Current-Commutated Chopper

Definition: Current Commutated Chopper also known as Resonant-Pulse Commutated Chopper is a type of chopper that allows the thyristor to get commutated by the use of an oscillatory circuit.

It is known to be a reliable commutation technique.

Introduction

We all have the basic idea that choppers are circuits that possess turn-on and turn-off mechanisms. Commutation corresponds to turning the thyristor off. Previously, we have seen that commutation can be of two types:

1. Forced Commutation: In this technique, external elements such as L and C are used to turn off a conducting thyristor. It can be sub-classified as:

• Voltage Commutation
• Current Commutation

2. Load Commutation: In this technique, a conducting thyristor is brought to a non-conducting state if the load current becomes zero either because of the nature of load circuit parameters or the current is transferred from conducting thyristor to another device.

One of the important characteristics possessed by current commutation is that the diode is connected in antiparallel arrangement with the main thyristor that leads to reverse biasing of the main thyristor due to the drop across the diode.

It is to be noted here that the voltage drop is nearly 1 volt thus current commutation is a quiet flow than the voltage commutation.

Circuit of Current Commutated Chopper

The figure down below represents the circuit diagram of the current commutated chopper:

Here T_M is the denotation for the main thyristor while T_A represents auxiliary thyristor. D₁ and D₂ are two diodes along with inductor L and capacitor C. Also, there is a freewheeling diode represented by FD and a charging resistor R_c.

Similar to the voltage-commutated chopper which we have discussed recently the circuit operation considerations are as follows:

• The value of the charging resistor must be high.
• The current flowing through the load will be of invariable nature.
• The thyristors and diodes must show ideal behavior.

Here in the above circuit i_c is the commutating current, i_TM is the current through the main thyristor, while i_FD is the current through the freewheeling diode and i_o represents the load current. The operation of this circuit is somewhat similar to the voltage-commutated circuit in the sense that the energy required to commutate the circuit is provided by the capacitor.

Also, the capacitor is charged through input voltage V_dc. Also, when T_M is in conducting state, the commutating circuit will be in an inactive state. The commutation process begins when T_A is turned on.

Working of Current-Commutated Chopper

Consider the circuit shown above. The circuit operation begins with the charging of the capacitor up to the peak of the supply input and the path through which charging of the capacitor takes place will be +V_dc – C – R_c – (-V_dc). Initially, the main thyristor T_M is triggered at t = t_0, and at this time, T_M will be in the conducting state. This allows the current to pass through the main thyristor and appears at the load. Hence V₀ = V_dc.

Now let us see how the process of commutation takes place.

So, at instant t = t₁, in order to commutate T_M, the auxiliary thyristor i.e., T_A is triggered. As soon as T_A is triggered, it comes into conducting state then oscillatory current i_c begins to flow through the components +C – T_A – L – (-C) of the circuit. Further, at the instant t₂, the current through the capacitor will get reversed but will be maximal. Hence, at t₂, V_c = -V_dc. Due to the negative polarity across the capacitor, T_A will get reversed biased hence will be commutated while T_M will be unaffected and so in this case the current through the load will be i₀ while V₀ = V_dc.

Now, as T_A is turned off and T_M is still in the conducting state at instant t₂, the oscillatory current ic will flow through +C – L – D₂ – T_M. This means that the oscillating current will flow through the main thyristor this time and not through D₁.  Here, in this case, the direction of flow of ic will be opposite to that of the previous direction and so there will be a decrease in the current flowing through the main thyristor i.e., i_TM. The direction of flow of oscillating current is reverse to the flow of load current i₀ through T_M therefore, i_TM = I₀ – I_C.

At instant t₃, the oscillating current ic will reach i_0, and therefore, i_TM = 0, this leads to turning off of the main thyristor at t₃. From the above explanation, it is clear that oscillatory current turns off T_M thus it is referred to as a current-commutated chopper. And the voltage across the load will be V_dc. At this instant, T_M gets turned off and i_c exceeds i₀, hence after instant t₃, the load current is supplied by i_c. While the excess current i_c – i₀ will flow through diode D₁ and due to the drop across D₁, T_M will be reverse biased between the instant t₄-t₃ (i.e., t_q). At time instant t₄, if the value of V_c becomes more than the supply input V_dc then this forward biases the freewheeling diode.

Furthermore, at t₄, i_c becomes equivalent to i_0, and therefore the current through D₁ will become 0 thereby turning off diode D₁. However, at the same time, a constant current from V_dc to the capacitor to the inductor to diode D₂ and lastly to the load will flow. This charges the capacitor linearly and at instant t₅ the capacitor will get charged to source voltage. However, during t₅ – t₄ the current through the capacitor will be i₀. At t₄, D₁ is in an off state therefore V_TM = V_TA = V_c and so the load voltage V₀ will V_s – V_c.

However, at t₅, V_c = V_s, and as V₀ = V_s – V_c therefore, the load voltage will be 0 at this instant. At this instant, overcharging of capacitor above the supply input takes place, and this forward biases the freewheeling diode and it starts conduction. This allows the flow of load current i₀ through FD.

At this particular instant, the commutating current ic is non-zero and as C forms the connection through the load i.e., +V_dc – C – L – D_2, and overcharging takes place as energy from the inductor is transferred to the capacitor. However, at t₆, ic becomes 0, and the voltage across the capacitor exceeds the source voltage. In between t₅ to t₆, the load current i₀ will be the sum of commutating current (i_c) and current through FD (i_fd). When i_fd increases, then this leads to decaying of i_c hence resultantly i_c will be 0 therefore i_fd = i₀. Thus, this commutates the circuit and the overall turn off time will be t₆ – t₁.

The waveform representation for the same is shown below:

Advantages

1. The auxiliary thyristor TA will undergo natural commutation.
2. It is a reliable commutation technique till the load current is smaller than the peak commutation current.
3. The capacitor is in always charged state.

This is all about the current commutated chopper.