Mosfet reliability in Class D amplifiers

[Pierre]

Hello all.
This thread is a continuation of "Help with feedback", which I started months ago. It has derived in a interesting question that worths the pain we treat separately.
When testing some of my amplifiers, some of them have failed due to shorted mosfets.
Some of the members point to avalanche phenomena, others to the body diode and excessive dV/dt in unrated devices.

"analogspiceman" proposes that the dV/dt rating in repetitive avalanche mode is not given in all the mosfets, particullarly one of the ones I am using, NTP35N15. However, he said that FQPP46N15 is rated at about 6V/ns, although I can't find this data in the datasheet. However, a pair of them have also failed in the same way. Where did you find that value?

Others propose layout as the main cause: some bouncing can trigger false conduction and hence instant failure of both devices.

Any ideas are welcome.
Thanks.

Pierre

 

[Pierre]

Peter, I can't find info on the mosfet you previously used, the ones you say that worked very bad. What was the problem with them, did they overheat or simply failed in a similar way as mine?

BTW: the mosfets you say you use are TO247, but the photos you posted show TO220 mosfets, what are them?

Best regards

 

[hypnopete]

Pierre,

I had problems with getting a clean gate drive waveform, which resulted in shoot-though etc. They died in a much more spectacular fasion i think, they totally exploded =) .

Yep, very observant of you, they are actually the same mosfet, just in different packages. I opted for the TO-220, as it was a bit smaller, but has a lower peak power disipation rating as expected, in my situation this wasn't a problem.

Regards

Peter.

 

[Pierre]

You had a worse gate waveform with the other mosfet but the same layout? Perhaps that had a bigger Qgate?

Best regards
Pierre

 

[hypnopete]

I don't think they did, it might have been more to do with the Millar charge. I am not sure though, also the output slew rate was quite slow in comparison.


I had a fair bit of trouble keeping the fets cool as well, at frequencies above 30 or so kHz.

Peter

 

[classd4sure]

Hello,

I posted this already but It seems to be the best app note going on this topic, well explained and even providing worked examples to evaluate repetitive avalanche SOA.

Here's the link again:

http://www.semiconductors.philips.com/acrobat_download/applicationnotes/AN10273_1.pdf

I've seen it written by someone who knows alot about how to handle mosfets that the zener drain-gate feedback technique can be troublesome with fets rated at greater than 100V, because the parasitics of the devices in that range lead to oscillation well into the Mhz range, and you wind up with greater than 20V on the gate, causing spurrious turn on.

Ever seen this problem? Of course I'm not sure how relevant that is because of the always advancing technology..

Regards
Chris

 

[Kenshin]

Keep in mind that the reapplied dv/dt limit is never exceeded by too strong of turn off drive (one need only limit the turn on drive). This is best done with a simple dv/dt feedback circuit from drain to the appropriate spot in the gate drive. Simply increasing the gate resistor will serve to limit maximum dv/dt, but it also slows down the already slow enough parts of the transition, thereby unnecessarily increasing switching losses.

why strong turn off never exceed the dv/dt limit?

 

[analogspiceman]

Originally posted by analogspiceman
Keep in mind that the reapplied dv/dt limit is never exceeded by too strong of turn off drive (one need only limit the turn on drive). This is best done with a simple dv/dt feedback circuit from drain to the appropriate spot in the gate drive. Simply increasing the gate resistor will serve to limit maximum dv/dt, but it also slows down the already slow enough parts of the transition, thereby unnecessarily increasing switching losses.

Originally posted by Kenshin
Why would strong turn off never exceed the dv/dt limit?

One should never say "never", but the mosfet dv/dt problem generally only occurs in a totem pole structure where one of the mosfets' body diodes is first conducting substantial current at the end of dead time and then is immediately (i.e., before it fully recovers) forced, by sudden turn on of the other mosfet, to support too much voltage too quickly, thereby inducing a destructive secondary breakdown in the parasitic bipolar/diode region of the mosfet. Turn off drive speed to the totem pole's "passive" device simply doesn't matter because its voltage is pinned by the current flowing in the reverse direction through its own body diode.

 

[Pierre]

About the power section schematics I posted. Someone asked why I had a shottky diode in parallel with the the low side mosfet and not in the upper side one.
It is there to protect the driver from negative spikes at VS output (referred to COM pin, which is connected to the upper leg of the current sense resistor) If I remember well it is recommended in the IR2110 datasheet or some app.note.

Is it bad placed there?

Have you guys found any other thing worth reviewing or modified? In particular, I think it is ok to connect COM pin to the upper leg of the current sense resistor instead of VSS, as it is there where the source of the LS mosfet is connected, right?

Best regards,
Pierre

 

[dmfraser]

FET Drive

I'll give you a hint on what the big guys use. They have a PNP transistor between gate and source to discharge the gate source capacitance faser, hence shutting off the FET faster. This makes it run cooler.

 

[Kenshin]

One should never say "never", but the mosfet dv/dt problem generally only occurs in a totem pole structure where one of the mosfets' body diodes is first conducting substantial current at the end of dead time and then is immediately (i.e., before it fully recovers) forced, by sudden turn on of the other mosfet, to support too much voltage too quickly, thereby inducing a destructive secondary breakdown in the parasitic bipolar/diode region of the mosfet. Turn off drive speed to the totem pole's "passive" device simply doesn't matter because its voltage is pinned by the current flowing in the reverse direction through its own body diode.

when the low side FET is pulling in current to VSS, turn it off suddenly, and the output voltage of switching stage will rise to VDD suddenly due to the output inductor ...does this matter?

 

[phase_accurate]

No it does indeed help switching in some cases. But we have to keep two cases in mind:

1.)
The output current is lower than the idle current (i.e the triangular current flowing in the output coil when an input signal is absent - maybe someone knows a better expression for this ) and then the inductor helps us with switching and even the body diode of the FET that is switching off doesn't fet into conduction. Only the body diode of the FET that is about to turn on gets some foprward voltage and therefor some current flowing. But this is not a problem because this current is taken over by the FET itself and it is therefore harmless.

2.)
The inductor current is exceeding the peak value of the aforementioned "idle current". In this case it can happen that the body diode of the FET that is turned off is getting into conduction, which is causing a reverse-recovery current-spike.

This spike can be minimised (or even completely eliminated in theory) by clever timing of the FET drive-signals.

Regards

Charles

 

[Pierre]

Following the discussion about mosfet selection, I have been surprised that Hypex's Ucd400 amplifiers use the "very advanced power mosfet" (in their own words) FDP42AN15A0.
When I have read its datasheet, I have found that it is the kind of mosfets that look perfect in the datasheet but isn't completely rated for avalanche nor dv/dt. It is rated at 150V, so one could say they have skimped a bit, as the maximum voltage of the module is about +/-65V if I am not wrong.

Yes, it has low gate charge and Rds(on), but its _single pulse_ avalanche energy is 90mJ, compared with the 460mJ rated in the IRFB38N20D, for example.

Surprisingly though, the module seems to be reliable.

So, how can one know "a priori" if one mosfet is going to be suitable and reliable for a Class-D amplifier?

One thing that has been cleared is that one mustn't go to dV/dt higher than the rating for the body diode (that isn't present in a lot of datasheets, on the other hand).


Thanks!

 

[Pierre]

Adding more info on the issue, LCAudio ZAP Pulse 2.0 modules seemed to use IRF640N from IR, right?

They have a higher Rds(on), about 150mohms, but have very low gate charge, etc, _AND_ are avalanche rated and have a 5V/ns dv/dt. They are rated at 200V instead of 150V.
From the reliability point of view, I think they are a good choice, although the Rds is a bit high.

However, perhaps the newer modules have other mosfet, could anyone tell what are they using?

Thanks!

 

[phase_accurate]

They have a higher Rds(on), about 150mohms, but have very low gate charge

This is still better than the original 640 (without suffix N) that had even 180 milli Ohms. The types with suffix N of the 530, 540 and 640 have all slightly improved Rds(on), trr and dV/dt values compared to the "original" ones.

Regards

Charles


P.S. How is your amp, Pierre ? If you want to try to "fry" some 640s (without suffix N) then I'd have some in my drawer that you can have if you like.

 

[Pierre]

A lot of thanks for the offering, Charles. You are really kind.
They are quite cheap and easy to get here, however, I would like to try another ones with lower Rds(on).
Nevertheless, I suspect that my current mosfets (FQP46N15) are quite good, they have dV/dt=6V/ns, and are avalanche rated (if I am not wrong) with single and repetitive energies of 650 and 21mJ, respectively. They have acceptable gate charge (around 70-100nC) and very low Rds(on). Looking at theif datasheet, they seem very roughed, don't they?

One question that has just come to my mind: When the amp failed, I had two modules running at +/-40V on 4 ohm speakers, and I have 7A fuses in each rail. Is it possible that the failure was caused by _one_ blown fuse that produced a misbehaviour in the circuit, causing a shoot-through current and destroyed both mosfets? Just thinking loud, that shouldn't cause both mosfets to fail and only in one of the modules...

My amp works very well apart from that esporadic and inexplicable failures. :-(

Best regards,
Pierre

 

[subwo1]

Pierre, since it sounds like you were driving two speakers independently rather than in parallel or series from a bridge amp, I tend to think that one amp failure should not cause the other to fail. But one possible though unlikely explanation is that if you send the signal through a set of mono amplifiers out of phase to counter power supply pumping, then one amp not operating could cause the other to pump a supply up, leading to the MOSFETs breaking down and passing avalanche current.

 

[Pierre]

No, both amps were operated with independent speakers and with stereo signal, not out of phase.
The supply was +/-45V at idle, and my mosfet were 150V, I haven't experienced problems with pumping so far.

Anyway, I was only thinking loud, I don't think it was due to a failing fuse as one of the amps survived without damage, and I can't find a way in that blowing a fuse can lead to failure of BOTH mosfets. BTW: the one that survived had FQP46N15 mosfets, the other had NTP32N15 from On-Semi.
BUT: the one that failed had previously broken a pair of FQP46N15 in the lab, but as I didn't have more, I put the NTPs. Perhaps the FQPs failed due to an accidental short in the lab... perhaps not! Some more tests are needed!

The thing is: do that mosfets seem rough enough to you?

BTW: The speakers were quite big, a pair of 2x15"+tweeter JBL's TR225.

 

[phase_accurate]

The supply pumping could indeed lead to such a failure if both amps are fed in parallel. This could even happen to a single amp as well if I do a little more thinking. This could only be prevented by having capacitors of significant size AFTER the fuses AND under-/over- voltage lockout.

Regards

Charles

 

[Pierre]

Next time I will monitor the VCC and VSS rails to see if they grow, but I am afraid they don't do too much.
I used 22.000uF per rail (after the fuses, of course) and music (normal LF content, not 20Hz sines or the like Anyway, climbing from 45V to 75V seems too much to me, mainly because I haven't seen that: even in my lab tests with resistive loads, I got 20Hz at 265W while observing that the rails _decreased_ due to transformer losses and 50Hz ripple, instead of having overvoltage.

Besides, the logical thing is that overvoltage should have lead to failure of both modules, right?

Thanks for your help