# Another freewheel diode question



## jehan12413 (Feb 4, 2010)

I noticed one of the old curtis controller cores I have uses 4 MUR2020 diodes. These have a rating of 20A but the controller was rated at 275A. When I modify controllers I have always gone under the assumption that the diodes must be able to handle at least as much current as the switches. These would have had a total current rating of 80A so my question is why did it not blow up? By trying to at least equal the switch rating am I using an unnecessarily large number of diodes? Thanks


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## Tesseract (Sep 27, 2008)

The MUR2020 is a dual rectifier with common anode (very rare configuration). I'd guess that both cathodes are connected together on the board to run them in parallel. But yeah, this is why the Curtis - and many other controllers of similar size - don't really put out the current they claim, or at least not for any longer than a few milliseconds without blowing up.


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## jehan12413 (Feb 4, 2010)

Tesseract,
Thanks for the answer, 4 diodes did seem to be a pretty low rating to me.
One more question, as an experiment I am thinking of building one using some higher rated diodes (600v, 80A) which I found in a TO 247 case. The reverse recovery is listed as 85 ns, is this too slow? If I recall correctly the MUR 2020 is 35 ns. Thanks


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## Tesseract (Sep 27, 2008)

Sure, 85ns is still plenty fast enough for the Curtis... if it takes less than 500ns to switch its MOSFETs on I'd be very surprised.

This is, however, something you should verify by looking at the drain-source waveform, in particular the time it takes to drop from Vbatt to 0V (turn-on time). The controller has to be connected to an inductive load (motor) and not a light bulb for this measurement to be accurate, btw.


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## ngrimm (Oct 19, 2007)

I was under the impression that as a general rule, the freewheel diodes carry their highest current when the mosfets are at 50%. And that is why the don't need the same current capacity as the mosfets. Is this true?


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## jehan12413 (Feb 4, 2010)

Tesseract said:


> Sure, 85ns is still plenty fast enough for the Curtis... if it takes less than 500ns to switch its MOSFETs on I'd be very surprised.
> 
> This is, however, something you should verify by looking at the drain-source waveform, in particular the time it takes to drop from Vbatt to 0V (turn-on time). The controller has to be connected to an inductive load (motor) and not a light bulb for this measurement to be accurate, btw.


Thanks again. I'll take a look as soon as I get som free time to work on it some more. Just ordered some of the TO-247 type to try out. I figured I'll try mounting them sideways higher up on the heatsink that way I can bend the leads 90 deg and use the appropriate holes in the circuit board. This will however lengthen the lead length between the diode and switch about 1/2". I just hope that isn't enough of an increase to cause problems. Guess I'll find out once it's finished.


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## Tesseract (Sep 27, 2008)

ngrimm said:


> I was under the impression that as a general rule, the freewheel diodes carry their highest current when the mosfets are at 50%. And that is why the don't need the same current capacity as the mosfets. Is this true?


Nope - but the input capacitors see the highest amount of ripple current at 50% duty cycle, so that may be what you are thinking of.

If the switches are operating with duty cycle D then the freewheeling diodes are operating with duty cycle 1-D. This even applies when the switch is totally off - if the motor somehow turns into a generator (by, e.g., letting the car roll backwards when it is in a forward gear) a strong jerk will be felt from the freewheeling diodes suddenly becoming forward biased enough to conduct.


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