Step Down Chopper (Buck converter)

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In Step Down Chopper (Buck converter) Output Voltage is less than input voltage.  (Vo < Vs).

This is achieved by switching of a semiconductor device. The decrease in output voltage depends on the switch off and on time. The thyristor in the circuit acts as a switch. When thyristor is ON, supply voltage appears across the load and when thyristor is OFF, the voltage across the load will be zero. Stepdown chopper is known as a buck converter.

Recommended Reading StepUp chopper (Boost converter) http://www.electricalidea.com/2016/05/22/chopper/

So here is how it works. The switch is turned on and off periodically: ton is the time it is on, toff is the time it is off and T=ton+toff is the period. We can also define duty cycle as D=ton/T the fraction of the period the switch is on. For the sake of simplicity, we assume that the ripple in the output voltage is so small that we may consider it constant during a cycle.

When the switch is on, the power supply is connected to the inductor and the diode is reverse polarized.

Buck Converter (Step down chopper)

When the chopper is ON –

Vs = (VL+Vo), VL = Vs – Vo, Ldi/dt = Vs – Vo, LΔi/Ton = Vs + Vo
Thus, peak-to-peak current load is given by,
Δi = (Vs – Vo / L) Ton

Where, Vs = Supply Voltage, VL = Voltage across Inductor , Vo = Output voltage

When the chopper is OFF, polarity reversal and discharging occurs at the inductor. So, The current passes through the free-wheel diode and the inductor to the load. This gives,
Ldi / dt = Vo.………………………………………………………………………………………………(i) Rewritten as −LΔi / Toff=Vo
Δi=Vo(Toff / L)……………………………………………………………………………………….(ii)
Equating equations (i) and (ii) gives;
((Vs−Vo) / L ) Ton = (Vo/L)To
(Vs−Vo)/Vo = Toff / T
Vs / Vo = (Ton−Toff) / Ton

The above equation gives; Vo = (Ton/T)Vs = DVs
Equation (i) gives − Δi= (Vs−DVs/L)DT from D=Ton / T
= (Vs − (1−D) D ) / Lf Where, D = Duty cycle
f = 1 / T = chopping frequency

So, Output Voltage of Step Down Chopper (Buck converter)
Vo = (Ton/T)Vs = DVs

Current and Voltage Waveforms

For a step down chopper the voltage output is always less than the voltage input.


Buck Converter waveform

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