Protecting 12V DC input

[deephaven]

I have a board which uses a switching regulator with a Vin MAX if 20V. I run this with a nominal input voltage of +12V which is normally fed from an unearthed 12V mains power supply - the usual 12V 1A wall wart switching reg or similar. The input does need to handle up to 17V on the occasion that the source is a freshly charged battery pack. I want to provide protection for the situation where the user plugs the 12V in from the unearthed power supply when the supply is already plugged into the mains. This means it has to protect the input of my board from the induced AC volts on the connector which might touch the +12V input pin before the 0V connects. Looking at the specs of TVS diodes, it is difficult to find one which is safely above the 17V so that it doesn't normally turn on, but still provides the 20V protection needed by the regulator. Any suggestions on a good choice of protection device?

[Mangozac]

This is not an uncommon problem actually! Indeed the voltage range between when the TVS diode starts conducting and when it clamps is quite high.

Earlier in the year an application I designed had a similar configuration as you. I ended up using a zener and MOSFET configured as a drain follower on the input to the switching regulator. Plus a TVS diode on the input to the circuit. The MOSFET drain is connected to the source (and TVS diode), the drain is connected to the switching regulator input. The gate is connected to the drain via a resistor and to ground with the reverse biased zener diode.

This circuit effectively acts as a more precise voltage clamp, but still allows the TVS diode to take the brunt of the power dissipation for higher voltage spikes. It wasn't the most amazing solution (and you end up wasting a small amount of power in the MOSFET) but it worked for that application.

I'd certainly like to hear how others deal with the problem!