Author Topic: TVS diode selection for 400v transistor  (Read 13536 times)

Offline John123

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TVS diode selection for 400v transistor
« on: March 28, 2020, 12:59:08 AM »
Hello again, I'm in lockdown and have gone flyback mad!

What TVS reverse stand off and breakdown voltage would you guys select for a 400v transistor?

I've gotten myself confused when it comes to TVS datasheets. I'm running a TV flyback transformer like in the pic at 12v, it's actually quite efficient with nothing getting that warm drawing about 2 amps from the 12v supply and a 2 inch arc (still working on 24v mode). Frequency is about 15khz.

The TVS will not conduct under normal operation as the capacitor is selected to limit the peak voltage to something sensible (about 170v peak right now when the output is unloaded), the TVS is just added protection for unloaded flyback output and capacitor charging situations.


However by reducing the capacitor value and allowing the primary voltage to peak higher I can get more hv on the output without needing to increase the supply voltage, so I want to select a TVS which protects the transistor but doesn't conduct under normal operation.

But here's where I'm stuck, I've currently got a 200v TVS in there and when I retuned the capacitor to allow for more peak primary voltage the TVS clamps much harder than I expected. I'm looking at the datasheet for the P6KE200 and was expecting maximum peaks of 274v, but reality it clamps pretty close to the 200v breakdown rating rather than shooting up that large exponential curve.


I'm guessing the energy in the circuit isn't enough to push the TVS to such a high clamping level? And if that's the case what TVS voltage can I safely go up to for protecting my 400v NPN transistor (MJE13007)? Maybe the 300-350v standoff versions would be best suited, they're pretty expensive so I wanted check before ordering something which might not be a good choice, then there's the whole transistor safe operating area to further complicate things.

tl;dr which TVS diode breakdown would be a good choice for protecting a 400v transistor switching 15khz tv flyback at 2 amps average primary current?

Thanks.
« Last Edit: March 28, 2020, 01:17:14 AM by John123 »

Offline ritaismyconscience

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Re: TVS diode selection for 400v transistor
« Reply #1 on: March 28, 2020, 01:02:41 AM »
You want higher voltage across primary for obvious reasons but the voltage at peak current is still smaller than 400. The 250V one definitely will work, but you could probably go up to 300 to 350V because the current probably won't be close to 1A.

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #2 on: March 28, 2020, 01:09:39 AM »
You want higher voltage across primary for obvious reasons but the voltage at peak current is still smaller than 400. The 250V one definitely will work, but you could probably go up to 300 to 350V because the current probably won't be close to 1A.

Yeah I that's what I'm getting at, the energy in the primary appears to be low enough that the TVS does a pretty good job at clamping near its breakdown voltage (under forced conditions by reducing the capacitor value), I think 210v was the highest I saw using a 200v TVS and the TVS doesn't really get warm doing that.

But the TVS diodes all have such a wide clamping range in the datasheets that it makes selection confusing for amateurs like me. Like I said under normal operation it won't be allowed to conduct, but I still want it to protect the transistor from going over 400v in the event of something going wrong, the oscillator section is still pretty experimental so lots to go wrong.
« Last Edit: March 28, 2020, 01:13:32 AM by John123 »

Offline davekni

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Re: TVS diode selection for 400v transistor
« Reply #3 on: March 28, 2020, 04:44:17 AM »
Yes, worst-case specifications for TVS diodes don't allow tight clamping.  I've purchased quite a range of TVS diodes, finding them handy to have for my personal stock.  All have been quite close to their nominal voltage at low current and room temperature.  So, a P6KE300 is likely to have a breakdown right around 300V.  However, that voltage will depend on current, and especially on temperature.  The clamping voltage and current is specified with a total energy that heats the diode to its maximum 150C or 175C.  As with most silicon avalanche-breakdown voltages, it goes up with temperature.  So, the P6KE300's worst-case clamping voltage of 414V might not quite protect your 400V FET.  That depends heavily on the TVS current pulse duration and repeat frequency - in other words how hot the TVS diode gets.  Also, if your FET is running hot, it's breakdown voltage will be above 400V.  And, some FETs have some avalanche energy capability.  Overall, P6KE300 is probably the best choice.  Definitely not P6KE350.  Use the P6KE250 if you want to be extra safe with clamping voltage.  Still, be careful about letting the TVS do too much clamping, keeping it under its average power dissipation rating.  Or, go to 1.5KE300 for more power and higher peak current capability.
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Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #4 on: March 29, 2020, 07:52:58 AM »
Thanks, other than the higher power rating are there any other tradeoffs between the P6KE and 1.5KE versions? I'm guessing extra capacitance and leakage current. Then there's uni polar vs bi-polar, there's going to be tradeoffs between them too I take it.

It's actually a 400v NPN bipolar transistor (MJE13007) which I'm using hence the extra measures with the TVS being taken, it forms part of a cheap and nasty spare parts flyback driver (apart from the TVS).

Update: Found a P6KE250CA and when that is allowed to clamp it clamps pretty sharply around 258v, safe to proceed to the 300v version then?
« Last Edit: March 29, 2020, 11:58:06 AM by John123 »

Offline davekni

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Re: TVS diode selection for 400v transistor
« Reply #5 on: March 29, 2020, 09:08:39 PM »
The 258V clamp voltage will increase if the diode is required to clamp repetitively, enough to get it hot.  But that is a great ratio from 250V rating to 258V clamp even for single-pulse, a good low-impedance part.

Yes, I'd go for 300V.  It isn't quite guaranteed worst-case (414V), but likely to be fine.

The best I can tell, the 1500W diodes are equivalent to 2.5 paralleled 600W diodes.

The only real down-side to bidirectional TVS diodes that I'm aware of is slower clamping in the ns range due to forward recovery time of the forward-biased diode.  Capacitance is generally lower for bidirectional TVS diodes due to being two in series.  The clamping speed should be no issue for your use, since the flyback capacitor limits voltage slew rate already.
David Knierim

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #6 on: March 30, 2020, 01:16:47 PM »
Quote
It's actually a 400v NPN bipolar transistor (MJE13007)

The datasheet quotes VCEO = 400V and VCES = 700V. What do you have to do to get the 700V rating? Is it as simple (!) as having the driver clamp the base voltage to 0V during the off time?

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Re: TVS diode selection for 400v transistor
« Reply #7 on: March 30, 2020, 04:30:55 PM »
The 258V clamp voltage will increase if the diode is required to clamp repetitively, enough to get it hot.  But that is a great ratio from 250V rating to 258V clamp even for single-pulse, a good low-impedance part.

Yes, I'd go for 300V.  It isn't quite guaranteed worst-case (414V), but likely to be fine.

The best I can tell, the 1500W diodes are equivalent to 2.5 paralleled 600W diodes.

The only real down-side to bidirectional TVS diodes that I'm aware of is slower clamping in the ns range due to forward recovery time of the forward-biased diode.  Capacitance is generally lower for bidirectional TVS diodes due to being two in series.  The clamping speed should be no issue for your use, since the flyback capacitor limits voltage slew rate already.

Thanks again! I learned something here.

BTW don't you just hate it when circuits perform better as a point to point rats nest of wiring and alligator jumpers than on a nice PCB?

Quote
It's actually a 400v NPN bipolar transistor (MJE13007)

The datasheet quotes VCEO = 400V and VCES = 700V. What do you have to do to get the 700V rating? Is it as simple (!) as having the driver clamp the base voltage to 0V during the off time?
Yeah I think it is something to do with that, in my circuit the base is clamped to just under its negative breakdown voltage via some zener diodes.

It's a bit confusing at any rate, during my testing I accidentally went over the 400v rating before putting the TVS on and it broke down and went short instantly. It seems very fragile to breakdown compared to other bipolar transistors I tried which'll go into a destructive clamping mode and get very hot.
« Last Edit: March 30, 2020, 05:18:33 PM by John123 »

Offline davekni

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Re: TVS diode selection for 400v transistor
« Reply #8 on: March 30, 2020, 06:35:13 PM »
Yes, Vces (700V) just needs the base pulled down to the emitter voltage to sink the collector-to-base leakage current.  I'm rusty on my semiconductor physics, but having the base below emitter voltage by almost Vbe breakdown might actually make things worse than 0Vbe.  I need to consult my brother, as he is more versed in such matters.  I'd suggest clamping the base voltage to one diode-drop below emitter just to be safe.  There's no value in going farther negative.

The Vces is typically a static (DC) test.  There may also be issues during turn-off, when there's still some minority carriers in the base region (base storage charge).  Again, I'll need to think about this more and/or consult my brother.
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Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #9 on: March 30, 2020, 07:20:08 PM »
having the base below emitter voltage by almost Vbe breakdown might actually make things worse than 0Vbe.  I need to consult my brother, as he is more versed in such matters.  I'd suggest clamping the base voltage to one diode-drop below emitter just to be safe.  There's no value in going farther negative.

Thing is it eats into the output of the flyback if I just clamp it with a single diode, there needs to be some wiggle room there to prevent this from happening. The zener + diode idea was to clip it just before the emitter-base reverse breakdown happens (not related to the TVS in my original question).

BTW are TVS diodes better than zeners for low voltage repetitive waveform clipping in the hundreds of milliamps range? I'm using an 8.2v 1N5344B in anti series with a UF4004 diode between base and emitter to clip before the emitter-base junction does.
« Last Edit: March 30, 2020, 07:23:41 PM by John123 »

Offline davekni

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Re: TVS diode selection for 400v transistor
« Reply #10 on: March 31, 2020, 04:44:05 AM »
The MJE13007 data sheet has a graph called "Maximum Reverse Bias Switching Safe Operating Area", figure 7.  It shows the allowed current at turn-off vs. Vce, with different graphs for different levels of Vbe, 0V, -2V, and -5V.  I see that it's better with more negative Vbe.  Hadn't ran into that situation before.  It may improve farther to -9Vbe, but it's also possible that the curve would reverse direction and allow lower Vce at -9Vbe.  If the Vce punch-through voltage is only slightly above the 700V avalanche-breakdown voltage for Vces, then the more negative base could bring the punch-through breakdown voltage below 700V.  So, -5Vbe is good, and -9Vbe MIGHT be better.  (Punch-through is when the reverse-biased depletion regions for C-B and B-E meet in the middle of the base region.  It's a different voltage breakdown mechanism than the more common avalanche breakdown.)

Notice that -9Vbe is the maximum allowed.  An 8.2V nominal zener plus diode forward drop will be typically about 9V.  Worst case it could be a bit higher.  For a one-up hobby project, that's likely fine.  Might drop to a 7.5V zener if making more than one.  One other consideration from my brother Dan:  Many bipolar transistors have their properties, particularly beta, degrade when operated with reverse Vbe for extended periods.  This is true especially when Vbe is close to breakdown voltage.  Dan's experience is with high-frequency BJTs, not power devices, so this may or may not be significant for this device.

That zener looks plenty capable for several hundred mA.  No need to use a TVS instead.
David Knierim

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Re: TVS diode selection for 400v transistor
« Reply #11 on: April 01, 2020, 10:29:06 AM »
Wow thanks for the deep level info!

I can confirm what your brother said about the gain degrading, originally the MJE13007 measured a gain of 25 on my cheap chinese tester, but now its only 19 (permanently). Definitely some reverse breakdown damage being done, although it hasn't further degraded below 19.

I'm starting to think the MJE13007 is a standout case, I just swapped it for an MJ15003 to test (retuned for less peak collector voltage of course!) and noticed the flyback output didn't change if I used the zener diode on the base in series with the UF4004 (as opposed to just the UF4004 on its own clamping it to 0.7v negative).

In fact the circuit with an MJ15003 performs much better than the MJE13007 ever did (less waste heat too), more output voltage for a lower peak collector to emitter voltage.

I notice the MJ15003 is designed for linear applications such as audio amplifiers and boasts a large safe operating area, whilst the MJE13007 is technically geared toward switching power supplies. In ATX power supplies I've seen the MJE series used in they've always got them in pairs with base drive transformers supplying base current, makes me wonder if the weird reverse base voltage effects are down to this design requirement.

Still seems weird that an audio output transistor performs better than a SMPS part, I guess it's down to how the cycle is terminated via the transistor coming out of saturation or core going into saturation.

I think my topology might technically be some kind of ringing choke converter or blocking oscillator.
« Last Edit: April 01, 2020, 12:27:21 PM by John123 »

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Re: TVS diode selection for 400v transistor
« Reply #12 on: April 01, 2020, 08:24:06 PM »
Do you have any MJE13007 parts you haven't used yet?  You could try it at -5V instead of -9Vbe.  The beta degradation is tied to reverse BE leakage current, which typically rises sharply as you approach the -9Vbe spec limit.  I don't know if switching speeds or any other parameters degrade with beta.  You might find that an MJE13007 part operated within recommended conditions works well.

Yes, MJE13007 is an unusual part relative to my experience.
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Re: TVS diode selection for 400v transistor
« Reply #13 on: April 02, 2020, 11:40:55 AM »
Do you have any MJE13007 parts you haven't used yet?  You could try it at -5V instead of -9Vbe.  The beta degradation is tied to reverse BE leakage current, which typically rises sharply as you approach the -9Vbe spec limit.  I don't know if switching speeds or any other parameters degrade with beta.  You might find that an MJE13007 part operated within recommended conditions works well.

Yes, MJE13007 is an unusual part relative to my experience.

I've got 2 MJE13009's untouched by my transistor abusing hands, I guess I could try them and just adhere to a strict negative turn off bias from the get go with suitable zeners.

Do you think the gain degradation happens instantly over a couple cycles or gradually over the course of a few minutes, seems a bit odd how according to my chinese tester the gain dropped from 25 to 19 and no more after that. Its like its reached an equilibrium, maybe more reverse turn off current would push it further.

Unclamped the reverse voltage e-b acted like a zener with very poor regulation, I think I saw -11.3v across it at one point with a 12v supply.

Update: I finally plucked up the courage to scope the circuit with an arcing flyback connected, firstly you were very right about the 8.2v zener being too high. I'm seeing it clamp at 8.6v + UF4004 so yeah too much for the transistor, I've dropped to a 7.5v zener now and it clamps the base with some safety margin.

Secondly the MJE13007 seems to be very slow to turn off in this circuit without additional base drive components, the MJ15003 works great out the box and produces a much higher dv/dt rate for more power on the output. The peak drain voltage can be the same but the rate of collapse seems to be key for bigger arcs.
« Last Edit: April 02, 2020, 06:56:16 PM by John123 »

Offline davekni

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Re: TVS diode selection for 400v transistor
« Reply #14 on: April 02, 2020, 08:20:25 PM »
John,

Probably best to switch to a part that is working well.  Turn-off speed is certainly important for flyback use.  The MJE13007 inductive turn-off characteristics are specified with -2.5A base drive.  With lower base-drive currents, other transistors designed for higher-frequency might function better.  (My guess as to why -5V is better for this part is that internal base resistance requires that much voltage to get the -2.5A recommended for inductive load turn-off.)

I don't know if the beta degradation has a plateau or not.  In general it's a cumulative process, so more use should make it worse.  Since the goal is usually to avoid degradation, data on extended degradation probably isn't valuable enough to measure.
David Knierim

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Re: TVS diode selection for 400v transistor
« Reply #15 on: April 06, 2020, 08:04:52 PM »
Thanks!

btw a question about hv flyback transformer safe unloaded output voltages, is there a safe maximum in relation to their rated final anode voltage where it won't stress the insulation too much? As they're getting scarce around here and CRT TV's and monitors are nearly all gone, I don't want to overvolt and damage my remaining good ones like I've done in the past and destroyed them chasing after the big volts.

Let's say the HR diemen equivalent flyback is rated for 25kv, which in terms of arc length is just over 2cm between pointed needle electrodes (ignition, not drawn out). What would be a safe maximum ignition distance I could push it to, 3cm? I had another 25kV rated flyback jumping a 7cm gap for a brief time before the insulation failed, but that was for like 10 seconds, I guess the max safe distance is going to be lower for continued operation.

I can't really tell until it's too late lol. Are pins and arcing to the core a good indication of over voltage stress? One of them starts trying to arc from around the HV out area to the core at 35kV unloaded.

Here is the chart I'm following
« Last Edit: April 06, 2020, 08:30:52 PM by John123 »

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Re: TVS diode selection for 400v transistor
« Reply #16 on: April 06, 2020, 10:07:11 PM »
John,

For how far to push voltage, I don't have any good advice.  There's likely an exponential function of life time vs. voltage, perhaps something like 10th the life for every 10% above rated voltage.  But I have no idea on the actual factor, and it likely is a steep function of temperature as well.

Concerning pins arcing, I'm guessing there's a pin that needs to be grounded.  Many CRT flyback transformers have an internal high-value resistor from either the HV output or from a lower-voltage tap (ie. after the first diode in the output winding/diode string) to a pin.  This pin is used with an external usually-adjustable resistor for focus voltage and/or other grid voltages.  )Some flybacks have the adjustment POTs integrated with the flyback.)  The resistance is 268meg on one part I checked.  I think some parts had two such resistors, but none I have now do.  This pin should be tied to ground, or through a resistor to ground to measure output voltage.  If left floating, it does tend to arc, and that may be internal rather than external, causing failure even below rated output voltage.
David Knierim

Offline petespaco

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Re: TVS diode selection for 400v transistor
« Reply #17 on: April 07, 2020, 12:13:11 AM »
I wonder if electron microscopes might use/still use a flyback transformer.
I googled:
flyback transformer for electron microscope
and got several hits for companies that appear to make and/or sell them.

We used to have a machine that worked sorta like an electron microscope, at 20,750 volts (supposedly, just below the level where x-rays would become a problem).  Instead of scanning, it wrote characters about 120 microns high onto our special dry processed microfilm  (3M).  At the time, one of my jobs was to make sure that the flyback transformers were up to spec.   We had them made by Setchell-Carlson in St.Paul, Mn., in small quantities, of course.
I visited their lab a couple of times to work out specific issues.  They really knew what they were doing.

FYI, in case you are interested, here's a webpage that describes that machine:
https://spaco.org/History/3M-Graphic-Systems-Hardware-History/115EBR.htm

Pete Stanaitis
---------------
 

Offline John123

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Re: TVS diode selection for 400v transistor
« Reply #18 on: April 07, 2020, 12:38:59 AM »
Ah thanks! My thinking with leaving it floating was to reduce loading on the output end.

The pin for the bottom of the divider resistors and HV return (ABL) pins 11 and 7 are about 1cm apart so that makes for about 10-15kV between them when driven hard, this is for a TV flyback. I think this is the schematic.


Although it could also connect to the final anode like this one, this is my thinking for the reduced performance of monitor flybacks when we use them in these homemade drivers. The HV out is always being loaded down (this schematic is also from a TV model however).


Do PC monitor ones have any differing drive requirements such as frequency and drive waveform, maybe the core material is of a different composition. I've noticed they never seem to put out as much when external primary coils are wound on the exposed part of the core for low voltage driving. Like you're lucky to get more than an inch ignition out of a monitor flyback for the same amount of input power as a TV flyback putting out 2 inches +.

I wonder if electron microscopes might use/still use a flyback transformer.
I googled:
flyback transformer for electron microscope
and got several hits for companies that appear to make and/or sell them.

We used to have a machine that worked sorta like an electron microscope, at 20,750 volts (supposedly, just below the level where x-rays would become a problem).  Instead of scanning, it wrote characters about 120 microns high onto our special dry processed microfilm  (3M).  At the time, one of my jobs was to make sure that the flyback transformers were up to spec.   We had them made by Setchell-Carlson in St.Paul, Mn., in small quantities, of course.
I visited their lab a couple of times to work out specific issues.  They really knew what they were doing.

FYI, in case you are interested, here's a webpage that describes that machine:
https://spaco.org/History/3M-Graphic-Systems-Hardware-History/115EBR.htm

Pete Stanaitis
---------------
Wow that looks interesting, cool website too! What sort of thing went into designing and coming up with those HV flyback transformers? I assume they were specific and unique to every CRT or else there wouldn't be so many different makes, models and variants out there!
« Last Edit: April 07, 2020, 12:54:59 AM by John123 »

Offline SteveN87

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Re: TVS diode selection for 400v transistor
« Reply #19 on: April 07, 2020, 01:25:47 AM »
This might be of interest (I bought one a while back):

https://www.amazon.co.uk/ZREAL-Flyback-Transformer-Engraving-Cutting/dp/B07F6FRX6C

No data supplied and couldn't find much on the web.

Haven't destroyed used it yet, but have done some measurements:

Primary inductance: 7.7 mH
Primary DC resistance: 0.6 ohm
Core: probably 3C90, Le=186mm, Ae=215 mm^2, no gap

The core is just held in place with a bit of hot melt, so it's quite easy to remove. I added a 0.85 mm (per leg) spacer, which makes the primary inductance 475 uH. With that gap, a separate winding of 22 turns measured 92 uH.

As a lockdown exercise, I'm simulating a general-purpose flyback driver; I might build one and try it with this victim flyback...


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.

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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 »

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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

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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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[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
March 20, 2024, 04:09:59 AM
post Re: 160mm DRSSTC II project | Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Benjamin Lockhart
March 20, 2024, 01:13:23 AM
post Re: Phase Lead Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Keybored
March 20, 2024, 12:45:16 AM
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[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
March 20, 2024, 12:30:30 AM
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[Dual Resonant Solid State Tesla coils (DRSSTC)]
Benjamin Lockhart
March 19, 2024, 11:12:24 PM
post Re: 160mm DRSSTC II project | Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Late
March 19, 2024, 09:47:49 PM
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[Dual Resonant Solid State Tesla coils (DRSSTC)]
Late
March 19, 2024, 09:44:19 PM
post Phase Lead Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Saattvik24
March 19, 2024, 06:52:09 PM
post Re: 160mm DRSSTC II project | Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
March 19, 2024, 05:02:44 PM
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Mads Barnkob
March 19, 2024, 05:01:41 PM
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[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
March 19, 2024, 04:31:02 PM
post Re: 160mm DRSSTC II project | Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
March 19, 2024, 03:59:54 PM
post Re: Benjamin's DRSSTC 2 in progress
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Benjamin Lockhart
March 19, 2024, 06:41:39 AM
post Re: Welcome new members, come say hello and tell a little about yourself :)
[General Chat]
davekni
March 19, 2024, 04:05:49 AM
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OmGigaTron
March 18, 2024, 09:08:35 PM
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2020-Man
March 18, 2024, 09:07:35 PM
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Twospoons
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MRMILSTAR
March 18, 2024, 03:51:33 PM
post Re: 160mm DRSSTC II project | Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Late
March 18, 2024, 02:59:46 PM
post Re: 160mm DRSSTC II project | Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Late
March 18, 2024, 02:33:25 PM
post Can I Trust This Super Cheap Site?
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2020-Man
March 18, 2024, 11:02:12 AM
post Re: Where's all this voltage coming from?
[Spark Gap Tesla Coils (SGTC)]
Twospoons
March 18, 2024, 02:36:11 AM
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Mads Barnkob
March 17, 2024, 07:42:55 PM
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Michelle_
March 17, 2024, 04:15:14 PM
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Michelle_
March 17, 2024, 05:05:04 AM
post Re: Where's all this voltage coming from?
[Spark Gap Tesla Coils (SGTC)]
davekni
March 17, 2024, 04:50:51 AM
post Re: 2x Panasonic Inverter Microwaves - what to salvage, dangers?
[General Chat]
Twospoons
March 17, 2024, 04:45:17 AM
post 2x Panasonic Inverter Microwaves - what to salvage, dangers?
[General Chat]
Michelle_
March 17, 2024, 04:17:51 AM
post Where's all this voltage coming from?
[Spark Gap Tesla Coils (SGTC)]
Terry
March 17, 2024, 01:29:32 AM
post Re: DRSSTC Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
March 17, 2024, 12:33:06 AM
post Re: DRSSTC Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Keybored
March 16, 2024, 08:46:20 PM
post Re: Bleeder resistor for MMC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hydron
March 16, 2024, 08:39:24 PM
post Re: DRSSTC Questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hydron
March 16, 2024, 08:21:44 PM
post Best forum for vacuum tube amplifiers?
[General Chat]
yourboi
March 16, 2024, 08:20:13 PM

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