Problem with CAB008M12GM3 in three phase LLC application

Duclair

We have an application featuring CAB008M12GM3 module in a three phase LLC configuration with gate driver UCC21710-Q1. The setup seems to works ok when running in a 2Ph configuration with 90deg phase shift light load, but when I try running it with a 3Ph PWM, with 120deg phase shift, the gate driver signals an overcurrent fault. I have a hard time understanding why. the OC detection circuit for UCC21710-Q1 in our application is setup for a threshold of 4V (estimated).

the switching frequency when the trip occurs is around 160kHz, the tank's resonant frequency is around 140kHz. the deadband is increased at light load, along with the switching frequency, with a minimum of 400ns.

Please, do you have any idea of what could be causing this?

Thanks.

Forum_Moderator

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JHayes

Hi Duclair,

Do you have any waveforms you are willing to share? Also, can you provide more details about your system - heatsink/coldplate, Rg value, expected current/power level?

Does the trip just occur at the 160kHz operating point? does 3ph it work properly up to that power level?

Thanks,

Jonathan

Duclair

Hi Jonathan,

Thanks for your reply. I wasn't able to get some waveform to support my question. The protection circuit seems to works just fine, in fact, my modules would have fried up multiple times if it wasn't for it . This problem was solved by changing the control algorithm to use burst mode at light load on 3ph instead of deadband control. it looks like deadband control did not work because 3ph LLC behaves in some respects like a (synchronous) interleaved converter where with small duty cycle and in discontinuous conduction mode, there is a risk of the transformer/inductor being shorted with the DC bus + and - creating an increase of current at a very high rate.

Best regards,

Duclair.

TBhatia

Hi, I hope that this answered your question. I will close this discussion for now but if you have a follow up question, please "Start a New Discussion" and we would be glad to support you further.