9.2.2.4 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 a proper MOSFET based on a tradeoff between conduction loss and switching loss. For a top-side MOSFET, FOM is defined as the product of the MOSFET's 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's on-resistance, R_DS(on), and the total gate charge, Q_G.

Equation 17.

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

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's on-resistance R_DS(on), input voltage (V_IN), switching frequency (F), turnon time (t_on) and turnoff time (t_off):

Equation 18.

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 switching loss. The MOSFET turnon and turnoff times are given by:

Equation 19.

where

• Q_SW is the switching charge,
• I_on is the turnon gate driving current,
• and I_off is the turnoff gate driving current.

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

Equation 20.

The gate driving current total 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:

Equation 21.

The conduction loss of the bottom-side MOSFET is calculated in Equation 22 when it operates in synchronous continuous conduction mode:

Equation 22.

If the SRP-SRN voltage decreases below 5 mV (the charger is also forced into non-synchronous 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 of the freewheeling current goes through the body diode of the bottom-side MOSFET. The maximum charging current in non-synchronous mode can be up to 0.9 A (0.5 A typical) for a 10-mΩ charging current sensing resistor, considering the IC tolerance. Choose a bottom-side MOSFET with either an internal Schottky or body diode capable of carrying the maximum non-synchronous mode charging current.

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

Equation 23.

where

• Q_{g_total} is the total gate charge for both the upper and lower MOSFETs at 6V V_REGN.