SLUS892D December 2009 – December 2019 BQ24610 , BQ24617
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 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:
where
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.
where