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DRSSTC design questions

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Physics Junkie

Wed Dec 26 2012, 05:03PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Steve Conner wrote ...

The heatsink should be connected to the same ground as the driver circuitry. The negative terminal of the DC bus should be connected to this same ground via a small class Y capacitor.

If you don't do this, enough RF voltage can appear across the IGBT insulators and gate drive transformers to arc them over, with spectacular results.

This is the only thing that really matters in DRSSTC grounding.

I'm interested that you brought this up because I've never been quite sure as to what type of line filter/EMI suppression capacitor to use. Class X1,X2,Y1,Y2,X/Y. I know the difference between X & Y and X1 & X2, but I've never found what the difference is between Y1 and Y2. Do you happen to know if it matters which to use?

RateReducer: The diagram you posted is a typical configuration. I would just follow that.

Steve Conner

Wed Dec 26 2012, 06:12PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I have no idea of the difference between Y1 and Y2, I just go to my junk box and pick a cap with lots of safety approval logos stamped on it.

I always connect the secondary base and strike rails together, to what I call "RF ground". I've run my DRSSTC with this RF ground grounded separately, or joined to the mains ground, and it seems to make no difference.

The only scenario I worry about is a separate RF ground that is poor and high impedance. Enough RF voltage could appear on it to flash over to the primary or anything else nearby.

Physics Junkie

Wed Dec 26 2012, 06:33PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Steve Conner wrote ...

I have no idea of the difference between Y1 and Y2, I just go to my junk box and pick a cap with lots of safety approval logos stamped on it.

Agreed. As long as it will protect me from getting shocked to death

As for the RF ground, I do the same thing even for my sstc's secondary. I buy a 4-5 foot rod for a few bucks from my local hardware store and stake it into the ground as deep as possible. Where I live, the soil becomes solid clay and rock at just about 4-5 feet depth, so thats my depth limit but it still works fine. The only difference I've noticed is that when I ground everything together, there is heavy interference picked up by any nearby phones, computers, Tv, etc. Doing things like disabling the keyboard, touch screens, remotes, internet...

Dr. Dark Current

Fri Dec 28 2012, 10:53AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The TVS diodes don't do anything, take for example the 1.5KE400CA, if you look in the datasheet you see it is rated for 3A peak and the voltage rises to >550V. You would need to use 200 of them in parallel to do anything and the capacitance would be collosal...

dex

Fri Dec 28 2012, 11:40AM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

Dr. Dark Current wrote ...

The TVS diodes don't do anything, take for example the 1.5KE400CA, if you look in the datasheet you see it is rated for 3A peak and the voltage rises to >550V. You would need to use 200 of them in parallel to do anything and the capacitance would be collosal...

Putting diodes in parallel? Sorry,won't work

Dr. Dark Current

Fri Dec 28 2012, 12:46PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Why not? If you put the transient voltage suppresion diodes in parallel, you increase the current rating and decrease the dynamic resistance.

RateReducer

Fri Dec 28 2012, 02:46PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

Diodes in parallel does not work because the breakthrough voltages of the diodes are not exactly the same. The diode with the smallest breakthrough voltage gets conducting at first and carries all the current.

Dr. Dark Current

Sat Dec 29 2012, 10:29AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Take for example the 400V TVS diode, 1.5KE400. It starts conducting somewhere above 400V but the voltage rises to >550V at 3A. The dynamic resistance is pretty large. That's the main reason I said putting more in parallel would help, because it decreases the dynamic resistance and the voltage pulse magnitude. If you put more 400V TVSs in parallel, and give them a 600V pulse, I'm pretty sure the currents will more or less even out.

But, take a look at this product:

It can conduct a 10,000A pulse with the voltage rising no more than 150 volts! A little difference to the 3A pulse the 1.5KE400 can take

Steve Conner

Sat Dec 29 2012, 11:05AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I agree with Dr. Dark Current, I never bothered to use TVS. If you have a nice low-inductance layout, the IGBTs' own antiparallel diodes will clamp spikes, returning them to the DC bus.

dex

Sat Dec 29 2012, 12:04PM

Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147

Parallelling more diodes in order to increase total current capability isn't good idea.
If one measures a foreward voltage drop for a bunch of a particular diode, all being in the same batch ,manufacturer,and claimed characteristics, he finds each is slightly different at a given current. This changes in a quite a nonlinear way for other currents.When diodes are put in parallel, the diode with the lowest forward voltage drop will conduct the most current.Matter of fact better the diodes' characteristic (more steep) the worse is mismatch in current sharing.In the cases of higher power or more continious run with almost identical diodes in parallel, if the temperature of some diodes were even slightly hotter than the others, more of the current would go through the most hotter ones. And these ,with more current, would heat up more rapidly, lowering its conductance even more, and after a short time few diodes carry most of the current while others stay practically current-free.. There is way to improve this situation.To put a low value series resistor in front of each diode .The voltage drop across this series resistor will tend to equal out the differences in the diodes. Needless to say this is an unefficient soloution and pain in a** if you have many diodes.And considering high frequency signals as in switching power supply or protection circuits add even more problems the answer is no.Way much better is to use one monster diode.

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