Tool/software:
Hi Everyone,
We are using UCC28070 for a boost PFC and I have a strange issue - from a certain input voltage the chip does not PWM and VAO is stuck at 0V:
Input Voltage Range - 170Vrms ~ 528Vrms (VINAC gain is 1/217 V/V)
Output Voltage - 800V (VSENSE gain is 1/266 V/V)
Up to input voltage of 524Vrms (741V on the output since its a boost) the chip is able to start-up, SS is ramping up and output voltage follows up to 800V, at 525Vrms SS still ramping up but VAO is stuck at 0V so output voltage does not rise to 800V.
What we tried:
- We checked 6V VREF - its good.
- We though maybe at higher input voltage something is happening with VINAC so we decreased the VINAC gain so VINAC will be lower but it did not fix the issue.
- We though maybe at this output level VSENSE is 2.8V so maybe it was something with the slew-rate correction so we decreased the gain of VSENSE so it will be below 2.8V before start-up but it also did not fix the issue.
- We have nothing to pull down the VAO (except for the feedback resistor and capacitors) so it must be something internally.
- If we apply input voltage >525Vrms and then decrease to 520Vrms the Chip does turn on and output voltage ramp up and if we increase input voltage back to >525Vrms the chip still operated and working (VAO does not go down to 0V).
1. Is there any special reason why at certain input voltage the UCC will not boot up (even though SS did finish ramping, and 6V Vref is good)?
2. What even more strange is some units are able to turn on while some units does not.
Your help is highly appreciated and needed.
Thank you.