Flyback diodes for fans

PMU
1

Hi, I have 2 radiator fans setup on a PMU16 using outputs 2 and 15 and PWM control using a table based off coolant temp and whether AC is on, and i get these circuits saying shutdown overheat. I did install flyback diodes, i think i used 1n5408 diodes only because these were recommended in the documentation for something i found, but i don’t think they survived. Can you recommend the correct diodes to use? I can’t remember the exact specs of the fans, but they have a max current of around 20A and are overheating at 75% DC drawing around 14A.

Edit; the fans are set at 100Hz

2

Those diodes are for an average current of 3 A.
That’s not enough in this case.

The average current of the diode should be at least half of the maximum load current. With the load current of 20 A, the diode should be rated at 10 A or more.

Another rating is the maximum reverse voltage, which must be 30 V or more.

This is one example of a diode that will work. It is a Schottky diode, which means it has a lower voltage drop (lower power loss).

Here are the filtered results of all Schottky diodes that meet those requirements.

Lowering the frequency will also result in less heat.

3

Is there a way to know the optimum frequency to set?

4

There is no optimum.

Higher frequency allows for smoother operation and lower audible noise.

Lower frequency generates less heat in the switching device and less electrical noise.

5

Thankyou. Once again, excellent customer support.

1 Like
6

Sorry, one last question. Am i correct that these would not be required on a PMU24 on outputs 1 and 16?

7

Yes, that’s correct.