SNVSCL2 Data sheet

SNVSCL2 December 2024 LM51770

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9.2.1.11 MOSFETs QH2 and QL2

The output side MOSFETs QH2 (Q3) and QL2 (Q4) see the output voltage of 16 V and additional transient spikes at SW2 during switching. Therefore, QH2 and QL2 must be rated for 25 V or more. The gate plateau voltages of the MOSFETs must be smaller than the minimum input voltage of the converter, otherwise, the MOSFETs may not fully enhance during start-up or overload conditions.

The power loss in QH2 in buck mode of operation is approximated by:

Equation 35.

P_{COND (QH 2)} = {I_{OUT}}^{2} \times R_{DS, On (QH 2)}

The power loss in QL2 in the boost mode of operation consists of both conduction and switching loss components given by Equation 36 and Equation 37, respectively:

Equation 36.

P_{COND (QL 2)} = (1 - \frac{V_{IN}}{V_{OUT}}) \times {(I_{OUT} \times \frac{V_{OUT}}{V_{IN}})}^{2} \times R_{DS, On (QL 2)}

Equation 37.

P_{SW (QL 2)} = \frac{1}{2} \times V_{OUT} \times (I_{OUT} \times \frac{V_{OUT}}{V_{IN}}) \times (t_{r} + t_{f}) \times f_{SW}

The rise (t_r) and the fall (t_f) times can be based on the MOSFET data sheet information or measured in the lab. Typically, a MOSFET with smaller R_DSON (lower conduction loss) has longer rise and fall times (larger switching loss).

The power loss in QH2 in the boost mode of operation is shown in Equation 38:

Equation 38.

P_{C O N D (Q H 2)} = \frac{V_{I N}}{V_{O U T}} \times {(I_{O U T} \times \frac{V_{O U T}}{V_{I N}})}^{2} \times R_{D S, O n (Q H 2)}

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9.2.1.11 MOSFETs QH2 and QL2