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ZHCSTC5B October 2023 – June 2024 LM51772

PRODUCTION DATA

封装选项

机械数据 (封装 | 引脚)

RHA|40
- MPQF135D

散热焊盘机械数据 (封装 | 引脚)

RHA|40
- QFND660

订购信息

zhcstc5b_oa

9.2.2.11 MOSFETs QH2 and QL2

The output side MOSFETs QH2 (Q4) and QL2 (Q3) see the output voltage of 48V and additional transient spikes at SW2 during switching. Therefore, QH2 and QL2 must be rated for 58V 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 53.

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 54.

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)}

and, respectively:

Equation 55.

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 below:

Equation 56.

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)}