# How to calculate the power rating of zener diode for flyback protection?

I'm trying to calculate the power rating for the zener diode I'm using on my circuit for flyback protection.

For example, if a flyback event occurs, the cathode of the zener diode is having a 30V and 500mA momentarily before the diode conducts and dissipate the power to ground, how should I rate my zener diode power as it would be 15W at the cathode? The current will only be a pulse instead of a constant current. Most of the zener diode are rated to 1W or less. Thanks.

• How often can the pulse occur? You can imagine that one 1 mJ event in a minute would deliver less power into the diode than a hundred 1 mJ events in a second. Commented Feb 14, 2020 at 21:37
• If the 24V supply is not subject to "pump up" from flyback energy then returning a lower voltage zener to the 24V rail will greatly reduce dissipation. You will probably need to add a series diode. | Flyback current starts at inductor current at turnoff and energy is 0.5.L.i^2 Commented Feb 14, 2020 at 23:22
• As shown, the zener is superfluous and does zilch. Commented Feb 15, 2020 at 11:12