SLUSA75B July 2010 – January 2020 BQ24650
PRODUCTION DATA.
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}.
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}):
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:
where
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}):
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:
The conduction loss of the bottom-side MOSFET is calculated in Equation 22 when it operates in synchronous continuous conduction mode:
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.
where