HELP calcul power losses of FETs driving 3 phase BLDC motor

a3930 application note

Hello masters of electronics.

Does anybody know how to calculate power losses for 3 Half H bridges driving a Brushless DC motor (3 phase). This would include switching power losses and conduction power losses and body diode reverse recovery losses.
I use 3 X IRF7341Q dual FETs from International rectifier. These are only N channel FETs and their gates are driven by A3930 from Allegro. Allegro driver has charge pumps for the High side FETs and is driving the FETs through syncronous rectification. I don't have any clamping for the inductive kick from the motor, and I don't know the inductance of the motor.
I have 2 app notes from IRF but not much time to study them.
AN 967 it seems to me complicated with too much details, and for AN 1048 I couldn't find the ICalc they are talking about.
If anybody could give me a step by step guidance, I would greatly appreciate it. I know I should do this by myself but I am in a terrible time crunch and I have other things to resolve.
Any input/advice will be greatly appreciated.
I can provide further details.

power driving 3

The second AN

a3930

Anyone? Pleeeease.

bldc losses

yes i know how to do this. you need to determine the 3-phase current at a steady-state operating condition.

then the datasheet for your mosfet and the switching frequency of your inverter. that is it.

do you have these things?

how to know the steadystate.. you mentioned BLDC motor. so think of the rated speed, steady-state. that will produce a VOLTAGE because of the magnets sweeping past the stator.

your inverte rmust create a votlage that is at least this much. so this will define your modulation index.

now you have a modulation index and the rest falls into place. can you figure it out from here?

mosfet losses depend on the AVERAGE current at a given modulation index then (I_avg_at_ma)^2*Ron

a mosfet switching losses are dependent on how fast you switch on/off and the bus voltage.

see this application note for more info:
**broken link removed**

so really now what you need is the formula that calculates 3phase power from an inverter. pretty standard.. has a sin(theta). i used to have this in my notes somewhere but can't find it handy.. oh well, i'm sure you can google it.

Mr.Cool

Added after 9 minutes:

the reverse recovery is a bit more difficult because it is very non-linear at low current. i.e. the losses are significant at low current and dominate the device power dissipation.

there is a good approx. equation in the notes for infineon's software "IPOSIM". unfortunately the website says the software is "currently unavailable".. so i'll just upload the document here so you can read it. skip to the reverse recovery section. i know it is for IGBTs, but the reverse recovery losses for MOSFET is calculated in the exact same way. i like infineon's approximation for this loss, but you can use whatever method you want.

calculpower

Hi Nikeplato!

I am also going with A3930 for BLDC controller development.
And I have some issues with it.
As you have already gone with A3930, pls let me have some guideline to it.
My development system has following components and specs:
BLDC motor : 24 volt, 6 Amp, 4000 RPM, line-line inductance 1 miliHenry
Controller: A3930, Mosfets are IRFR2405
sense resistor = 0.055 ohm
RC oscillator frequency = 35 Khz with Rosc= 39 k

My issue is My Mosfets and motor both get heat upto considerable extent which we can't easily touch consistently eventhough current is not more than 3.5 ampere with 0.055 ohm sense resistor.

Hope to have favorable reply.

Thanks & Regards!