Hi All,
So I have two large (25A @ 12v) fans that I am controlling via PWM and some MOSFETS. They are working but I am seeing 62v PTP with the scope at the Fets output. I have a schottky diode across the fan terminals at the fans, but am still getting the 62v spike briefly.
Better way?
briefly.
(can you elaborate on that since you have a scope ? What is your idea of "briefly" ?
?
Why do people think there is some necessity or advantage of using a Schottky diode for this purpose?
If the diode is across the fan terminals and there is significant length of cabling between the diode + fan and driver, the spike may be due to inductance in the cables. All the more so if you happen to have included interference suppression inductors in the lines.
If you want to protect the driver from spikes, it makes sense that the "kickback" diode is fitted in the driver board itself. Don't forget to check the waveform on the supply rails also.
(can you elaborate on that since you have a scope ? What is your idea of "briefly" ?
No I can't at this time... I will when I get it back on the bench. It is a good question, and I figured someone was going to ask when I was typing the original question. Is it unrealistic to expect to clamp the spike totally or am I always going to have some?
Why do people think there is some necessity or advantage of using a Schottky diode for this purpose?
I usually use them for the low voltage drop, but in this case it was just what I had in the parts bin. That is one reason I am asking is it the correct type to be using? My understanding is a faster diode will perform better in this application?
The better spot for the diodes would be at the driver?
Due to the size of your fans 25A @ 12V 300W?? ... I would use a snubber circuit.
02660:
I usually use them for the low voltage drop, but in this case it was just what I had in the parts bin.
Well, that's as good a reason as any.
02660:
That is one reason I am asking is it the correct type to be using? My understanding is a faster diode will perform better in this application?
I don't think so. As best I know, distinctions between diode "speed" refer not to how fast they turn on, as all do so very quickly, but as to how fast they turn off, due to the persistence of carriers in the junction region.
02660:
The better spot for the diodes would be at the driver?
Absolutely. As in the article dlloyd cites, it's not just the motor, but the cabling, and you even have to account for the cabling to the power supply, so to protect the switching device, you have to have a "stiff" local return (capacitor) for the kickback diode to work to since it is connected not to ground but to the positive rail, otherwise the spike appears on the supply rails as well, which is what I suggested you test for to start with.