Hello,
I would like to ask you about UCC256403A.
(Question 1)
A PFC circuit is used in the front stage of this device.
Since the PFC output voltage oscillates at a period twice the frequency of the AC power supply input voltage, I am aware that the LLC switching frequency also fluctuates.
At present, the switching frequency fluctuates between 100kHz and 108kHz at normal temperature and rated load.
Since no filter is inserted in the output of the PFC, it is assumed that the LLC input voltage will fluctuate and the switching frequency will also fluctuate.
Do you agree with this recognition?
(Question 2)
I have confirmed that the switching frequency fluctuates significantly in an environment of -20°C.
The switching frequency fluctuates between 96kHz and 110kHz, probably because the low temperature increases the ESR of the PFC's output electrolytic capacitor and increases the ripple in the PFC's output voltage.
Is it reasonable for this change to occur?
Is there a way to reduce the variation in constants around the device?
I checked the PFC switching waveform and LLC phase margin, but there were no particular problems.
I did LLC phase compensation to speed up or slow down the response, but there was no difference in switching frequency variation.
I also changed the value of the voltage divider capacitor on the VCR pin, but it had no effect.
Best regards,
