# How to determine PWM Frequency of a MOSFET to drive a BLDC motor?

What are the factors determining the PWM Frequency of the MOSFET driver circuit that drives a BLDC motor.

Is there any equation for PWM calculation?

For eg. I need to control the speed of a BLDC motor- (PMSM ME0201013001) with spec, 48 V, 5 kW (100 A continuous), 5000 rpm, 0.013 mH p-p inductance, 22 mOhm resistance.

I plan to use STH315N10F7-6 as my MOSFET which has Rds(on)max=2.3 mOhm @ Vgs=10 V and is driven by a Pre-driver L9907.

Since my power electronic knowledge is not so strong, I need to ask, how does the MOSFET circuitry (two half bridges in each phases of BLDC) produce a Very High Current of 100 A in Motor Phases?

Circuit reference is from an evaluation board of ST Microelectronics.

I have seen some posts in Stack but that does not work out for me! (Sorry!!)

The frequency used for PWM to drive a motor is a design choice.

Choosing a very low frequency can result in:

• the motor making audible sounds

• the motor might not run very smoothly

Choosing a very high frequency can result in:

• the induction of the motor coils preventing the motor from working

• more switching losses in the motor driver circuit, it might become hotter which is bad.

So selecting the right frequency is a choice that is done by the system designer. It is a compromise between everything listed above and sometimes other factors as well.

how does the MOSFET circuitry ...100 A in Motor Phases?

It does not produce the current. The current that flows is also a result of the total design. Only if the supply can deliver at least 100 A, the MOSFETs are capable of conducting 100 A, the MOSFETs are cooled well enough to support 100 A, the circuit board supports 100 A, all series resistances are low enough to support the flowing of 100 A and finally the motor is designed to draw up to 100 A, will 100 A flow. That 100 A will not just flow by itself, a lot of conditions need to be satisfied to make such a large current flow in a motor driver.

• Thanks for the information. But I have to move some how forward! Any helping article or websites would be helpful Commented Jan 15, 2019 at 14:10
• Yeah, well, as explained, not everything in electronic design can be described in a formula. <all Asians run away scared and crying>. Also designing an 48 V, 100 A motor driver with not so strong power electronic knowledge will be quite a challenge. I would consider it a challenge and I have over 25 years of experience in electronics. Commented Jan 15, 2019 at 14:18
• @Bimpelrekkie , do you think you can rewrite your comment so as to be less potentially offensive re: "all Asians?" Commented Jan 17, 2019 at 17:49

My quick mental calculator says..

Voltage sags to 37% @ 1T and ~ 5% @ 10T =L/Rtot.

, 0.013 mH p-p inductance, 22 mOhm resistance.

Thus Tmin = 10 * 13u/22m=6us

If Tmin is at 1 or 99% duty cycle f max= 1/600us = 1.7kHz

If Tmin is 10% or 90% then f max= 17kHz

But if Rs effective of clamp diode is higher than 22mOhm f max can be raised.