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Zener diode dissipation for protection.

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davelectronic:
I'm trying to work out if I can increase a zener diodes power dissipation ability for circuit over voltage protection. I don't have room in two power supply projects to add in a crowbar circuit. So I wanted to use a fuse and zener diode, just incase the series pass transistors where to fail short circuit. But the zener diodes I have are rated at 16 Volts and 5 watts power rating. I think if the 16 Volt 5 watt zener does reverse conduct it would probably burn up before it blows the fuse. I might not be right, but I think I need a series current limiting resistor for the zener diode. Really I don't want to power up radio equipment unless I have some over voltage protection. Years ago I had a retail Altai PSU push out over 20 Volts and destroy a radio. So I need a solution that doesn't take up much space inside the power supply case. Does that sound right about using a resistor to prevent the zener diode burning up ? The maximum current that the power supply will deliver is 12 Amps, at 12.60 Volts DC. I need a solution if one or more series pass transistors fail short circuit. Any ideas and help appreciated. Thanks for reading.

davelectronic:
Just found something, but don't know if its useful or not, that was using a BJT to shunt the zener diode. But its power rating was raised by the beta of the transistor. Any thoughts if that's possible for a 12 Amp 12 volt power supply over voltage protection ?

David Hess:
I will assume that you mean a TVS diode because they are designed to have overload capability.  True zener/avalanch regulator diodes are not as robust.

A big TVS diode can work to blow a fuse but needs to be sized properly, or the TVS diode will short, although that of course will blow the fuse also.

I have done it with a zener or TVS diode and a big bipolar power transistor when I needed a higher power rating than a zener or TVS diode will allow.  But if you have room for a transistor and zener, then you have room for an SCR and zener.

davelectronic:
It was a zener diode and fuse I was referring to, I hadn't given TVS diode any thoughts. I will snap a picture inside the power supply to give an idea on amount of space I have inside the PSU. I've had a few issues with a crowbar circuit firing randomly, it's a real pain it kept doing this. I had included the capacitor to stop it firing randomly if the voltage spiked. I'm just looking for an over voltage protection circuit that just works, and figured a zener diode and fuse could fill that brief, but quickly realised the zener diode power rating would be a problem. So was looking for a solution to increase the 5 watt zener diodes current handling rating.

David Hess:

--- Quote from: davelectronic on May 14, 2021, 03:20:42 pm ---It was a zener diode and fuse I was referring to, I hadn't given TVS diode any thoughts.
--- End quote ---

TVS diodes are zener diodes that are processed and specified to handle surge currents.  They no longer make big zener diodes like they used to because they were replaced with TVS diodes.  There is no reason not to use a TVS diode in place of a zener diode.

The processing difference is the same as that used to make avalanche rated rectifiers.  The junction is made more uniform so that hot spots do not form which would allow localized thermal runaway.

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