- 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 turns on when a positive voltage is applied between the gate and cathode terminals. This transition from forward blocking to conduction mode has a specific duration, known as the turn on time of an SCR, divided into delay(td), rise time(tr), and spread times(ts).
Delay Time of SCR
After application of gate current, the thyristor will start conducting over a very tiny region. Delay time of SCR can be defined as the time taken by the gate current to increase from 90% to 100% of its final value Ig. From another point of view, delay time is the interval in which anode current rises from forward leakage current to 10% of its final value and at the same time anode voltage will fall from 100% to 90% of its initial value Va.
Rise Time of SCR
Rise time of SCR in the time taken by the anode current to rise from 10% to 90% of its final value. At the same time anode voltage will fall from 90% to 10% of its initial value Va. The phenomenon of decreasing anode voltage and increasing anode current is entirely dependent upon the type of the load. For example if we connect a inductive load, voltage will fall in a faster rate than the current increasing. This is happened because induction does not allow initially high voltage change through it. On the other hand if we connect a capacitive load it does not allow initial high voltage change through it, hence current increasing rate will be faster than the voltage falling rate.
High increasing rate of dia/dt can create local hot spot in the device which is not suitable for proper operation. So, it is advisable to use a inductor in series with the device to tackle high dia/dt. Usually, the value of the maximum allowable di/dt is in the range of 20 to 200 A per microsecond. Note that you can learn more by studying some of the basic electronics questions we have here at Electrical4U.
Spread Time of SCR
The spread time of an SCR is the duration for the anode current to increase from 90% to 100% of its maximum while the anode voltage reduces to its minimum. During this phase, conduction expands across the cathode, fully activating the SCR. The spread time is influenced by the cathode’s cross-sectional area.
Turn OFF Time of SCR
Once the thyristor is switched on, and the anode current exceeds the latching current, the gate loses control. To turn off the SCR, the anode current must fall below the holding current, and carrier charges in the four layers must be cleared before reapplying forward voltage. Thus, the turn off time of an SCR is the interval from when the anode current falls to zero until the device returns to its forward blocking mode.
- Reverse Recovery Time.
- Gate Recovery Time
Reverse Recovery Time
It is the interval in which change carriers remove from J1, and J3 junction. At time t1, anode current falls to zero and it will continue to increase in reverse direction with same slope (di/dt) of the forward decreasing current. This negative current will help to sweep out the carrier charges from junction J1 and J3. At the time t2 carrier charge density is not sufficient to maintain the reverse current hence after t2 this negative current will start to decrease. The value of current at t2 is called reverse recovery current. Due to rapid decreasing of anode current, a reverse spike of voltage may appear across the SCR. Total recovery time t3 – t1 is called reverse recovery time. After that, device will start to follow the applied reverse voltage and it gains the property to block the forward voltage.
Gate Recovery Time
After sweeping out the carrier charges from junction J1 and J3 during reverse recovery time, there still remain trapped charges in J2 junction which prevent the SCR from blocking the forward voltage. This trapped charge can be removed by recombination only and the interval in which this recombination is done, called gate recovery time.
