Sizing a zener diode for flyback path

[JackofallTrades]

This is a follow up to the question asked here:

https://www.edaboard.com/threads/312697/



An N-MOSFET is switching current to a solenoid. Supply voltage is 24V. The MOSFET is rated to 60V. The solenoid has a resistance of 4ohms, thus giving an actuated current of 6A.

When the MOSFET is shut off, large back-EMF spikes are produced as the solenoids field is collapse. Adding a flyback diode across the solenoid snubs the spikes, but also introduces an unacceptable delay in the solenoid disengaging after the MOSFET is switched off.

It has been suggested a zener diode in the flyback circuit will allow the field to collapse more quickly.

For the zener voltage, to minimize delay, I should pick a value close to my transistor rated voltage - the supply voltage - the forward drop of the normal diode. For this circuit to leave some margin for the MOSFET, let's assume I select a zener voltage of 60-24-1-5 (for safety) = 30V zener diode.

My question then is what current does the zener need rated at? Initially I would think since the actuated current through the solenoid is 6A, the initial off current through the zener is 6A, leading to a hefty 180W dissipated. This seems excessively large. Indeed, I do not even see 180W rated zener diodes at Digikey for purchase.

If indeed the analysis is correct, what are some other ideas for speeding up the field collapse timing?

 

[crutschow]

A zener can carry a momentary pulse of many times its average power rating. The average power dissipated by the zener is determined by the frequency of the switching and the inductance of the solenoid so you need to know that to determine how large a zener you need.

If this average power dissipated is too high then you may have to use a lower voltage zener or a resistor and diode rather than a zener.

 

[Tunelabguy]

It also matters how often the solenoid will be switched off. It if it going to be switched off 1000 times a second, then the heat could build up in the zener. But if it is going to be switched off less than 5 times per second, with typical solenoid inductance, the duty cycle would be so low that heating would not be a problem. The only spec you would need to worry about would be the instantaneous pulse rating. You may find that some zeners have a separate rating for pulse current. And then you just need to find one that can carry 6 Amps instantaneous.

 

[BradtheRad]

Snubber networks often include a resistor. You can install a value of 1 to 20 ohms, inline with the zeners, to absorb some of the energy from the solenoid.

A higher ohm value drives up the voltage spike. (This is expected with an inductor.) And the resistor will absorb the bulk of the energy.

OTOH if you use a lower ohm value, the spike is a lower voltage. And the zener absorbs the bulk of the energy.

There is also the option of a series capacitor and resistor, acting as a snubber network.