A chopper is basically a power semiconductor device used to obtain a desired dc voltage from an unregulated dc voltage source. The chopper circuit employs a thyristor that is turned ON and turned OFF at the required instant to get desired output. But the most undesirable feature of the thyristor is it cannot turn OFF by itself.

Thus, in order to turn OFF or commutate thyristor, there are some commutation circuits that are broadly classified into two groups,

• Load Commutation and,
• Forced Commutation.

Load Commutation :

In load commutation, the thyristor is turned OFF by reducing the current flowing through it to zero. This can be achieved either by load circuit parameters that bring thyristor current to zero or by diverting current flow away from the thyristor path.

Forced Commutation :

In this technique, energy storage elements such as inductor and capacitor are connected additionally in the circuit. Using these external elements the forced commutation can be achieved in two ways,

• Voltage Commutation – In voltage commutation, a reverse voltage is applied across the thyristor using a charged capacitor. So that conducting thyristor reverse-biases and turned OFF.
• Current Commutation – In current commutation, a current pulse greater than the load current is applied to the thyristor in the reverse direction. Due to this reverse current, both load and reverse currents cancel each other, and thus the net current flowing through the thyristor becomes zero, thereby turning OFF the thyristor.

In this article, let us learn about circuit construction and working of voltage commutation technique in chopper or voltage commutated chopper. This type of chopper circuit is also called impulse commutated or classical or parallel-capacitor turn OFF or oscillation chopper.

Circuit of Voltage Commutated Chopper :

The below shows the circuit diagram of voltage commutated chopper. It consists of a main thyristor T_M and auxiliary circuit that compresses components inductor (L), capacitor (C), auxiliary thyristor (T_A), diode (D) for commutating the main thyristor T_M.

A freewheeling diode D_f is connected across the load to freewheel the current due to load inductance. In the above circuit load current I_O is assumed to be constant, and zero losses in thyristors and diodes.

Working of Voltage Commutated Chopper :

To simplify the understanding of the operation of voltage commutated chopper, the entire operation is divided into four different modes. Before starting the chopper circuit, the capacitor C must be charged to positive source voltage V_dc with the upper plate as positive polarity and lower plate as negative polarity as shown in the above circuit.

The charging of capacitor can be done in either of the following two ways,

• To connect extra circuit that compresses of a switch (S) and resistance (R_C) as shown above. The switch (S) is closed until the capacitor gets charged to the source voltage through the path V_dc⁺ → C → S → R_C → V_dc^–. Once the capacitor voltage becomes equal to source voltage i.e., V_C = V_dc, the switch is opened and is remained open during the entire operation of the chopper.
• Another way of charging the capacitor is by triggering auxiliary thyristor T_A so that capacitor gets charged to V_dc through the path V_dc⁺ → C → T_A → load → V_dc^–. Once the charging current becomes zero, T_A is turned OFF and thus V_C will become equal to V_dc.

Now let us see the operation of the chopper in different modes.

Mode I :

In this mode of operation, the main thyristor T_M is triggered at an instant of time t₀, and thus the load is connected to the source. Now load voltage V_O follows source voltage V_dc.

Once T_M starts conducting the current in the circuit flows through two paths. The load or output current I_O constitutes one path that flows through the path V_dc⁺ → T_M → load → V_dc^–, and commutation current I_C constitutes another path that flows through C⁺ → T_M → L → D → C^– as shown below.

Now after a while capacitor voltage, V_C reaches zero from its initial value, whereas, capacitor current I_C reaches its maximum value. Now, I_C decreases from its maximum value and becomes zero at instant t₁, and the capacitor starts charging in the opposite direction to -V_dc. This reverse voltage -V_dc in the capacitor which is varying as a function of cosine is held constant (at -V_dc) by diode D.

This mode ends at an instant t₁ and the state of the circuit at the end of mode will be,

Mode II :

In this mode of operation, the main thyristor will remain in the conduction state and the auxiliary thyristor will be in a nonconduction state. Since there is no change in the circuit, the load current flows through the main thyristor similar to the path in mode I as shown below. This condition continues until t₂, till then the circuit state will remain the same as that of mode I.

Mode III :

This mode starts at desired instant t₂ when auxiliary thyristor T_A is triggered. As T_A is triggered, it starts conducting and the capacitor voltage to which it is charged i.e., -V_dc, appears across the main thyristor T_M.

Thus main thyristor T_M becomes reverse-biased due to reverse voltage and it gets turned OFF. Hence load current flowing through thyristor T_M becomes zero at t₂ and now the load current flows through the auxiliary thyristor T_A in the path V_dc⁺ → C → T_A → load → V_dc^– as shown below.

The load voltage V_O at instant t₂ is given by the sum of source voltage V_dc and the voltage across the capacitor V_C i.e., V_O = V_dc + V_dc = 2V_dc. Now, the load voltage decreases from 2V_dc to V_dc at a certain instant of time, and finally, the load voltage becomes zero at t₃.

As V_O decreases, the voltage across the capacitor also decreases. Then after V_C charges linearly from -V_dc at t₂ to V_dc at t₃ since load current is assumed to be constant. At the end of this mode, the circuit state will be,

Mode IV :

This mode is from instant t₃ where V_C = V_TA = V_dc, V_O = 0, and capacitor current becomes zero. Thus auxiliary thyristor T_A is turned off naturally and capacitor C is charged to a higher value at t₃.

Due to slight overcharging of the capacitor, the freewheeling diode D_f is forward biased and it conducts. As a result, the load current I_O flows through the load and freewheeling diode as shown above. From t₃ to t₄, the voltage through the auxiliary thyristor becomes slightly negative since the capacitor is overcharged at t₃. In this mode, the state of the circuit will be,

The below shows the related voltage and current waveforms for voltage commutated chopper.

At instant t₄ main thyristor is triggered again and the process described from t₀ to t₄ repeats again. Since the main thyristor is commutated by impressing a reverse voltage obtained from the charged capacitor, the circuit is said to be called Voltage Commutated Chopper.

Disadvantages of Voltage Commutated Chopper :

Though the circuit configuration of voltage commutated chopper is simple. It suffers from the following drawbacks,

• Initially, an additional circuit is required to charge the capacitor before switching ON the chopper circuit.
• At the instant when the commutation of the main thyristor is initiated by triggering auxiliary thyristor, the output voltage becomes double the source voltage.
• When the value of load current is very low, the discharge time of the capacitor will be high. Thus turn OFF time of the circuit increases, thereby limiting the switching frequency of the chopper.
• This type of chopper circuit cannot work if there is no load connected to it. Because the capacitor which is charged to -V_dc in mode II cannot come to +V_dc in mode IV. Thus commutation in the circuit fails.
• The main thyristor carries both load and commutation current which means the thyristor is to be overrated.