Author Topic: TVS Diode Questions (Rule of thumb maybe?)  (Read 970 times)

Offline Andy

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TVS Diode Questions (Rule of thumb maybe?)
« on: April 02, 2019, 10:04:00 AM »
Aloha Doooods!

Maybe I'm looking at all the wrong places but I can't find the information I assume I need so here I am.
I can't find any information on selecting a TVS voltage.
I understand that large portion is based off your stray inductance in your leads so each person may get a different outcome or drastically different outcome....
However, I was hoping there would be a general rule of thumb (assuming I planned carefully and kept things respectably close) for choosing
a TVS voltage based off the Bus Voltage...  Maybe plus ** volts or a typical 110% for example.
When I finish laying out my Bus I'm going to want to fire this pig up with or without those TVS.
So I would like to order some now and I noticed people don't mention them a lot and only refer to the Zeners typically.
I tried to reference peoples chosen TVS with their FET's datasheets and their bus voltage and only see they choose a voltage higher than their bus.
I put a couple project specs below. I have other questions I'm working on writing out and will post them once they are in a good order and detailed enough.

Mahalo for the help!

Current Project:
(I bought a little box of these 60N60C2D1's. So those will be what I'm using here)
I attached my JavaTC results as I have it built. (I ordered a 40cm x 10cm Toroid top load which lowers Fres down to about 220kHz)
My buss is 110 Volts through a 7.5Kva Transformer up to 270vac then through a full bridge rectifier.
I do want to be able to just Run the 110v rectified for initial testing on my variac but I know the Buss voltage will affect the MMC calculations but I'll probably just build interchangeable Bus setups to allow for 2 different bus voltages and put the transformer on the ass end of the variac for now.
(My issue is that I live in Hawaii so other than outdoors, my only room I can test this in is my living room.. .I learned a hard way trying to verify all my numbers in my lab which is a 10' Square room with a healthy size "L" shaped lab bench which is why I need to test it in my family room where I have a 110v and 240v mains outputs. But carrying out my 110-270v transformer that weighs 130 lbs Just sucks and I don't think I should toss in 240 volts with streamers jumping about 2' up to a ground wire hanging from my ceiling fan. Lol.)

Offline Mads Barnkob

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Re: TVS Diode Questions (Rule of thumb maybe?)
« Reply #1 on: April 03, 2019, 01:47:01 PM »
Steve Ward wrote a little passage about this on his website: http://stevehv.4hv.org/drsstc_design.htm

Quote
Use Transient Voltage Suppressors (TVS) from collector to emitter of each IGBT.  The TVS should typically be the 1.5kW type.  I use strings of the 1.5KE220CA seriesing up as many as needed.  They are rated to stand off 220V, so for 600V IGBTs I use 2 in series.  For my 1200V IGBTs I use 4 in series for 880V clamping.  Note that the TVS do not start to clamp "hard" until you exceed their voltage rating by some %, check the spec sheet for the exact breakdown voltage.  Keep the TVS physically very close to the IGBT terminals.  The TVS are there to catch the very fast transients that the decoupling capacitors may not catch.  IGBTs do not tolerate overvoltage very well at all!

Use TVS or zeners on the IGBT gates.  If there is a voltage transient across the C-E junction, and the gate has some capacitance with respect to the collector, this transient will appear on the gate as well (its capacitively coupled through).  I use 30V TVS of the 600W type.  Be sure to get the bi-polar TVS.  Since we are driving the gates at their limitations, we need to be extra careful not to damage them, so TVS or zeners go a long way to protect them.
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Offline Andy

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Re: TVS Diode Questions (Rule of thumb maybe?)
« Reply #2 on: April 05, 2019, 03:53:30 AM »
Steve Ward wrote a little passage about this on his website: http://stevehv.4hv.org/drsstc_design.htm

Use Transient Voltage Suppressors (TVS) from collector to emitter of each IGBT.  The TVS should typically be the 1.5kW type.  I use strings of the 1.5KE220CA seriesing up as many as needed.  They are rated to stand off 220V, so for 600V IGBTs I use 2 in series.  For my 1200V IGBTs I use 4 in series for 880V clamping.  Note that the TVS do not start to clamp "hard" until you exceed their voltage rating by some %, check the spec sheet for the exact breakdown voltage.  Keep the TVS physically very close to the IGBT terminals.  The TVS are there to catch the very fast transients that the decoupling capacitors may not catch.  IGBTs do not tolerate overvoltage very well at all!
Quote

Yes I've read this but don't feel I got the answers I'm looking for with that.

Quote
Use TVS or zeners on the IGBT gates.  If there is a voltage transient across the C-E junction, and the gate has some capacitance with respect to the collector, this transient will appear on the gate as well (its capacitively coupled through).  I use 30V TVS of the 600W type.  Be sure to get the bi-polar TVS.  Since we are driving the gates at their limitations, we need to be extra careful not to damage them, so TVS or zeners go a long way to protect them.

Well I'm using 30v 5w zeners tied anode to anode between G & E (I assume that's the same as bi-polar).  Which is easy to choose based on FET's the datasheet.
However, I also did research on the differences in Zeners and TVS and that's where this got frustrating..
I've found many voltage ratings of TVS, not just 220v.
So they TVS's can take that current on the rails but the transient voltage creates a max current that really depends on the voltage you choose and the lead inductance your layout has given meaning if I have a lot of lead inductance then I would need a higher peak current for the given wave period.
I'm very compulsive about learning all of this and why each components were chosen. TVS diodes could be used as Gate protection but based on my professors (youtube creators) there is a different application for each of them.
With that said, I understand no datasheet will give parameters directed to this unusual design. But I want to know how to calculate to order the perfect TVS for my project.
Not that the ones you referenced won't work or even that I won't buy that exact one..... but I feel this is a topic that's avoided probably due to not knowing how another person will build their bus and the inductance given. But there has to be a way to narrow down the actual values needed.
and since the TVS are tied C to E, which is only Half bus voltage on a half bridge, would that mean I should choose half of my bus voltage TVS for each FET?


Lastly- to clarify, bi-directional is the same as if I tie 2 anodes together of the uni-directional type correct? So I've made a bi-directional Zener with my 2 uni-directional zeners? And would the TVS work in the same way?

Here are a couple videos I watched. Maybe this doesn't apply to this design but inductance in excess leads can give an excess surge current.. agree? Meaning I should choose TVS with voltage and max surge current for my specific bus layout/voltage while choosing a Zener based on the max voltage tolerance of the Fet's Gate.

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Re: TVS Diode Questions (Rule of thumb maybe?)
« Reply #2 on: April 05, 2019, 03:53:30 AM »

 


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