Electronics > Repair

300W inverter repair - problem with H-bridge

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Harry_22:
Hi!

I can’t understand why you have -13.2 volts on the trace (trigger mark in the second picture)? Please clarify where the probe and ground were connected.

bborisov567:
Probe is connected between source and gate on the top mosfet. I should have inverted the trace. These -13.2 v are actually 13.2 V of Vgs.

Harry_22:
I tried to simulate the circuit in Microcap but the upper T6 opens too slowly. I had to increase the period from 25 µs to 100 µs so that the peak reaches 240V.
I don't see any problems with the diodes, but it looks like the circuit is slightly different.

bborisov567:
I also simulated the circuit on LTSpice and results are looking really good. Yesterday i had some more time to test and it turns out that if connect the probe gate-source the impulses are looking really clean 12 v p-p, but if i connect it source to gate (gnd to gate and active probe end to source) then i observe the oscillation shown before. I believe there might be some issue with the oscilloscope grounding to the grid that causes the bad measurement.

BennoG:

--- Quote from: bborisov567 on October 24, 2024, 05:41:05 am ---I also simulated the circuit on LTSpice and results are looking really good. Yesterday i had some more time to test and it turns out that if connect the probe gate-source the impulses are looking really clean 12 v p-p, but if i connect it source to gate (gnd to gate and active probe end to source) then i observe the oscillation shown before. I believe there might be some issue with the oscilloscope grounding to the grid that causes the bad measurement.

--- End quote ---
Never put the gnd clip on anything else then the gnd of your DUI.
Even when you have everything doucopled.
Use 2 probes and substract the value in the scope.
Or use a differential probe.

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