Snubber circuit design

I just had a look at the Epcos application notes. They specifically recommend using their varistors for clamping spikes due to switching off an inductive load. Take a look: (sorry for the long URL, I'll break it across a few lines, since I don't know if I'll mess things up otherwise.)

Certainly, if a varistor is used to divert a lightning strike then at best only one operation can be expected before the varistor becomes useless, but I do not see any mention of a slower wear-out mechanism in the Epcos application notes, except the warning that the varistor must not be allowed to reach an excessive temperature.

I believe that the OP intends to put a large motor-run type capacitor in parallel with the varistor so that it will only be in unusual circumstances that the varistor will carry the full welder current.

Chris

I thought that it could simply flow into the capacitor through the resistor.

There are two kinds of events, really, a transient switch off during bridge switching, and a complete shutdown (turn off) of the bridge. Note that most complete shutdowns would happen while NOT under power, for obvious reasons. I cannot be switching AC switch off while welding.

I suppose that shutdowns due to arc extinguishing are rare and softer.

For transient switch offs...

Suppose that, for worst case example, 300 A is flowing into the circuit. Then a switching even occurs and the circuit opens for 2 microseconds (a very long time for IR22141SS that can likely be reduced severalfold).

That means that if the current continues to flow without any reduction, into the capacitor, without change, the charge of the capacitor would be

300 * 2E-6 = 0.0006 coulomb.

A 10 uF capacitor charged to 0.0006 coulomb would have a voltage across its leads of 0.0006/10e-6 = 60 volts. That's quite survivable, especially in an RCD snubber.

That makes perfect sense. I have the snubber diagrams right in front of me, RCD has a diode in parallel with resistor. Right?

What kind of diode should serve as that D. It should survive currents up to, say, 300-400 A and voltages of a few hundreds of volts.

Also, if I read you correctly, a MOV is not necessary, is that right?

I also measured inductance of my reactor. It seems to be about 1 mH.

I used this technique

and used both high and low Ohms on my decade resistor. High ohms were useless, I did it more for curiosity.

i

You are 100% right. I will use a relatively large capacitor, which should be no problem with an RCD type snubber (if I am not mistaken). It would not make starting conditions severe and would not make starting welding arcs make a sound of a .22. Then the varistor would be for catching emergency situations, etc, and not strictly necessary.

i

that means you have to give the inductor current somewhere to commutate to, whilst the bridge is off.

thats not gonna work so well. that R carries the full load current, so will need an impressive peak pulse power rating.

an RCD snubber, with a monstrous D, is probably what you want.

you can read more languages than I can.

they may not mention the most important part - the wear-out characteristics of MOVs. basically they can clamp a finite amount of Joules, then they go short circuit. Then (often within a few hundred microseconds) they go *BANG*

applications using MOVs to clamp repetitive surges are generally best suited for breaking MOVs.

find a manufacturer (eg Seimens, or whatever they are called now) that actually makes the things, and read their app notes.

Cheers Terry

Got it. Are any particular ones more suitable? I looked at digikey and found too many dazzling choices.

I see. It's more for protection when "shit happens", like electrical ground. It does not normally see any use.

CORRECTION, it is about 2-5 mH, see my another post Measured INDUCTANCE of my reactor. I used some wrong data when I made my previous post.

Thanks. I looked up the splat test. I am not quite sure how would I measure current with my oscilloscopes. Tek 2465 and 475. I kind of like this idea though, that's definitely the way to go.

I did some calculations of capacitance of snubbers for given parameters. The I used formula from Fuji

Cesp = L*Io^2 / (Vcep - Ed)^2

L inductance Io normal current Vcep snubber peak voltage Ed DC supply voltage

The results are mind boggling. My calculations suggest that I need about 50,000 uF (!) for 250V caps. Either my math is wrong somewhere, or else I am missing something very big.

The above calculation applies to complete turnoff of the H bridge. Which should almost never happen under load. I assume that conditions of arc extinguishing are much milder.

Complete turnoff at high current, I think, should happen rarely. Maybe never. In those instances, a snubber may provide appropriate protection.

In regular operation, turn off time would be very short, perhaps 0.5 uS or so. To absorb 0.5 uS's worth of high current, I need a lot less capacitance. (microfarads)

So, I can say that I am quite confused.

i

yep^2

not strictly speaking, no.

not necessarily much use for your inductor, as it only pumps very low current thru the inductor. If the core material permeability varies strongly with H, then this will give an incorrect reading - invariably an over-estimate. google groups "splat test" for a trick that doesnt have that problem.

Cheers Terry

pass. its gotta carry the right amount of current, and have an appropriate voltage rating - at least peak snubber cap voltage.

use a pulse generator (555 timer) and a decent FET, a current shunt and a low-voltage supply with shitloads of capacitance - ensure Ecap >> Einductor at measured current.

the latter.

inductor energy Eind = 0.5*L*I^2 initial cap energy Ecap_init = 0.5*C*Vinit^2 final cap energy Ecap_final = 0.5*C*Vfinal^2

Efinal = Einit + Eind

0.5*C*Vfinal^2 = 0.5*C*Vinit^2 + 0.5*L*I^2

C*Vfinal^2 = C*Vinit^2 + L*I^2

C*[Vfinal^2 - Vinit^2] = L*I^2

C = L*I^2/[Vfinal^2 - Vinit^2]

so yeah, there's a bit of a mistake in your numbers, because the equation is wrong.

yeah Q = CV = It, can assume I is constant. So 200A for 0.5us = 100uC =

100V across 1uF. You've also got stacks of volts to play with....

the typo in the fuji app note doesnt really help, does it :)

Got it.

I think that I can buy 15 Panasonic ERZV20D271 varistors, they would provide max voltage of 225 VDC, clamping voltage of 455 VDC, and 135 joules each. I would parallel them to get appropriate joule rating.

The hope is that these varistors would never actually get a chance to conduct current. The H bridge should not be stopped at full load unless someone moves the current switch handle during welding, which is unlikely. I hope that interruptions of the welding arc would happen more slowly and thus the highest peak voltages/currents would not occur. I may be mistaken.

For a diode, I could use a 300U60AMA diode, 300 A, 600 V rated. It's kind of pricey, maybe I can parallel several cheaper diodes? Such as

3-4 Fairchild diodes RURU10060?

I see. I may do that. Good idea.

Thanks, you are right. Still, the required capacitance is tremendous.

Very nice to know, makes life easier.

Thanks Terry. Your thoughts about component selection for the snubber (see my text in the middle of the message)will be appreciated...

i

I was kinda shocked by the price of diodes for 300+ amps and thought about something.

In my spare parts pile, I have a big heavy TT170N silicon controlled rectifier:

It is rated for 1,200V, 350 amps. Can I just use it as a diode in an RCD snubber if I supply proper voltage to its gate continuously to turn it on?

i