> Tom Rauch wrote:
> >> There are ways to design around this problem, but the only reliable
> >> way is to use a higher-rated device and/or a lower rail voltage.
> >
> >.......or to clamp the shunt side of the series resistor with a
> >large zener with a capacitor across. You can use a zener clamp rated
> >below the maximum voltage breakdown of the shunt device, and above
> >the highest desired regulated voltage.
> >
> It doesn't make sense to spend money on a high-power, high-voltage
> zener (don't forget the heatsink) when you can uprate the shunt MOSFET
> for much less cost.
Why would you need a high power zener? The zener doesn't
regulate anything, it is just a safety device in case you get a
millisecond or two glitch.
It is a hard clamp for quick transients. It will fail-short if a
reasonable size diode is used and your fancy regulator goes bad.
As a matter of fact, the LAST thing you would want is a zener
clamp that is very large because it would allow the screen to run
too high with poor regulation if the normal regulator failed.
Another transient protection system is a conventional power
rectifier with the cathode feeding a high-value electrolytic with a
high resistance bleeder. This even works very well for lightning
protection if you clamp negative transients to ground.
73, Tom W8JI
W8JI@contesting.com
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