Thyristor Function and Circuit Applications

Key Takeaways

• Thyristors are a three-pin, three-junction semiconductor device that adds a switch to the forward conduction of a diode.

• The key to understanding thyristors is the hold current, which provides a reduced voltage requirement for forward conduction when gate-toggled and is the minimum threshold to maintain forward conduction.

• Thyristors provide numerous control benefits to power circuitry.

Thyristors can come in various packages for assemblies and system integration.

The diode is the most fundamental unit of circuit control, acting analogous to a check valve in fluid systems by only permitting a directional flow of current from anode to cathode under normal conditions. This seemingly simple description belies a world of potential circuit implementations: power conversion, polarity and over-voltage protection, temperature sensing, and more. However, the diode is limited in its ability to actively control and respond to changes in operating conditions, as its conduction or nonconduction fixes during layout and installation. A thyristor remedies this flaw with conduction patterns that can respond during in-circuit performance.

Thyristor Function: A Switch-Controlled Diode

A thyristor is a semiconductor device built from four alternating p- and n-doped layers. On the die, this requires NPN and PNP bipolar junction transistors (BJTs) tied at the same doped regions; one pair of p-type and n-type regions will combine, with the remaining n-type and p-type region that are unaccounted functioning as the total anode and cathode of the joint structure. This arrangement either ties the PNP's collector to the NPN's base or the NPN's emitter to the PNP, with the pin from the p-doped region closest to the cathode becoming a switch for the diode. Connecting two BJTs in this manner has the total effect of a diode with gate action (much like a transistor), where normal diode operation cannot proceed without a gate bias to enable the diode at the base pin. As a result, the thyristor has an extra operating mode compared to a diode:

• Reverse blocking - Driving a positive potential difference from the cathode to the anode blocks current until the voltage is significant enough to overcome the insulator effect and enter the breakdown region. A triggering bias on the gate pin during reverse bias conditions has no effect.

• Forward blocking - Underneath the breakdown voltage, applying a positive potential difference from the anode to the cathode without enabling the gate pin creates an insulation effect that prevents conduction. At the breakdown voltage of the gate pin’s p-n junction, conduction begins (known as the latching current). It continues so long as the bias from the anode to the cathode is enough to support the holding current, significantly reducing biasing requirements compared to the latching current.

• Forward conduction - An alternate I-V curve pathway uses a biasing voltage on the gate pin and a forward bias to reach the holding current without first reaching the voltage requirements of the latching current. The gate bias is a trigger: while the current meets or exceeds the holding current, there is no need to bias the gate continually. 

Turning a thyristor off requires suppressing the forward conduction so the current falls below the self-maintaining holding current. By reverse biasing the anode-cathode junction (usually accomplished with the discharge of a second thyristor to the anode), the current falls below this critical threshold in a process known as forced commutation. However, there is a time frame where a forward bias reapplied to the anode-cathode junction without a triggering bias voltage will resume forward conduction; to prevent an accidental state resumption, designers need to check the datasheet for the minimum commutation turn-off time. A newer sub-class of thyristors, called gate turn-off (GTO) thyristors, can turn off conduction with a negative bias at the gate pin.

What Circuits Benefit From Thyristor Inclusion?

Thyristor applications tend to focus on high-voltage and high AC conditions. As a switch-controlled diode, the thyristor can selectively allow conduction that could otherwise overwhelm rectification circuitry due to high-order harmonics. Zero-point (or synchronous) crossing is one method for correcting this, where a thyristor only conducts at or very close to zero volts. The sudden inrush of current and associated high voltages, if a diode were to turn on close to the peak of a periodic waveform, would carry significant noise from high-frequency spikes. This action switches between conduction and nonconduction cycles, which can vary depending on the average power needs of the system – effectively a slowed-down pulse width modulation.

However, the thyristor is not limited to AC circuits or those with high current and voltage. Many low-current/voltage circuits and DC applications benefit from a switch-controlled diode:

• DC switching - A thyristor roughly functions as a starter in some DC motor setups, where the circuit will not complete and conduct until the thyristor switches over to forward conduction. During conduction, this start switch will have no effect; an open switch in parallel or a closed switch in series will have to close or open (respectively) to interrupt the current flow through the thyristor long enough to fall below the holding current and reset to its isolation station. While this may seem more complex than a single switch to control motor operation, the advantage of the thyristor is the low bias voltage requirements on the gate pin allow for computer control, allowing automated control of current leveraging.

• Crowbar - A crowbar circuit offers overvoltage control for power supplies. When a sensor detects voltage above maximum safe operating values, a thyristor between the supply output and ground turns on and conducts, blowing a fuse and opening the circuit.

Cadence Has Solutions for Any Topology Imaginable

Thyristors are a fusion of semiconductor construction and function, combining BJTs at the die level to make a switch-controlled diode that has uses in controlling power systems. The isolation, safety, and control measures provided by thyristors, alongside their broad current and voltage applicability, make these devices suitable for many circuit configurations. Whatever the topology, designers need simulation environments that can thoroughly characterize components and overall circuit performance. Cadence’s PCB Design and Analysis Software gives design teams all the tools for pre-production electronic development. Coupled with the fast and powerful OrCAD PCB Designer, ECAD has never been more seamless.

Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. To learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.