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Old 7th March 2005, 12:56 PM   #21
subwo1 is offline subwo1  United States
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The FQP46N15 MOSFETs are superior to the NTP32N15 ones, I think. What I meant is that if two amps are supplied by a single supply or a +/- dual supply, and the input, stereo or mono, to one them is inverted in respect to the other, the bass reproduction of the two amps will draw power through MOSFETs on both rails simultaneously so long as the inputs have not been independently tone-altered in the bass region of the audio spectrum. The output is put back in phase simply by reversing the speaker connections from one amp. Charles has mentioned this method in the past.
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Old 7th March 2005, 01:01 PM   #22
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Besides, the logical thing is that overvoltage should have lead to failure of both modules, right?
Not necessarily since one FET might fail at some Volts lower than the other one.

But the fact that your caps come after the fuses rules the supply pumping (for being the cause) out to almost 100 %.

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Charles
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Old 7th March 2005, 01:16 PM   #23
Pierre is offline Pierre  France
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Thanks, subwo1. I knew what you meant. But no, there was no pumping compensation at the moment of the failure.

Charles, you are right. Sorry if I insist too much, but I don't think that the supplies have increased more than 45V unless it is too fast that I can't see it with a several ms/div scale. (that's not the nature of pumping, anyway)

Assuming that the rails are +/45V, the mosfets have Vds(br)=150V and that there is no supply pumping, is avalanche still a possibility?

Pierre
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Old 7th March 2005, 01:38 PM   #24
subwo1 is offline subwo1  United States
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You're welcome, Pierre. It seems not to be from pumping. I think it could be from two MOSFETs in a totem being gate-enhanced at the same time. I see two possible causes of that scenario. One is that the Miller effect is turning one back on because its drain voltage rise has exceeded allowable limits. The way to prevent failure from this problem usually involves slower or delayed turn-on of the opposite MOSFET until the first can get itself off and the voltage flown back by the inductor to the opposite power supply rail.

Another is that your MOSFET drive is being triggered spuriously or randomly a minimum of one time, since once is enough to cause cross-conduction, we know. I wonder what other possible causes of the failure there are. I probably left out something.
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Old 7th March 2005, 01:43 PM   #25
Pierre is offline Pierre  France
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Perhaps when connecting inductive loads (i.e. speakers), the PWM waveform has higher spikes at turn off, and it eventually reaches an unallowable Vds value? That could cause one mosfet to fail to shortcircuit due to avalanche, so in the next cycle the other fails too due to overcurrent.

Next time I connect the speakers I will check that waveform and compare it with the resistive load case (where almost no overshoot was found).

If there is another waveform that could be useful to measure, please tell me, I will be doing tests this weekend hopefully.

Pierre
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Old 7th March 2005, 03:51 PM   #26
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Hi Pierre, I once seemed to attribute to the inductance of the speaker more villainy than it deserved as far as hardship on the MOSFETs themselves. I think now that, generally, its impedance is too high at the switching frequency to put too much relative strain on them. It may contribute to power supply pumping some, though it doesn't seem to affect even that nearly as much as the filter inductor. But it is good you were able to rule out power supply pumping.

In the vein of the two other ideas I mentioned before, I think there could be a chance that at near maximum duty cycle, one MOSFET could be conducting nearly all the time. But when it switches off, it sends the diode on the opposite rail into freewheel conduction. But before that diode's MOSFET gate enhancement delay times out, the lower MOSFET turns back on again before the diode has time to recover. This explanation reminds me of one analogspiceman gave in the past. The question seems to be then, was the output at or near clipping at the time of failure?
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Old 7th March 2005, 07:22 PM   #27
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It failed while playing music, but it was not clipping I think.
However, how can that be corrected if it is the cause, adding more dead-time?

Your explanation about speaker inductance seems sensible: although it can make things very bad if the loop is not properly compensated, it shouldn't do harm to the PWM waveform itself.

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Old 7th March 2005, 07:49 PM   #28
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You're welcome. Since the top MOSFET gate charging process aborts in this scenario before it gets beyond V(th), if the deadtime were increased more time would be given for the freewheeling diode to start tapering off its conduction. However this option may not be optional since it can increase distortion greatly.

A better alternative may be to limit the input to the amp to keep the duty cycle from getting too close to 100%. If it were simply limited to 98%, there should be enough time for the associated MOSFET to begin conduction and short the voltage across the body diode. Preventing the amp from clipping prevents the filter resonance from making an appearance and adding to distortion, which is related to control loop retaining its effectiveness along the lines of your comment.

Chris mentioned a possibly ideal way which I think is hard to actually implement. It is to prevent the freewheel diodes from conducting by having the timing of MOSFETs precise enough to avoid the body diodes from ever forward biasing. This way would give the minimum distortion too. But this way would seem very hard for the DIYer to implement, but I suspect that the Hypex UcD amps achieve it.

One thing in your favor which should thwart one of the potential appearances of one of the three cross-conduction failure modes, namely the second one, is that the IR2110 has internal logic which prevents both outputs from being driven on simultaneously. That feature is one that I like about the chip. I blew plenty of MOSFETs from that occurrence before I found the IR chip.
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Old 7th March 2005, 08:26 PM   #29
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It is to prevent the freewheel diodes from conducting
This can also be done using a technique developed by Brian Attwood. He constructed the output coil as an auto-transformer that feeds two fast-recovery diodes so that the body diodes never get into conduction state.

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Charles
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Old 7th March 2005, 09:01 PM   #30
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Below you can see what I am talking of.

Regards

Charles
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