Self-powered solid state relay using digital isolators

A circuit comprising:

• an output terminal;
• a solid-state relay (SSR) having first and second outputs configured to be coupled to a transformer, wherein the solid-state relay includes:
• a first transistor having first and second current terminals;
• a diode;
• and
• a second transistor having first and second current terminals, the second current terminals of the first and second transistors coupled together, wherein the diode is coupled between the second current terminals of the first and second transistors and ground;
• a rectifier having an output, and first and second inputs respectively coupled to the first and second outputs of the SSR, the rectifier configured to provide a first voltage to the output terminal responsive to the SSR being in an off state;
• a current transformer-based power supply having an input coupled to the output of the rectifier, the current transformer-based power supply configured to be coupled to the transformer, the current transformer-based power supply configured to provide a second voltage to the output terminal responsive to the SSR being in an on state;
• and
• control circuitry having an input and an output, the input of the control circuitry coupled to the output terminal and the output of the control circuitry coupled to the SSR, the control circuitry configured to control the SSR.

A circuit comprising:

• an output terminal;
• a solid-state relay (SSR) having first and second terminals, wherein the SSR includes:
• a first transistor having first and second current terminals;
• and
• a second transistor having first and second current terminals, the second current terminals of the first and second transistors coupled together and to an anode of a diode;
• a half-bridge rectifier having a rectifier output, the half-bridge rectifier configured to be operably coupled to the first and second terminals of the SSR to provide a first voltage on the rectifier output responsive to the solid-state relay being in an off state;
• a transformer adapted to be operably coupled to the first and second terminals of the solid-state relay responsive to the solid-state relay being in an on state;
• and
• a current transformer-based power supply coupled to the rectifier output and to the transformer, the current transformer-based power supply is configured to:
• generate a second voltage for the output terminal responsive to the solid-state relay being in the on state using a current through a secondary winding of the transformer;
• and
• provide the first voltage to the output terminal responsive to the solid-state relay being in the off state and using a current output from the rectifier output, wherein the current transformer-based power supply includes:
• a transistor coupled to the output terminal, the transistor having a control input;
• and
• a comparator having first and second inputs and an output, the output of the comparator is coupled to the control input of the transistor, wherein the first input of the comparator is coupled to the output terminal, and the second input of the comparator is configured to be coupled to a reference voltage source, and wherein the comparator is configured to control the transistor responsive to a comparison of the second voltage on the output terminal to a threshold set by the reference voltage source.