13.24.2 Bootstrap Capacitor

The high-side driver bootstrap capacitor needs to power the high-side driver and gate for 1/3 of the motor electrical period for a 3-phase BLDC motor operating in Six-Step mode.

Let:

MOSFET Driver Current = 300 mA

PWM Period = 50 µs (20 kHz) Minimum Duty Cycle = 1% (500 ns) Maximum Duty Cycle = 99% (49.5 µs)

V_IN = 12V

Minimum Gate Drive Voltage = 8V (V_GS)

Total Gate Charge = 130 nC (80A MOSFET)

Allowable VGS Drop (V_DROP) = 3V

Switch RDSON = 100 mW Driver Internal Bias Current = 20 µA (IBIAS)

Solve for the smallest capacitance that can supply:

• 130 nC of charge to the MOSFET gate
• 1 Megohm gate source resistor current
  • Driver bias current and switching losses

Q_MOSFET = 130 nC

Q_RESISTOR = [(V_GS/R) × T_ON]

Q_DRIVER = (I_BIAS × T_ON)

TON = 49.5 µs (99% DC) for worst case

Q_RESISTOR = Q_RESISTOR

Q_DRIVER = 20 µA × 49.5 µs = 0.99 nC

Sum all of the energy requirements:

• C = (Q_MOSFET + Q_RESISTOR + Q_DRIVER)/V_DROP
  • C = (130 nC + 0.594 nC + 0.99 nC)/3V
  • C = 43.86 nF

Choose a bootstrap capacitor value that is larger than 43.86 nF.