Thyristor Triggering or SCR Triggering

An Thyristor is a latching semiconductor switch used in power electronics. The silicon-controlled rectifier (SCR) is the best-known member of this device family. It controls current in one direction and can block a forward voltage until it receives a valid turn-on stimulus.
Applications of power electronics use switching devices to convert and control electrical energy. The appropriate semiconductor depends on the voltage, current, switching frequency and control method. Common choices include power diodes, Bipolar Junction Transistors (BJTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), insulated-gate bipolar transistors (IGBTs) and gate turn-off thyristors (GTOs).

Thyristor is the name of a semiconductor device family. A conventional SCR has four alternating P and N layers and three junctions, J₁, J₂ and J₃, formed from N type semiconductor and P type semiconductor material. Its three terminals are the anode, cathode and gate. With the anode positive relative to the cathode, the SCR remains in its forward-blocking state until a trigger initiates regenerative conduction. It then latches on if the anode current reaches the specified latching current.

Thyristor triggering means changing the device from its blocking state to its conducting state. An SCR can turn on through a gate signal, forward breakover, excessive rate of voltage rise, heat or light. Gate triggering is the normal controlled method for a standard SCR; several of the other mechanisms are usually unwanted.

1. Voltage Thyristor Triggering: With the gate open, increasing the forward voltage above the device’s breakover voltage can switch it on. Designers normally keep the working voltage below this rating because breakover turn-on can expose the device to high instantaneous power and damaging current.
2. Thermal Thyristor Triggering: Higher junction temperature increases leakage current and the current gains represented by α₁ and α₂. When (α₁ + α₂) approaches unity, regenerative action can turn on the SCR without a gate command. This is an abnormal condition rather than a practical control method. Inadequate thermal design can damage the device.
3. Light Thyristor Triggering: In a light-activated SCR (LASCR), incident light creates charge carriers in a light-sensitive region and initiates turn-on. A suitable optical source, including a LASER, provides electrical isolation between the control and power circuits.
4. dv/dt Triggering: A rapid rise in anode-to-cathode voltage produces displacement current through the SCR’s junction capacitances. If this current is high enough, the device can turn on without a gate command. The datasheet’s critical off-state dv/dt rating defines the limit below which spurious turn-on should not occur under its stated test conditions.
5. Gate Triggering: This is the usual controlled method of thyristor triggering. A positive gate-to-cathode current at or above I_GT, the specified trigger requirement, turns on a forward-biased SCR. Once the anode current has risen above the latching current, the gate signal can be removed. A standard SCR then remains on while its anode current stays above the holding current; its gate does not provide normal turn-off control.

When an SCR is forward biased, a positive gate-to-cathode current can start regenerative turn-on.

The waveform shows anode current after the gate pulse begins. Under the convention shown, total turn-on time t_on runs from 10% of the final gate current, 0.1I_g, to 90% of the final on-state current, 0.9I_T. It is the sum of delay time t_d and rise time t_r. In equation form, t_on = t_d + tr. Delay time runs from 0.1I_g to 10% of on-state current, 0.1I_T. Rise time covers the increase from 0.1I_T to 0.9I_T. Exact thresholds and test conditions depend on the device datasheet.
The following points matter when designing a gate-trigger circuit.

1. Supply enough gate current for the specified duration and temperature range. The gate pulse can end after the anode current has exceeded latching current, but it must remain within the device’s peak and average gate ratings.
2. Keep gate-to-cathode voltage and current within their forward and reverse datasheet limits. A reverse-biased anode does not create a valid forward-conduction command.
3. Make the gate pulse long enough for anode current to exceed latching current I_L. Inductive loads may delay the current rise and therefore require a longer pulse or a train of pulses.

A conventional SCR cannot normally be turned off by applying a negative gate signal. Its anode current must fall below holding current IH for enough time to recover its forward-blocking capability. This happens naturally at an AC current zero or through a commutation circuit in DC service. Holding current is the minimum anode current that keeps a fully conducting SCR on without gate drive under the stated test conditions.

Latching current IL is the minimum anode current required to keep the SCR conducting immediately after the gate signal is removed. After the device has fully latched, its anode current may fall below IL without turn-off. Conduction ends when the current falls below holding current IH. For a conventional SCR, the specified latching current is normally greater than the holding current, and both values vary with operating conditions such as junction temperature.