LMG3525R030-Q1: LDO5V fails when operating in a totem-pole PFC

While testing today, we saw a similar spike in the switch node voltage at the beginning of each line cycle. The FLT trigger was too short for the microcontroller to act, causing the low side GaN FET to fail eventually. Again it seems like the LDO5V fails, when looking at the FLT signal, this is not a surprise. In these figures:

C1 (yellow) is the gate signal of the low side GaN FET, measured at the microcontroller

C2 (red) is the FLT signal from the low side GaN FET, measured at the microcontroller

C3 (blue) is the voltage at the switch node relative from DC-, measured with a differential probe 

C4 (green) is the inductor current (positive current is current flowing into the AC side of the circuit) (The first figure shows the average inductor current)

Do you maybe have an idea how we can prevent this from happening? I will add a detailed description of the circuit and some more measurements tomorrow.

Hi Thijs,

This seems to be an issue with the LDO5V and not IDM. Could you please try de-soldering the LDO5V and adding a resistor from LDO5V to the digital isolator LDO5V?

If this does not work, try a slower slew rate.

Thanks,

Noah

Dear Noah, 

Thank you for your quick reply. We added a 180k resistance between RDRV and GND to lower the slew rate and a 47E resistance between the LDO5V and VDD1 of the digital isolator. Furthermore, we replaced the 12V isolated DC-DC converter with an alternative with a lower isolation capacitance (2.5pF instead of 40pF). 

Even though these changes improved the situation, they did not solve the problem. The totem-pole rectifier was able to operate without glitches at 14.1VAC/25VDC, but there were again some glitches with operation at 28.3VAC/50VDC. This led to failure of the low side FET after a few line cycles. 

Did we use the correct resistor values for the changes? Is there something we are missing? If needed, I can provide some more information about the control scheme and the hardware design. 

Hi Thijs,

Thank you for the update with your issue. Let me review this with the team and I will get back to you soon.

Thanks,

Noah

Dear Noah,

Thank you again for your reply. The datasheet of the LMG3425R030 makes it clearer how the device behaves in IDM. 

I see that the LMG3522R030 is not in stock anymore on the website. Would you be able to provide us with some samples for development? We would need around 10 pieces.