Thanks to everyone for the information. The FETs I am using are IRF3205's. I had these doubled up ( 8 of these in the H-bridge) to avoid the need for large heat sinks. These diodes in these FETs are rated 1.3 volt drop at 59amps, and the FET itself is .008 ohm. The spec sheet says typical reverse recovery time for the diode is 110nS, and turn on time is "negligible". These numbers are very close to the numbers Brent used in his example calculation, so my max heat dissipation should occur at 100% duty cycle. That would be (10amps per FET)^2 * .008 = 0.8 W, which is still OK for a 220 case without heat sink. So it looks like I am OK without adding large external diodes. -Nicholas ----- Original Message ----- From: "Brent Brown" To: Sent: Thursday, August 17, 2000 4:46 AM Subject: Re: [EE]: Freewheeling or EMF diodes question > > 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 > > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu