Author Topic: TVS diode selection for 400v transistor  (Read 1521 times)

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #20 on: April 07, 2020, 02:00:26 AM »
Nice find! Back in the 4hv days a couple members had some similar looking units custom made.

I recommend this driver for a controllable way of reliably driving them in flyback mode, it was designed by Jan Martis of 4hv.

Bipolar drive designs like the ZVS and bridges are fun and all, but they put a lot of stress on the internal diodes and often destroy them in the process. CRT screens for salvage are getting harder to find so it makes sense to take extra care of them these days in my option.

Use a more modern MOSFET rated for higher voltage, I use a 600v low rds on one and a smaller primary capacitor (used 100nF for 24v in) and allow the peak drain voltage to peak higher and faster for more output voltage.

Measure the peak drain voltage with a voltmeter and high impedance RCD snubber across the drain and source pins, 4.7nF, 10 MEG ohm and UF4007 is what I used, that way you don't have to risk blowing your scope up if something goes wrong or arcs over (unless you've got a HV probe). The 4.7nF cap charges to the peak of the flyback waveform and lets a regular multimeter read it as DC.

For my 600v fet I like to keep it below 500v peak drain to source volts, the 200v rating of IRFP260 is a bit limiting imo unless it's a really high voltage flyback with a large number of secondary turns, speeding up the fets turn off with the diode helps but there was a point were it was too fast and made mine oscillate.

Had to put a resistor in series (10 ohm) with the mosfet gate turn off diode or else it went unstable. That 5k power limit pot can be made smaller for lower power if needed (I have a switch which lowers it to 2k), it controls the peak primary current threshold. At its default its tuned for 35v input.

Place a low ESR capacitor of a few 1000uF across the primary +, mosfet and current sense resistor loop even if you're using an already regulated and filtered power supply, the peak currents are very high and performance is reduced without it. The loop area should be small with all components no more than a few centimeters away from each other.

Using the UC3845 version it can be powered from a 12v source as the UVLO thresholds are lower, there are newer versions of these chips called UCCx8C4x too but make sure they're the <50% duty cycle versions.
« Last Edit: April 07, 2020, 02:49:51 AM by John123 »

Offline davekni

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Re: TVS diode selection for 400v transistor
« Reply #21 on: April 07, 2020, 02:52:19 AM »
John,

"Do PC monitor ones have any differing drive requirements such as frequency and drive waveform, maybe the core material is of a different composition."

Yes, PC monitors are higher frequency, often much higher.  The flyback frequency is normally matched to the horizontal deflection frequency.  The horizontal deflection coil is typically in parallel with the flyback primary, so share the same drive circuitry.  TVs were generally 15.734kHz (PAL) or 15.625kHz (NTSC).  Computer monitors can be up to 100kHz, or even above for very high-end models.  So the flybacks tend to have fewer turns and higher voltage/turn, and use higher-frequency ferrite materials.
David Knierim

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #22 on: April 07, 2020, 02:59:28 AM »
John,

"Do PC monitor ones have any differing drive requirements such as frequency and drive waveform, maybe the core material is of a different composition."

Yes, PC monitors are higher frequency, often much higher.  The flyback frequency is normally matched to the horizontal deflection frequency.  The horizontal deflection coil is typically in parallel with the flyback primary, so share the same drive circuitry.  TVs were generally 15.734kHz (PAL) or 15.625kHz (NTSC).  Computer monitors can be up to 100kHz, or even above for very high-end models.  So the flybacks tend to have fewer turns and higher voltage/turn, and use higher-frequency ferrite materials.

Wow, I had no idea! The core material thing probably explains a lot of the performance differences people online have noted about them, so as an analogy it's like trying to use a switching transformer on 50/60hz mains? The core and windings just aren't designed for it.

My driver currently tops out at 60khz so looks like I need to add another adjustment pot for some experiments  ;D
« Last Edit: April 07, 2020, 03:06:19 AM by John123 »

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #23 on: April 07, 2020, 03:21:55 AM »
Quote
I recommend this driver for a controllable way of reliably driving them in flyback mode, it was designed by Jan Martis of 4hv.

I've seen the video of that driver in operation - very impressive!

This is the schematic of the simulation I've been doing:



And some waveforms (from a slightly earlier version):



It's a work-in-progress and as it's just a simulation, I've gone for a 50+ quid IGBT with 3600V/70A, running at 2800V/60A. The fun occurs half way through the 5 msec simulation when a 50k resistance is switched onto the output. At that point, some 850W average is dissipated in that poor 50k resistor.

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #24 on: April 07, 2020, 03:38:12 AM »
Yeah apparently that was with one hell of a good Sony flyback, he said most fail way before those output voltages are reached. Apparently he fiddled with the core gap too and many won't approach those kind of output voltages anyway with a primary limited under 200v peak to protect the IRFP260 fet in the schematic (hence why I used a 600v one).

lol! So 50k is roughly the same resistance as a current limited arc channel I take it. There was another simulation of a LOPT I saw and it had about 50pF of self capacitance on the output, not sure if that's helpful or not.

Does that £50 IGBT exist only in the simulation or have you got one for real? I think my fet cost £10 several years ago but there are probably much cheaper and better options available these days from places like RS components.

In Jans schematics I posted, I wonder how critical the 470pF and 470 ohm resistor and capacitor are on the current sense pin. Would changing their values change how the circuit behaves?
« Last Edit: April 07, 2020, 07:01:54 AM by John123 »

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #25 on: April 07, 2020, 12:44:56 PM »
Quote
Does that £50 IGBT exist only in the simulation or have you got one for real?

Simulation only. My track record of IGBT longevity isn't great  :-[

Quote
There was another simulation of a LOPT I saw and it had about 50pF of self capacitance on the output, not sure if that's helpful or not.

Yeah, I haven't got round to adding parasitic capacitances/inductances to this simulation yet...
When I do actually build one, it'll use a junk box NGTB20N120IH and the ZREAL flyback (and a much lower peak current!).

Quote
In Jans schematics I posted, I wonder how critical the 470pF and 470 ohm resistor and capacitor are on the current sense pin. Would changing their values change how the circuit behaves?

They might be for both noise rejection and stability, so more aggressive filtering might have the side effect of making it unstable.
The 8-pin version of the UC3844 has common power and current sense grounds; just using careful layout/star grounding might not be enough on its own.

Your HR Diemen datasheets reminded me of something - you can sometimes find a pair of taps on the deflection output winding that can be used as a low-voltage primary. I found a low-resistance winding with 15 uH inductance once, which worked well with a 12V driver and a peak current of about 8A. There's the added bonus of slightly better coupling than an "other leg" winding.

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #26 on: April 07, 2020, 06:23:47 PM »
How come you keep destroying IGBTs lol? Tesla coils? I started plastering protection all over my power switching devices and using peak current sensing, when I started doing this sort of thing in my teens blowing fets were an endless source of frustration when you'd read posts by people on 4hv running theirs with no protection and relying on avalanche ratings.

In the datasheet it says its for filtering out spikes on the current sense signal due to capacitive coupling, but many of the schematics I see online for these IC's use 1k and 470pF, some even used a 100pF. Just wondering if the 1nF + 1.2k I used is having any adverse effects, the fet has pretty large input capacitances and I think there was a largeish spike or something I was trying to filter out (built it years ago so can't really recall). The gate drive signal between gate and source is pretty clean and I put small ferrite beads on the gate and drain pins.

The link between power and driver ground run underneath that resistor with the currents for the gate drive running underneath it, wonder if that could be a problem.

Do the low voltage taps give much better performance than the own wound primaries? I'd always assumed they were for lower currents.

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #27 on: April 08, 2020, 12:53:15 AM »
Quote
How come you keep destroying IGBTs lol?
I was OK with MOSFETs - I bought 10 IRFP450s from Maplin for £3 each in the 90's and never had a problem in LOPT or ignition coil flyback circuits (with RC snubbers). As soon as I substituted IGBTs, they started popping. I gave up on them until fairly recently.

Quote
Do the low voltage taps give much better performance than the own wound primaries? I'd always assumed they were for lower currents.
The 15 uH taps measured 0.1 ohm. I can't get a vernier on the wires emerging from the potting resin, but they look to be about 0.5 mm thick. Didn't do a comparison of internal vs home-wound performance, sorry.

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #28 on: April 08, 2020, 07:53:19 PM »
hmm that's odd, did you use external reverse diodes across the IGBT?

Ah Maplin, that's a blast from the past now. In the years leading up to its closure in my area the best fet they sold was the IRF540.

They're still about online but the components are gone and they appear to be selling mostly gadget tat.

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #29 on: April 09, 2020, 12:19:57 PM »
Quote
hmm that's odd, did you use external reverse diodes across the IGBT?

No, the clamps were dissipative so the diode should not have conducted anyway.

I think it was more a case of relying on the '450's good avalanche performace compared to the IGBT used at the time (BUP213), combined with under-snubbing and youthful (ish) exuberance  ;D

Offline petespaco

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Re: TVS diode selection for 400v transistor
« Reply #30 on: April 09, 2020, 05:38:35 PM »
Quote
I was OK with MOSFETs - I bought 10 IRFP450s from Maplin for £3 each in the 90's and never had a problem in LOPT or ignition coil flyback circuits (with RC snubbers). As soon as I substituted IGBTs, they started popping. I gave up on them until fairly recently.

Could the problem with the IGBT's be rise time?
I read someplace that IGBT's normally aren't used at frequencies over about 20 kHz.   I assume that means that they don't/can't turn on as fast as a Mosfet in a given application.  And, if 20 kHz is the upper limit, approaching the speed might lead to  poorer performance.  And, come to think of it. you didn't discuss heat sinking or cooling.
   
I also read that Mosfets are good to about 200 kHz.
With the ZVS induction heaters that I play with, it is easy to see that turn on times start stretching out  significantly at about 120 kHz.  And, when they do. heat starts building up pretty fast, as you would expect.

Pete Stanaitis
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Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #31 on: April 10, 2020, 01:20:11 AM »
Quote
I read someplace that IGBT's normally aren't used at frequencies over about 20 kHz.

I had two drivers (using current sensing - absolutely no 555 timers) - one for ignition coils and one for LOPTs running at about 200 Hz and about 17 kHz respectively, so I don't think that was the problem. The ignition coil version had a stored energy of about 360 mJ per cycle and the IGBT I substituted (BUP213) had a maximum rating of 22 mJ - so if the snubber wasn't up to the job, a good chunk of that energy could have ended up in the IGBT, especially with no secondary load.

My best-performing ignition driver used a bipolar device from a scrap TV (which survived similar drive circuitry and energy levels). It produced 80mm sparks and needed lots of extra insulation on the coil turret, but it wasted 25 W in the base drive circuit. The MOSFET version solved the base drive power, but had to have less output voltage due the 500 V Vds rating of the IRFP450. The IGBT version should have been the best of both worlds, but after popping a few, I gave up!

Fast forward about 25 years and my lockdown challenge is to beat the bipolar version with an IGBT and keep said IGBT alive.

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Re: TVS diode selection for 400v transistor
« Reply #32 on: April 10, 2020, 02:05:07 AM »
For both the ignition-coil and flyback transformer versions, I'd suggest using a resonant capacitor across the IGBT S-D (or across the transformer primary - which is equivalent if there's good bypassing from supply go ground).  With the proper capacitor size (for the transformer primary inductance and your peak drive current) the unloaded flyback voltage can be made just under Vce breakdown.  Normal TV flyback circuits include such a capacitor, as do old mechanical-breakpoint ignition circuits.
David Knierim

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Re: TVS diode selection for 400v transistor
« Reply #33 on: April 10, 2020, 09:38:15 AM »
Do TVS diodes work well in series? Could save a bit of cash if they do.

Offline johnf

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Re: TVS diode selection for 400v transistor
« Reply #34 on: April 10, 2020, 09:53:19 AM »
I use unidirectional ones to assemble super high voltage zeners 2 -6 kV they work fine no balancing resistors or caps, only cooked a few whole chains of them due to over current.
I did try a chain of bi directional ones and had voltage stability issues I do not know why but went back to the tried and true unidirectional types.

I did put the unidirectional types on one of our SMU's and did a plot from reverse breakover to forward conduction. In reverse mode the leakage was extremely low - a couple of pico amps until very near the rated conduction point. Because of this I got a patent and published papers on using them to self bias secondary electron  suppression electrodes in ion implanters and ion transport systems in general, (by using the secondary electrons to pump up the suppression electrode).
In using them for series high voltage zener work you have to pay very close attention to the continuous dissipation figures and allow a resonable overhead.
 In some of my systems I have had to use 50 volt ones to allow a few tens of milliamps of conduction current had to change out the 400 volt ones that were cooked.

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #35 on: April 10, 2020, 12:34:17 PM »
Quote
For both the ignition-coil and flyback transformer versions, I'd suggest using a resonant capacitor across the IGBT S-D (or across the transformer primary

I'm now sold on that method, thanks.

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #36 on: April 10, 2020, 08:07:16 PM »
I use unidirectional ones to assemble super high voltage zeners 2 -6 kV they work fine no balancing resistors or caps, only cooked a few whole chains of them due to over current.
I did try a chain of bi directional ones and had voltage stability issues I do not know why but went back to the tried and true unidirectional types.

I did put the unidirectional types on one of our SMU's and did a plot from reverse breakover to forward conduction. In reverse mode the leakage was extremely low - a couple of pico amps until very near the rated conduction point. Because of this I got a patent and published papers on using them to self bias secondary electron  suppression electrodes in ion implanters and ion transport systems in general, (by using the secondary electrons to pump up the suppression electrode).
In using them for series high voltage zener work you have to pay very close attention to the continuous dissipation figures and allow a resonable overhead.
 In some of my systems I have had to use 50 volt ones to allow a few tens of milliamps of conduction current had to change out the 400 volt ones that were cooked.

Thanks! I'll stack some of the 250v unidirectional ones up for the new 600v fet I got in another thread, any harm in mixing and matching breakdown values? They're all from the same series of TVS, but for hobbyist one-off applications a ballpark breakdown figure should be good enough right?

Quote
For both the ignition-coil and flyback transformer versions, I'd suggest using a resonant capacitor across the IGBT S-D (or across the transformer primary

I'm now sold on that method, thanks.

Warning about PC monitor flybacks, they're much harder to tune and when they do put a half decent arc out they don't last very long!  ;D

Most of the monitor flybacks struggled to ignite much over an inch even with tuning and primary coil experimentation, but the one I tried last night with something silly like 5 primary turns gave some 7cm arcs igniting at 12v 50 watts input and even longer when stretched out, that lasted for about 5 seconds until there was a funny noise and now it's back to sub-1 inches.

I guess I shorted something internally, maybe inside the resistor-capacitor network monitor flybacks seem to have. At their longest point they were somewhat capacitive looking sparks but not the loud bangs you often get from the big capacitor inside them. It was either a small capacitance or a large impedance in series limiting the max capacitive discharge current.

Wish i'd got a video for you guys as it was incredible output for a monitor flyback, I tried the same tuning techniques with the other monitor flybacks but no dice. The only thing which I can think of is it had a much fatter core than the others.

« Last Edit: April 10, 2020, 08:31:34 PM by John123 »

Offline davekni

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Re: TVS diode selection for 400v transistor
« Reply #37 on: April 10, 2020, 09:34:23 PM »
I've also had no issues with series strings of TVS diodes, including mis-matched voltages.  Just make sure that the highest voltage part in the string can handle the current, both average (power dissipation) and peak.

Ran into one case of series TVS diodes commercially, in a Power Integrations gate driver for IGBT bricks.  They are used to clamp Vce spikes by turning the gate back on a bit when needed.  Their ASIC handles turning the gate pull-down off momentarily during clamping.
David Knierim

Offline T3sl4co1l

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Re: TVS diode selection for 400v transistor
« Reply #38 on: April 11, 2020, 10:52:36 PM »
TVSs in series are fine.  Higher voltage devices are stacked dies -- internally in series.

They're alright in parallel too, at least under surge conditions, if maybe not so much at lower currents.

Tim

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #39 on: April 27, 2020, 02:19:10 PM »
Goal achieved using the resonant capacitor method - no longer destroying IGBTs!

Ignition coil: Lucas DLB704, IGBT: NGTB20N120IH. Small jacob's ladder, supply 24V @6A

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Re: TVS diode selection for 400v transistor
« Reply #39 on: April 27, 2020, 02:19:10 PM »

 


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post Re: Freewheeling diodes are burning (Now other problems)
[Solid State Tesla Coils (SSTC)]
hip424
May 24, 2020, 02:31:39 PM
post Re: Working Principle Of Neon Transformer Circuit
[Transformer (Ferrite Core)]
iraquois
May 24, 2020, 02:08:53 PM

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