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SLVAF76 August 2021 TPS62901 , TPS62902 , TPS62902-Q1 , TPS62903 , TPS62903-Q1

2.1 Conduction Losses in the MOSFET

The conduction loss in the MOSFET inside the buck converter is the power loss due to the on-resistance (R_DS(ON)) of the high-side FET and low-side FET as they conduct current. As the high-side FET is on and the low-side FET is off, the conduction loss of the high-side FET can be estimated from the output current, on-resistance, and on-duty cycle (D). As the high-side FET is off and the low-side FET is on, the conduction loss of the low-side FET can be estimated from the output current, on-resistance, and off-duty cycle (1-D). Use Equation 1 and Equation 2 to estimate the power loss on both the high-side and low-side FETs. Equation 1 and Equation 2 provide a good estimate of the FET conduction power losses and how the R_DS(ON) contributes to efficiency:

Equation 1.

C o n d u c t i o n_L o s s_H_F E T = {I O U T_r m s}^{2} \times R D S O N_H \times D

Equation 2.

C o n d u c t i o n_L o s s_L_F E T = {I O U T_r m s}^{2} \times R D S O N_L \times (1 - D)

D is duty cycle and can be calculated as VOUT/VIN. Choosing a converter that has low R_DS(ON) is key to improve efficiency of the system. The TPS62903 has very low R_DS(ON), the high-side FET on resistance is 62 mΩ and the low-side FET on resistance is only 22 mΩ. This helps on the reducing the conduction loss.