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"Big Bad" the CW coil

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Marko
Mon Aug 30 2010, 11:20PM Print
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
november 2011 - Time for some updates!

I've already posted some of progress in the thread titled "CWDRSSTC". Research has been done and some results can be summarized, though some new questions have appeared too.

1. Mosfets - shortly said, they turned to be crap for this application, do not use Mosfets for a high powered SSTC application. Reason? RDS-ON, and it's high positive temperature coefficent. DC load tests have shown that 44N50's I used before tend to reach as much as 200 miliohms at higher temperatures (80-90C).
So far I have ran close to 60A RMS with 30N60 IGBT's, which would result in over 700W dissipation per halfbridge had I used mosfets - and this is just from conduction losses alone!

Ofcourse I was very wary of using IGBT's at first since datasheet predictions of hard switching losses looked quite grim. Still they survived punishment at 10kW mark with hard switched primary current at nearly 60A RMS!

I suspect important reason for this was that IGBT's only really switch the primary magnetizing current, which was never more than about 30A peak in my case while the load component of current is sinusoidal and soft switched.

But after newest advancements, I wouldn't really recommend this to anyone unless there's a particular need to be a sadist towards the silicon.

Here be an example of the hard switched run at 10kW:




Obviously one would want to get rid the inverter of hard switching and unnecessary reactive power handling, which can be achieved using a dual resonant topology. A series resonant primary circuit would solve all the problems, but I thought I would require extremely high tank impedance in order to keep the current at a safe level. This would mean acquiring a high voltage, high reactive power tank capacitor as well as using many more turns on my primary which was not within my budget at time - so I acquired a large ferrite transformer to step the bridge voltage down, and used a 125nF CDE cap DRSSTC MMC as my resonant cap.

The results were very surprising - with 27:9 transformer ratio, the output was actually much less than with teh initial hard switching setup! I lowered the primary somewhat and got a slight increase in power output.

Primary current didn't even trip 40A limit during this run (with hard switched runs, it went over 80A peak!)



This is a very interesting result, because it means that if I reconfigured my H bridge to work as two parallel half bridges, I could implement series resonant topology without primary modifications! Why the coil draws relatively little power in this mode compared to DRSSTC's remains a question to ponder!

Some pics of the transformer and the cap:


1321120011 89 FT128184 P4090078 Large

1321120011 89 FT128184 P4090079 Large



More updates soon!



---old posts---

Hello all -

I haven't been around for a while - and I'm finally back with this project to share with you.

I'll start with some pictures, some questions, and update the thread over time. This project has started ages ago and I have actually forgotten a lot of it.

Current state of the coil - had it's first light, but died the other day due to unknown reasons. The mosfets died cold, just few tens of seconds into the run. I was way over confidentand didn't expect this to happen so early. Finally it was rebuilt with IGBT's and tested again.

Desired results: CW and interrupted runs from 3 phase supply with draw of up to 10kW with purpose of sowing general chaos. Lower power runs with half and full-wave rectified single phase.

Clues so far are:

1. my linear power supply. I used a 44VA 14V transformer, which I suspect may not be up to a task. I don't know how much can I trust a digital meter near the coil, but I measured the voltage to sag to about 11V when the coil is sparking - not looking good at all. I'm going to use a bigger transformer to see if the problem goes away. Or perhaps I should just use a 5A switching power supply? I originally intended that, and am not completely sure why I changed my mind for a linear supply now.

2. Sanity? I'm actually not sure whether I just pushed the TO247 mosfets too much. But I don't believe I can only get 40cm of CW spark from these! I'll post the part numbers of the mosfets when I find their bag tomorrow.


Here be the driver circuit schematic:
1283213526 89 FT1630 Schematic


As you can see, I expressed minimalism the design, in attempt to minimize the number of problems that could occur.
I used a linear regulator with 2N3055 to provide 12V at several amps required by the circuitry and fans.
I decided the UCC37322 would make most reliable drivers and sufficient because of relatively low operating frequency. (<200kHz).
A current transformer samples the secondary base current, and is connected to pads CT1, CT2 in schematic. 12V zeners provide clamping, and schottky's prevent them from forward-conducting. Note that this arrangement provides symmetric drive to both UCC chips - I didn't use any additional buffers since CT signal is already clamped into nearly perfect squarewave.
The problem remains that this circuit is not self-starting - I solve this by introducing a TL494. It's purpose is to provide weak drive to the UCC's, and is easily overriden by CT signal when it gets strong. I can regulate the amount of signal by a potentiometer to +12V.


Now some construction pictures.

Starting point - CNC milled wooden frame:



Winding the coil:

1283210442 89 FT0 3 Winding Setup

1283210442 89 FT0 4 Wound

This H-bridge was created by one of my previous incarnations... most about it that I can't see right now, I've forgotten - don't ask me what mosfets are being used - I don't know! I suspect they are big and begin with FDH... Mosfets are pressed directly onto large CPU heatsinks with pieces of Al profile, with a thin layer of arctic silver thermal compound. I wanted to get out as much of allowable dissipation as humanly possible with TO247 devices.
EDIT: The devices are FDH44N50. Link2

1283210442 89 FT0 6 Bridge1

I used a single GDT made of RG174 cable per mosfet - for an really overkill gate drive.

1283210442 89 FT0 Bridge Another

Whole thing starting to take shape:

1283210442 89 FT0 8 Mock

For primary supports, I used polypropylene rod cut in 6 pieces and tapped on a lathe. 6mm copper pipe was formed and zip-tied into shape.

1283210442 89 FT0 Primary Large

I decided tomake the low voltage section as a rack I can get out of the coil easily - hence the additional piece of wood.

1283210442 89 FT0 Board And Faceplate

Inside of a high voltage section - it's not completed yet, but fine enough for now.

1283210442 89 FT0 Rectifier Large

The whole thing nearing final shape:

1283210442 89 FT0 Setup

And yesterday, I finally got some sparks. I used a large three phase variac and feed to power the coil, producing some rather amazing CW sparks - due to 3 phase rectifier. I brought the power nearly to a full, nothing exploded, so I took some pics... Primary was still tapped on 6 turns, as I was cautious - still the CW sparks were at least like 40 cm long. I'd be grateful if someone could approximate input power by size of this spark - since I haven't tried to measure it yet.

1283210442 89 FT0 P1240996 Large

Due to obvious corona around the edge of secondary on this pic, I stopped the runs for the day - I didn't want to risk a breakout from an edge of a winding which would be very damaging. This had to be taken care of.

1283210442 89 FT0 P1240997 Large


I built this corona ring which I mounted on the top of the secondary. I ran the coil again, only to blow the mosfets several tens of seconds into run. Heavily frustrated, I didn't take any more pics. I'm clueless about why this happened - since I've put so much work into this coil to make it buletproof. I haven't changed anything on the coil from the day before, apart from adding this ring and changing the breakout point from stainless steel to brass. Stainless had too low thermal conductivity and tended to melt and burn under spark.

1283210442 89 FT0 Ring

1283210442 89 FT0 Ring Finished


So much for now -

Marko
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Linas
Tue Aug 31 2010, 06:14PM
Linas Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
I like bridge , it looks good :)
also very cute GTD (white one ) cheesey (or whatever that is )
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Steve Conner
Tue Aug 31 2010, 08:21PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hey Marko, looks great! smile

The power level is probably quite extreme. I've never seen a breakout point glow so bright that it causes lens flares. If you're running off 3 phase power, the DC bus voltage will be practically pure DC, so the sparks will be shorter for a given power throughput/level of MOSFET abuse. So maybe they just died of excessive power.

Also, isn't the FDH44N50 a 500V MOSFET? If you put 3-phase mains into a 6-pulse rectifier you get 560V DC out. So if you turned the variac "quite far up" you probably got quite close to the voltage rating. And your bridge layout has lots of stray inductance, because you had to put the MOSFETs far apart to make room for the heatsinks. So that's another possibility, maybe they avalanched themselves to death.

It doesn't matter whether you connect the variacs in star or delta, 100% on the dial will still give you 400V line to line and 560V out of your rectifier.

Finally, what about common-mode voltage getting into your CT? If there's a lot of RF voltage between your secondary base ground and your driver circuit ground, then it could couple through the CT's interwinding capacitance and give your gate driver chips a surprise.

One last thing, is your power supply RF proof? I once had a bench power supply that would increase its output voltage when it picked up RF, and destroy my circuit.
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Marko
Wed Sept 01 2010, 01:25AM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,

Linas: The cute toroidal coil is a feedback current transformer. Also it's not white, but pink!

Steve:

Bad news first. I did some DC load tests on the mosfets (I don't know why I didn't before) to see how they behave. I bolted one to same heatsink I used in the coil, and connected it straight to 5V rail of a large server power supply while holding the gate on. I was surprised to see the mosfet only drew about 24 amps in steady state - indicating a hot on-resistance of over 200 miliohms. Total dissipation was about 100W, and heatsink only got warm despite I used 2x less powerful fan than those in the coil.

Conclusion: satisfactory performance of heatsink, unsatisfactory performance of mosfets. After looking at thermal resistance characteristics in datasheet, projections of performance at high power appear grim. It's not unrealistic that mosfets will need to carry 30amps in operation in the coil - and this would already amount to 180W of just the conduction losses. Adding switching losses that may be in excess of 100W, it looks impossible to avoid junction temperature going nuclear with total thermal resistance of 0.5 k/W (junction to case + case to heatsink) - 300W of dissipation would result in 175C on the die even if I could keep the heatsink at constant 25C!

This made me wonder whether I pushed the limitations of what any discrete mosfet can do - their resistive nature makes them inherently unsuitable for large current switching.

I'm left with choice of either using a large number of parallel devices, or switching to IGBT's.

In comparison, while these mosfets rated at 44A would dissipate >180W just from conduction losses, and IGBT dropping 2V would dissipate only 60W - three-fold improvement.

I'm thinking of simply replacing the mosfets from the coil with some 30N60's - they are actually faster apart from turn-off delay time. Inductive hard switching may be worse on them, but I still think they'll be much more rugged than the mosfets.


Regarding other things: I didn't turn my variac too far up, I marked it and took special care about it. I never allowed more than 400V DC on the bridge, which is the rating of my DC block caps.

Finally, what about common-mode voltage getting into your CT? If there's a lot of RF voltage between your secondary base ground and your driver circuit ground, then it could couple through the CT's interwinding capacitance and give your gate driver chips a surprise.

They are the same ground! ill I could never find a statisfactionary RF ground, and it's a real headache to me - I ground all my coild to neutral line. Once I used the water tap as a ground, only to realize it's getting very hot and could heat water boiling in it.

As far as I know, bad ground is only bad for my electronic devices - none of which are close to garage - if anyone has a theory about how it might cause mosfet death, please let me know!

One last thing, is your power supply RF proof? I once had a bench power supply that would increase its output voltage when it picked up RF, and destroy my circuit.

I suppose it is, though I'll probably use an SMPS for next tests. Or should I be afraid of the SMPS for the same reason, and just take a bigger transformer? I could even get rid of regulator and run with unregulated supply.

Marko

EDIT: Looks like I found what the problem was. After disassembling the bridge I realized one heatsink had hardly any thermal grease on it - this mosfet had to have really poot thermal contact! I'm surprised it lasted even this long. EIther the mosfet or the heatsink might have had poorer flatness, and the amount of grease I applied was just insufficient. I believed a thinner layer of grease would be beneficial - in the end I really just pushed this faith way too far.

I replaced the mosfets with IGBT's now anyway, going to try tomorrow. I doused them with as much grease I could and bolted them hard to the heatsinks.

1283386012 89 FT1630 P1271106 Medium

1283386012 89 FT1630 P1271110 Medium
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Marko
Fri Sept 03 2010, 03:08PM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,

After decision to replace the mosfets with igbt's, I ran the coil again on lower power levels, but this time with much better preparation. I put an amp meter onto one phase and slowly increased the power, leaving the coil at one power setting for few minutes to see how it behaves. I cranked the variac to about 50V - and was startled to see that the coil already draws nearly 10amps per phase just to produce a puny 10cm CW spark! Any my primary was already tapped at the last rurn. I increased the voltage a bit more and left the coil like that. After a few minutes, I noticed strong burning smell from the coil follower by smoke from the bridge section. I could witness the quite fat audio cable I used for primary connection melting and sliding out of it's insulation.


1283526501 89 FT95545 P1281113 Large

1283526501 89 FT95545 P1291115 Large

1283526501 89 FT95545 P1281112 Large

After replacing the damaged cable with battery cable, I ran for a while longer. I kept my nose high to notice any burning smells from the coil. And indeed after several minutes they did appear - but this time not from the cable, but from a zip tie at the end of my primary! As I stopped the run, it just fell off.
This wouldn't be unusual, if the primary wasn't only barely warm after the run - nowhere close enough to melt the strip! Also, it appears like heatink only occured in a very localized spot - which was not a spot in contact with the primary!



1283526501 89 FT95545 P1291116 Large

The missing tie wasn't a big deal,and I continued the run for some more, but watching closely - as soon as the smell appeared again, I shot off - now I could clearly see what was happening - on a zip tie just after the first one - yet it leaves me completely puzzled for now. I'm suspecting the black plastic maybe somehow conductive for HFHV and perhaps much more conductive when melted,starting a chain reaction which ultimately blows holes in the plastic.
1283526501 89 FT95545 P1291120 Large


I'll try some white zipties to see whether they're immune to this problem.

Marko
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Luca
Fri Sept 03 2010, 05:56PM
Luca Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
Hi Marko,
maybe it's just dielectric loss of the plastic of the ties, which at high level of electric field can heat up and eventually melt the plastic itself...
Anyway, it's impressive...

Regards,

Luca
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Marko
Fri Sept 03 2010, 10:48PM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Luca

While I was running the coil again, the wooden ring on the top of my primary arced and caught fire. The secondary is mostly undamaged, mostly thanking pure luck. It turns out that wood is far too conductive to use in the place where I did, and brass screws made it even worse. It looks like electric field between primary was strong enough to cause invisible streamers, but which were still hot enough to melt the plastic zipties in their path. Above the first ziptie which is closest to the wood, a carbonization spot appeared. Another black spot few cm eated away the wood until it ran away into fierce fire and arced to the secondary.

I'll look into replacing the ring and bolts completely with plastic, although the primary seems quite stable even without any.

Marko
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...
Sat Sept 04 2010, 03:03AM
... Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
A very nice coil!

It reminds me of my CW coil, I got ~6" out of it at 3kw DC input, so getting ~15" with upwards of 10kw sounds about right.

I found that using a piece of tungsten (I use the electrode out of a large xenon lamp) that was loosely connected to a piece of copper gave a reliable topload. The W would get glowing white hot, but the poor thermal contact to the copper prevented it from melting, and the W could handle the high temperatures.

I hope you can sort out the overheating issues, are you sure its not jut the primary getting hot due to eddie/i^2r losses? You could always run some water through it shades Those fdh44n50's are awesome mosfets, the die is the size of the entire mounting plate!
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Steve Conner
Sat Sept 04 2010, 10:29AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Glad to hear you've had no more silicon deaths!

You mentioned that you got heavy current draw even at low voltages. This is quite normal, Richie and I have seen it too. As you keep increasing the voltage, the current will level out.

The reason is that the spark clamps the top voltage to an almost constant value. As you increase the power, the plasma gets bigger, but the voltage doesn't really go up much. The resonator is a 1/4 wave transformer, so the constant voltage characteristic transforms to a constant current at the base.

Wood and brass screws? Time to replace those and sow some more general chaos for us, plz. tongue
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Marko
Sat Sept 04 2010, 04:26PM
Marko Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
... wrote ...

A very nice coil!

It reminds me of my CW coil, I got ~6" out of it at 3kw DC input, so getting ~15" with upwards of 10kw sounds about right.

I found that using a piece of tungsten (I use the electrode out of a large xenon lamp) that was loosely connected to a piece of copper gave a reliable topload. The W would get glowing white hot, but the poor thermal contact to the copper prevented it from melting, and the W could handle the high temperatures.

I hope you can sort out the overheating issues, are you sure its not jut the primary getting hot due to eddie/i^2r losses? You could always run some water through it shades Those fdh44n50's are awesome mosfets, the die is the size of the entire mounting plate!

Hi peter -

Meh, I hoped to acheive at least like 20inch CW sparks without going over 10kW :O Looks like I'm really underrating the amount of power CW sparks can eat.

I dislike the breakout points made of metals with poor thermal conductivity, because they glow so bright that they're uncomfortable to watch. I found brass to be by far best electrode material - copper has higher thermal conductivity but is oxidized in the air very fast. I've ran my brass breakout point for probably half a hour now and it's still sharp! I have to see what higher power levels will do to it, though.

Regarding the primary heating - no, it's just getting warm by now, at low power levels. And I also don't expect the primary current to increase significantly with increase in voltage. It was a wise decision to use a copper tube for the primary - it dissipates heat excellently with some air blown on it from beneath, which was not true for the audio cable with rather thick low-melting-point insulation.


Regarding mosfets - these were the largest ones I could come by easily. I cracked a dead one open, and the die is indeed so big that it barely fits in a package - I presume that it's probably the maximum of silicon one can place into a TO247 package. Still they don't seem to have all that great Rds ON, as they get hotter it seems to approach like 200miliohms! I might be passing up to like 40A through the mosfets. This would lead to 320W of dissipaton just from conduction alone! This dissipation is rather unmanageable for the package and just it's internal thermal resistance.

In comparison an IGBT dropping 2V would only dissipate 80W from conduction of the same current - a 4 fold improvement. Also, the 30N60's I have seem to have their forward voltage drop decrease with increase in temperature - to like 1.5V typ at 100C!

Use of igbt's over mosfets is certainly going to result in increased switching losses, though. Looking at the datasheet, just summing the turn on and off energies makes a grim prognosis. But I believe the losses in a sstc will actually be much less due to zero voltage switching. The igbt's also swithc the current at significantly less than it's peak value - they basically only switch the magnetizing current of the primary.

For now, the devices seemed rather cold after the runs - much colder than what mosfets felt after 50W DC load tests.

Still, I want to find a way to closely monitor the die temperature. I was considering to implant an SMT thermistor into the igbt - just underneath the collector leg, after perhaps drilling a small hole in the plastic to bury the NTC. I would solder the NTC down to the copper and using a floating supply (a battery) with a regulator and current meter to monitor it's resistance. This would provide me a great deal of knowledge about how much torture are the power devices taking in the operation.

I have some questions for you, Peter, since you seem to have a lot of experience with power devices - I was wondering what are the most powerful devices I could find in a TO247 package, for use in this coil. I would prefer either fast igbt's, or mosfets with really low Rds ON in SUPER TO247 package. I know there are such devices that have dies so large that they can't even put a hole on the to247 package. I've also seen 'elongated' ixys packages without holes which improve the dissipation even further.
I'd appreciate any ideas and possible means of acquiring some of such devices without too much pain.

Steve: yes, I've noticed that phenomenon long time ago. When secondary has little spark load it has very high Q and just tends to suck a lot of power from the primary circuit. I didn't expect it to draw over 10A so early though - my breaker and variac are both rated 10A - this was something like maximum I ever wanted to draw from the variac. Still I'm apparently going to increase the breaker to 16A and just pretend it doesn't matte rto the variac. It's so huge that I don't think it will bother it much anyway. I could lower my primary a bit down as well...

Marko



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