Question

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

Answer 1

### Thermal Triggering Method of SCR

The thermal triggering method is a technique used to turn on a Silicon Controlled Rectifier (SCR) by applying heat to it. SCRs are normally in the off state, but they can be triggered into conduction by supplying heat to their junctions. In this method, a temperature rise causes the SCR to become conductive and allow current to flow through it.

Here’s how it works:

1. Temperature Increase: When heat is applied to the SCR, the temperature at the junction rises.
   

1. Breakdown of Junction: The increase in temperature causes the semiconductor material's conductivity to increase. This affects the gate junction of the SCR, causing the device to turn on and allow current to flow.
   

1. Thermal Runaway: If the temperature rises beyond a certain limit, it could result in the SCR turning on, even without any external triggering signal. This is often considered an undesirable behavior in SCRs because it can lead to thermal runaway, where the SCR might continue conducting uncontrollably.
   

Different Triggering Methods of SCR

There are several methods to trigger an SCR into conduction, and they can be broadly classified into the following categories:

1. Gate Triggering:
   

   - A small voltage or current is applied to the gate terminal of the SCR to trigger it.
   - This is the most common and efficient method of triggering an SCR. Once the gate current is applied, the SCR turns on and allows current to flow between the anode and cathode.

1. Thermal Triggering:
   

   - As explained earlier, heat is applied to the SCR, causing the junction temperature to rise and triggering the SCR into conduction.

1. Light Triggering:
   

   - This method uses light, typically in the form of an LED or laser, to trigger the SCR.
   - The SCR used in this case is usually a photonic SCR (photoSCR), which can be triggered by light. The intensity of the light affects the SCR’s ability to conduct current.

1. Voltage Triggering:
   

   - In this method, a high voltage is applied across the anode and cathode of the SCR.
   - When the voltage exceeds a certain threshold, the SCR becomes conductive.

1. Pulse Triggering:
   

   - This involves applying a pulse of current to the gate terminal of the SCR.
   - The duration and magnitude of the pulse can control the triggering and turn the SCR on.

1. dv/dt Triggering:
   

   - This method relies on the rate of change of voltage (dv/dt) across the SCR.
   - If the voltage changes too quickly (at a very high rate), the SCR may accidentally trigger on due to the induced current created by the rapid voltage change. This is why SCRs are designed to have a dv/dt rating.

1. Current Triggering:
   

   - This method applies a small current to the gate terminal of the SCR. Once the current reaches a certain value, it triggers the SCR into conduction.

In summary, gate triggering is the most commonly used method, while others like thermal and light triggering offer specialized uses depending on the application.