Selecting a zener diode for solenoid flyback suppression

Thread Starter

Involute

Joined Mar 23, 2008
97
I'm low-side driving a 24V, 1.2A DC solenoid with a 30V MOSFET. I'd like to protect the MOSFET with a zener diode across the source and drain (cathode to drain, anode to source). If I select, say, a 27V zener, how do I spec the power rating? Is there a way to calculate how much current will be flowing through it when the MOSFET shuts off and the flyback voltage rises above 27V? My MOSFET and (randomly selected) 500 mW zener work fine when driving a 24V, .8A solenoid, but using the 1.2A solenoid fries the zener. Thanks for any tips.
 

Thread Starter

Involute

Joined Mar 23, 2008
97
Well, to start with, the initial current thru' the diode will be the current that was flowing thru' the Solenoid at the time of shut off of the MOSFET.
I assume there’s something more going on, otherwise the .8A solenoid would be pushing 27V x .8A = 21.6W through the 500mW zener and it would fry.
 

Thread Starter

Involute

Joined Mar 23, 2008
97
A regular diode works across the solenoid’s terminals. If necessary I could do that, but it’s inconvenient since there are 40 solenoids I’m dealing with and they’re off of the PCB. A zener, in theory, can be placed across the MOSFET on the PCB which is more convenient from an assembly standpoint.
 

Sensacell

Joined Jun 19, 2012
2,631
A regular diode can be placed on the board too.

Sure, a diode right on the terminals is better, but it will also work just fine on the board.
Just connect the Diode cathode to the solenoid supply rail. It's the identical connection to placing it on the coil, but physically separated.
 

Thread Starter

Involute

Joined Mar 23, 2008
97
A regular diode can be placed on the board too.

Sure, a diode right on the terminals is better, but it will also work just fine on the board.
Just connect the Diode cathode to the solenoid supply rail. It's the identical connection to placing it on the coil, but physically separated.
Sure, but that assumes I have the solenoid supply rail on the board. I can put it there if I have to, but I’d prefer not to. Plus, I’d like to understand this zener issue.
 

ArakelTheDragon

Joined Nov 18, 2016
1,350
A regular diode can be placed on the board too.

Sure, a diode right on the terminals is better, but it will also work just fine on the board.
Just connect the Diode cathode to the solenoid supply rail. It's the identical connection to placing it on the coil, but physically separated.
A regular didode should be placed. What for does he need the zener?

1n4148 or 1n4007 should do.
 

Thread Starter

Involute

Joined Mar 23, 2008
97
A regular didode should be placed. What for does he need the zener?
I’m controlling 40 solenoids. Each requires a MOSFET and a diode on the PCB. Using a regular diode requires snaking an additional trace (for the solenoid power rail) through the layout, making it a lot more complicated. I don’t need to do that with a zener.

The protection principle is different. With a regular diode you’re dissipating the flyback energy through the coil. With a zener you’re sending it to ground, bypassing the MOSFET.
 

ebp

Joined Feb 8, 2018
2,332
Can you find a spec for or measure the inductance of the solenoid? The inductance of a solenoid can vary quite a bit depending on the position of the armature since it is a big player in the magnetic path.

You already know about the peak current issue, but the inductance of the solenoid determines the total energy that must be handled (energy in joules = 1/2 LI^2 - L in henries and I in amperes).

Ordinary zeners generally aren't well specified in terms of handling high transient power. Usually all you get is an average power rating, so you're left doing some guessing in terms of transient. Zeners made specifically for transient voltage suppression are much better specified.

It all (well almost all) comes down to temperature rise in the actual semiconductor die. That depends on the physical size of the die, but also how the die is connected to the metal structures that form the leads. If a power pulse is very short, the heat doesn't have time to travel and the comes down to the specific heat of the die itself and possibly the metal to which it is intimately attached (e.g. a MOSFET is usually soldered to a fairly substantial copper "slug" which can absorb significant heat in the short term; an axial-lead diode in a glass package with alloy leads gets little benefit from the metal leads because they don't conduct heat well).

This isn't very helpful at this point. I would consider small transient suppressor diodes from companies such as Vishay or Littelfuse. The SMF series from Littelfuse might be OK. Unfortunately, this sort of part will cost considerably more than a simple diode across the coil, but ...
 

Sensacell

Joined Jun 19, 2012
2,631
Unless I need a rapid current decay on turn-off, i would always use a simple diode.

Using a zener is problematic, you need to be sure the drain voltage does not overshoot the FET's maximum voltage.
If the duty cycle is high, the zener can dissipate substantial power.

I would never let a PCB layout issue dictate circuit design, you are trading a simple problem for a tricky one.
 

ebp

Joined Feb 8, 2018
2,332
Although it isn't common, there is another option, though I would be leery of using it with a 30 V FET operating at 24 V because of the small margin.

A small, inexpensive zener in series with a signal diode can be placed between the drain and the gate of the FET. This requires that the current sinking ability of the gate driver be fairly low. As the drain voltage rises as the FET begins turns off, the zener will conduct, keeping the FET in the linear range and dissipating the energy stored in the inductor in the FET's channel resistance. The practicality of this depends on the FET's transient thermal impedance (transient power handling ability) which is something that is generally very well specified. Of course the zener voltage must be chosen with consideration to the gate voltage and rated drain-source breakdown voltage. The signal diode (1N4148 class) is required to prevent the zener from conducting in forward mode, which would make a Baker clamp of sorts.

Though they are no longer made as far as I know, there was a time when you could buy power Darlington transistors, intended for automotive iginition use, with the zeners built in. The intent was that if the energy stored in the ignition coil couldn't go into a spark, it had to go somewhere, and dissipating it in the transistor was practical.
 
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crutschow

Joined Mar 14, 2008
25,669
I second ebp's suggestion to use a Zener plus regular diode between the drain and gate of the MOSFET, but you should use a MOSFET with a higher voltage rating.
Using a 30V MOSFET with a 24V supply doesn't give you much margin.
I suggest at least a 50% margin or a MOSFET with a ≥40V rating.
 
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crutschow

Joined Mar 14, 2008
25,669
Below is the LTspice simulation of the circuit.
It keeps the gate voltage at about 3V to clamp the drain voltage to ≈37V with a 33V Zener.

Note that a resistor in series with the gate source is required for the circuit to work.

How rapidly are the solenoids operated?

upload_2018-11-14_9-48-4.png
 
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