SLUSFH5 May 2024 BQ25856-Q1
ADVANCE INFORMATION
Refer to the PDF data sheet for device specific package drawings
Four external N-channel MOSFETs are used for a synchronous switching buck-boost battery charger. The gate drivers are integrated into the IC with 5 V of gate drive voltage. An external gate drive voltage can be provided directly into the DRV_SUP pin for increased efficiency.
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, RDS(ON), and the gate-to-drain charge, QGD. For the bottom side MOSFET, FOM is defined as the product of the MOSFET's 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. Taking buck mode operation as an example the power loss is a function of duty cycle (D=VOUT/VIN), charging current (ICHG), MOSFET's on-resistance (RDS(ON)_top), input voltage (VIN), switching frequency (fS), turn-on time (ton) and turn-off time (toff):
The first item Pcon_top represents the conduction loss which is straight forward. The second term Psw_top represents the multiple switching loss items in top MOSFET including voltage and current overlap losses (PIV_top), MOSFET parasitic output capacitance loss (PQoss_top) and gate drive loss (PGate_top). To calculate voltage and current overlap losses (PIV_top):
The MOSFET turn-on and turn-off times are given by:
where Qsw is the switching charge, Ion is the turn-on gate driving current, and Ioff 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 (QGD) and gate-to-source charge (QGS):
Gate driving current can be estimated by REGN voltage (VREGN), MOSFET plateau voltage (Vplt), total turn-on gate resistance (Ron), and turn-off gate resistance (Roff) of the gate driver:
To calculate top MOSFET parasitic output capacitance loss (PQoss_top):
To calculate top MOSFET gate drive loss (PGate_top):
The bottom-side MOSFET loss also includes conduction loss and switching loss:
The first item Pcon_bottom represents the conduction loss which is straight forward. The second term Psw_bottom represents the multiple switching loss items in bottom MOSFET including reverse recovery losses (PRR_bottom), Dead time body diode conduction loss (PDead_bottom) and gate drive loss (PGate_bottom). The detail calculation can be found below:
PGate_bottom can follow the same method as top MOSFET gate drive loss calculation approach.