Question

Explain the thermal triggering method of SCR. Enlist different triggering methods of SCR.

Answer 1

**Thermal Triggering Method of SCR**

Thermal triggering of a Silicon Controlled Rectifier (SCR) involves using temperature changes to turn the SCR on. Here's a detailed explanation:

1. **Principle**: In the thermal triggering method, the SCR is activated by increasing the temperature in its gate region. As the temperature rises, it causes a change in the internal characteristics of the SCR, leading to a reduction in the breakover voltage (the voltage required to trigger the SCR into conduction).

2. **Mechanism**:
   - **Temperature Coefficient**: The SCR's gate and junctions have temperature coefficients that influence their electrical characteristics. As the temperature increases, the junction characteristics change, affecting the device's behavior.
   - **Thermal Runaway**: In some cases, the increase in temperature causes a positive feedback loop where the SCR becomes more conductive. This process, known as thermal runaway, leads to the SCR turning on.
   - **Heating Element**: A heating element or resistor is often placed near the SCR to control the temperature. By carefully managing the temperature, the SCR can be triggered.

3. **Applications**: Thermal triggering is less commonly used in modern applications due to the advent of more precise and controllable triggering methods, but it can still be seen in specific applications where temperature-based control is desired.

**Different Triggering Methods of SCR**

SCRs can be triggered into conduction using various methods, each suited for different applications and requirements:

1. **Gate Triggering**:
   - **Principle**: A small voltage or current applied to the gate terminal of the SCR can trigger it into conduction.
   - **Application**: Commonly used due to its simplicity and control.

2. **Pulse Triggering**:
   - **Principle**: A pulse of current or voltage is applied to the gate or gate-cathode junction of the SCR to turn it on.
   - **Application**: Used in digital circuits and precise timing applications.

3. **Thermal Triggering**:
   - **Principle**: The SCR is triggered by increasing the temperature in its gate region, leading to a change in its electrical characteristics.
   - **Application**: Used less frequently but can be applied where temperature-based control is needed.

4. **Light Triggering** (Optically Triggered SCR or TRIAC):
   - **Principle**: Light exposure, typically from an LED or laser, triggers the SCR by generating carriers in the gate region.
   - **Application**: Used in opto-isolators and remote triggering applications.

5. **Voltage Triggering**:
   - **Principle**: A voltage is applied across the SCR that exceeds the breakover voltage, causing it to conduct.
   - **Application**: Often used in power electronics where precise control over the triggering voltage is possible.

6. **Current Triggering**:
   - **Principle**: A specific amount of current is passed through the SCR to trigger it into conduction.
   - **Application**: Used in situations where current control is preferable or easier to implement.

Each triggering method offers unique advantages and is selected based on the application's requirements for control, precision, and practicality.

Answer 2

### Thermal Triggering Method of SCR
In **thermal triggering**, an SCR (Silicon Controlled Rectifier) can be turned on by increasing the temperature of the device's junctions. When the temperature rises beyond a certain threshold, the leakage current in the reverse-biased junction (J2) increases significantly. This rise in leakage current may cause sufficient charge carriers to flow through the junction, leading to the breakdown of J2 and turning the SCR on.

However, this method is not reliable or controlled, and excessive heating may damage the SCR. It is generally avoided in practical applications because it lacks precision and can reduce the lifespan of the device.

### Different Triggering Methods of SCR

1. **Electrical Triggering (Gate Triggering)**:
   - The most common method.
   - A small positive voltage is applied to the gate terminal of the SCR relative to the cathode, causing it to turn on.

2. **Thermal Triggering**:
   - Involves increasing the temperature to turn on the SCR.
   - Not a preferred method for controlled operations.

3. **Light (Optical) Triggering**:
   - Used in opto-SCRs (photo SCRs).
   - A beam of light is focused on a sensitive area of the SCR, causing it to conduct.

4. **dv/dt Triggering**:
   - Occurs when the rate of change of voltage across the SCR (dv/dt) is high.
   - A rapid voltage increase across the device can create a current that triggers the SCR.

5. **Forward Voltage Triggering**:
   - When the anode-to-cathode voltage exceeds a certain breakdown value, the SCR turns on.
   - Similar to the avalanche breakdown in diodes.

Each method has its own practical application based on the need for control, reliability, and safety.