Hi all,
I’m repairing a Class A MOSFET amplifier with a blown channel. One N-channel output MOSFET was found shorted. Each channel uses three N-channel and three P-channel MOSFETs.
I initially replaced the failed device, but when checking the channel with thermal imaging, I noticed that one transistor was heating up a lot more than the other two. So I started to suspect the devices may not be well matched, or faulty.
To rule out if the issue is caused by mismatched MOSFETs (rather than something else in the circuit), I’m now measuring the two remaining original devices and comparing them to the replacement MOSFET.
I’m using a simple PassDiy MOSFET matching jig (I have attached a PDF to this post) with the following setup:
- Supply: 15 V
- Drain resistor: 56 ohm, 3 W
- All devices are mounted on a heatsink during testing
I did the measurements, and I found a strange behavior from one MOSFET in particular (an original device):
- On first power-up, Vgs read around 5.5–5.7 V and stayed there for the duration of the test.
- I then measured the other original device, which read ~4.2 V and didn't drift much.
- Then I measured the first MOSFET again (the one that first read 5.5-5.7V), and the Vgs read ~4.05 V. Incredibly confusing!
For reference, the new replacement MOSFET measured 4.5 V under the same test conditions and was the most stable, with the almost no drift in Vgs voltage.
My questions:
1. Could this behavior indicate a faulty or degraded original device?
2. Could it explain why one transistor heats up significantly more than the others in the amplifier?
Has anyone seen this kind of behavior when matching output MOSFETs during a repair?
Any help would be very much appreciated.
Thanks!
I’m repairing a Class A MOSFET amplifier with a blown channel. One N-channel output MOSFET was found shorted. Each channel uses three N-channel and three P-channel MOSFETs.
I initially replaced the failed device, but when checking the channel with thermal imaging, I noticed that one transistor was heating up a lot more than the other two. So I started to suspect the devices may not be well matched, or faulty.
To rule out if the issue is caused by mismatched MOSFETs (rather than something else in the circuit), I’m now measuring the two remaining original devices and comparing them to the replacement MOSFET.
I’m using a simple PassDiy MOSFET matching jig (I have attached a PDF to this post) with the following setup:
- Supply: 15 V
- Drain resistor: 56 ohm, 3 W
- All devices are mounted on a heatsink during testing
I did the measurements, and I found a strange behavior from one MOSFET in particular (an original device):
- On first power-up, Vgs read around 5.5–5.7 V and stayed there for the duration of the test.
- I then measured the other original device, which read ~4.2 V and didn't drift much.
- Then I measured the first MOSFET again (the one that first read 5.5-5.7V), and the Vgs read ~4.05 V. Incredibly confusing!
For reference, the new replacement MOSFET measured 4.5 V under the same test conditions and was the most stable, with the almost no drift in Vgs voltage.
My questions:
1. Could this behavior indicate a faulty or degraded original device?
2. Could it explain why one transistor heats up significantly more than the others in the amplifier?
Has anyone seen this kind of behavior when matching output MOSFETs during a repair?
Any help would be very much appreciated.
Thanks!
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