What Is a MOSFET Output?
MOSFET Output relays are equipped with an LED, a light receiving element, and a MOSFE inside, and operate in the same way as conventional relay components using a combination of these elements.
Conventional mechanical relays were controlled ON/OFF by passing an electric current through a built-in coil and operating an electrical contact point by the magnetic force of the coil. In contrast, MOSFET Output relays emit light by passing a current through a built-in LED, which uses the light to generate an electromotive force in a photodetector. The MOSFET is operated by that electromotive voltage, enabling control similar to that of a conventional relay.
Similar photo MOSFET Output relays include photocouplers, phototransistors, phototriacs, and SSRs (Solid State Relay). These are collectively referred to as contactless relays or semiconductor relays, and are used differently depending on the application.
Applications of MOSFET Output Relays
MOSFET Output relays are often used in circuits that require a large current, or in phototriacs and SSRs that are good at turning alternating current on and off, rather than in photocouplers, phototransistors, and other applications.
MOSFET Output relays use light to transmit signals from the primary side to the secondary side, and since the primary and secondary sides are completely isolated electrically, they are driven by commercial power supplies or high voltage power supplies with small signals, such as microcontroller circuits. Therefore, they are used in circuits that turn on and off relatively large power loads of several A or so, and in bridging circuits such as motor drivers.
Principle of MOSFET Output Relays
When a signal current is applied to the input terminal, an LED emits light and the light-receiving element receives the light, generating an electromotive voltage. This voltage causes the gate voltage of the MOSFET to rise and the state between the source and drain of the two MOSFETs turns ON.
Then, current can flow through the source-drain of the MOSFET on the side with the higher voltage opposite the voltage between the output terminals, and through the parasitic diode of the MOSFET in the next stage. As a result, current can flow between the output terminals regardless of the electrical polarity between them.
Structure of MOSFET Output Relay
A photo MOS relay consists of three elements: an LED, a light-receiving element such as a photodiode, and a MOSFET Output.
The MOSFETs feature a structure in which two circuits are arranged in opposite directions, each connected by a parasitic diode.
Other Information on MOSFET Output
1. Application to Full-Bridge Circuits
For example, when trying to make a motor driver, it is common to construct a circuit using a full-bridge circuit. However, the biggest concern with this circuit is that when the power supply voltage is applied to both ends of the load as it is, the source voltage of the upper FET among the upper and lower FETs becomes equal to the power supply voltage. To turn on the upper FET from this state, it is necessary to prepare a separate voltage higher than the supply voltage and use that voltage to control the gate voltage of the upper FET.
However, since the MOSFET Output can operate as long as the built-in LED is lit, it can be directly driven by a signal of a few volts output from the CPU. It is also possible to completely isolate the power supply for the motor drive from the power supply for the control.
2. Types of MOSFETs
There are two types of MOSFETs: the type that is always OFF and the type that is always ON. The former can be used as make-contact relays and the latter as break-contact relays.
MOSFET Output relays can also be used as AC current switches by connecting two MOSFET sources facing each other. The conduction characteristics of photo MOSFETs are independent of the amount of input current, so that current ON/OFF control is possible even with minute input currents.