The thyristor, also called an SCR, is a widely used component in power control and switching applications. These devices handle switching of high voltages and currents and have replaced relays in many power-switching roles, although very high-voltage contactors are still used in some cases.
Thyristor/SCR basics
Thyristors operate differently from standard bipolar transistors or FETs. They have two main electrodes, the anode and cathode, which connect to the controlled power circuit, plus a third electrode, the gate, used by the control circuit to trigger the device.
In its equivalent model, an SCR can be represented by two interconnected transistors. Under initial conditions no conduction occurs between anode and cathode. A current applied to the gate switches the internal transistor so the SCR conducts in the forward direction. Conduction remains even after the gate current is removed, so the gate provides a triggering pulse.
To stop conduction, the anode-to-cathode voltage must be reduced below a threshold so that one or both internal transistors go into cutoff. Then the device unlatches and must be retriggered to conduct again.
Synthesizing these points: an SCR conducts only in one direction and must be retriggered on each conducting half-cycle when used with AC. When fully on, the forward voltage drop across a thyristor is typically about 1 V up to its rated current. The device remains conducting while the anode current remains above the holding current IH. If current falls below IH, the SCR turns off; therefore DC and some high-inductance AC circuits require a method to force the device to turn off.
Gate circuitry
Gate resistors are commonly added to prevent gate overload and to avoid false triggering. Typical designs include two resistors in the gate circuit.
R1 limits gate current to an acceptable level, chosen to provide sufficient current to trigger the SCR without stressing the gate junction. The second resistor, R2, is a gate-to-cathode resistor (sometimes labeled RGK) that reduces gate sensitivity and helps prevent spurious triggering. Some SCR packages include an internal RGK; consult the manufacturer datasheet to determine whether an external resistor is required.
Gate drive requirements, trigger timing, and the duration of gate excitation needed to latch the device depend on the chosen triggering method. Careful gate design is crucial to avoid false triggers while ensuring reliable firing when required.
SCR trigger methods
Understanding the different trigger mechanisms helps ensure the SCR fires only when desired. Common trigger methods include:
Gate triggering
Gate triggering is the most common and straightforward method. The SCR must operate below its breakdown voltage and with margin for transients to prevent forward-breakdown or avalanche triggering. A positive gate-to-cathode voltage produces gate current that injects charge into the internal p-layer, lowering the forward-breakdown voltage. Higher gate current reduces the forward-breakdown voltage and accelerates turn-on.
A simple gate trigger arrangement uses a resistor to limit gate current and an RGK resistor to prevent false triggering. The RGK bypasses some internal gate current and raises the required anode current to latch conduction, which increases dv/dt immunity, raises latching and holding currents, and can reduce turn-off time tq.
Forward-voltage (avalanche) triggering
This occurs when the anode-to-cathode forward voltage causes avalanche breakdown of the internal junction J2. When J2 breaks down at the forward-breakdown voltage VB0, carriers flow and the SCR latches on. This method is generally undesirable because repeated or excessive avalanche can damage the device; circuits should be designed to avoid it and to limit voltage spikes.
dv/dt triggering
A rapid rise of anode voltage relative to cathode can induce internal currents sufficient to trigger the SCR without gate current. dv/dt triggering is an important failure mode to consider because transient voltages across the device can cause unintended or intermittent firing.
Thermal triggering
Thermal triggering can occur if junction leakage and the voltage across J2 heat the junction, increasing leakage and creating a positive feedback loop that leads to conduction. This effect is more likely at elevated device temperatures and should be considered in thermal and reliability assessments.
Light triggering
Light-activated SCRs (LASCRs) are used where electrical isolation between trigger source and power circuit is required, such as in high-voltage systems. A light pulse generates carriers in the internal P region and triggers conduction. Special SCR structures and light-guiding features maximize photogenerated carriers, and optical fibers often deliver light to the activation region. LASCRs are used in high-voltage switching centers where high isolation and low-level drive are needed.
DC SCR circuits
SCRs are often used to switch DC loads such as motors or lamps. A basic DC SCR circuit can use a small switch to apply gate current and latch the device on, then a separate switch in the anode circuit to interrupt current and turn the SCR off.
In the illustrated circuit, S2 is the gate switch and S1 is an anode-circuit switch. Closing S2 provides gate current via R1 and triggers the SCR; current then flows through the load until the anode circuit is opened by S1. S1 must be rated for full load current, while S2 only needs to carry gate current. R2 is included to reduce sensitivity to noise and prevent false triggering.
Basic AC SCR circuits
With AC supplies, the SCR will be reverse-biased during the negative half-cycle, so it turns off naturally each half-cycle and does not require a separate turn-off switch.
When the gate switch is closed, the circuit waits until sufficient forward anode voltage appears in the positive half-cycle and the gate circuit supplies enough current to fire the SCR. The SCR conducts for the remainder of that positive half-cycle until the anode-to-cathode voltage falls below the level needed to sustain conduction. The process repeats on the next positive half-cycle if the gate switch remains closed. Note that a single SCR in this simple arrangement cannot deliver more than 50% of the AC power to a resistive load because it does not conduct during the negative half-cycle.
Phase control using gate timing
Power delivered to an AC load can be controlled by delaying the gate firing angle within each positive half-cycle. A common method uses an RC timing network and a variable resistor to control the delay before a diode conducts and applies the gate pulse.
In the phase-control circuit, an RC network charges each half-cycle and the gate is triggered only when the capacitor voltage reaches the diode threshold. Adjusting the variable resistor changes the RC time constant and thus the firing angle, varying the portion of each half-cycle during which the SCR conducts. An additional series resistor limits the minimum resistance so the diode and gate receive acceptable gate current. For full 0° to 180° control of the firing angle, the gate waveform phase must be adjustable across that range.
Crowbar overvoltage protection using SCRs
Thyristor crowbar circuits provide overvoltage protection by shorting the supply to clamp voltage when an overvoltage condition is detected. Although crude, this method is effective for protecting supplies from catastrophic overvoltage, such as when series regulator elements fail. Proper design must consider the energy and stress imposed on the crowbar and the supply wiring.
