What is PhotoMOS?
In the heart of modern electronic devices, a component called the PhotoMOS optocoupler-type solid-state relay is quietly changing the way we control power. By combining optoelectronics with semiconductor technology, it replaces mechanical contacts with light beams and silent operation instead of mechanical vibrations. This “electronic switch” has become indispensable in industrial automation, energy management, and precision instruments. Not only does it solve the common problems of traditional relays—like wear and slow switching—it also shines with outstanding stability in extreme conditions such as high temperatures, corrosion, and strong vibrations. This article will take a closer look at what PhotoMOS relays are, how they work, their advantages, and where they're used.
Catalog
I. What is PhotoMOS?
PhotoMOS (photo MOS relay) is a type of solid-state switch built on semiconductor structures. Its core consists of an LED on the input side and a power MOSFET on the output, linked through a photosensitive element that provides electrical isolation and signal transmission. Unlike traditional mechanical relays that use an electromagnetic coil to drive physical contacts, PhotoMOS relays have no moving parts at all—they're contactless. This key difference gives them the safety isolation of an optocoupler and the power control capability of a MOSFET in one package.
Structurally, PhotoMOS uses a gallium arsenide infrared LED on the input, paired with a solar cell array on the output side, all sealed in a translucent resin that maintains at least 0.4mm of physical separation. This design allows them to withstand insulation voltages up to 5000 Vrms (like Panasonic's HSSR-DA01 series), while keeping signal accuracy intact. The output stage uses a vertical channel DMOSFET, with the source and drain on opposite sides of the wafer, enabling high-voltage, high-current control.
II. How Does It Work?
The working process of a PhotoMOS relay is a precise conversion of light, electricity, and mechanical energy:
· Photoelectric Conversion: When a control current (typically 3-7mA) flows into the input terminal, the infrared LED lights up. It's highly sensitive—some models start working with as little as 0.5mA.
· Light Transmission: The infrared light passes through the transparent isolation layer and hits the solar cell array. The solar cells convert the light into about 10 volts of driving voltage for the MOSFET gate.
· Power Control: The generated voltage charges the MOSFET gate. When the gate voltage exceeds the threshold, the MOSFET turns on, allowing load current to flow. Turning off the LED drops the gate voltage, switching the MOSFET off.
What makes this so valuable is that energy transfer happens purely through light, meaning there's no direct electrical connection between input and output. This completely eliminates ground loop interference and high-voltage leakage risks. Plus, the MOSFET's voltage-controlled nature allows for very low on-resistance (down to 0.09Ω) that can adjust dynamically, providing smooth switching.
III. Features & Benefits
· Electrical Isolation: Input and output are electrically isolated via optical coupling, greatly improving noise immunity.
· High Reliability: No mechanical contacts means no wear and tear or contact failures, resulting in a long lifespan.
· Fast Response: Very short switching times suit high-speed control needs.
· Low Power Consumption: Minimal input current reduces overall energy usage.
· Compact Size: Small and lightweight, ideal for space-constrained designs.
IV. Typical Applications
Thanks to these strengths, PhotoMOS relays are vital in several key areas:
· Solar inverters: Safely switching on the high-voltage DC 1500V side; battery packs in energy storage systems can be isolated within milliseconds during faults; charging stations rely on PhotoMOS for precise leakage current detection. Panasonic, for example, has developed high-voltage resistant models for multi-level protection in solar AC/DC systems.
· Industrial automation: ICP DAS integrates eight PhotoMOS relays in the I-87069PW module to directly drive solenoid valves and contactors, cutting out delays and failure points from intermediate relays. In food and beverage production, its sealed design resists corrosive gases, greatly reducing downtime.
· Medical equipment: Used in devices like blood analyzers to avoid sparks that could ignite flammable gases; telecom equipment uses models like HSSR-DA01 for routing signals at 400V/120mA; test equipment (e.g., IC handlers) benefit from their billion-cycle operation life, lowering maintenance costs.
V. Conclusion
With the rise of SiC and GaN power devices, PhotoMOS optocoupler solid-state relays have become indispensable in modern electronic control systems. The latest generation pushes beyond 600V/5A ratings while innovating in impedance matching and thermal management. Looking ahead, PhotoMOS relays will play an even bigger role in smart manufacturing, IoT, and smart home applications—making power control safer, faster, and more reliable than ever before.
