Switching power supply with optocoupler

Optocouplers provide electrical isolation while enabling safe switching of high-voltage or high-current circuits using a low-voltage control signal.

Function and Principle

An optocoupler, also called an opto-isolator, transfers electrical signals between two electrically isolated circuits using light, typically from an LED to a phototransistor. In power switching, the input side (LED) is driven by a low-voltage control signal, while the output side (phototransistor) switches the high-voltage or high-current load. This isolation protects sensitive control electronics, such as microcontrollers, from dangerous voltages.

Switching Operation

When used as a switch, the phototransistor operates in saturation and cut-off, similar to a BJT transistor switch. The forward current through the LED determines the collector current of the phototransistor via the Current Transfer Ratio (CTR), which is the ratio of output current to input LED current. To ensure proper switching, the LED current must be sufficient to saturate the phototransistor, allowing it to fully conduct and minimize the collector-emitter voltage.

The phototransistor can switch loads connected either to the collector or emitter, with the output polarity inverted depending on the configuration. However, general-purpose optocouplers are limited to tens of milliamperes, so they are typically used to drive intermediate stages like MOSFETs or TRIACs in power circuits.

Applications in Power Electronics

In switch-mode power supplies (SMPS), optocouplers are widely used to provide reinforced isolation between the high-voltage AC input and the low-voltage DC output. They enable feedback control from the output to the primary side while maintaining safety standards such as DIN EN 60747-5-5 (VDE0884),. Proper selection ensures compliance with insulation requirements and long-term reliability.

Design ConsiderationsCTR Selection: Choose an optocoupler with a CTR sufficient to drive the load or the next switching stage.

LE

D Forward Current: Must be high enough to saturate the phototransistor. Load Resistor: Determines the output voltage and current; must match the phototransistor characteristics. Isolation Rating: Reinforced insulation is recommended for line-voltage separation, while basic insulation may suffice for non-touchable low-voltage circuits.

Temperature and Aging: CTR can vary with temperature and LED aging, so design margins are necessary.

Summary

Optocouplers in power switching enable safe, isolated control of high-voltage circuits using low-voltage signals. They operate as switches by saturating the phototransistor, with performance determined by CTR, LED current, and load configuration. They are essential in SMPS, microcontroller interfaces, and other applications requiring electrical isolation and safe switching of power devices.