Hello, we have purchased your MP9931 chip and designed a BUCK circuit based on the typical circuit in your datasheet, with an input of 48V, a switching frequency of 200kHz, and an output of 12V 20A.Four layer board, with GND in the middle two layers.
During the power on test, we gradually increase the load from 0A (load in CC mode). When the load reaches around 1.8A, the output voltage will oscillate, triggering the load OSC protection mechanism. Afterwards, we changed the switching frequency from 200kHz to 100kHz, reduced the output capacitance, and obtained the same test results. We also changed the working mode of the chip from PSM to FCCM, but the situation still hasn’t improved.
The waveform is shown in the following figure (red: SW, orange: TG, blue: BG, green: output voltage).
We speculate that the protection mechanism of the chip was also triggered, but the current limit we designed is 100mV, and the on resistance of the lower tube is 1.57m Ω. The corresponding current protection threshold should be around 60A, and the 1.8A constant current load is far from reaching this threshold.
We found that when the output voltage suddenly drops to 0, the conduction time of the last gate signal BG of the lower tube will be longer than that of the BG signal when it is stable; When the output voltage remains stable at 12V, the amplitude of the signals from SW and TG will slowly decrease. I don’t know if it’s related to this.
I hope to receive your answer, thank you.