Voltage transients on low voltage power lines can sometimes attain amplitudes many times the nominal voltage level. That behavior often calls for protection against the application of improper power levels. The usual way to protect sensitive circuitry against overvoltage is to add parallel clamps. Fuses or other current-limiting devices precede these clamps’ high energy absorption capability. Other cases require the use of high-voltage series protection (instead of parallel clamps)because of the difficulty in resetting or replacing fuses, an inaccessible operating environment, or the need for uninterrupted operation. The series-protection circuit of
Figure 1 turns off the power switch using a series-connected, high-voltage, nchannel MOSFET power switch, Q1, and a fast overvoltage detector. The power switch and series-connected power rectifier, D1, protect the load against high-voltage transients and continuous overvoltage as high as ±500V of either polarity.
In the circuit, which powers loads as heavy as 1A from a nominal 12V power line, a high-side-switch driver, IC1, biases the power switch fully on. You can increase the maximum load current by changing D1 and Q1. To guard against low supply voltage, IC1 includes an undervoltage-lockout feature that allows operation only when the line voltage is greater than 10V. To protect against overvoltage, the circuit includes a three-transistor, no-bias-current, 50nsec-operation overvoltage detector that triggers when the input voltage reaches approximately 20V. At that time, Q4 “crowbars” the gate of the power switch to ground, turning it off hard. Rising overvoltage first turns on zener diode D2, which protects the IC by clamping the voltage across it to approximately 18V. Zener current flows through the 2.2kΩ resistor, producing a base voltage that turns on Q2. That action initiates a rapid sequence: Q3 turns on, which turns on Q4, which turns off Q1 by quickly discharging its gate capacitance.
You can demonstrate the circuit’s performance by applying a 150V transient to the supply voltage while the circuit output is delivering 1A at 12V (
Figure 2)The internal impedance of the transient source is 1Ω, and the rise time of the applied voltage is 1µsec. The circuit draws 20µA during normal operation, including 3µA by the undervoltage-lockout, voltage-sensing divider and 17µA by IC1. If your design needs high temperature operation, note that the gate-current output of IC1 is relatively limited. Your design calculations for high temperature should also pay close attention to leakage currents that the other circuit components contribute.
