MP24830 - oscillator out of whack?

Hello.

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?

MP24830 gate buck-boost|504x434

Hi Kaykay:

Welcome to our community. Can you please send me the schematic of your converter. Also I have few questions:

1)How many LED’s do you have in series at the output?
2)What is the full range on the Input voltage?
3)Is your design goal to regulate 1.6A current into the LED load?
4)If the load is not LED but resistive in nature then is the goal to create a constant current regulator for 1.6A CC at the output?

Thanks,

Hi. Thanks for replying.

Schematic attached. To run in buck I remove D2 and short across LK1 (this is a long split in a plane, not just a small resistor pad). To buck-boost I install D2 and remove LK1.

  1. The final design has 6 high-power LEDs at the output - 6.4A at 37V. At the moment I have increased the FB resistors to regulate at 1.6A (1/4 of final output) and substituted a smaller resistive load that I can modify between 2.5 and 30 ohms.

2.By design 35 to 50V.

  1. No, 6.4A into LED load but am trying first with smaller load.

  2. No, LED load. But apart from the start up with minimal current draw at less that LED turn on voltage why is there a difference?

Thank you.