# How to calculate switching frequecncy [duplicate]

I am trying to control a brushed DC motor with PWM control through a MOSFET, below is the basic circuit for an idea. The PWM signal is given from a microcontroller to the gate driver IC which drives the MOSFET.

What is the nominal switching frequency? How to calculate the PWM switching frequency for this motor control circuit?

simulate this circuit – Schematic created using CircuitLab

• It's probably not a good idea to use a 100V MOSFET to switch 180VDC through the load. (Avalanche!) Nov 14, 2013 at 13:27
• @Madmanguruman:Actually I am using IRFP460 forgot to update the MOSFET PartNo.
– siva
Nov 14, 2013 at 13:56
• D1 shouldn't be a Zener diode. A Zener makes no sense here. It would be nice if it could be a Schottky diode, but at this voltage you probably have to settle for a fast silicon diode. Nov 14, 2013 at 15:49

A quick and dirty ballpark estimate is 5 or 10 times the max motor speed.

For example, if your maximum motor speed will be 6000RPM:

$\dfrac{6000~RPM}{60~sec/min}=100~\text{rotations per second or Hz}$

$100\text{Hz} \cdot 5 = 500\text{Hz}$

This may or may not be okay for your application since that frequency falls in the range of human hearing. If this is a concern, 20kHz to almost always a safe starting point.

Being a DC motor, it really wants to be driven by a DC voltage. So it would be more accurate to base it on the characteristics of the motor. A motor is typically modeled as a resistor and an inductor in series which equates to a first-order inductive low pass filter so we're in luck. As long as the PWM period is lower (the frequency is higher) than the time constant of the filter, the motor will essentially see DC.

So if the time constant is:

$\tau=\dfrac{L}{R}$

And:

$\tau=\dfrac{1}{f}$

then your frequency needs to be higher than:

$f>\dfrac{1}{L/R}$

Use that as a starting point and then measure the current ripple. Decide how much current ripple is acceptable and increase the frequency as needed.

• Do we have to care about conduction type? either it is continuous or discontinuous like we do in SMPS ? May 20, 2023 at 19:42