SNVSB97A December 2018 – September 2021 LM34936-Q1
The input side MOSFETs QH1 and QL1 need to withstand the maximum input voltage of 30 V. In addition, they must withstand the transient spikes at SW1 during switching. Therefore, QH1 and QL1 should be rated for 60 V or higher. The gate plateau voltages of the MOSFETs should be smaller than the minimum input voltage of the converter, otherwise the MOSFETs may not fully enhance during startup or overload conditions.
The power loss in QH1 in the boost mode of operation is approximated by:
The rise (tr) and the fall (tf) times are based on the MOSFET datasheet information or measured in the lab. Typically, a MOSFET with smaller RDSON (smaller conduction loss) will have longer rise and fall times (larger switching loss).
The power loss in QL1 in Buck mode of operation is shown in Equation 30: