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.
Based on the voltage output, choppers can be classified as
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.
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.
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 −
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
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
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 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
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,
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.
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.
When the chopper is switched ON, inductor L will become charged by the source voltage Vs. Therefore, Vs = VL.
When the chopper is switched OFF, the inductor’s polarity reverses and this leads to the discharge through the diode and the load.
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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