Switching or ON OFF Characteristics of SCR or Thyristor

Contents

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Key learnings:

SCR Definition: A Silicon Controlled Rectifier (SCR) is defined as a semiconductor device that switches and controls power in circuits, turning on by a gate signal and off by cutting the power
Turn ON Process: The turn on time of an SCR involves several stages—delay, rise, and spread time—each critical for the device to reach full conduction.
Turn OFF Process: Turning off an SCR requires the anode current to drop below a specific threshold and involves clearing internal carrier charges for proper shutdown.
Load Impact on SCR: The type of load connected to an SCR—whether inductive or capacitive—affects how quickly the device’s voltage and current levels adjust during operation.
Recovery Times: SCR recovery times, including reverse and gate recovery, are essential for the device to return to its blocking state and be ready for the next cycle.

Turn ON Time of SCR

A forward-biased thyristor can be triggered by applying sufficient gate current between its gate and cathode. The required gate drive is specified by current and voltage limits in the device data sheet. The transition from forward blocking to conduction takes a finite turn-on time, often described by delay time t_d, rise time t_r and spread time t_s.

Delay Time of SCR

After the gate pulse begins, conduction starts near the gate region. Delay time of SCR is the initial part of the switching interval before the main anode current has risen appreciably. Gate current is denoted by I_g. A common waveform definition measures delay time from a stated point on the gate-current pulse to the point where anode current reaches 10% of its final value, while anode voltage falls to about 90% of its initial value V_a. The exact thresholds and test circuit should come from the relevant data sheet.

Rise Time of SCR

Rise time of SCR is commonly measured while anode current rises from 10% to 90% of its final value. Over the same transition, anode voltage falls from about 90% to 10% of its initial value V_a. The observed waveforms depend on the source, load, stray elements and test circuit. An inductive load opposes rapid change in current, while a capacitive load opposes rapid change in voltage. These properties affect the relative rates at which SCR voltage falls and current rises.

A high di_a/dt can concentrate current near the initial conducting area and overheat the device before conduction spreads. A series inductor may be used to limit di_a/dt where the circuit requires it. There is no universal allowable range: use the maximum turn-on di/dt, gate-drive requirements and test conditions specified for the selected SCR. You can review related concepts in these basic electronics questions.

Spread Time of SCR

The spread time of an SCR describes the final part of turn-on as the conducting area expands across the device. In the usual simplified waveform, anode current rises from about 90% towards its final value while anode voltage approaches its on-state value. Device geometry, gate drive, anode-current rise and junction temperature affect this process.

Turn OFF Time of SCR

Once the thyristor has latched and its anode current remains above the holding current, removing gate drive does not turn it off. The external circuit must reduce anode current below the holding current, usually to zero. It must then keep the device reverse-biased or otherwise provide enough recovery time. The specified turn-off time is the interval after anode current reaches zero during which stored charge clears before forward voltage can be reapplied at the stated rate without retriggering.

Reverse Recovery Time.
Gate Recovery Time.

Reverse Recovery Time

During reverse recovery, stored charge is removed from the outer junction regions J₁ and J₃. After t₁, the anode current has crossed zero, and reverse current can sweep charge from J₁ and J₃. It reaches its peak near t₂ and decays after t₂. The current at t₂ is the peak reverse recovery current. Circuit inductance and a rapid current change can produce a reverse voltage transient, so the commutation and snubber components must stay within device ratings. In this simplified description, the interval t₃ – t₁ is the reverse recovery time. Reverse-current recovery alone does not mean the SCR can yet block reapplied forward voltage.

Gate Recovery Time

After charge is removed from junctions J₁ and J₃ during reverse recovery time, residual charge around the inner junction J₂ must recombine before forward blocking is restored. This later interval is sometimes called gate recovery time in simplified texts. Modern data sheets normally specify the total circuit-commutated turn-off time tq under defined current, reverse voltage, temperature and reapplied dv/dt conditions. Designers should use that specified tq rather than adding informal recovery intervals.