Hello everyone,
I was repairing a Class D Type 4 amplifier with a simple output section failure, which had a dead FET on the low side. After removing that FET, I noticed it’s running very hot at idle with a 4-ohm load connected.
I replaced all the FETs, but the issue remains.
The gate drive is not perfect but not terrible either. However, the PWM output from the optocoupler on the low side looks pretty bad.
Additionally, the rails have some nasty noises.
Any help would be greatly appreciated. 🙏
*Sorry, I misplaced the LS and HS
I was repairing a Class D Type 4 amplifier with a simple output section failure, which had a dead FET on the low side. After removing that FET, I noticed it’s running very hot at idle with a 4-ohm load connected.
I replaced all the FETs, but the issue remains.
The gate drive is not perfect but not terrible either. However, the PWM output from the optocoupler on the low side looks pretty bad.
Additionally, the rails have some nasty noises.
Any help would be greatly appreciated. 🙏
*Sorry, I misplaced the LS and HS
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If the low-side device fails as a short-circuit (a usual for MOSFETs), the next pulse that comes in would take with it the high side device as well. Thus, replacing only the low-side one would simply repeat the same failure, only that the next time it'd be the high-side blown and so on...
Any SC protection is usually not fast enough to protect the MOSFETs, as the PWM pulses are really quick.
Always replace both devices of the same leg together with adjacent devices from the same strip / tube if possible.
I think the high-side might be partially failed, or the gate drive itself was faulty which caused the primary failure.
I tried replacing the both banks with the same batch of mosfets but the issue still remains.
While attached to the heatsink, it reaches 70 C within 10 mins of idle doing nothing
Always check the gate-drive immediately after removing the blown devices. Do not replace them unless the gate-drive is clear.
MOSFETs are rugged when the gate-drive is good but many a time the gate-drive (particularly the isolated ones) could have bad timing leading to failure of the leg.
Partial shoot through could be reason for the heat, due to bad gate-drive.
MOSFETs are rugged when the gate-drive is good but many a time the gate-drive (particularly the isolated ones) could have bad timing leading to failure of the leg.
Partial shoot through could be reason for the heat, due to bad gate-drive.
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Thanks for your help.
The shoot-through was a result of a bad choice of input for the first 2N4401. I changed the resistor from 6.8k to 10k, the feedforward capacitor from 280pF to 1nF, and removed the capacitor between pin 6 and 8 of the optocoupler. These changes seem to have solved the problem.