### Commutation Techniques of SCR (Silicon Controlled Rectifier)
Commutation refers to the process of turning off a conducting SCR (Silicon Controlled Rectifier). Once an SCR is turned on and conducts current, it will continue conducting until the current flowing through it drops to zero or reverse biases. The process of forcing an SCR to turn off is essential in many electronic circuits. There are different methods to accomplish this, called **commutation techniques**.
SCRs are widely used in power electronics applications, such as rectifiers, inverters, motor speed control, and switching power supplies, where the ability to turn off the SCR efficiently is crucial.
### Two Major Types of Commutation
Commutation techniques can be broadly classified into two categories:
1. **Natural Commutation (Line Commutation)**
2. **Forced Commutation**
#### 1. Natural Commutation (Line Commutation)
Natural commutation occurs when the AC voltage supply itself naturally reduces the current to zero. It is commonly used in **AC circuits**, particularly in rectifiers, where the current periodically drops to zero due to the alternating nature of the AC waveform.
**How it Works**:
- In an AC circuit, the voltage and current change direction periodically.
- When the current reverses direction, the current through the SCR drops to zero and the SCR turns off.
- The SCR remains off until a new gate pulse is applied in the next positive half cycle of the input voltage.
**Example**: In rectifiers like half-wave or full-wave rectifiers, the SCR automatically turns off when the current crosses zero at the end of each half cycle of the AC waveform.
**Advantages of Natural Commutation**:
- No additional circuitry is required.
- Simple and cost-effective.
- It works well with AC systems where natural current zero exists.
**Limitations**:
- It is only applicable in AC circuits.
- It cannot be used in DC circuits where the current never naturally goes to zero.
#### 2. Forced Commutation
Forced commutation is used in **DC circuits** or in cases where there is no natural current zero crossing. Since the SCR will not turn off automatically, external circuitry is needed to force the current through the SCR to zero, thus turning it off.
There are several types of forced commutation techniques, each using external components like capacitors, inductors, and other switches to control the current through the SCR.
**Forced commutation techniques** can be classified into the following types:
### Types of Forced Commutation Techniques
1. **Class A: Load Commutation (Self Commutation)**
2. **Class B: Resonant Pulse Commutation**
3. **Class C: Complementary Commutation**
4. **Class D: Impulse Commutation**
5. **Class E: External Pulse Commutation**
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### 1. **Class A: Load Commutation (Self Commutation)**
In this method, the SCR is turned off automatically by the characteristics of the circuit itself, particularly in resonant circuits. This is typically used in circuits with **capacitive or inductive loads** that create oscillations.
**How it Works**:
- A resonant LC circuit is used, where the load itself oscillates, causing the current to pass through zero.
- The zero current in the circuit turns off the SCR.
- The natural oscillations of the LC circuit are used to turn off the SCR.
**Applications**:
- Load commutation is widely used in high-frequency inverters and other resonant circuits.
**Advantages**:
- No need for additional switching devices.
- Suitable for high-frequency applications.
**Disadvantages**:
- Limited to resonant circuits.
- The circuit design is complex and frequency-dependent.
---
### 2. **Class B: Resonant Pulse Commutation**
In this method, the SCR is turned off by creating a resonant pulse in an LC circuit connected in parallel with the SCR.
**How it Works**:
- A parallel LC circuit (inductor and capacitor) is connected across the SCR.
- When the commutation capacitor is charged and switched to the SCR, it forces a resonant pulse.
- The pulse momentarily forces the current through the SCR to zero, thus turning it off.
**Applications**:
- Commonly used in chopper circuits, inverter circuits, and DC to AC converters.
**Advantages**:
- Suitable for medium to high-power circuits.
- Can turn off the SCR at any point during conduction.
**Disadvantages**:
- Requires additional components like inductors, capacitors, and sometimes diodes.
---
### 3. **Class C: Complementary Commutation**
This technique uses two SCRs (or thyristors) in a complementary fashion. One SCR turns off the other by reversing the current flow or providing a path for current to commutate.
**How it Works**:
- Two SCRs are connected in such a way that when one SCR is triggered, the other SCR turns off.
- The turning on of one SCR provides the reverse voltage across the other, forcing it to turn off.
**Applications**:
- Used in inverter circuits, especially in **DC to AC conversion** applications.
**Advantages**:
- Simple and reliable method.
- Can handle high power.
**Disadvantages**:
- Requires multiple SCRs, increasing the complexity and cost.
---
### 4. **Class D: Impulse Commutation**
In this method, an external **commutation circuit** is used to apply a large reverse current through the SCR to turn it off.
**How it Works**:
- A commutation capacitor is charged in one direction and then discharged through the SCR in the opposite direction.
- This reverse current opposes the flow of current through the SCR, forcing it to turn off.
- A diode is often used to prevent the forward current from continuing after the SCR is turned off.
**Applications**:
- Used in choppers and inverters where quick switching is required.
**Advantages**:
- Can be used to turn off the SCR very quickly.
- Works in DC circuits.
**Disadvantages**:
- The complexity of the circuit is higher.
- Additional components like capacitors, inductors, and diodes are needed.
---
### 5. **Class E: External Pulse Commutation**
This technique uses an external pulse, usually from another SCR or a power transistor, to create a reverse voltage that turns off the conducting SCR.
**How it Works**:
- A second SCR or transistor is used to apply a reverse voltage pulse across the conducting SCR.
- This reverse voltage causes the current through the SCR to fall to zero, turning it off.
- The external pulse source is carefully timed to ensure proper commutation.
**Applications**:
- Often used in **DC motor control** circuits, inverters, and power supply circuits.
**Advantages**:
- Can precisely control the turn-off time.
- Suitable for circuits that need precise timing.
**Disadvantages**:
- Requires external pulse circuitry, increasing the overall complexity.
- Timing must be precise, making it sensitive to errors.
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### Conclusion
Each commutation technique for SCR has its own set of advantages and disadvantages, and the choice of technique depends on the application, whether it involves AC or DC, the power level, and the complexity of the circuit.
- **Natural commutation** is ideal for AC circuits with a natural current zero point.
- **Forced commutation** techniques, like Class B or Class D commutation, are widely used in DC circuits where external control is needed to turn off the SCR.
Understanding these techniques is essential for effectively designing and troubleshooting circuits that utilize SCRs in various power electronic applications.