Electronics > Repair
Inverter drive module overvolt fault
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Eamon:

--- Quote from: florentbr on November 09, 2022, 10:49:12 pm ---Check the voltage on the gate/drain of Q1/Q2 with and without load.

--- End quote ---

Eureka!
The voltage on the gates did not move one bit when turning on.  I had assumed that the PFC was turning on as the outputs from the MCU suggested that was so but I should have measured at the final target!

The MCU is driving pins 1 and 8 (Channel A and B enable pins) of the UCC27524 gate driver to 3.3v about two seconds after I send the compressor on signal, then just over a second later the MCU sends a ~48kHz 90% duty cycle 3.3v square wave signal to pins 2 and 4 (channel A and B Input) BUT output pins 7 and 5 remain at 0v. pin 6 (VDD) has ~13v and pin 3(Gnd) is 0v.




Output pins 7 and 5 are each connected through a 100Ω resistor to the gates of the two 34NM60N Power MOSFETs (Q1 and Q2 in the schematic).

13v VDD is within spec (-0.3v to 20v) and outputs should be VDD +-0.3v so I think it's safe to conclude that the gate driver is bad?


--- Quote from: florentbr on November 09, 2022, 10:49:12 pm ---Assuming the PFC is failing to pull the coils L1/L2 to ground with Q1/Q2, that would explain the ACPlateVoltage rising from 220v to 320v and why the InvertDCVoltage doesn't rise.

--- End quote ---
So a bad gate driver here fits with this theory too then I guess?

I'll source a replacement and get it changed over and provide an update once done.

In the meantime, a big thank-you to those who persevered with me through to this point, I've learnt heaps and it's been fun (and frustrating).

Special thanks to capt bullshot and florentbr
florentbr:

--- Quote ---so I think it's safe to conclude that the gate driver is bad?
--- End quote ---

I would change the driver and the mosfets since a leaky gate could have killed it.
It's usually the power switching components failing first followed by the driver.


--- Quote ---So a bad gate driver here fits with this theory too then I guess?
--- End quote ---

Yes it does since the voltage at the output of the bridge rectifier is supposed to decrease when the PFC is turned ON.
Since it doesn't, I suppose it's the reason why the software is reporting an over-voltage.
Check this simulation and pay attention to the output of the bridge rectifier with PFC ON and OFF:

https://www.falstad.com/circuit/circuitjs.html?ctz=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
capt bullshot:

--- Quote from: florentbr on November 10, 2022, 03:56:49 pm ---
--- Quote ---so I think it's safe to conclude that the gate driver is bad?
--- End quote ---

I would change the driver and the mosfets since a leaky gate could have killed it.
It's usually the power switching components failing first followed by the driver.

--- End quote ---
I can confirm that from others (colleagues) experience with power electronics. I'd at least check whether the gates are shorted to drain or source - if not, one cannot be totally sure there's no hidden damage that'll strike again somewhen in the future. At that particular job, it was an unspoken rule to replace the whole driver / power board in case of such kind of failure.
The other way 'round: In case the MOSFETs failed, they were shorted Drain-Source which obviously would blow fuses or release magic smoke in this configuration. So i'd bet they're still OK and prepared to lose my stake.


--- Quote ---
--- Quote ---So a bad gate driver here fits with this theory too then I guess?
--- End quote ---

Yes it does since the voltage at the output of the bridge rectifier is supposed to decrease when the PFC is turned ON.
Since it doesn't, I suppose it's the reason why the software is reporting an over-voltage.
Check this simulation and pay attention to the output of the bridge rectifier with PFC ON and OFF:

--- End quote ---
Eamon:

--- Quote from: florentbr on November 10, 2022, 03:56:49 pm ---I would change the driver and the mosfets since a leaky gate could have killed it.
It's usually the power switching components failing first followed by the driver.

--- End quote ---
Both power mosfets (Q1 and Q2) read exactly the same resistances in circuit:
source-drain=16.5k
gate-drain=27.7k,
gate-source=9.75k

DB2 = 12.8k

What about the power diodes (D1 and D2, EPU3006)? is it a case of better safe than sorry? They're about $10 each but a relatively small amount given the time invested so far, just more soldering :palm:.  Using my DMM Diode test function in circuit they each read 0.378v, but I assume the inductors are just a resistor at DC so they're effectively in parallel so that's about right? and in reverse the voltage just keeps climbing which I assume is because the diodes are good so not passing any current and that's just the effect of the capacitors charging.
florentbr:
You'll have to remove the mosfets to check them.

Since D1, D2 and DB2 are in parallel (coil is negligible), it won't show on a multimeter if one failed open. If either D1 or D2 was open, it would expose the mosfet to high voltage spikes well over the specified limit. I would just check them off circuit.
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