Hello,
I am working on the design of a 2kW PFC using the controller UCC2818. I am experiencing a problem during the startup.
When the PFC starts switching, there is not soft start at all. Indeed, the maximum input current is limited by the current peak protection. I assume this behaviour is related with the leading edge modulation and the fact that the output of the current error amplifier is a 0V at the startup.
Follow capture shows the start up at 240Vac.
Yellow- Bulk Voltage
Blue- Mosfet Gate
Pink – Soft Start pin
Green- Input current
I could reduce the problem implementing a circuit to decrease the current peak threshold as it is explained on SLUA321. However there are some points I don’t quite understand:
Why there is a SoftStart circuit?
The datasheet describes the SS as a way to increase the duty cycle slowly.
However, this is not the reality because during the start-up, the gate is controlled by the saturated current error amplifier and not by the voltage error amplifier. This can be observed on the first PWM pulses on the following capture:
Next capture shows the current through the mosfet at the startup (yellow channel).
Another question is regarding the voltage on pin Vaout. Based on the datasheet this voltage should be controlled by the voltage on the SS pin at the startup. However, what I can see is a voltage step on this pin when SS is still close to zero.
Vaout – Pink
I would appreciate any feedback regarding this issue.
Thanks
JGE
