BACKGROUND

Electromechanical relays or solid-state relays can be used to switch electrical loads. Electromechanical relays use mechanical contacts that make or break the electrical connection in a circuit, and the mechanical contacts are typically controlled using an actuator coil. The moving mechanical contacts, and wear and tear, limit the life of electromechanical relays, with the life span typically specified as a number of switching cycles. Solid state relays provide the functionality of electromechanical relay without any moving parts. Solid state relays use semiconductor switches to make or break the electrical connection. The absence of moving parts significantly improves the life of solid-state relays compared to electromechanical relays.

SUMMARY

A plurality of solid-state relays driven by a multi-channel isolator using a single isolated power supply is disclosed herein. In one example, a solid-state relay circuit includes an isolator circuit, a first output terminal, a second output terminal, and an output switch. The output switch is coupled to the isolator circuit, and includes a first transistor, a second transistor, and a diode. The first transistor includes a first terminal coupled to the first output terminal, and a second terminal coupled to an output of the isolator circuit. The second transistor includes a first terminal coupled to the second output terminal, a second terminal coupled to the second terminal of the first transistor, and a third terminal coupled to a third terminal of the first transistor. The diode includes a cathode terminal coupled to ground, and an anode terminal coupled to the third terminal of the second transistor.

In another example, a solid-state relay circuit includes a first output terminal, a second output terminal, an output switch, and an isolator circuit. The output switch includes a first transistor, a second transistor, and a diode. The first transistor is coupled to the first output terminal. The second transistor is coupled to the first transistor and the second output terminal. The diode is coupled to the first transistor, the second transistor, and ground, and is configured to block current flow from ground to the first transistor and the second transistor. The isolator circuit is coupled to the output switch and is configured to activate the first transistor and the second transistor.

In a further example, a solid-state relay circuit includes a first output terminal, a second output terminal, an output switch, a gate discharge circuit, a high impedance voltage translation circuit, and an isolator circuit. The output switch includes a first transistor, a second transistor, and a diode. The first transistor is coupled to the first output terminal. The second transistor is coupled to the first transistor and the second output terminal. The diode is coupled to the first transistor, the second transistor, and ground, and is configured to block current flow from ground to the first transistor and the second transistor. The gate discharge circuit is configured to shunt voltage at a gate terminal of the first transistor and at a gate terminal of the second transistor to a source terminal of the first transistor and a source terminal of the second transistor. The high impedance voltage translation circuit is coupled to the gate discharge circuit and is configured to boost drive voltage to and present a high impedance to the first transistor and the second transistor. The isolator circuit is coupled to the high impedance voltage translation circuit and is configured to activate the output switch.

In another example, a solid-state relay circuit includes a first output terminal, a second output terminal, an output switch, and a zero-crossing control circuit. The output switch comprising a diode bridge, a sense resistor, and a transistor. The diode bridge includes a first terminal coupled to the first output terminal, a second terminal coupled to the second output terminal, a third terminal, and a fourth terminal. The sense resistor includes a first terminal coupled to the fourth terminal of the diode bridge, and a second terminal. The transistor includes a first terminal coupled to the third terminal of the diode bridge, a second terminal, and a third terminal coupled to the second terminal of the sense resistor. The zero-crossing control circuit includes an input terminal coupled to the second terminal of the sense resistor, and an output terminal coupled to the second terminal of the transistor.

