9.2.2.5 Power MOSFETs Selection

Four 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 19 V - 20 V input voltage.

Figure-of-merit (FOM) is usually used for selecting proper MOSFET based on a tradeoff between the conduction loss and switching loss. For the top side MOSFET, FOM is defined as the product of a MOSFET's on-resistance, R_DS(ON), and the gate-to-drain charge, Q_GD. For the bottom side MOSFET, FOM is defined as the product of the MOSFET's 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), MOSFET's on-resistance (R_DS(ON)), input voltage (V_IN), switching frequency (f_S), turn on time (t_on) and turn off time (t_off):

Equation 7.

The first item represents the conduction loss. Usually MOSFET R_DS(ON) increases by 50% with 100°C junction temperature rise. The second term represents the switching loss. The MOSFET turn-on and turn-off times are given by:

Equation 8.

where Q_sw is the switching charge, I_on is the turn-on gate driving current and I_off is the turn-off gate driving current. If the switching charge is not given in MOSFET datasheet, it can be estimated by gate-to-drain charge (Q_GD) and gate-to-source charge (Q_GS):

Equation 9.

Gate driving current can be estimated by REGN voltage (V_REGN), MOSFET plateau voltage (V_plt), total turn-on gate resistance (R_on) and turn-off gate resistance (R_off) of the gate driver:

Equation 10.

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

When charger operates in non-synchronous mode, the bottom-side MOSFET is off. As a result all the freewheeling current goes through the body-diode of the bottom-side MOSFET. The body diode power loss depends on its forward voltage drop (V_F), non-synchronous mode charging current (I_NONSYNC), and duty cycle (D).

The maximum charging current in non-synchronous mode can be up to 0.25 A for a 10-mΩ charging current sensing resistor or 0.5 A if battery voltage is below 2.5 V. The minimum duty cycle happens at lowest battery voltage. Choose the bottom-side MOSFET with either an internal Schottky or body diode capable of carrying the maximum non-synchronous mode charging current.