DC motor PWM flyback diode
DC motor PWM flyback diode
(OP)
I have a simple circuit for controlling the speed of a 12V 10A dc motor using 1 mosfet on the low side. For cost reduction I opted for no heatsink and chose a mosfet with the lowest RDSon. This worked well with no much heating.
However I am getting a lot of heat in the flyback diode. I tested different types HER602, MUR820, MUR1620 but all of them heated up after a few minutes. Since all diedes have a forward drop of about 1V, I cant think of a way to solve this without adding a heatsink.
Any help please?
However I am getting a lot of heat in the flyback diode. I tested different types HER602, MUR820, MUR1620 but all of them heated up after a few minutes. Since all diedes have a forward drop of about 1V, I cant think of a way to solve this without adding a heatsink.
Any help please?





RE: DC motor PWM flyback diode
The speed of the diode is also an important parameter. The built-in diode in the MOSFET is fast, compared to most other diodes.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: DC motor PWM flyback diode
The diodes I tested have 35ns recovery time which are fast enough. In fact I tried a mosfet instead of the diode and that heated much faster.
I will try a lower frequency and see the results. Thanks.
RE: DC motor PWM flyback diode
Remember, you run without any heat sink. That means you have already left trodden roads and may need to deviate also from standard switching frequency to make the thing work properly.
Yes, low frequencies can produce audible noise. If you cannot tolerate that, you will probably have to surrender and use a heat sink.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: DC motor PWM flyback diode
I had to go down to 250Hz to get it working with no much heating at no load. However when loading motor the diode starts to heat again. Will have to test it at normal working load and see how it goes.
RE: DC motor PWM flyback diode
RE: DC motor PWM flyback diode
How much current passes through the flyback diode? If the motor takes 10A how much will the diode take back - continuous and pulsed? If I can work with 2A diodes then I may consider paralleling otherwise I have to opt for heatsink.
Another question - I dont have any current monitoring at the moment - just a fuse. The mosfets can handle 100A and the fuse is 15A so they should be safe enough right? But if I monitor the mosfet/diode temperature, will that have the same effect as monitoring the current - temp/current should be proportional right?
RE: DC motor PWM flyback diode
Benta.
RE: DC motor PWM flyback diode
So the question is - if schottky diode is better than the Ultra fast diode why do the latter exist? Or when should one be used or the other?
Still need to know how big diode should be for a 10A motor please.
RE: DC motor PWM flyback diode
RE: DC motor PWM flyback diode
Upsides to Schottkys:
Low forward voltage
Nice reverse recovery characteristics (capacitive, no stored charge)
Downsides:
Leakage current (not a problem in a 10 A application
Reverse voltage normally below 100 V, unless you start using exotic materials like SiC.
Cheers,
Benta.
RE: DC motor PWM flyback diode
Back to the motor flyback current. With the scope connected across the motor at 20kHz I have almost perfect square wave with a few ns ringing at the edges. On the OFF period I have a negative 0.4V which means that the diode is conducting. How can I determine the current passing through the diode please?
At 250Hz the wave form becomes a little complicated to explain.
RE: DC motor PWM flyback diode
RE: DC motor PWM flyback diode
If you cannot find a resistor, you can make one from a piece of wire. Make sure the wire is heavy gauge so it doesn't heat and change resistance. Wire inductivity is not a problem at the switching frequencies you are dealing with. If you want to reduce inductivity you can fold the wire and twist it. That reduces inductivity at least ten times. For better results, use a coaxial shunt or a high speed DC clamp.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: DC motor PWM flyback diode
Will get a resistor and have a try.
RE: DC motor PWM flyback diode