Chopper Circuit: Principle of Operation, Types and Applications

A chopper circuit is known as DC to DC converter. As compared to the transformers of the AC circuit, choppers are used to step up and step down the DC voltage. They change the fixed DC voltage to the variable DC voltage. Using these, the DC voltage supplied to the devices can be adjusted to the required amount.

Devices Used in Chopper

• Low power application: IGBT, Power BJT, Power MOSFET etc.
• High power application: Thyristor or SCR.

Principle of Operation

1. Step Down Chopper

The principle of operation of the chopper can be understood from the circuit diagram below. The circuit consists of a semiconductor diode, a resistor, and a load. For all types of chopper circuits, the output voltage value is controlled by the periodic closing and opening of the switches used in the circuit. The chopper can be viewed as an ON/OFF switch that can rapidly connect or disconnect the source to load connection. Continuous DC is given as a source to the chopper as V_s and chopped DC is obtained across the load as V₀.

Output Voltage and Current Waveforms

The output voltage and current waveforms of a chopper circuit are shown below. From the voltage waveform, it can be seen that during the period of T_ON, the load voltage V₀ is equal to the source voltage V_s. However, when the interval T_OFF occurs, the DC voltage step-downs to zero, thus making the load short-circuited.

In the current waveform, it can be seen that during the interval T_ON, the load current rises to the maximum value. During the interval T_OFF, the load current decays. In T_OFF condition the chopper is off so, the load voltage becomes zero. However, load current flows through the diode FD, making the load short-circuited.

Thus, the chopped DC voltage is produced at the load. The current waveform is continuous which rises during the TON state and decays during the T_OFF state.

2. Step Up Chopper

Step up chopper or boost converter is used to increase the input voltage level of its output side. Its circuit diagram and waveforms are shown below in the figure:

Operation of Step-Up Chopper

When CH is ON it short-circuits the load. Hence output voltage during T_ON is zero. During this period inductor gets charged. So, V_S = V_L

Where ΔI is the peak-to-peak inductor current.

When CH is OFF inductor L discharges through the load. So, we will get the summation of both source voltage V_S and inductor Voltage V_L as output voltage, i.e.

Now, by equating (iii) and (iv),

(V_S / L) × ​​T_ON​ = {(V_O​−V_S​​) / L} × T_OFF​

⇒V_S​ (T_ON​ + T_OFF​) = V_O​ × T_OFF

​⇒V_O ​= (TV_S​​ / T_OFF​) = V_S / (T−T_ON​) / T

Therefore average output voltage, V_O​=V_S​​ / (1−D)​​

As we can vary T_ON from 0 to T, so 0 ≤ D ≤ 1. Hence V_O can be varied

from V_S to ∞. It is clear that the output voltage is always greater than the input

voltage and hence it boosts up or increases the voltage level

Difference Between Step Up And Step Down Chopper Circuit

Types of Chopper Circuit

The main element based on which choppers are categorized is the semiconductor used in the chopper circuit. Based on the positioning of this semiconductor, choppers can be made to work in any of the four-quadrant conditions. Depending on the quadrant of operation choppers are categorized as Type A, B, C, D, and E.

• Type-A chopper works in the first quadrant. In this chopper, the voltage, and current both are positive and flow in the same direction. Power from source to load and the average output voltage is less than input DC voltage.
• Type–B chopper works in the second quadrant. Here the load voltage is positive and the current is negative. Power flows from load to source. This chopper is also known as a step-up chopper.
• Type–C chopper is formed by parallel connection of Type A and Type B choppers.
• Type-D chopper is the two quadrant type B chopper and Type E chopper is the fourth gradient chopper.

Type-A Chopper First–Quadrant Chopper

This type of chopper is shown in the figure. It is known as the first-quadrant chopper or type A chopper. When the chopper is on, V₀ = V_S and as a result, the current flows in the direction of the load. But when the chopper is off V₀ is zero but I₀ continues to flow in the same direction through the freewheeling diode FD, thus the average value of voltage and current say V₀ and I₀ will be always positive as shown in the graph.

In a type A chopper the power flow will be always from the source to the load. As the average voltage, V₀ is less than the dc input voltage V_S.

Type-B Chopper or Second Quadrant Chopper

In type B or second quadrant chopper, the load must always contain a dc source E. When the chopper is on, V₀ is zero but the load voltage E drives the current through the inductor L and the chopper, L stores the energy during the time Ton of the chopper.

When the chopper is off, V₀ = (E+ L. (di/dt)) will be more than the source voltage V_S. Because of this, the diode D2 will be forward biased and starts conducting and hence the power starts flowing to the source. No matter whether the chopper is on or off the current I₀ will be flowing out of the load and is treated as negative.

Since V_O is positive and the current I₀ is negative, the direction of power flow will be from load to source. The load voltage V₀ = (E+L .di/dt ) will be more than the voltage V_S so the type B chopper is also known as a step-up chopper.

Type-D Chopper or Two Quadrant Type-B Chopper

The circuit diagram of the type D chopper is shown in the figure below. When the two choppers are ON, the output voltage V₀ will be equal to V_S. When V₀= – V_S the two choppers will be OFF but both the diodes D1 and D2 will start conducting. V₀, the output voltage, will be positive again when the choppers are turned on and T_ON will be more than the turn-off time TOFF, as shown in the waveform below. As the diodes and choppers conduct current only in one direction the direction of the load current will be always positive.

The power flows from source to load as the average values of both V₀ and i₀ are positive. From the waveform, it is seen that the average value of V0 is positive thus the fourth quadrant operation of type D chopper is obtained. From the waveforms, the Average value of output voltage is given by

V_O​=(V_S​T_ON​−V_S​T_OFF​)/T=V_S​×(T_ON​−T_OFF​)/T

Must Read Working Principle of Chopper Circuit

Jones Chopper Circuit Working Principle

• Jones Chopper is an example of a Voltage Commutated Chopper.
• Morgan Chopper is an example of the Current Commutated Chopper.

The Jones chopper circuit is shown below. Here T1 is the main thyristor and
T2 is an auxiliary thyristor.

• Jones chopper is an example of a voltage commutation chopper.
• The Jones chopper circuit is frequently used for the speed control of DC motors.
• Observe that the portion of the circuit is enclosed in the blue lines.
• It is the commutating circuit for the main SCR T1.
• A charged capacitor (C) is switched by an auxiliary SCR T₂ and the autotransformer (Tr) provides commutation of T₁.
• In this chopper, for varying the output voltage either t_ON or t_OFF of the main thyristor T₁ can be varied.

Role of Diode

• This diode is to used carry the inductive current when T1 is turned OFF.
• Thus the diode D1 prevents the load from the high voltages appearing across it.

Morgan Chopper Circuit

Morgan Chopper Circuit Working Principle

• When SCR T1 is switched off, the charging of the capacitor is done through the (+)Vdc – C – SR – LOAD – Vdc (-) path and the saturable reactor is in positive saturation.
• While SCR T1 is switched on, the voltage across the capacitor appears across the saturable reactor and core flux flows from positive saturation to negative saturation.
• When the core flux reaches negative saturation, the discharging of the capacitor is done through the (+)C – T1 – SCR – C(-) path.
• The capacitor and saturable reactor make a resonance circuit and its discharge time is equal to π √ LsC.
• The Ls is inductance after saturation.
• The voltage across the capacitor becomes – Vdc in a very short time and this voltage appears across the saturable reactor.
• The discharging of the capacitor is done through SCR T1 and inductance after saturation in the reverse direction when the core flux reaches positive saturation.
• As this discharging current passes through SCR T1, it will switch off SCR T1 and forward-biased diode D1.
• When SCR T1 is switched off, the load current passes through the freewheeling diode.
• The turn-on time of the SCR T1 is always constant due to the constant time of core saturation and it depends upon √ LsC.
• The average output voltage can be changed by changing only the output frequency.
• Turn on-time of the SCR is equal to the time for positive saturation to negative saturation and again positive saturation of the saturable reactor.

This type of chopper circuit assures better commutation and has better performance for lower values of chopper frequencies.