Daily Archives: May 18, 2023

What are MOSFET Relays

For certain applications, especially for high-power switching, traditional electromagnetic relays are still a popular choice. However, with the advent of solid-state relays, particularly MOSFET Relays, this trend is now shifting for a growing range of applications. In addition, with IoT growing exponentially, and 5G networks moving the trend towards shrinking form factors, engineers are forced to fit more powerful devices with higher functionality into smaller spaces. That means, they must also find better ways of improving power efficiencies through improved switching speeds.

Modern IT infrastructure, such as switching power supplies and DC-DC converters, presents engineers with specific design challenges. MOSFET relays help to address these challenges as their characteristics are superbly suited to several key applications.

Although the name includes the word relays, MOSFET relays are actually electronic circuits rather than relays and feature an input and an output side. The input side comprises a PDA or photodiode dome array, along with an LED or Light Emitting Diode. The output side comprises a FET or Field Effect Transistor block, with a control circuit bridging the two.

To activate a MOSFET relay, a current must flow through its input LED and turn it on. The PDA then converts the light from the LED to a voltage. The control circuit uses this voltage to drive the output block. This action turns on the double MOSFETs, present in the output block, allowing them to pass either AC or DC loads bi-directionally.

Unlike electromagnetic relays, MOSFET relays have no moving parts. Therefore, the latter can withstand vibration and physical shock without suffering damage or malfunction. Ideally, the MOSFET relay should perform indefinitely, operate silently, and cause very little electrical interference, provided it is under proper use.

While MOSFET relays can handle a wide range of input voltages, they consume very little power and do not arc during operation. That makes this solid state relays eminently suitable for working in hazardous environments. While enabling the switching of both AC and DC signals, solid state relays minimize surge currents. A physical comparison with electromagnetic relays reveals MOSFET relays to be considerably smaller, occupying less space on printed circuit boards, and consuming very low power.

Certain characteristics of MOSFET relays offer advantageous implications in electronic applications. For instance, they offer low output capacitance, implying an improvement in switching times with better isolation characteristics for load signals at high frequencies. The presence of an LED at the input implies optical isolation between the input and output circuits offering a better physical or galvanic isolation. The on-resistance for MOSFETs is low, implying increased switching speeds and low power dissipation when switching high currents. Being solid state, MOSFET relays have no hysteresis when switching from the on-state to the off-state and vice versa. These relays have high linearity, ensuring there is no signal distortion when switching. Therefore, MOSFET relays are equally suitable for analog and low-level signal switching.

The above characteristics of the MOSFET relay make them ideally suitable for a wide range of applications. These include use in energy-related equipment, telecommunications, factory automation, amusement equipment, security equipment, medical equipment, automated test equipment, and much more.