GZPA Class D 4000W with IRFP264

[Pafi]

Originally Posted by Eva

Well, you get the idea now, but switching events don't last that long, a few hundred nanoseconds would be more accurate.

It would be accurate, if it was that short, but it is much more, even with these easily drivable FETs. Look at the gate waveform!

Yes, charging the gate to the full drive voltage can take a few microseconds,

Read what I wrote! I wrote about miller charge only. Do the calculations yourself! Or just look at the picture of mgm2000ro! With the original devices the gate is NEVER charged up fully.

Consider that the PCB is good only up to 50A/us per MOSFET or so,

This is more than enough here.

so building up the current has to take some time (achieved just with 100 ohm gate resistors and relying on finite transconductance).

But here is not only 100 ohm/gate, but a common 220 ohm for the global gate!

[Andrew Eckhardt]

Seems like having an 8V sawtooth on the gate at idle wouldn't be the best indication of linearity over the full modulation range, even with logic level gates. Fix it?

[Pafi]

Originally Posted by Andrew Eckhardt

Cheap cross conduction limiting?

Maybe, but this is still insane.

Though with that much resistance there's really no need to double the NPN drivers. Is that a verified resistance value?

The power dissipation of gate-driver doesn't depend on the gate resistor. With smaller R the current is higher, but the impulse is shorter.

I don't know if it is really that high, but gate waveform shows it is guite high really (maybe not 220 ohms), but the high side must be much stronger (judged from output waveform). B.I.G should check it!

You might be able to get away with things at 55kHz that just wont go at 550kHz, if you're used to looking at full bandwidth switching amps.

The question is: do we still have to limit the efficiency if the output devices are replaced by much better ones?

Originally Posted by darkfenriz

Turn-on current is the range of 80 to 100mA

If we belive 2V/div for the first picture and R192=220 ohms, then turn on current can't be more than 40 mA.

[Andrew Eckhardt]

I was thinking that at 200 ohms the power of the gate driving would all be lost in the resistor. (Assuming the waveform on the emitters is still square)

It might be interesting to see what happens by trying to warm up those NPN's a little bit.

Is there any/enough dead time from the modulator output?

[Andrew Eckhardt]

With 100R and a diode on each gate I'd probably try actually shorting those emitter resistors and see what happens to the supply current. Depending on what you have for a power supply you could try decreasing slowly so yo don't get any surprises. Of course you might want to make sure the gate supply isn't too high for that.

[B.I.G]

it looks like my driver board has R192=150 ohms and R191=100ohms R190=830ohm R189=880ohm and my driver transistors get very hot . without a fan i have yet to run them for more than a few seconds beacuse i was afraid of blowing them up.

my board does not use mpsa92 but ktc3228/kta1275

i can try the old light bold in series with the +12V but i think the driver bipolars will get fryed cause they already get hot...

[Eva]

Before talking about speeding up gate drive, please check the huge distance between the gate drivers and the MOSFET... Consider potential ground loops too... And don't forget the unequal current sharing that arises in such a layout when gate turn-on resisance is lowered and di/dt becomes more dependent on source lead (and PCB) inductance than on transconductance.

Slow gate drive is used for a good reason

Then look again at gate waveforms. Are you considering the effects of source lead inductance? The positive glitch indicates the beginning of conduction (charging of switching node capacitance, positive di/dt on Ls), then the negative glitch indicates that switching has been completed (capacitance charged, current drops, negative di/dt on Ls).

Throwing some low value resistors at the PCB is very easy, all newbies like to do that (I once thought it was good too).

Note that IRFP4229 does not start to conduct some current until 5.5V or so and is fully on at 6.5V

[Andrew Eckhardt]

Yes indeedy there are some mightly large kinks in those waveforms. It's all fun and games until the B+ current starts to rise.

[B.I.G]

i think what Eva is telling us is that is a poor design leave it that way or else ... i will just replace the irf3205 and try again but first i need to fiind some heatsink clamps or drill the heatsink and put screws .... the hard way....

i actualy liked how it worked on my woofer ...

Yes indeedy there are some mightly large kinks in those waveforms. It's all fun and games until the B+ current starts to rise.

than it starts to smell ... that is the smell of money burning with a thick smoke too


later edit: is IRL3705ZPBF any good for replacing irf3205 ? i can not find any in stock ... so it will take a lot more time till i get some irf3205...

[Andrew Eckhardt]

It looks like something is starting to turn on at around 2.3V and then the drains are dragging the gates back into the ground against that high drive resistance. But hey, it probably is the only way that amp it will work.