Minimizing destructive HV spikes on square wave push-pull MOSFET drive to transformer

You can place a ferrite bead on a leg of the FETs too. Right at their entry to the PCB. We also got results from these practices.

Ours were about a quarter inch long. Maybe slightly less.

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For one thing, the OP has confused leakage inductance with magnetizing indu= ctance. That transformer primary has nowhere near 180u leakage inductance. = Then his unnecessary and inordinate slew rate control on the FET gates is t= he root of their overheating and eventually destruction.

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Actually the PWM output is not designed for push-pull but two pins a = touch of code and a pair of and gates can steer the PWM output to suit your system. PIC uCs are kind of known for cycle by cycle PWM, add a couple = of lines for the alternating drive for push pull.

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Fred, thanks for the idea with the diode clamp. I had tried something = like=20 that before, but it seems to work quite well. So I have added a linear=20 current limiter to the battery, set at about 80 amps, but I also changed = the=20 output capacitors to what they actually are, 3300 uF each, with 0.016 = ohms=20 ESR. I found that it will take about 300 mSec for the output voltage to = come=20 up to 300V, and during that time the capacitors seem to be dissipating = about=20

300 watts each!

Actually, it will work without those capacitors, since the square wave = has=20 only about 1 uSec during which the transformer is not being driven. I = needed=20 them previously when I had them connected in a doubler configuration to = work=20 on 12 VDC. So I should be able to use something much smaller, and/or use = the=20 capacitors in the VF drive. But I think I may need an inductor to ease = the=20 peak current in them as well.

In some ways it's amazing that the components lasted as long as they = did.=20 The capacitors have 0.5*300*300*6600=3D 297 W-Sec so at 24V that would = be 12=20 amp-seconds, but since the original simulation showed the peak voltage = being=20 reached at about 50 mSec it would be 240 amp-seconds. If I limit the = current=20 to 80 amps it should reach the desired voltage in about 297/(24*80) =3D =

154=20 mSec. But if the capacitors are also dissipating 300 watts each during=20 charge, that would be added, and there would be more like 900 = watt-seconds,=20 so the peak might be reached at 450 mSec. Of course real components do = not=20 always match the simulator (or vice-versa), and the more accurate the=20 simulation the longer it takes. I'm running it now and it seems to be=20 stabilizing at about 270 mSec, but it took probably 10-15 minutes. I'll = see=20 what it looks like when it reaches 350 mSec at which point the PWM = stops.

Wow, the RAW file is 1 GB! Looking at the last 20 mSec, the input power = is=20

783W, output is 276W, the MOSFETs are 9.7W, the two series pass are 23W=20 each, and there is still 214W in each of the output capacitors! At this=20 point the input current has dropped to 33A.

"Where has all the power gone, long time passing..." Bob Dylan, PP&M

Actually, though, this is still "reactive" power, because the capacitors = are=20 still charging. The actual ripple current is about 1 amp, so the real = power=20 is minimal. I needed to continue the simulation until full charge was=20 reached. Looking at the first 20 mSec of startup, the capacitor current = is=20 just 3.7A. The series pass current limiters are dissipating about 900W, = so=20 that might be a problem. But they will only do so for a short time, and=20 should gradually drop to the 23 watts at 250 mSec or so.

At least the linear regulator reduces the transients during turn-on, = since=20 the MOSFETs only see about 3 volts at first. It may be better to design = a=20 switching current limiter, but I'm trying to reduce complexity and = enhance=20 reliability, so a brute force linear circuit might be acceptable.

Thanks,

Paul

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Here's my modified ASCII = file =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

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SYMBOL ind2 112 240 R0 WINDOW 0 45 35 Left 2 WINDOW 3 41 61 Left 2 SYMATTR InstName L2 SYMATTR Value 180=B5 SYMATTR Type ind SYMATTR SpiceLine Rser=3D100u SYMBOL ind2 240 176 M0 WINDOW 0 21 -5 Left 2 WINDOW 3 -9 113 Left 2 SYMATTR InstName L3 SYMATTR Value 32m SYMATTR Type ind SYMATTR SpiceLine Rser=3D10m SYMBOL nmos -208 384 R0 SYMATTR InstName M1 SYMATTR Value IRFZ44N SYMBOL nmos -48 384 R0 SYMATTR InstName M2 SYMATTR Value IRFZ44N SYMBOL voltage -976 400 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=3D8m SYMATTR InstName V1 SYMATTR Value 24 SYMBOL diode 384 288 M270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value MUR460 SYMBOL diode 320 224 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D3 SYMATTR Value MUR460 SYMBOL polcap 480 288 R0 WINDOW 3 24 64 Left 2 SYMATTR Value 3300=B5 SYMATTR InstName C1 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=3D400 Irms=3D30 Rser=3D0.016 Lser=3D0 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SYMATTR Value .003 SYMBOL diode -976 352 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D8 SYMATTR Value 1N4148 SYMBOL res -704 400 R0 SYMATTR InstName R4 SYMATTR Value 220 SYMBOL pnp -736 304 M270 WINDOW 0 51 63 VLeft 2 WINDOW 3 78 85 VLeft 2 SYMATTR InstName Q2 SYMATTR Value 2N2907 SYMBOL diode -912 352 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 -21 32 VBottom 2 SYMATTR InstName D9 SYMATTR Value 1N4148 SYMBOL npn -752 208 R270 SYMATTR InstName Q1 SYMATTR Value ZTX849 SYMBOL npn -752 64 R270 SYMATTR InstName Q3 SYMATTR Value ZTX849 TEXT 32 88 Left 2 !K1 L1 L2 L3 0.998 TEXT -536 576 Left 2 !.tran 0 400m 0 2u startup TEXT 216 -24 Left 2 ;Primary 2x8 turns 2V/turn at 600 Hz=20

250msec is not short in the eyes of a transistor. When it comes to the SOA that counts almost as "DC". You aren't really planning on those li'l ZTX849, are you? That would be one quick *PHUT* and they'll turn into a puff of black smoke :-)

The way to handle this is via your controller. It should issue very short pulses during start-up, slow repetition rate. If you can't do short pulses then leave out lots of cycles until the caps are at nominal voltage, then throw the "virtual clutch" and go full throttle. This requires no extra parts.

Also, C10 could gradually charge up despite R6. Might want to zener that.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

I was going to use some MJ11029s that I have in some quantity. But I did = not=20 look carefully enough at the SOA, so it should be limited to about 15A = at=20

20V which it will see under start-up conditions. Then I considered using = a=20 P-MOSFET, such as IRF4905, which is rated 55V and 74A, and 20 mOhms so = it=20 will dissipate "only" 18 watts at 30 amps, but its SOA limits it to 20A = at=20 20V for a 10mSec pulse. So I'd need four of them to be safe. And then = the=20 power will be only about 5 watts total.

Since I already have a lot of these, maybe I can just cobb together a = brute=20 force current limiter just so I can proceed with other parts of the = design.=20 It might be a handy unit for other projects where I want to use = batteries=20 protected with a circuit breaker but also current limited to reduce the=20 total "destructive" power available. I could also probably add some = smarts=20 that would latch the series pass transistors off if there were a = persistent=20 short and not just a big capacitor bank.

Actually, I don't really need the big 3300 uF capacitors with a square = wave=20 and FWB. But I will have to consider the capacitors inside the VF drive. =

With 440 uF they charge up within 25 mSec with a current limit of 70A. = But=20 it's still 1200W for the first 10 mSec, and 660W over 30 mSec. So I'm=20 beginning to see the magnitude of the problem.

One problem is that the PIC16F684 does not work for push-pull PWM using = its=20 PWM module. I'm using it as a half-bridge with 50% duty cycle. I tried a =

start-up by using a timer ISR to get 12% PWM with 250 uSec pulses and 4 = kHz,=20 but the waveform was rather crappy. However, I could try even much = shorter=20 pulses and maybe add some inductance to limit the rate of rise somewhat. =

Then maybe measure the drain currents and use an interrupt to turn off = the=20 pulse for a while. I'll have to think about that and maybe try fiddling = the=20 PIC code a bit more. I really should be monitoring the drain current = anyway,=20 as well as the battery voltage so it doesn't try to start before I have=20 enough voltage to drive the gates properly.

If I want to use the iron-core toroid, then I may have to live with its=20 leakage inductance and lower frequency limit, and use a switching=20 pre-regulator. It would be essentially a buck converter, and I could use =

something like 100 kHz so I could get 50A peak at 5 uSec and 24V with a =

2.5=20 uH choke. Something like this:=20

Good idea, but if the voltage builds up, R6 will pull more current and=20 power. Unless the spikes have a lot of power and high repetition rate, I =

don't see a problem. But a zener wouldn't hurt. Maybe a TVS.

Thanks,

Paul=20

[...]

One of the many reasons why I never warmed up to using uCs for such jobs. A dedicated switcher chip will do a much better job and most are only a buck fifty.

Maybe you could disable the port for many cycle, let one through, suppress again, and so on?

Monitoring is one of the best insurances against a major kablouie.

Looks a little wimpy but might survive :-)

It's cheap insurance, considering the molten solder and plastic shrapnel that could be splattering about if one of the big FETs avalanches.

--
Regards, Joerg

http://www.analogconsultants.com/

(snipped)

Paul, the simulation is showing Q2 (2N2907) dissipating 20 watts.

John

That was just to approximate the behavior of the darlington PNP I was = going=20 to use. But there were other problems with the brute force approach and = I=20 would need to use many in parallel to stay with their SOA. So I have=20 simulated a design using an inductor and a series MOSFET (and other=20 components) to current regulate the load with minimal losses. It seems = to=20 work pretty well, finally. LTSpice doesn't have many good P-channel = power=20 MOSFETs in their library. So I used a high side gate driver and a good = size=20 NMOS. I'll post a separate thread with more details. It may be a handy=20 circuit to use when charging big capacitors from low impedance sources = like=20 batteries.

Thanks,

Paul=20

Check Fairchild, they've got tons of SPICE models. You may have to become a "club member" there but it's free.

--
Regards, Joerg

http://www.analogconsultants.com/