Question

Draw construction of SCR using two transistor model. Explain its operation.

Answer 1

The Silicon Controlled Rectifier (SCR) is a semiconductor device that functions as a switch, allowing current to flow in one direction when triggered. The SCR can be understood using a two-transistor model, which helps illustrate its operation and characteristics. Let's break down the construction, working, and operation of the SCR using this model.

### Construction of SCR Using Two-Transistor Model

1. **Basic Structure:**
   - The SCR is made of four layers of semiconductor material (P-N-P-N), forming three junctions (J1, J2, J3).
   - It consists of two P-type (positive) materials and two N-type (negative) materials arranged alternately: P1-N1-P2-N2.

2. **Transistor Representation:**
   - The SCR can be represented using two transistors: Q1 (NPN) and Q2 (PNP).
   - **Transistor Q1 (NPN):** Comprises the N-type material (N1) as the collector and the P-type material (P1) as the base.
   - **Transistor Q2 (PNP):** Uses the P-type material (P2) as the collector and the N-type material (N2) as the base.

3. **Feedback Mechanism:**
   - The two transistors are interconnected: the emitter of Q1 is connected to the base of Q2, and the emitter of Q2 is connected to the base of Q1.
   - This interconnection creates a positive feedback loop, essential for the SCR's operation.

### Operation of SCR

1. **Off State (Blocking State):**
   - In the off state, no gate signal is applied, and the SCR behaves like an open circuit.
   - Both transistors Q1 and Q2 are in the cutoff region, preventing any current flow through the device.

2. **Triggering the SCR:**
   - To turn on the SCR, a small positive voltage (gate current) is applied to the gate terminal.
   - This gate current feeds into the base of Q2, allowing Q2 to conduct.
   - When Q2 conducts, it provides current to the base of Q1, turning Q1 on as well.

3. **On State (Conducting State):**
   - Once both transistors are conducting, the SCR enters the on state, allowing a large current to flow through the device.
   - The SCR remains in this conducting state even if the gate current is removed, due to the positive feedback mechanism between the two transistors.

4. **Turning Off the SCR:**
   - To turn off the SCR, the current flowing through it (anode current) must drop below a certain threshold, known as the "holding current."
   - This can be achieved by reducing the supply voltage or increasing the load resistance.
   - When the current falls below the holding current, both transistors switch off, returning the SCR to its off state.

### Summary

- **Structure:** The SCR is represented by two interlinked transistors (NPN and PNP) in a four-layer configuration.
- **Operation:**
  - **Off State:** No current flows; both transistors are off.
  - **On State:** A small gate current triggers conduction, maintaining it through positive feedback.
  - **Turning Off:** The device can be turned off by reducing the current below the holding current.

The two-transistor model provides a clear understanding of the SCR's operation, emphasizing its unique ability to switch on and off, making it widely used in power control applications, such as in motor drives and light dimmers.

Answer 2

The two-transistor model is a widely used method to understand the operation of a Silicon-Controlled Rectifier (SCR). It helps in visualizing the behavior of the SCR by breaking it down into two interconnected bipolar junction transistors (BJTs). Below is the construction and explanation of the SCR using the two-transistor model:

### 1. **Construction of SCR Using Two-Transistor Model**

An SCR consists of four layers of semiconductor material, forming three PN junctions. These layers are named as follows:

- **Anode (A):** Connected to the P-type layer.
- **Cathode (K):** Connected to the N-type layer.
- **Gate (G):** Connected to another P-type layer closer to the cathode.

In the two-transistor model, the SCR can be broken down into two transistors:

- **Transistor Q1 (P-N-P):** The first transistor is a PNP type.
- **Transistor Q2 (N-P-N):** The second transistor is an NPN type.

These transistors are connected in such a way that:

- The **collector of Q1** is connected to the **base of Q2**.
- The **collector of Q2** is connected to the **base of Q1**.
- The **emitter of Q1** is connected to the **anode**.
- The **emitter of Q2** is connected to the **cathode**.
- The **gate** terminal is connected to the **base of Q2** (NPN transistor).

### 2. **Operation of SCR Using Two-Transistor Model**

#### **Step-by-Step Operation:**

1. **Initial Condition (Off State):**
   - When the gate terminal is not triggered, both transistors Q1 and Q2 are in the cutoff region (no current flows through them).
   - The SCR is in the off state, and no current flows from the anode to the cathode.

2. **Gate Triggering (Turning On the SCR):**
   - When a small gate current is applied to the gate terminal, it forward biases the base-emitter junction of transistor Q2 (NPN).
   - This causes transistor Q2 to turn on, allowing current to flow from its collector to its emitter (from the base of Q1 to the cathode).

3. **Positive Feedback Mechanism:**
   - The current flowing through the collector of Q2 forward biases the base-emitter junction of transistor Q1 (PNP), turning Q1 on.
   - When Q1 turns on, it allows current to flow from its collector to its emitter (from the anode to the base of Q2).
   - This current further enhances the base drive of Q2, which in turn allows more current to flow through Q2.
   - This regenerative feedback causes both transistors to conduct heavily, resulting in a large current flowing from the anode to the cathode.

4. **Latch-On State:**
   - Once both transistors are fully turned on, the SCR enters a latch-on state, where it remains conducting even if the gate current is removed.
   - The SCR will continue to conduct as long as the current through it remains above a certain threshold (holding current).

5. **Turning Off the SCR:**
   - To turn off the SCR, the current flowing through it must be reduced below the holding current.
   - This can be achieved by reducing the load current or by using an external commutation circuit.

### **Diagram:**

Below is a diagram representing the construction of an SCR using the two-transistor model:

```
      Anode
        |
       ---
       |   \
   |----->| Q1 (PNP)
   |       \
   |     ---
       |  
      |----->|
       |
      Q2 (NPN)   ---------> Gate
       |          |
        \        ---
       |       |---
        Cathode
```

### **Explanation:**

- **Q1 (PNP):** When turned on by the gate signal, it provides base current to Q2.
- **Q2 (NPN):** Once turned on, it provides base current to Q1.
- This positive feedback ensures that the SCR remains in the conducting state until the current is reduced below the holding threshold.

### **Conclusion:**

The two-transistor model of an SCR provides a clear understanding of how the device operates. By using the positive feedback between two transistors, the SCR can latch onto the conducting state, making it a highly efficient and effective device for controlling high-power applications.