I previously asked about switching losses in the MP24830 - I seem to have sorted that out a bit with a FET with lower gate charge and things are working ok in buck mode. However in buck-boost, which I select by the addition of a solder link and extra diode on the PCB, things go into disarray. Most obviously I see the FET gate being driven with a frequency well in excess of the set 120kHz. It’s as though the MP24830 is shutting down and restarting!
In my circuit I have FB set for an output of 1.6A and putting it into a 10ohm load. I wind the input voltage up from zero and start seeing activity on the FET gate at around 17V in. Unintuitively as I raise Vin the duty cycle seems to increase! Till I hit around 21V and get the mishmash that can be seen on the attached image. If I increase the load (ie decrease resistance) to 3ohm the activity starts again around 15V in but the funny stuff does not start till around 35V. In this latter instance I see good regulation at the output during that time with commensurate diminishing duty cycle.
Can you provide any clue for the cause of this behaviour?