The current commutated chopper is a type of forced commutated chopper that uses an oscillatory current in order to turn OFF the main thyristor. The oscillatory current flowing through the chopper must be greater than the load current.

Similar to the capacitor in a voltage commutated chopper, the oscillatory current to commutate the main thyristor in this chopper is obtained from a charged capacitor. This type of chopper circuit is also known as Resonant-Pulse Commutated Chopper. Let us see the circuit diagram, working and related waveforms of the current commutated chopper.

Circuit of Current Commutated Chopper :

The below shows the circuit configuration of the current commutated chopper. It consists of main thyristor T_M and auxiliary thyristor T_A. The components inductor (L), capacitor (C), T_A, diodes (D₁ and D₂) forms the commutating circuit in order to turn OFF the main thyristor.

In the above chopper circuit,

• The load current I_O is assumed to be constant.
• The internal resistance of thyristors and diodes is assumed to be zero.
• The resistor is assumed to be so large that during commutation of the thyristor, it remains as an open circuit.

Working of Current Commutated Chopper :

For convenience, the working of the current commutated chopper can be explained in five modes. Before initiating the operation, the capacitor in the circuit must be charged to the source voltage. This can be done by closing the switch (S) and the charging current flows through the path V_dc⁺ → C → S → R → V_dc^–. The charge that is stored in the capacitor is used for the commutation process.

Once the capacitor is charged, the switch is opened and the main thyristor T_M is triggered at instant t = t_o. Once T_M starts conducting, load voltage V_O follows source voltage V_dc, whereas, load current I_O remains constant.

Mode I (t₁ < t < t₂) :

Mode I starts when the auxiliary thyristor T_A is triggered which is at instant t = t₁, and the commutation process starts at this instant. Now, an oscillatory current I_C of magnitude (V_dc sin ω_ot)/ω_oL is set up in the circuit and it flows through the path C⁺ → T_A → L → C^– as shown below.

This oscillatory current increases as a function of sine and reaches its negative maximum value and finally becomes zero at instant t₂. Now, at instant t₂ capacitor voltage reverses completely i.e., V_C = -V_dc, capacitor current I_C changes its direction of flow in the auxiliary thyristor T_A due to which T_A is reverse biased and turned OFF.

Also in this mode, from t₁ to t₂ main thyristor T_M is neither triggered nor commutated. So, load current and load voltage remain as I_O and V_dc only. At t = t₂, the upper plate of the capacitor becomes negative and the lower plate to positive.

Mode II (t₂ < t < t₃) :

As soon as the auxiliary thyristor T_A gets turned OFF at t₂, oscillatory current I_C flows through the path C⁺ → L → D₂ → T_M → C^– as shown below.

Here, I_C flows through T_M since it is still in a conduction state. As T_M is ON, the ON-stage voltage drop of T_M appears across D₁ and it is reverse-biased, thereby not allowing to flow I_C through it. The net current flowing through thyristor T_M will be (I_O – I_C), since load current I_O is flowing in the opposite direction to that of I_C.

At instant t = t₃, I_C increases and reaches I_O due to which the current flowing through main thyristor T_M becomes zero (i.e., I_TM = I_C – I_L = 0) and hence it is commutated.

Mode III (t₃ < t < t₄) :

In this mode of operation, with the commutation of main thyristor T_M, I_C increases and reaches a value higher than load current I_O as shown in waveforms below. Hence, this excess current (I_C – I_O) starts flowing through the diode D₁ as shown below.

Due to the flow of current, there will be a drop in voltage across D₁. This voltage appears across T_M as reverse voltage as they are in parallel and hence keeps T_M in OFF state for the time t_q (shown in the waveform).

At instant t = t₄, if V_C > V_dc then the free-wheeling diode (D_F) gets forward biased and starts conducting. However, if V_C < V_dc, then the circuit operates in Mode IV.

Mode IV (t₄ < t < t₅) :

This mode starts at instant t = t₄, at which I_C reduces to I_O. Because of this the current flowing through the diode D₁ becomes zero, and hence it is reversed biased and turned OFF. After t = t₄ the load current will be constant and follows the path V_dc⁺ → C → L → D₂ → load → V_dc^– as shown below.

At instant t₅ the capacitor is again charged to source voltage V_dc with upper plate positive and lower plate negative.

Mode V (t₅ < t < t₆) :

At instant t₅, the capacitor charges at a higher rate, and it gets charged to a value greater than the supply voltage source. Due to overcharging of the capacitor, free-wheeling diode D_F gets forward biased and starts conducting as shown below.

At this instant, since D_F is conducting, load voltage V_O becomes zero, and I_C reaches zero at instant t₆. Then after diode D₂ is reverse biased and turned off. Now, the load current I_O flows through the freewheeling diode D_F until the main thyristor T_M is triggered again. These five modes will repeat again when T_M has triggered again. The below shows the voltage and current waveforms.

In the above chopper operation, we can notice that in mode II, the main thyristor is commutated by applying a reverse current pulse generated by the capacitor. Hence the chopper circuit is called as Current Commutated Chopper. This type of chopper circuit is widely used in traction cars.

Advantages of Current Commutated Chopper :

• The charging of the capacitor will be always done in the correct polarity.
• Commutation is possible until peak commutating current is maintained greater than the load current.
• In this chopper auxiliary thyristor is naturally commutated.