Output overvoltage protection circuit

In test and measurement applications, it is necessary to provide overvoltage protection for the outputs of amplifiers, power supplies, and similar components. The traditional way to accomplish this task is to add series resistance in the output node and clamping diodes across the supply line or other threshold voltage (Reference 1 and Figure 1). This resistance greatly reduces the current output capability and the output voltage swing into low-impedance loads. Another solution is to use a fuse or other current-limiting device, which has advantages over the high energy absorption capabilities of these clamping circuits. When the voltage drop across source resistor R6 is greater than the gate threshold voltage of depletion-mode MOSFETs Q1 and Q2, the Figure 2 circuit acts as a bipolar current source, thereby limiting the current through the clamping diodes (Reference 2). The disadvantage of this solution is the large power dissipation in the series elements during overload conditions.

A reasonable approach is to disconnect the amplifier output node from the output terminals for a period of time when an overload voltage is present at the output terminals. For decades, engineers have used electromechanical relays to perform this series disconnect in audio power amplifiers, but for different reasons: for speaker protection. SSRs (solid-state relays), including optoelectronic, photovoltaic, OptoMOS, and PhotoMOS devices, are suitable for the task of disconnecting moderate current loads because of the galvanic isolation between the control and load terminals (Reference 3).

The series protection circuit in Figure 3 uses a series-connected high-voltage SSR to cut off the output of the amplifier.

When the reference voltage threshold is negative, it causes the IC2 or IC3 comparator to change its output state, turning off the SSR IC4 through the logic device IC5. Figure 4 shows a simple circuit to implement this scheme.

The circuit in Figure 4 requires only a few external components and uses an SSR for output overvoltage protection. The rising overvoltage turns off the two transistors in IC2, cutting off the current flowing through IC3 to control the LED. Relay IC3 opens, protecting the amplifier and clamping diode. The circuit was tested with a series of Clare, Matsushita Electronic Works, and Panasonic SSRs, some with internal current protection and some without. The power line voltage is ±15V; R10, R11, and R12 set the trigger level to ±16V. Omitting R11 can move the trigger level to ±14.5V. When the protection circuit is working, the SSR's turn-off delay is 100ms to 200ms for a 0.5V overvoltage protection relay, and the delay will be shorter at higher overvoltages. Note that when using low on-resistance SSRs, the peak current through the clamping diode can be quite large.