# How do I size a zener diode for use as a freewheel diode for a ZVS induction heater?

My electronics experience is "hobbyist" and I'm in some new territory with this question, so forgive me if this question turns out to have an easy answer or prove to be dumb.

I have a basic ZVS induction heater circuit:

It runs on 12VDC and works fine for light duty work, similar to a wax carving tool heater. Each heating event lasts 5-10 seconds, max.

I activate it via a momentary switch controlling a MOSFET relay, which handles the high current. It is not included in the above schematic for simplicity.

After a heating event, there is an inductive voltage spike of up to 50V when the momentary switch is opened.

Due to the oscillation inherent in a ZVS circuit, I didn't think I could use a standard freewheel diode to bleed off this excess voltage. Instead, after some reading, I opted to use pair of 24V zener diodes, wired in reverse, as per this thread:

These are diodes D5 and D6 in the above circuit, and they are in the return/recovery path. They work very well for this function and reduce the voltage spike to a MOSFET-friendly 20V.

Finally, my question:

How do I determine the correct wattage for these diodes for the power they must withstand in this function?

The heater draws 9A at full power (as measured on my Klein inductive current meter.)

I had initially spec'd 24V, 1W zeners for this function, but it seems that my math was way off, as they have been failing, and I suspect it is because they are being pounded with potentially 9A of current when the heater is operating normally.

How do I determine the correct wattage zener to use for this function?

Can someone point me to the correct specs and calculation, and which are specific zener specs I should seek out?

• It seems likely that 12V may be too low a voltage rating for the zeners. What voltage relative to ground do you see during operation ? Apr 29, 2019 at 7:39
• @RussellMcMahon : I am using 24V zeners (1N4749A) for voltage protection. During normal operation, the voltage will rise to 28V, and I see a brief transient spike to as high 32V upon release of the momentary switch, as measured where zener D6 connects to inductor L2 in my schematic. Apr 29, 2019 at 8:09
• @redeyedjim tvs SHOULD BE GOOD. zENERS ALSO SHOULD NOT BE TERRIBLE. dO YOU KNOW THE MAIN INDUCTOR INDUCTANCE? As someone noted. the energy ne3ded to be handled in a spike is <= i^2 x Ltank at switch off. This gives the maximum single pulse energy dump you need to survive assuming the clamp effectively cools completely between spikes. Apr 30, 2019 at 7:22
• @redeyedjim (2.) Anode to Anode OR Cathode to Cathode. Either way one forward conducts while the other is in zener breakdown. Voltage is increased over Vz by about 1 volt from the forward conducting zener. In this case the change in voltage is not going to make a substantial difference. (1). Energy stored in inductor = 0.5 X l X I^2 ~= 3 mJ = tiny. At resonance the energy transfers to and from the capacitors so 0.5 .L.i^2 = 0.5.C.v^2 so you'd expect Vpeak resonant to be ~~= 29 Volts = about what you see ! :-). ... May 1, 2019 at 7:09
• As long as the zeners do not conduct during operation and only absorb the energy as a transient spike then a zener or TVs that can tolerate 9A peak and absorb a few millijoules will suffice. May 1, 2019 at 7:09

24V zeners are liable to be far too low in voltage rating.
At 28V a 24V zener will be drawing "immense" current.

Your 24V zener draws 10 mA at 24V - dissipation is already about 50% of rated continuous.
At 28V current may be 100 mA++ - 2.8 W dissipation and magic smoke country.

Place a say 0.1 ohm or even 1 ohm in series with a zener to allow you to measure the current it is passing.
At 10 mA a 1 ohm would drop 10 mV - what do you get during normal operation?

This datasheet is not your for your device but close enough - Look at fig 3 on page 3!!!

This high quality precision extrapolated diagram shows a 22V zener operating at 26V - 4V above its rated voltage. Your 24V zeners if operated at 28V will fare similarly. For a while .

• Neat extrapolation! :-) Apr 29, 2019 at 13:15
• @winny :-) ..... Apr 29, 2019 at 14:00