8.1.12.2 High-Side Power MOSFET

The next set of equations are used to calculate the losses associated with the high-side MOSFET.

Equation 25.

P_{CND_HS} is the conduction loss of the high-side MOSFET during the D interval. this equation includes a self heating coefficient of 1.3 to approximate the effects of the R_DS(on) temperature coefficient. R_{DS(ON)_HS} is the drain to source resistance, I_OUT is the output current and D is the duty ratio. P_{SW_HS} is the switching power loss during the high-side MOSFET transition time. V_IN is the input voltage, f_SW is the switching frequency, and t_r and t_f are the rise and fall times of the switch-node voltage, respectively. P_{TOT_HS} is the total power dissipation of the high-side MOSFET.

The gate charge of the high-side MOSFET greatly affects the turn-on transition time, and therefore efficiency. Furthermore, consider the ratio of switching loss to conduction loss associated with the high-side MOSFET. If the duty ratio is small and the input voltage is high, it is beneficial to tradeoff Q_G for higher R_DS(on) to avoid high switching losses relative to conduction losses. If the duty ratio is large and the input voltage is low, then a lower R_DS(on) MOSFET in tandem with a higher Q_G may result in less power dissipation.