Schottky diode

[VinceB]

Hey guys

In a previous discussion, I was advised to use Schottky diodes across each dump load to prevent voltage spikes from damaging my power FETs. The thing is I haven't got a clue what spec. to look for. The battery bank is 12 V and each dump load is rated 2 ohms. I have no idea what the inductance value would be, so I can't predict what the voltage spike will be like. The type of resistor is wirewound.

Basically, what current rating should I look for?? And what voltage value?

Thanks a lot

[Nando]

Vince:

You do not need Schottky diodes, Standard Silicon Diodes will do.

The diodes need to be connected from the Positive of the battery ( the Cathode of the diode) and the Drain of the FET connect the Anode of the diode.

You have 12 volts and 2 ohms dumps, that is 6 amps each dump when ON, a standard 1N4004 ( 1 amp diode) will suffice for each dump load you have.

Though the dump load is 6 amps you are only suppressing a possible spike due to the dump load being turned OFF and that is just done once in a blue moon and this diode can take single spikes of several decades of amps ( 60 amps minimum ).

I am assuming that the dump load switches around few times a second.

Nando

[ghurd]

Would it be better or safer to go to a 3A or 6A?

Pretty cheap upgrade.

G-

[Nando]

My comment for 1 amp minimum is right, though if he has 3 or 6 amps diodes then OK.

I was establishing the energy requirements with 1 amp diode.

Nando

[VinceB]

Hey Nando,

Thanks for the reply. The switching frequency is actually around 500 Hz.

That's why I am afraid the voltage spikes will be much higher.

Are standard silicon diodes fast enough for this purpose?

Thanks again for all your support guys

[Flux]

I can't see any point in using a standard diode, use a ultrafast or schottky.

With a perfectly resistive load there will be no need for any diode. In real life nearly all resistors are wound as a coil and have significant inductance. There will inevitably be fast spikes if you switch at 500Hz with a well driven mosfet with fast transition time. The energy may be too low to damage the fet, but why take a chance.

The energy recovered by the diode will be very tiny and 1A rating should be adequate but then again a 3A one will cost you no more and may better stand abuse that you may inflict on it by doing things you don't understand.

I would choose any ultrafast or schottky with over 30v rating and a current rating of 3 to 5A. It may be that a RC snubber would do all you want but unless you have a scope and know to use it to look at fast rise pulses then you will never know.

Flux

[(unknown)]

I would definitely go 3 amp. The most common failure I saw at the casino were blown diodes on the horizontal fets. We used to replace many of them with 6 amp diodes to prevent repeat failures and circuit boards going up in smoke. The switching speed was quite a bit higher but it is an area I wouldn't skimp on.

[Nando]

One needs to be practical.

Standard diodes are good for 1KHZ without any problems,

With 6 amp current, the spike pulse is 1000 Hz at 6 amps with a duty cycle of around 5 microseconds out of 2 milliseconds.

Though one can use Schottky if they are 30 volts it is just a more difficult to find than a Silicon.

Also, you can use, a regular 5 to 10 watts transistor, tie the base to the emitter ( becomes the Cathode) then Collector is the Anode and one can attain the same results, make sure that the NPN transistor is at least 30 volts breakdown.

A PNP power transistor, if used, the emitter becomes the Anode and the Collector the cathode.

Nando

[RobC]

Fast or ultrafast 6 amp diodes are easy to come by and not expensive. I usually buy ten to twenty at a time from Digikey. RobC