> I agree with you it pays to do the math - the juice is always right!
>
> Cheerful regards,
>
> Bob

Thanks for your comments Bob, I think you guys are right in that
50% duty cycle is the time of worst power dissipation in the diode.

Let me scratch my brain and think about this duty cycle thing
again, in terms of power dissipation for each device. Consider
current to be the same during on and off phases because of the
motors inductance:-

For an example, using NDP7050's, assume 75A at 100% duty
cycle. FET is on continuously, power is 75A^2 x 0.01ohms = 56W.
EMF diode is doing nothing so power = 0W.

At 50% duty cycle lets assume 37.5A (this is quite an assumption
as it depends on the motor characteristics and frequency, but
should suffice for the calculations). Power in MOSFET, neglect
switching losses, is 37.5A^2 x 0.01ohms x 50% = 7W. Diode
power is 37.5A x 0.9V x 50% = 17W.

Lets put it in a table:-

For 100% duty cycle FET power = 56W, diode power =   0W
For  75%  duty cycle FET power = 24W, diode power = 13W
For  50%  duty cycle FET power =   7W, diode power = 17W
For  25%  duty cycle FET power =   1W, diode power = 13W
For    0%  duty cycle FET power =   0W, diode power =  0W

Hmm, interesting. I must admit that it took me a while to beleive
that the diode dissipation would be worst at 50%. Yes the diode
dissipation is higher than the FET some of the time, but hey - the
figures are still pretty realistic and relative.

Using expensive and bulky shottkey power diodes in parallel with
the FETs would reduce power dissipation a little, but mostly only
around the 50% duty cycle point. You need to ask yourself in a
given application where the motor is run the most.

Cheers everyone and goodnight, Brent.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile: 025 334 069
eMail:  brent.brown@clear.net.nz

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