Power Electronics Choppers - Power Electronics

What are Choppers in Power Electronics?

In power electronics, hopper will use high speed for connecting and disconnecting from a source load and a fixed DC voltage will be applied rarely to the source load by constantly triggering the power switch ON/OFF. Time period where the power switch stays ON or OFF is known as the choppers ON and OFF state times, respectively.

Choppers will be mostly applied in electric cars, wind and solar energy conversion and DC motor regulators.

Symbol of a Chopper

chopper_symbol

Classification of Choppers

Based on the voltage output, choppers can be classified as

  • Step Up chopper (boost converter)
  • Step Down Chopper(Buck converter)
  • Step Up/Down Chopper (Buck-boost converter)

Step Up Chopper

Average voltage output (Vo) in a step up chopper will be greater than the voltage input (Vs). Below figure displays a step up configuration of a chopper.

step_up_chopper.jpg

Current and Voltage Waveforms

V0 (average voltage output) will be positive when chopper is switched ON and negative when the chopper is OFF as shown in the waveform below.

current_and_voltage_waveform.jpg

Where

TON – time interval when chopper is ON

TOFF – time interval when chopper is OFF

VL – Load voltage

Vs – Source voltage

T – Chopping time period = TON + TOFF

Vo is given by −

formula.PNG

When the chopper (CH) is switched ON, the load will be short circuited and therefore, the voltage output for the period TON will become zero. Additionally, the inductor will be charged during this time. This will give VS = VL

formula.PNG

Δi = is the inductor peak to peak current. When the chopper (CH) is OFF, discharge will occur through the inductor L and therefore, the summation of the Vs and VL is given as shown below

formula.PNG

Above equation shows that Vo can be varied from VS to infinity and it also proves that the output voltage will always be greater than the voltage input and therefore, it will boost up the voltage level.

Step Down Chopper

Step down chopper is also called as a buck converter. In this chopper, the average voltage output VO is less than the input voltage VS. When the chopper is ON, VO = VS and when the chopper is off, VO = 0

When the chopper is ON

formula.PNG

Circuit Diagram

circuit_diagram.jpg

Here, FD is free-wheel diode.

When the chopper is OFF, polarity reversal and discharging will occur at the inductor and the current will pass through the free-wheel diode and the inductor to the load. This gives,

formula.PNG

Current and Voltage Waveforms

Current and voltage waveforms are given below

For a step down chopper, the voltage output will always be less than the voltage input. This is shown by the waveform below.

chopper_current_and_voltage_waveform.jpg

Step Up/ Step Down Chopper

Step Up/ Step Down Chopper is also called as a buck-boost converter and it helps in increasing or decreasing the voltage input level. Below diagram shows a buck-boost chopper.

step_down_chopper.jpg

When the chopper is switched ON, inductor L will become charged by the source voltage Vs. Therefore, Vs = VL.

formula.PNG

When the chopper is switched OFF, the inductor’s polarity reverses and this leads to the discharge through the diode and the load.

Hence,

formula.PNG

D can be varied from 0 to 1. When, D = 0; VO = 0

When D = 0.5, VO = VS

When, D = 1, VO = ∞.

Therefore, in the interval 0 ≤ D ≤ 0.5, output voltage will vary in the range 0 ≤ VO < VS to get step down or Buck operation. While, in the interval 0.5 ≤ D ≤ 1, output voltage will vary in the range VS ≤ VO ≤ ∞ to get step up or Boost operation.

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