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, rDS(on), and the gate-to-drain charge, QGD. For a bottom-side MOSFET, FOM is defined as the product of the MOSFET ON-resistance, rDS(on), and the total gate charge, QG.
The lower the FOM value, the lower the total power loss. Usually lower rDS(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 = VOUT/VIN), charging current (ICHG), the MOSFET ON-resistance tDS(on)), input voltage (VIN), switching frequency (fS), turnon time (ton) and turnoff time (toff):
The first item represents the conduction loss. Usually MOSFET rDS(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 (QGD) and gate-to-source charge (QGS):
Total gate-driving current can be estimated by the REGN voltage (VREGN), MOSFET plateau voltage (Vplt), total turnon gate resistance (Ron), and turnoff gate resistance (Roff) 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 Qg_total reduces the IC power loss to avoid thermal shutdown.