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Q:
Could you please briefly tell me which MOSFETs are on/off and which MOSFETs are switching when in each mode?
A:
In Buck Mode, Q3 is on always and Q4 is off always and Q1 and Q2 are switching. In Boost mode, Q1 is on always and Q2 is off always and Q3 and Q4 are switching.
When VIN is near VOUT, the converter will boost for a few cycles then buck for a few cycles.