Over-voltage protection system for a 7 - 20 A Load with 4 - 5.5 V output from PSU
I was thinking along the same lines as Huisman &mdash; an electronic circuit breaker for the input that's triggered by the output overvoltage &mdash; and came up with the following circuit.







simulate this circuit &ndash; Schematic created using CircuitLab

The 555 is a good building block for this, because it contains a flip-flop and a couple of voltage comparators. Here's a brief circuit description:


R1, D1 and C1 establish the power supply and reference levels for the 555 at 5.1V. R1 can supply about 5mA.
R2 and C2 create a trigger pulse that makes sure that the 555 starts up in the "triggered" state.
M1 is the main power switch, and Q1, R3, R4 and R5 drive it. Q1 serves as a switchable 1-mA current sink, which develops 10V across R3 to drive M1's gate, independent of the actual supply voltage. When the 555's "discharge" pin is grounded, this circuit is switched off.
R5 and R6 establish the trip point for the output voltage. The 555 will shut off the output if the "threshold" pin goes above \$\frac{2}{3}5.1V = 3.4V\$, and with these resistor values, that means if the output voltage goes above 6.0V.
C4 provides a 22&micro;s time constant to filter out any fast glitches that might cause "nuisance" trips.


To restart the circuit, either interrupt the input power, or temporarily ground the "trigger" pin of the 555 (e.g., with a momentary switch).