What Are Photothyristors?

Photothyristors are four-layer semiconductor switching devices that are triggered into conduction by incident light rather than by a conventional electrical gate signal. Like standard thyristors, they operate as latching switches: once turned on, they remain conductive until the current through them falls below a minimum holding threshold, regardless of whether the initiating light pulse has ceased. The substitution of an optically sensitive gate region for a wired gate terminal provides galvanic isolation between the control circuit and the power circuit, an advantage in electrically noisy or high-voltage environments.

The device family includes the light-activated silicon controlled rectifier (LASCR), which controls current in one direction, and the phototriac, which handles bidirectional AC current. In all variants, the active region is a specially enlarged base-collector junction exposed through a transparent window in the device package. Photogenerated carriers in that junction supply the gate current required to trigger the four-layer pnpn structure into its low-impedance on state. The photothyristor can also function as an optical switch in signal-level optocoupler circuits, though its primary engineering significance lies in power control.

Light-Activated Silicon Controlled Rectifiers

The LASCR is the most commonly used photothyristor. Its gate sensitivity is determined by the area of the optically exposed junction and by the minority carrier lifetime in the silicon, both of which are engineered to be larger than in a purely electrically triggered SCR. A short pulse of light, typically from a light-emitting diode coupled through an optical fiber, delivers enough photocurrent to initiate regenerative switching. Because the triggering path is entirely optical, the control electronics can sit at a different electrical potential from the power device, eliminating the need for bulky isolation transformers or high-voltage gate driver boards. This property makes LASCRs attractive for AC motor speed control, light dimming systems, and controlled rectifier bridges, where the power thyristor may operate at hundreds of volts above the control reference.

The sensitivity of the device to spurious light must be managed in practice: shielding the package from ambient illumination prevents unintended triggering. A gate resistor connected between the gate and cathode is often included to shunt photocurrent generated by stray light below the triggering threshold while still allowing a focused control pulse to trigger the device.

Light-Triggered Thyristors in High-Voltage Systems

Direct light-triggered thyristors (LTTs) represent the high-power end of the photothyristor family and are used specifically where many devices must be connected in series at kilovolt-class potentials. In high-voltage direct current (HVDC) transmission, converter valves require stacks of thyristors rated at tens of kilovolts, with each device floating at a different potential. Electrically triggered variants require elaborate isolated gate drive electronics at every level; LTTs replace those electronics with a single optical fiber run from a ground-potential controller to each optically gated device, greatly simplifying valve construction and improving reliability. The triggering pulse is a laser-generated infrared pulse with peak power in the hundreds of milliwatts, delivered through a low-loss fiber.

Modern LTTs for HVDC incorporate integrated self-protection functions, including overvoltage breakover circuits that trigger the thyristor if the forward voltage exceeds a safe limit, as described in IEEE Xplore conference proceedings on direct light-triggered thyristor valve technology. Individual devices rated at 4 to 8 kV blocking voltage and thousands of amperes average current are produced commercially by manufacturers including Infineon and ABB. An application note from Infineon on triggering LTTs describes the optical power budget, fiber selection, and timing requirements. The use of LTTs for HVDC and static var compensation systems is also covered in IEEE Xplore papers on light-triggered thyristors for HVDC.

Applications

Photothyristors have applications in a wide range of disciplines, including:

• HVDC transmission converter valves requiring series-stacked optically triggered switching
• Static var compensation and flexible AC transmission systems
• AC motor speed control and industrial power converters
• Light dimming and power regulation for commercial and residential loads
• Optocoupler isolation in signal-level and medium-power control circuits