# How do I calculate the max pwm frequency my h-bridge can handle?

my configuration:
H-bridge with 4 n-ch mosfets (IRF1405) with a gate capacitance of 5480pF and turn on delay time 13nS. I am using a hip4081 as my driver which has a peak current of 3.8A! This is how I am calculating the max pwm frequency my circuit can handle:

V=IR---(1)
t=RC---(2)


from (1) R=V/I plug in (2) => t=(V/I) * C where
V=Gate Voltage (10V)
I=Max gate current (3.8A)
C=Input gate capacitance (5480pF)

plugging these values I get t = 14.4 nS. now this is the time it will take to charge the gate capacitance and so my pwm frequency should be < 1/t?

EDIT
Power = 150 - 450 W Voltage = 24VDC I = P/v = 7A - 19A

• I think you may have to add informations about load. Resistive load may allow high frequencies, but adding some capacitance or inductance may change everything. – Kamil Apr 22 '14 at 23:04
• Please read the data sheet for the hip4081. "The HIP4081A can switch at frequencies up to 1MHz". Your calculation give an approximate turn-on time for the FETs. You want the actual conduction time to be a good deal longer than this if you want decent efficiency. – WhatRoughBeast Apr 23 '14 at 0:59
• Is it a brushed DC or PM? Usually for motors, PWM frequency doesn't need to be too high ... 20 or 30kHz. – gsills Apr 23 '14 at 3:37
• it is a DC motor. – rashid Apr 23 '14 at 13:13