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Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
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.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
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.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
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
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
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.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
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.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
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.
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
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?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
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). ?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
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.
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PC Power Supply Hacking - 50V Output
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.