Circuits protect outputs against overvoltage
Adding a solid-state relay and control circuitry to an amplifier's output provides overvoltage protection.
Dimitri Danyuk, Kiev, Ukraine; Edited by Charles H Small and Fran Granville -- EDN, October 11, 2007
In test-and-measurement applications, you must provide overvoltage protection for the output terminals of amplifiers, power supplies, and similar components. The conventional way to accomplish this task is to add series resistors with the output node along with the clamping diodes to power-supply rails or other threshold voltages (Reference 1 and Figure 1). This resistor significantly reduces current-output capability and the output-voltage swing with low-resistance loads. The alternative approach is to use fuses or other current-limiting devices, which precede these clamps’ high energy-absorption capability. The circuit in Figure 2 works as a bipolar current source when the voltage drop across source resistor R6 becomes larger than the gate-threshold voltage of depletion-mode MOSFETs Q1 and Q2, thus limiting the current through the clamping diodes (Reference 2). The drawback of this approach is high power dissipation on series components during the overload condition.
A reasonable approach disconnects the amplifier-output node from the output terminals for the period when the overload voltage exists on output terminals. Engineers for decades have used such serial disconnection by means of electromechanical relays in audio power amplifiers but for a different reason: loudspeaker protection. SSRs (solid-state relays), including optoelectronic, photovoltaic, OptoMOS, and PhotoMOS devices, suit the task of load disconnection at moderate current levels because of galvanic isolation between the control and the load pins (Reference 3).
The series-protection circuit of Figure 3 disconnects the amplifier-output terminal using a series-connected, high-voltage SSR. Raising the output voltage above the positive-reference-voltage or below the negative-reference-voltage threshold causes either the IC2 or the IC3 comparator to change its output state and turn off SSR IC4 through AND logic element IC5. Figure 4 shows the simple circuit realization of this approach.
The circuit in Figure 4 requires only a couple of external components to use an SSR for output-overvoltage protection. Rising overvoltage turns off both transistors in IC2, interrupting current flow through the control LED of IC3. Relay IC3 opens, protecting the amplifier and clamping diodes. The circuit was tested with a handful of Clare, Matsushita Electronic Works, and Panasonic SSRs with and without internal current protection. The power-supply rails are ±15V; R10, R11, and R12 set the triggering levels and are equal to ±16V. Omitting R11 shifts the triggering levels to ±14.5V. The SSR turn-off delay in protection-circuit operation is 100 to 200 µsec for relays with 0.5V overvoltage protection and becomes slightly shorter with higher overvoltage. Note that the peak current through clamping diodes can be rather high with low-on-resistance SSRs.
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