Help , Class D Amp mosfet - gate troubles.

Thread Starter


Joined Oct 3, 2012
Hello, I have a problem with the resistance R79 (attached image). So I hypothesized a malfunction of the mosfets. I connected the oscilloscope to the outputs of the amplifier, provided a wave (if I remember correctly, sinusoidal 440Hz) and a load of 8 Ohm. The result is what you see in the image, and in detail.
WhatsApp Image 2020-07-10 at 16.45.04 (1).jpeg
Detail for Left and right Channel:
WhatsApp Image 2020-07-10 at 17.53.16.jpeg
WhatsApp Image 2020-07-10 at 18.03.57.jpeg
I checked the gate and it was very disturbed. (I have no photos unfortunately). I removed the mosfets and rechecked the gates and the result is the following.
WhatsApp Image 2020-07-10 at 20.09.16.jpeg

The output of Q6 is apparently good, that of Q5 is disturbed.
The noise is present on the power supply of both the driver (TC4423EPA) and the photocoupler (HCPL-0611) consequently on the signa. It is correct before the optocoupler (TP14 - 15) . I reconnected the source and drain only of the mosfets q5 and q6) to recheck the power supply with the correct load and both burned (source driain shorted). Is there a valid reason why the two mosfets connected without the gate burnt out or were already defective and burnt out completely? Source drain voltage is 98v for q5 and -98 for q6.
Would it be possible to replace the q5 and q6 mosfet with a diode to test the power supply and the gate (as if it were the diode inside the power mosfet) without risking destroying other mosfets or other components? If yes, how do I size it?
Another thing, when I carried out the tests ( the mosfets were already burnt and therefore shorted) at the gate of q6 the signal appeared continuous at q6 and pulsed at q5 but much less noised. So the noise can depend on the lack of mosfet or do I have to look for other causes? Thank you, see you soon.