Transient positive voltage spikes killing MOSFET drivers

[MarkM]

I really appreciate all of the great info!  Thanks for the replies.  I'm going to go through everything and research.  Some of this is over my head ATM.

[nuno]

The application I was simulating was 5 - 10KW DC motor control (<80V). High currents and high ESR -> heat on the electrolytic caps.

I've seen the "double cap level" recommendations for this application, but I could see in simulation that, under certain conditions, the bulk eletrolytics can resonate with the ceramic/film ones; the problem with this is increased RMS current on the caps. And it's hard to actually measure this (at least at my hobby level), so how can you be sure it's not happening?

I've also seen some designs with only film, but that translates to too little capacitance to leave me unworried about battery - DC link caps loop inductance.

Due to the power involved in this application is hard to protected from all cases, and so in my limited knowledge, I prefer to keep things simple and add tolerance.

[T3sl4co1l]

The application I was simulating was 5 - 10KW DC motor control (<80V). High currents and high ESR -> heat on the electrolytic caps.

Yikes!  Pile on the caps, indeed!

I've seen the "double cap level" recommendations for this application, but I could see in simulation that, under certain conditions, the bulk eletrolytics can resonate with the ceramic/film ones; the problem with this is increased RMS current on the caps. And it's hard to actually measure this (at least at my hobby level), so how can you be sure it's not happening?

If they are resonating, there is voltage somewhere, and there is current somewhere [else].  The voltage ripple need not be much because the impedance will be low, but the current can be detected easily with an inductive probe.

If you can guarantee the correctly damped combination of ESR and impedance by design, that's probably fine.

I've also seen some designs with only film, but that translates to too little capacitance to leave me unworried about battery - DC link caps loop inductance.

Due to the power involved in this application is hard to protected from all cases, and so in my limited knowledge, I prefer to keep things simple and add tolerance.

For something like that, about all you can do is pile on electrolytics -- the voltage is too low for films anyway, but you might consider the low-voltage equivalent of them, which is aluminum polymer (equivalent in terms of ESR time constant and energy density).  With such low impedances, it's fairly unlikely that you'll get resonances, but it can still be checked in the simulator, assuming some typical wiring inductances and ESR.  And maybe even PCB resistance..

And for the battery end, you can also invoke some rough estimate of battery ESR (which depends on all sorts of things, such as charge state and ionic diffusion, worse for some types), which might not help much, but you can also look at using TVSs to clamp what spike remains (of course, it needs to be a bit larger than your average SMAJ size TVS... have fun with that?).

Tim

[nuno]

I've seen the "double cap level" recommendations for this application, but I could see in simulation that, under certain conditions, the bulk eletrolytics can resonate with the ceramic/film ones; the problem with this is increased RMS current on the caps. And it's hard to actually measure this (at least at my hobby level), so how can you be sure it's not happening?

If they are resonating, there is voltage somewhere, and there is current somewhere [else].  The voltage ripple need not be much because the impedance will be low, but the current can be detected easily with an inductive probe.

Only if you mean one of those utterly expensive "proximity" current probes, because otherwise there isn't space for any other kind of (current) probe. The voltage ripple is indeed very small, and the frequency is high (extra problem for those special current probes).

If you can guarantee the correctly damped combination of ESR and impedance by design, that's probably fine.

Problem may be getting the specs from the caps.

For something like that, about all you can do is pile on electrolytics -- the voltage is too low for films anyway, but you might consider the low-voltage equivalent of them, which is aluminum polymer (equivalent in terms of ESR time constant and energy density).  With such low impedances, it's fairly unlikely that you'll get resonances, but it can still be checked in the simulator, assuming some typical wiring inductances and ESR.  And maybe even PCB resistance..

I'll have to look more carefully at polymers, because at the 1st look I got the impression that they are too "sensitive" compared to electrolytics.

And for the battery end, you can also invoke some rough estimate of battery ESR (which depends on all sorts of things, such as charge state and ionic diffusion, worse for some types), which might not help much, but you can also look at using TVSs to clamp what spike remains (of course, it needs to be a bit larger than your average SMAJ size TVS... have fun with that?).

Yes, just a little larger, lol. From the simulations, the worst thing is battery - controller (DC link caps) wiring inductance.

[T3sl4co1l]

Only if you mean one of those utterly expensive "proximity" current probes, because otherwise there isn't space for any other kind of (current) probe. The voltage ripple is indeed very small, and the frequency is high (extra problem for those special current probes).

Expensive?



Two turns on the end of some RG-174, so what?

I'll have to look more carefully at polymers, because at the 1st look I got the impression that they are too "sensitive" compared to electrolytics.

How so?

The capacitance is stable, the ripple, ESR and lifetime are specified, and the breakdown voltage is conservative.

Apparently, they breakdown much like film caps do, self-healing at about double the rated voltage.

The only downside seems to be the wear mechanism, which leads to failure (ESR rising I think???) at high temperatures.  The culprit is ingress of humidity, which degrades the polymer.  As this is a single order diffusion mechanism, lifetime should be quite good at lower temperatures (which are easier to achieve, thanks to the low ESR).

I don't use them all that much, mainly because their ESR is too low!   I don't have many applications where electrolytic ESR (or an estimate thereof) isn't enough.  But they're perfect for a really low impedance application like yours.  If you don't mind the price of course...

Tim

[nuno]

Expensive?
(...)
Two turns on the end of some RG-174, so what?

Ah, that... I was thinking about those that look like a pen.
I see what you mean, there will be high current in this application (so the source signal is strong) and we just want to see if it's ringing so no need to have an absolute amplitude measurement... makes sense, thanks for the tip!

The capacitance is stable, the ripple, ESR and lifetime are specified, and the breakdown voltage is conservative.

Apparently, they breakdown much like film caps do, self-healing at about double the rated voltage.

The only downside seems to be the wear mechanism, which leads to failure (ESR rising I think???) at high (...)

Ok, I'll have a better look at them.