Failure during PFC+LLC integration

[biswaIITH]

Hii,

I am working on a 750 W battery charger project.
PFC topology

• Boost topology with UCC28180 control IC from Texas instruments.
• Switching frequency is 105khz

DC-DC topology

• LLC half bridge converter(Split cap configuration) with secondary synchronous Mosfet.
• Digital control technology is implemented using C2000 series MCU
• Secondary side synchronous Mosfet drive is implemented through UCC24624.
• Gate driver is UCC27714
• Operating frequency range is from 93khz to 200khz
• Resonant frequency is 153khz
• Lm= 110uH,Lr = 27uH ,Cr= 40nF
• Dead time is around 220nSec

Problem statement

• During the standalone load test of individual converter, both converters are working perfectly fine with desired performance.
• When I am integrating both PFC & LLC , & apply 230Vac at the input there is a gate driver & mosfet failure of LLC converter at the starting itself with out any load.
• LLC mosfet pulses are applied with a particular frequency(250khz) before 230V is applied in integration testing
• For standalone LLC testing , i am applying the pulses (250khz) to both high & low side mosfets first & then apply 390 V at the input

Is it happening due to absence in softstart mechanism for LLC ?? If it is the reason, then why it is not failing in standalone testing

 

[mtwieg]

Hard to give specific advice, the failure is probably related to very specific implementations details, and you don't provide a schematic. Or any waveforms.

• LLC mosfet pulses are applied with a particular frequency(250khz) before 230V is applied in integration testing
• For standalone LLC testing , i am applying the pulses (250khz) to both high & low side mosfets first & then apply 390 V at the input

What exactly do you mean here? Does the LLC converter use a PWM controller chip, or are you directly defining the PWM waveforms to the LLC on startup? What provides the low voltage power supply to the PWM controller and gate drivers? In a real implementation it will have to be derived from the 390V bus, but it sounds like you're providing an external supply (otherwise how could the MOSFETs be driven before AC is applied).

 

[cupoftea]

• Operating frequency range is from 93khz to 200khz
• Resonant frequency is 153khz

Are you sure youre not operating it in the capacitive region, where the diodes may get severe reverse recovered?...your min freq is a long way below your res freq.

 

[biswaIITH]

Are you sure youre not operating it in the capacitive region, where the diodes may get severe reverse recovered?...your min freq is a long way below your res freq.

Yes . There is no problem. Operation at minimum frequency is well above the capacitive mode.

Is it happening because of wrong or absence of soft start mechanism in LLC converter while integrating?

Should I power on each stage independently i.e. first PFC is powered on, 390Vdc is built in the bulk capacitor & then enable LLC with appropriate soft start mechanism???

--- Updated Mar 18, 2023 ---

Hard to give specific advice, the failure is probably related to very specific implementations details, and you don't provide a schematic. Or any waveforms.

What exactly do you mean here? Does the LLC converter use a PWM controller chip, or are you directly defining the PWM waveforms to the LLC on startup? What provides the low voltage power supply to the PWM controller and gate drivers? In a real implementation it will have to be derived from the 390V bus, but it sounds like you're providing an external supply (otherwise how could the MOSFETs be driven before AC is applied).

1. LLC converter uses C2000 processor to implement the control logic.
2. Power supply to the gate driver is independently given via isolation flyback supply.

Is it happening because of wrong or absence of soft start mechanism in LLC converter while integrating?

Should I power on each stage independently i.e. first PFC is powered on, 390Vdc is built in the bulk capacitor & then enable LLC with appropriate soft start mechanism???

 

[mtwieg]

Soft start implementation might be the issue, but there's no way for us to tell. The LLC being controlled by a MCU also complicates things. You're going to have to give much more detailed info about how you're testing this. For example:

1. Power supply to the gate driver is independently given via isolation flyback supply.

This doesn't explain anything. I was asking about how the power to the control circuits and the PFC/LLC is sequenced. Perhaps in initial testing, the control circuit supply is independent from the PFC/LLC, and is always applied before power is applied to the PFC/LLC. That might work fine, but in a more realistic scenario the control power comes after the PFC/LLC. In this case, you may have problems if your hardware/firmware doesn't default to a safe state.

Let me ask this: when testing with just the LLC, have you tried resetting the MCU during operation? Or otherwise forcing its code to re-initialize its peripherals and control variables?