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What is Chopper?

A chopper circuit comprises numerous types of electronic switching circuits used in power control and signal applications. A chopper is a static device that converts fixed DC input voltage to a variable DC output voltage. It is a high-speed ON/OFF semiconductor switch. It may be thought of as the DC equivalent of an AC transformer since they behave identically.

In power electronics applications, since the switching element is either completely on or completely off, its losses are low, and the circuit can provide high efficiency. However, the current supplied to the load is discontinuous and may require smoothing or a high switching frequency to avoid undesirable effects.

In signal processing circuits, the use of a chopper stabilizes a system against the drift of electronic components. The original signal can be recovered after amplification or other processing by a synchronous demodulator that essentially un-does the “chopping” process.

What is Chopper

Chopper is fed through a constant DC voltage source, and its output is variable DC voltage. The average value of output DC voltage may be less than or higher than the input DC voltage source.

A chopper is a DC equivalent to an AC transformer having a continuously variable turn ratio. Like a transformer, it can be used to step up or step down the fixed DC input voltage. On this basis, there are two types of the chopper: Step-up and Step-down Chopper. A chopper whose average value of DC output voltage is more than the fixed DC input voltage is called Step-up Chopper. At the same time, a chopper whose average value of DC output voltage is less than the DC input voltage is called a Step-down chopper.

Working Principle of Chopper

A chopper is a high-speed ON/OFF switch. It connected the source to load and disconnects the load from the source at a fast speed. The following image represents the simple circuit to show its working principle.

What is Chopper

In this circuit, the switch SW is a chopper. This switch can be made ON and OFF at a very high speed. In this way, the load may be connected and disconnected from the supply source Vs.

When the switch is ON, the load voltage is equal to the source voltage Vs, and when the switch is OFF, the load voltage becomes equal to ZERO.

Thus, a chopped voltage across the load is obtained. The output voltage, i.e., the voltage across the load, is shown in the below image.

What is Chopper

When the switch SW is made OFF, the load current finds its path through freewheeling diode D. Therefore, diode D acts as a short, and hence the voltage across the load becomes zero.

An inductor in the chopper is an essential thing. This inductor makes diode D forward biased when the switch SW is OFF.

Even though the switch SW is made OFF, the load current doesn’t become ZERO. Instead, it flows through the freewheeling diode, inductor L, and load. The load current is continuous, as shown below.

What is Chopper

It may be seen from the above waveform of the output current of a chopper that, during ON time, the current rises, whereas during OFF time, the load current io decays.

The time for which the chopper connects load from the source is called ON time (TON). Whereas the time for which load is disconnected from the source is called OFF time (TOFF).

Duty Cycle

A duty cycle is defined as the ratio of ON time to the total time. The symbol α denotes it. The total time is the sum of ON and OFF time.

Duty Cycle = TON / (TON+TOFF)

Assuming (TON+TOFF) = T, the duty cycle is given as below.

Duty Cycle, α = (TON / T)

Calculation of Output Voltage

The average output voltage of the chopper may found from the output voltage waveform. It is clear from the o/p voltage waveform that voltage Vo is available only for the TON time in the total time (TON+TOFF). Therefore, the average output voltage Vo may be calculated as shown below.

Vo = TON Vs / (TON+TOFF)

But, TON / (TON+TOFF) = α


Vo = αVs

Thus, the output voltage may control by controlling the duty cycle. This duty cycle may control in various ways, which we will discuss in the next post. And the output voltage is independent of the load current.

Classification of Chopper

The main classification of the chopper is DC chopper and AC Link chopper. Based on the commutation process, they are classified as natural commutated chopper and forced commutated chopper.

Forced commutated chopper is further classified as Jones chopper, Morgan chopper. Based on output voltage values, choppers are classified into the step-down chopper, step-up chopper, and step up/down chopper. Based on the power loss at switching time, choppers are classified as Hard switched and soft switched.

1. AC Link Chopper

In this classification of the chopper, the voltage inversion takes place. Here the DC voltage is converted into AC with the help of an inverter.

What is Chopper

Now, this AC is passed through step-down or step-up transformers. The output from the transformers is again converted into DC by a rectifier. AC link choppers are very bulky and occupy a large amount of space.

2. DC Chopper

DC chopper works on DC voltage. They work as step up and step down transformers on DC voltage. They can convert the steady constant DC voltage to a higher value or lower value based on their type.

What is Chopper

DC choppers are more efficient, fast, and optimized devices. These can be incorporated into electronic chips. They provide smooth control over the DC voltage.

Types of Chopper Circuit

There are mainly two types of chopper, Step-up and Step-down chopper. This classification is based on the average DC output voltage of the chopper. However, based on quadrant operation, a chopper may be classified into five different types: Class-A, Class-B, Class-C, Class-D, and Class-E chopper.

1. Step-up Chopper

It is a kind of chopper in which the average DC output voltage is more than the source voltage. In this chopper, power flows from load to source, and load contains a source of emf and should be inductive.

2. Step-down Chopper

A chopper whose average DC output voltage is less than the source voltage is called a step-down chopper. Power flows are always from source to load in this chopper.

Classification based on Quadrant Operation

The chopper is a semiconductor static device, which essentially means that the direction of current flow is restricted through it. But chopper circuit can be so modified that the operation of the chopper is achieved in any of the four quadrants.

This gives us a ground to classify the choppers. Based on quadrant operation, there are five types of choppers: Class-A, Class-B, Class-C, Class-D, and Class-E choppers.

  • Class-A chopper can only operate in the first quadrant. It means that the load current and load voltage are always positive. Load current always flows from source to load, and hence we can say that power is always transferred from source to load. The step-down chopper is an example of a Class-A chopper.
  • Class-B chopper is confined in the second quadrant. It means that the chopper’s output voltage will be positive, but the load current will be negative. Negative load current means io flows from load to source. Thus, the power will always transfer from load to source in this type of chopper. The step-up chopper is a Class-B chopper.
  • Class-C chopper can operate in both the first and second quadrant. The first quadrant operation acts as a step-down chopper, while in the second quadrant, it behaves as a step-up chopper. Class-C is obtained by parallelly connecting Class-A & Class-B chopper. This type of chopper is also known as the Two Quadrant Class-A chopper.
  • Class-D chopper can operate in the first and fourth quadrant. In this chopper, the direction of load current doesn’t reverse. Instead, the average DC output voltage (Vo) may reverse. Power can either be transferred from source to load or load to source. This chopper is also known as the Two Quadrant Class-B chopper.
  • Class-E chopper is a universal chopper and can operate in all four quadrants. Four choppers and four diodes are required to make the circuit of the Class-E chopper. The necessary condition for this chopper is that load must be inductive.

Applications of Chopper

Chopper circuits are used in multiple applications, such as:

  • Switched-mode power supplies, including DC to DC converters.
  • Speed controllers for DC motors
  • Driving brushless DC torque motors or stepper motors in actuators
  • Class D electronic amplifiers
  • Switched capacitor filters
  • Variable-frequency drives
  • DC voltage boosting
  • Battery-operated electric cars
  • Battery chargers
  • Railway traction

Chopper Circuit Control Strategies

For all the chopper configurations operating from a fixed DC input voltage, the average value of the output voltage is controlled by periodic opening and closing of the switches used in the chopper circuit. The following different techniques can control the average output voltage:

  1. Pulse-width modulation
  2. Frequency modulation
  3. Variable frequency, variable pulse width
  4. CLC control

In pulse-width modulation, the switches are turned on at a constant chopping frequency. The total time of one cycle of the output waveform is constant. The average output voltage is directly proportional to the ON time of the chopper.

The ratio of ON time to total time is defined as the duty cycle. It can vary between 0 and 1 or between 0 and 100%.

Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is used to encode a message into a pulsing signal. Although this modulation technique can encode information for transmission, its primary use is to control the power supplied to electrical devices, especially inertial loads.

  • The average voltage fed to the load is controlled by turning the switch between supply and load quickly. The longer the switch is compared to the off periods, the higher total power supplied to the load.
  • The PWM switching frequency has to be much higher than what would affect the load, which means that the resultant waveform perceived by the load must be as smooth as possible.
  • Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.
  • In frequency modulation, pulses of a fixed amplitude and duration are generated, and the average value of output is adjusted by changing how often the pulses are generated.
  • Variable pulse width and frequency combine both changes in the pulse width and repetition rate.

Chopper Amplifiers

One classic use for a chopper circuit is chopper amplifiers. These are DC amplifiers. Some types of signals that need amplifying can be so small that an incredibly high gain is required, but very high gain DC amplifiers are much harder to build with low offset and 1/¦{displaystyle f} noise, and reasonable stability and bandwidth. It’s much easier to build an AC amplifier instead.

A chopper circuit is used to break up the input signal to be processed as if it were an AC signal, and then integrated back to a DC signal at the output. In this way, minimal DC signals can be amplified. This approach is often used in electronic instrumentation, where stability and accuracy are essential. It is possible to use these techniques to construct pico-voltmeters and Hall sensors.

The input offset voltage of amplifiers becomes essential when trying to amplify small signals with very high gain. Because this technique creates a very low input offset voltage amplifier. This input offset voltage does not change much with time and temperature.

These techniques are also called “zero-drift” amplifiers because there is no drift in input offset voltage with time and temperature. Related techniques that also give these zero-drift advantages are auto-zero and chopper-stabilized amplifiers.

Auto-zero amplifiers use a secondary auxiliary amplifier to correct the input offset voltage of the main amplifier. Chopper-stabilized amplifiers use a combination of auto-zero and chopper techniques to give some excellent DC precision specifications.

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