Field Instrumentation

What is a DC Chopper (DC TO DC CONVERTER)?

CIRCUIT DIAGRAM OF CHOPPER

What is DC Chopper?

DC Choppers are static devices that change a set DC input voltage into a variable DC output value. Essentially, it functions as a high speed ON/OFF semiconductor switch. It can be viewed as the DC equivalent of an AC transformer because of how they both operate.

DC CHOPPER

The output of the DC chopper is variable DC voltage, which is fed through a source of constant DC voltage. The average output DC voltage value may be lower or higher than the source of the input DC voltage.

CIRCUIT DIAGRAM

Alternative term for a DC chopper is a dc-to-dc converter.

  • Greater efficiency,
  • Quicker response,
  • Less maintenance,
  • Smaller size, and
  • Smooth control

are all features of the converter.

  • Trolley cars,
  • Battery-operated vehicles,
  • Traction motor control,
  • Large-scale DC motor control, etc.

all frequently use DC choppers.

Additionally, they serve as DC voltage regulators and regenerative brakes for DC motors, which recycle energy back into the supply.

The DC voltage level at the chopper’s opposing side can be changed. DC choppers therefore provide the same function in DC circuit transfers as they do in ac circuits. As a result, it is also known as a DC transformer.

Working Principle of the DC Chopper:

A high speed ON/OFF switch is known as a DC Chopper. It quickly linked the source to the load and then disconnected the load from the source.

The switch SW in the circuit is a DC chopper. This switch can be turned ON and OFF very quickly. The load can be connected to and detached from the supply source Vs in this manner. The load voltage is equal to the source voltage Vs while the switch is ON, and it is equal to ZERO when the switch is OFF. This results in a chopped voltage across the load. The voltage is across the load or the output voltage.

EQUIVALENT CIRCUIT

It should be observed that the load current flows through the free-wheeling diode D when the switch SW is turned OFF. Since diode D now functions as a short, the voltage across the load is zero. A crucial component of the choppers is the inductor. Diode D is forward biased by this inductor while switch SW is off.

Additionally, it should be noticed that even when the switch SW is turned off, the load current does not stop flowing. Instead, it passes through the load, inductor L, and free-wheeling diode. The load current is actually constant.

WAVEFORM

As can be observed from the waveform of the output current of the DC chopper shown above, the load current increases when the power is on while decreasing when it is off.

On time, or TON, is the duration for which the load is connected to the chopper from the source. While TOFF, or the “OFF time,” is the duration during which the load disconnects from the source. Io decays due to the load.

Duty Cycle of the Chopper:

The ratio of ON time to total time is known as the duty cycle of a chopper. It is represented by the symbol. The sum of the ON and OFF times is the total time period.

Duty Cycle is equal to TON/( TON+TOFF)

α = TON / (TON+ TOFF)

The following duty cycle is presented assuming that (TON+ TOFF) = T.

Duty Cycle is equal to (TON / T)

α =(TON / T)

Calculating the output voltage:

The output voltage waveform can be used to determine the DC chopper’s average output voltage. The O/P voltage waveform makes it obvious that, for the entire (TON + TOFF) time period, voltage Vo is only available during the TON time period. As a result, the average output voltage Vo may be determined as illustrated below.

Vo equals TON Vs / (TON + TOFF).

However, TON /( TON + TOFF) =α

Hence,

Vo = αVs

As a result, altering the duty cycle might affect the output voltage. There are other techniques to control this duty cycle. It is also obvious from the calculation of output voltage that the output voltage is independent to the load current.

Types of DC Chopper:

DC Chopper are classified into different types, they are:

  • Step up DC Chopper
  • Step down DC Chopper
  • Multi-phase DC Chopper
  • Two quadrant class C DC Chopper
  • Two quadrant class D DC Chopper
  • Four quadrant class E DC Chopper
CHOPPER

Advantages of DC Chopper:

  • High performance
  • Regeneration
  • Adaptability in execution
  • Lightweight
  • Little size
  • rapid response
  • Dynamic reaction time

Disadvantages of DC Chopper:

  • Efficiency drops dramatically for high power applications due to large switching losses at high operating frequencies.
  • The supply voltage is doubled for the freewheeling diode.
  • Double the supply voltage is the peak load voltage.

Application of DC Chopper:

There are several uses for DC Chopper circuits, such as:

  • DC to DC converters is included in switched mode power supply.
  • DC motor speed controllers
  • Driving stepper motors or brushless DC torque motors in actuators
  • Electronic Class D amplifiers
  • Filters with switched capacitors
  • Drives with varying frequencies
  • DC voltage augmentation
  • Batteries-powered electric vehicles
  • Battery power sources
  • Locomotive traction
  • Controls for lighting and lamps

Some Useful Questions:

1. Define Step up chopper, Step down chopper?

In Step up chopper the DC output voltage will be is higher than the DC input voltage.

In Step down chopper the DC output voltage will be is lower than the DC input voltage.

2. Describe the various methods for chopper output voltage control?

The different method of chopper output voltage control is:

  • Variable frequency control (Frequency modulation control)
  • Current limit control
  • Constant frequency control (Pulse width modulation control)
3. Why is the DC chopper’s first quadrant referred to as forward drive?

The operation of the DC chopper in the first quadrant is referred to as forward drive since both the output voltage and current are positive.

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