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

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  • \$\begingroup\$ 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. \$\endgroup\$ Commented Feb 14, 2020 at 21:37
  • \$\begingroup\$ 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 \$\endgroup\$
    – Russell McMahon
    Commented Feb 14, 2020 at 23:22
  • \$\begingroup\$ As shown, the zener is superfluous and does zilch. \$\endgroup\$
    – Andy aka
    Commented Feb 15, 2020 at 11:12

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You don't mention the inductance of your load, but if it's not a drive motor for an aircraft carrier it probably won't store enough power to heat up your zener. Most likely, since you're driving it push-pull, the body diodes will provide flyback protection on their own, but the zener won't hurt anything. In any case, current from your 3.3-27V supply will dump into your 24V supply through the body diode before the Zener starts to conduct significantly. If you don't want to overtax the body diodes, you can put Schottky diodes across them.

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  • \$\begingroup\$ What if the high side (the PFET) of the push-pull get disconnected? The zener will start conducting, and what will be the rating needed? \$\endgroup\$
    – cy1125
    Commented Feb 17, 2020 at 14:16
  • \$\begingroup\$ The zener shouldn't start conducting significantly until you reach 30V, while according to your schematic the maximum voltage you're driving the load with is 27V. You'll only exceed 30V in the back-EMF scenario, which unless your inductance is very large will be too short to overstress the diode significantly. \$\endgroup\$ Commented Feb 19, 2020 at 17:19
  • \$\begingroup\$ So if there's an EMF/flyback event, and my voltage is more than 30V, how should I rate my zener power rating? Or is a TVS diode is better? \$\endgroup\$
    – cy1125
    Commented Feb 19, 2020 at 17:50
  • \$\begingroup\$ What is the inductance of your load? \$\endgroup\$ Commented Feb 19, 2020 at 19:57

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