Design Idea

Circuit offers series protection against power-line transients

Edited by Bill Travis

Alfredo Saab and Travis Eichhorn, Maxim Integrated Products, Sunnyvale, CA -- EDN, 10/14/2004

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, n-channel 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, 50-nsec-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 2.2-kÙ 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 1Ù, and the rise time of the applied voltage is 1 µsec. The circuit draws 20 µA during normal operation, including 3 µA by the undervoltage-lockout, voltage-sensing divider and 17 µA 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.

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