CMOS switches and solid-state relays (SSRs) are types of semiconductor switching devices but they differ significantly in structure, functionality, and applications. CMOS switches utilize pairs of p-type and n-type MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) to perform switching operations. SSRs utilize semiconductor components such as thyristors, triacs, or MOSFETs to achieve switching. These devices play crucial roles in modern electronic systems, each serving distinct functions across various applications.

CMOS switches use complementary MOSFET technology. They operate efficiently at low power and are used in battery-powered and mobile devices. These switches are used in low-power, digital, or analog circuits for switching signals. They enable multiplexing, signal routing, or switching between different circuit paths. CMOS switches feature low power consumption, high speed, and excellent integration in digital circuits. They are mainly used for switching electronic signals, rather than higher power loads.

Working Principle: The working of CMOS switches is based on the operation of MOSFETs. They utilize pairs of n-channel and p-channel MOSFETs that operate together to switch signals on and off.

A typical CMOS switch consists of two MOSFETs:

• NMOS Transistor: Conducts when a high voltage (logic 1) is applied to its gate.
• PMOS Transistor: Conducts when a low voltage (logic 0) is applied to its gate.

These two MOSFETs are connected in parallel, but they are complementary to each other, allowing one MOSFET to conduct when the other is off, and vice versa.

The operation is controlled by the input control voltage applied to the gates of the NMOS and PMOS transistors. When the control signal is HIGH (logic '1'), the n-channel MOSFET turns ON (it conducts) and the p-channel MOSFET turns OFF (it does not conduct). This allows the input signal to pass through the n-channel MOSFET to the output. When the control signal is LOW (logic '0'), the n-channel MOSFET turns OFF and the p-channel MOSFET turns ON. This allows the input signal to pass through the p-channel MOSFET to the output.

Solid-State Relays (SSRs) are used to switch electrical loads without mechanical movement. They are used to control high-power loads like motors, heaters, and lights without the need for moving parts. They use semiconductor components like transistors, triacs, or thyristors to switch on or off. These components allow for faster and quieter switching compared to mechanical relays.

Solid-state relays have no moving parts (hence called "solid-state"), have a long lifespan, and have higher reliability in harsh environments. They are also used in AC and DC applications depending on the design.

Working Principle: SSRs typically consist of three key sections. It consists of an input circuit that receives the control signal from a microcontroller, PLC, or switch. The second stage is an isolation stage that optically isolates the input from the output to protect low-voltage control circuits from high-voltage load circuits. The final stage is an output circuit that uses semiconductor switching components to control the high-power load.

The input circuit includes an LED (light-emitting diode). When a low voltage input control signal (in the range of 3 – 32 V DC) is applied, the LED is turned on. The LED's light activates a photodiode or phototransistor on the output side, allowing for optical isolation. This protects the low-voltage control circuit from the high-voltage load circuit. Optical isolation prevents electrical feedback from damaging sensitive control electronics. Once the output side receives the signal from the optical isolator, it activates a semiconductor switching component such as a triac or thyristor for AC load switching or a transistor or MOSFET for DC load switching. The semiconductor switch either opens or closes the circuit that controls the high-power load. When the relay is ON (closed circuit), the load such as a motor, heater, or light is powered. When the relay is OFF (open circuit), the load is disconnected, and no current flows through the circuit.

Click here to learn more about Power Switch ICs featured on everything PE.