10.2.1.2.4 Power MOSFETs Selection

Two external N-channel MOSFETs are used for a synchronous switching battery charger. The gate drivers are internally integrated into the IC with 6 V of gate drive voltage. 30-V or higher-voltage rating MOSFETs are preferred for 20-V input voltage and 40-V or higher-rating MOSFETs are preferred for 20-V to 28-V input voltage.

Figure-of-merit (FOM) is usually used for selecting the proper MOSFET based on a tradeoff between the conduction loss and switching loss. For a top-side MOSFET, FOM is defined as the product of the MOSFET ON-resistance, r_DS(on), and the gate-to-drain charge, Q_GD. For a bottom-side MOSFET, FOM is defined as the product of the MOSFET ON-resistance, r_DS(on), and the total gate charge, Q_G.

The lower the FOM value, the lower the total power loss. Usually lower r_DS(on) has higher cost with the same package size.

The top-side MOSFET loss includes conduction loss and switching loss. It is a function of duty cycle (D = V_OUT/V_IN), charging current (I_CHG), the MOSFET ON-resistance t_DS(on)), input voltage (V_IN), switching frequency (f_S), turnon time (t_on) and turnoff time (t_off):

The first item represents the conduction loss. Usually MOSFET r_DS(on) increases by 50% with 100ºC junction temperature rise. The second term represents the switching loss. The MOSFET turnon and turnoff times are given by:

If the switching charge is not given in the MOSFET data sheet, it can be estimated by gate-to-drain charge (Q_GD) and gate-to-source charge (Q_GS):

Total gate-driving current can be estimated by the REGN voltage (V_REGN), MOSFET plateau voltage (V_plt), total turnon gate resistance (R_on), and turnoff gate resistance (R_off) of the gate driver:

The conduction loss of the bottom-side MOSFET is calculated with the following equation when it operates in synchronous CCM:

If the SRP-SRN voltage decreases below 5 mV (the charger is also forced into nonsynchronous mode when the average SRP-SRN voltage is lower than 1.25 mV), the low-side FET is turned off for the remainder of the switching cycle to prevent negative inductor current.

As a result, all the freewheeling current goes through the body diode of the bottom-side MOSFET. The maximum charging current in nonsynchronous mode can be up to 0.9 A (0.5 A typical) for a 10-mΩ charging-current sensing resistor, considering IC tolerance. Choose the bottom-side MOSFET with either an internal Schottky or body diode capable of carrying the maximum nonsynchronous-mode charging current.

MOSFET gate-driver power loss contributes to the dominant losses on the controller IC when the buck converter is switching. Choosing the MOSFET with a small Q_{g_total} reduces the IC power loss to avoid thermal shutdown.