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Registered Member #8743
Joined: Thu Dec 13 2012, 04:18PM
Location:
Posts: 12
Today I tried to get first light on my first SSTC and got a blown up bridge instead.
I'm using Polonium-210's driver with Steve Ward's mini-SSTC's interrupter and bridge. MOSFETs are IRFP260Ns from feebay, GDT is 16 (or 17, depending on how you count it) turns of stranded CAT5 wire on a B64290L0618X830 core. Voltage divider caps are EPCOS MKT .68uF 250V. Bus uses a single 470uF 250V (measured 420uF) cap and a 35A rectifier. I'm also using a small 12V/2A supply instead of the transformer+rectifier+lm7812.
Secondary is 3"x7" (8.5" form) with 30AWG wire and a 2.5"x8" toroid. The toroid was made out of a swimming pool foam "stick" covered with thick tin foil - the tin foil has a lot of wrinkles and edges but I couldn't do better with what I had. A small needle thingy serves as breakout point. The primary is wound on top of the secondary, with a rolled up thick plastic sheet underneath. I used stranded 2.5mm^2 wire.
My house has no mains ground (all outlets have 2 pins) and it'd be hard to make a proper ground, which is why I used a tin foil counterpoise connected to the secondary base. I did test the connection and it seemed fine (55Ohms from counterpoise to toroid).
First I tried to run it with 6 primary turns, all I noticed was the sound of the bridge fan (used a CPU heatsink) and a "tec" sound at random times, which also seemed to come from the bridge. No breakout or (noticeable) corona anywhere. Ran for about 30s and the bridge got warm.
Then I tried with 5 primary turns and got a CFL lamp, which did light up some 40cm away. Still no breakout and the bridge got slightly hot.
The last time it ran with 4 turns, with the same result as before. After about 10s there was a bang, some sparks flew out of the bridge and I pulled the cord.
The bridge was very hot, after it cooled down I noticed the positive side MOSFET had blown on the right side. The source pin was gone and there was a hole on the sil pad, with a tiny black "hole" left on the heatsink. That MOSFET's gate resistor's coat was cracked even though it was on the other side of the transistor.
First I thought the culprit was my poor soldering on the bridge. Those thick copper wires don't seem to hold the solder at all, which made me think one of them must have gotten lose.
After a closer look I'm suspecting either the gate resistor or the mosfet shorted to the heatsink... On all pictures I've seen they blew on the front, while this one blew on the side.
I'm including pictures of the bridge, the whole setup and the blown up bridge.
Any help would be appreciated, specially: -Why it blew up and what can I do to avoid it blowing up again -Why there was no breakout before it blew up -Any tricks to solder the bridge properly (note that I only have an el cheapo soldering iron) -Can I reuse any of the bridge components (MUR860's, sil pads, caps)
Thanks and let me know if you guys need more info about the coil!
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Okay, first things first - SAFETY.
You need a good ground for safety - period. Either try to get a properly grounded outlet or make a good earth ground using a ground rod etc... You need to ground your enclosure, heatsink, etc... to earth ground for safety.
And it could be the counterpoise giving you issues. I know they can be tricky to do properly.
Are there no earth ground connections in your house?
Registered Member #8743
Joined: Thu Dec 13 2012, 04:18PM
Location:
Posts: 12
I agree safety must come first but unfortunately my house has no ground connections... the house dates back to the 50's and grounding wasn't a standard practice here in Brazil until much later. We will eventually have to rewire the whole house mains but that will be expensive and will most likely not happen this year.
I suppose I could try to DIY a ground rod but I won't be able to get it very deep (1.5m if I get lucky) and it'll be at least 10m away from the coil. I've heard that a distant ground is even worse than a counterpoise but I'm not really sure..
Secondary side of GDT's connections are too long, shorten them up as much as possible.
The distance between the bridge and the primary is far too long, shorten it up as much as possible as well.
I don't see any gate protection zeners, you might want to add them to eliminate any gate side issues.
Spin your fets around so their leads and the caps extend off the heatsink, not towards the center of it. You see the black char on the heatsinks? That indicates there was likely a flashover between it and something else. The entire high side winding of the GDT and everything connected to it (high side fet drain, resistor, zeners, fet source, etc) is at DC bus + rail potential and needs to be properly insulated for that voltage.
As for reusing components; remove and check the MUR diodes with a simple circuit to see if they're any good. I'm willing to bet they're fine as long as they are not shorted or blown open. The sil pads can be cleaned with rubbing alcohol and reused if they are not physically damaged. Check the caps for a short and for capacitance if you can, if they're fine then reuse them.
Edit: I just noticed you're using a somewhat long length of wire between the secondary LV end and the counterpoise. Place the secondary right on the aluminium foil, no box in between, and use a larger section of foil.
As for counterpoise vs. long distant ground, there are mixed results. My house has no earth ground either. Or should I say it had none before I put my own ground rod array in and ran a 6ga cable to it. There is about 45ft of wire between my ground rods and my coil, and I thoroughly tested using counterpoises but always found output was greatly reduced compared to using the long wire ground. So that is what I use.
Registered Member #8743
Joined: Thu Dec 13 2012, 04:18PM
Location:
Posts: 12
Ok, bridge construction looks alright.
wrote ... Secondary side of GDT's connections are too long, shorten them up as much as possible.
The distance between the bridge and the primary is far too long, shorten it up as much as possible as well.
Will do. As for the primary, to shorten the cable I'll have to define the amount of turns and won't be able to increase it. How many turns should I leave?
wrote ... I don't see any gate protection zeners, you might want to add them to eliminate any gate side issues.
I used no Zeners, should I use 15V "back to back"? Will order them today, seeing Ebay is my only source for components it'll take some 2 months to get here... I'll have to try first light again before that :P
wrote ... Spin your fets around so their leads and the caps extend off the heatsink, not towards the center of it. You see the black char on the heatsinks? That indicates there was likely a flashover between it and something else. The entire high side winding of the GDT and everything connected to it (high side fet drain, resistor, zeners, fet source, etc) is at DC bus + rail potential and needs to be properly insulated for that voltage.
Oh, I thought having them towards the center would allow the caps to benefit from the heatsink as well. Will spin it when I rebuild the bridge. The FET was insulated with a pad, unless that failed the only things that could have possibly shorted are the gate resistors or some loose solder.
wrote ... As for reusing components; remove and check the MUR diodes with a simple circuit to see if they're any good. I'm willing to bet they're fine as long as they are not shorted or blown open. The sil pads can be cleaned with rubbing alcohol and reused if they are not physically damaged. Check the caps for a short and for capacitance if you can, if they're fine then reuse them.
Managed to save one of the sil pads with isopropyl alcohol, the other one has a hole in it. I only have two more so I'd better make it work next time ;) Will check the diodes and caps as well.
wrote ... Edit: I just noticed you're using a somewhat long length of wire between the secondary LV end and the counterpoise. Place the secondary right on the aluminium foil, no box in between, and use a larger section of foil.
Will do! Do you thik twice as much foil would be fine? Can I place the other components on top of the foil with something to insulate?
wrote ... As for counterpoise vs. long distant ground, there are mixed results. My house has no earth ground either. Or should I say it had none before I put my own ground rod array in and ran a 6ga cable to it. There is about 45ft of wire between my ground rods and my coil, and I thoroughly tested using counterpoises but always found output was greatly reduced compared to using the long wire ground. So that is what I use.
Any tips on how to place/wire the rods?
wrote ... 1.5m ground will suffice. Dont worry about how far the coil will be away from grounding, but make it as short as possible.
Double check your phasing of the GDT
Cool, I think I'll try to place a rod!
I'm pretty sure the phasing is correct. After winding I left 3 markers on one side before twisting the pairs, as per the oneTesla manual. Then I connected the "+" phase thru a 0.1uF monolithic cap to the UCC37322 and the "-" phase to the UCC37321. The high FET got the "+" phase through a 5.1ohm resistor to the gate while the low FET got the "-" phase through the resistor to the gate.
Primary turns: In a SSTC the number of turns determines the coupling factor (you want it very high) and the current limiting impedance of the primary.
If you want to design for a certain load current for your TC you can use Ohms Law (I=V/R) and Inductive Reactance (XL= 2piFL) formulae to determine what impedance you need to draw a certain amount of current at whatever voltage your bridge will run on. I have had good results with 10t of 10ga on a 4" PVC form, but my coils always draw near 15A from 120v mains. If you did the same thing on 240V mains you would be drawing near 30A, so that would not be good. So first determine what current your bridge and breaker will be very happy at and work from there. More turns = more inductance = less current draw and more coupling.
Gate Zeners: I always use them. I find they're very valuable for protecting the fet gates, which seem to blow a lot easier than the DS side. Yep, 15v or 20v back to back (one diode will break down via zener voltage and the other will be forward biased when that happens, so you get Vz + Vf as the clamping voltage in either direction). They aren't 100% necessary, they just eliminate another source of failure. If your GDT and its secondary side terminations are good you should be fine without them.
I've never had any heating of my caps, even at considerably high RMS currents. If you pick good caps it should not be an issue. My bet is the gate resistor might have arc'd to the heatsink, swinging everything off the heatsink edge should take care of this too.
The larger the counterpoise the better. I wouldn't put any other components on the counterpoise as this can cause more problems than not. If your primary is exactly at the base of the secondary then in theory it should be at very low voltage in relation to earth, but this is not always the case. If you put the primary in the center of the secondary then both ends become the HV output. This is called a bipolar coil. So, just because the "LV end" is supposed to be near ground, doesn't mean it isn't floating at some voltage higher than it. Another reason why I don't like using counterpoises.
Ground rods: as many as you can do, as deep as you can do. Run individual wires from each rod to a central rod in the array, and then your main connection to that central node. This avoids ground loops and gives excellent conductivity.
GDT phasing: Primary side phasing errors can cause a failure to oscillate properly but generally do not blow up the bridge (at least mine has never failed from it). Just recently when changing drivers I had my primary side of GDT phased wrong. Switched it around and it was fine after. I did hear some clicking with it wrongly phased, not sure if it was just random though. Secondary side phasing errors (you want the low side fet to be 180deg out of phase of the high side) will cause cross-conduction where both fets are ON at the same time. This causes the DC bus to be shorted out by the fets, which usually results in them exploding. The easy visual way to check is if you hold the GDT so you're looking down on it's thin axis (perpendicular to the hole) the wire for the high side fet's gate should come from the top and the high side fet's source from the bottom. The low side's gate should come from the bottom and its source from the top.
Registered Member #3964
Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332
"Then I tried with 5 primary turns and got a CFL lamp, which did light up some 40cm away. Still no breakout and the bridge got slightly hot.
The last time it ran with 4 turns, with the same result as before. After about 10s there was a bang, some sparks flew out of the bridge and I pulled the cord."
-at what voltage? if @ 40% of max Vinput then, seems to me these following causes: 1. phasing of the transformers. (if not CT, then GDT, or Primary polarity - do test all combination to scrutinize phasing) -since you can light up some thing wirelessly, this means that you have power out, but not enough for corona, (i typically hear the ticking sound coming from my variac - or when 1 of my fet is already leaking(damaged) for some reason (e.g. full bridge, tank capacitor is problematic, and/or capacitor for filtering the supply have internal issues - seen on a tester good but when running a load/coil it causes unwanted spikes on the rails, when i replaced it, the coil's great.)
2. GDT driver, even when using UCCs, sometimes it bugs my design and will not work, i ended up using (level shifter circuit then to S.Wards UD gate driver output stage with P/N MOSFETs, works very well for me and it is very robust! - even work up to high gate C like CM300 -24H, of course i tried it with small fets - tuned to 400KHz which means fast and powerful driver.
3. ground (although many will get angry when i say this, but i ran my SSTC without grounding - well this is because it is very small, and i planned to play it whenever i want(sort of a demo), just plug it. output of 4 inch spark) , my grounding is the negative rails of my logic circuit plus the ATX power supply casing i removed from my old computer (used as a overall casing of my circuits) ., BUT do ground ur system for better system - not to mention, sparks are larger when grounded reach 5 inch plasma.
If your electronics are all good (driver circuit, MOSFETs are good, GDT material/phasing are correct, capacitor on bridge are good and correctly installed, bypass caps/TVS are intact closed to fets) then the solution you need to try is number 1 & 3.
Registered Member #8743
Joined: Thu Dec 13 2012, 04:18PM
Location:
Posts: 12
@Sig:
wrote ... Primary turns: In a SSTC the number of turns determines the coupling factor (you want it very high) and the current limiting impedance of the primary.
If you want to design for a certain load current for your TC you can use Ohms Law (I=V/R) and Inductive Reactance (XL= 2piFL) formulae to determine what impedance you need to draw a certain amount of current at whatever voltage your bridge will run on. I have had good results with 10t of 10ga on a 4" PVC form, but my coils always draw near 15A from 120v mains. If you did the same thing on 240V mains you would be drawing near 30A, so that would not be good. So first determine what current your bridge and breaker will be very happy at and work from there. More turns = more inductance = less current draw and more coupling.
So 6 turns of 2.5mm^2 right over the 3" form should draw about 10A from 120V? Strange, I always thought closer coupling meant less current draw but it does reduce inductance according to the calculator...
wrote ... I've never had any heating of my caps, even at considerably high RMS currents. If you pick good caps it should not be an issue. My bet is the gate resistor might have arc'd to the heatsink, swinging everything off the heatsink edge should take care of this too.
Yeah, that's my current guess as well... I'll rebuild the bridge from scratch with everything but the FETs on the outside.
wrote ... The larger the counterpoise the better. I wouldn't put any other components on the counterpoise as this can cause more problems than not. If your primary is exactly at the base of the secondary then in theory it should be at very low voltage in relation to earth, but this is not always the case. If you put the primary in the center of the secondary then both ends become the HV output. This is called a bipolar coil. So, just because the "LV end" is supposed to be near ground, doesn't mean it isn't floating at some voltage higher than it. Another reason why I don't like using counterpoises.
It's not exactly at the base due to my fear of accidentally removing the glue that holds the secondary wounds. It's about 1/4" above the last turn. I could lower it to about 1/8" if it's *really* needed.
wrote ... Ground rods: as many as you can do, as deep as you can do. Run individual wires from each rod to a central rod in the array, and then your main connection to that central node. This avoids ground loops and gives excellent conductivity.
Any specs on the wire?
wrote ... GDT phasing: Primary side phasing errors can cause a failure to oscillate properly but generally do not blow up the bridge (at least mine has never failed from it). Just recently when changing drivers I had my primary side of GDT phased wrong. Switched it around and it was fine after. I did hear some clicking with it wrongly phased, not sure if it was just random though. Secondary side phasing errors (you want the low side fet to be 180deg out of phase of the high side) will cause cross-conduction where both fets are ON at the same time. This causes the DC bus to be shorted out by the fets, which usually results in them exploding. The easy visual way to check is if you hold the GDT so you're looking down on it's thin axis (perpendicular to the hole) the wire for the high side fet's gate should come from the top and the high side fet's source from the bottom. The low side's gate should come from the bottom and its source from the top.
I haven't tested both primary side phases, will try that as well. I'm pretty sure the secondary side phasing is correct but will double check, just in case.
@julian:
wrote ... -at what voltage? if @ 40% of max Vinput then, seems to me these following causes:
170VDC, I don't have a variac and I'm pretty much stuck with that voltage.
wrote ... 1. phasing of the transformers. (if not CT, then GDT, or Primary polarity - do test all combination to scrutinize phasing) -since you can light up some thing wirelessly, this means that you have power out, but not enough for corona, (i typically hear the ticking sound coming from my variac - or when 1 of my fet is already leaking(damaged) for some reason (e.g. full bridge, tank capacitor is problematic, and/or capacitor for filtering the supply have internal issues - seen on a tester good but when running a load/coil it causes unwanted spikes on the rails, when i replaced it, the coil's great.)
Will try to reverse primary polarity as well, haven't tried that yet.
wrote ... 2. GDT driver, even when using UCCs, sometimes it bugs my design and will not work, i ended up using (level shifter circuit then to S.Wards UD gate driver output stage with P/N MOSFETs, works very well for me and it is very robust! - even work up to high gate C like CM300 -24H, of course i tried it with small fets - tuned to 400KHz which means fast and powerful driver.
I'd rather make it work with this driver, if I have to order more components it'll take a long time because my only source is feebay..
wrote ... 3. ground (although many will get angry when i say this, but i ran my SSTC without grounding - well this is because it is very small, and i planned to play it whenever i want(sort of a demo), just plug it. output of 4 inch spark) , my grounding is the negative rails of my logic circuit plus the ATX power supply casing i removed from my old computer (used as a overall casing of my circuits) ., BUT do ground ur system for better system - not to mention, sparks are larger when grounded reach 5 inch plasma.
Do you figure this would work with an isolated supply as well? Might try that..
I had a real hard time soldering the thick copper wires on the bridge, can I safely use stranded aluminum wire instead? That would be a lot easier to solder..
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SSTC bridge blew up
