 output is a fuzzy mess on the scope screen and the FETs heat up like mad but the output works, 13-30V adjustable, not sure about the current but a 100W 10ohm power resistor got pretty hot pretty fast.
What can be done for the oscillations?
Dago, Thu Aug 16 2007, 06:36PM
I tried modding an ATX-psu for higher voltages but I cant really get it to work.
I dremeled off all the connections to the main transformer secondaries (including center tap ground) and put up a full-wave (made of schottky diodes) rectifier + smoothing cap on the 12V winding. The feedback is taken from the cap to the TL494 pin 1 (which has a trimmer pot + resistor in parallel going to the ground) via a pot + 1k resistor. I also disconnected two diodes coming of the feedback line to... somewhere (not sure what was their purpose) but I presume atleast the other one was the "standby" powersupply override diode.
The PSU has some circuitry I'm not 100% clear on what its doing there, especially a LS168 BCD bidirectional counter chip next to the TL494. The power good signal wire goes there so I guess it has something to do with it. Two of its flip-flop outputs are connected to the TL494 pins 15, 14, 13. Any idea what its doing?
Well the thing didnt work at all with the full-wave rectifier (not 100% sure why... but possibly an already blown diode), but with a single rectifier diode it worked. But the thing is that the PSU oscillates like mad and makes a fairly loud screeching/sizzling noise and the TL494 output is a fuzzy mess on the scope screen and the FETs heat up like mad but the output works, 13-30V adjustable, not sure about the current but a 100W 10ohm power resistor got pretty hot pretty fast.
What can be done for the oscillations?
Re: PC Power Supply Hacking - 50V Output
Marko, Thu Aug 16 2007, 07:19PM
You did not use a buck inductor? Huh?
Marko, Thu Aug 16 2007, 07:19PM
I dremeled off all the connections to the main transformer secondaries (including center tap ground) and put up a full-wave (made of schottky diodes) rectifier + smoothing cap on the 12V winding. The feedback is taken from the cap to the TL494 pin 1
You did not use a buck inductor? Huh?
Re: PC Power Supply Hacking - 50V Output
thedatastream, Thu Aug 16 2007, 07:56PM
Yeah, you should really use the toroidal output inductor that came free with your hacked power supply. This should be between the rectifier diodes and the output filtering capacitor.
As to the oscillations, I found that I got oscillation if I didn't earth my secondary ground. Also, if your output capacitor is too small (or too large even) you can get oscillation fom that too.
James
thedatastream, Thu Aug 16 2007, 07:56PM
Marko wrote ...
You did not use a buck inductor? Huh?
I dremeled off all the connections to the main transformer secondaries (including center tap ground) and put up a full-wave (made of schottky diodes) rectifier + smoothing cap on the 12V winding. The feedback is taken from the cap to the TL494 pin 1
You did not use a buck inductor? Huh?
Yeah, you should really use the toroidal output inductor that came free with your hacked power supply. This should be between the rectifier diodes and the output filtering capacitor.
As to the oscillations, I found that I got oscillation if I didn't earth my secondary ground. Also, if your output capacitor is too small (or too large even) you can get oscillation fom that too.
James
Re: PC Power Supply Hacking - 50V Output
Dago, Fri Aug 17 2007, 03:42AM
Ah I didn't know the choke had other use than just for filtering.
I'll have to try the grounding too but I'm hoping to series these overclocked PSUs so I'm trying to avoid it.
Dago, Fri Aug 17 2007, 03:42AM
Marko wrote ...
You did not use a buck inductor? Huh?
I dremeled off all the connections to the main transformer secondaries (including center tap ground) and put up a full-wave (made of schottky diodes) rectifier + smoothing cap on the 12V winding. The feedback is taken from the cap to the TL494 pin 1
You did not use a buck inductor? Huh?
Ah I didn't know the choke had other use than just for filtering.
I'll have to try the grounding too but I'm hoping to series these overclocked PSUs so I'm trying to avoid it.
Re: PC Power Supply Hacking - 50V Output
thedatastream, Fri Aug 17 2007, 07:03AM
You may not be able to avoid it as the PCB corners have exposed copper on them to ground to the PEMs (threaded standoffs) on the chassis bottom. You could isolate these if you wanted to with some insulating washers.
Don't even think about disconnecting the earth wire to float the case.
James
thedatastream, Fri Aug 17 2007, 07:03AM
Dago wrote ...
I'll have to try the grounding too but I'm hoping to series these overclocked PSUs so I'm trying to avoid it.
I'll have to try the grounding too but I'm hoping to series these overclocked PSUs so I'm trying to avoid it.
You may not be able to avoid it as the PCB corners have exposed copper on them to ground to the PEMs (threaded standoffs) on the chassis bottom. You could isolate these if you wanted to with some insulating washers.
Don't even think about disconnecting the earth wire to float the case.
James
Re: PC Power Supply Hacking - 50V Output
Ken M., Sat Mar 28 2009, 11:16PM
Is it possible to take a standard atx\at psu and connect the +5 and the +12 in parallel with two diodes to prevent the psu from shuting down but allow more current?
Ken M., Sat Mar 28 2009, 11:16PM
Is it possible to take a standard atx\at psu and connect the +5 and the +12 in parallel with two diodes to prevent the psu from shuting down but allow more current?
Re: PC Power Supply Hacking - 50V Output
Electroholic, Sun Mar 29 2009, 03:38AM
by definition you can't put two voltage sources in parallel. It does depends on the construction of your psu, but my guess is it will blow up. The question is tho, why do you want to do it?
Electroholic, Sun Mar 29 2009, 03:38AM
by definition you can't put two voltage sources in parallel. It does depends on the construction of your psu, but my guess is it will blow up. The question is tho, why do you want to do it?
Re: PC Power Supply Hacking - 50V Output
Ken M., Sun Mar 29 2009, 03:20PM
I need a 12-24v 400w+ power supply for a new r\c battery pack charger, alot the users on the r\c forums say to just use the 12v line and ground but the problem is that in order to get at least a 400w 12v line one would either need to get a server psu or 650W+ pc psu, where as I knew that if you tried to use te 5v line and the 12v line in parralel it would shut down the psu since the voltage difference would make the 5v line look like a ground to the 12v line and draw too many amps there by tripping the protection circuits, but I tried to see if I could at least tie the 5v and 12v lines together with some diodes, not realy a current or power test but just to see if they could be connected without the psu shuting down. It let me power it on and I was 11.38v which is about right considering the vdrop of a diode, but I just wanted to see what thoughts you all had.
Ken M., Sun Mar 29 2009, 03:20PM
I need a 12-24v 400w+ power supply for a new r\c battery pack charger, alot the users on the r\c forums say to just use the 12v line and ground but the problem is that in order to get at least a 400w 12v line one would either need to get a server psu or 650W+ pc psu, where as I knew that if you tried to use te 5v line and the 12v line in parralel it would shut down the psu since the voltage difference would make the 5v line look like a ground to the 12v line and draw too many amps there by tripping the protection circuits, but I tried to see if I could at least tie the 5v and 12v lines together with some diodes, not realy a current or power test but just to see if they could be connected without the psu shuting down. It let me power it on and I was 11.38v which is about right considering the vdrop of a diode, but I just wanted to see what thoughts you all had.
Re: PC Power Supply Hacking - 50V Output
uzzors2k, Sun Mar 29 2009, 03:44PM
It works, but the diode on the 5V line will be reverse biased until the 12V line drops to 5V. So you can't squeeze any extra power out that way, because the 5V line will never contribute with any current.
uzzors2k, Sun Mar 29 2009, 03:44PM
It works, but the diode on the 5V line will be reverse biased until the 12V line drops to 5V. So you can't squeeze any extra power out that way, because the 5V line will never contribute with any current.
Re: PC Power Supply Hacking - 50V Output
Electroholic, Sun Mar 29 2009, 04:19PM
Well, if the 5V section is half wave rectified, I would probably turn then into full wave first and then trim it up by playing with the feedback network.
Electroholic, Sun Mar 29 2009, 04:19PM
Well, if the 5V section is half wave rectified, I would probably turn then into full wave first and then trim it up by playing with the feedback network.
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