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Step Down (Buck) Converter

In a switching converter circuit, the transistor operates as an electronic switch by being completely on or off. This circuit is known as a step down dc chopper circuit (Fig (a)).

Assuming the switch is ideal in Figure (a) above, the output is the same as the input when the switch is closed, and the output is zero when the switch is open. Periodic opening and closing of the switch results in the pulsed output shown in Figure (b). The average or dc component of the output (average voltage) is
{V_0} = \frac{1}{T}\int_0^T {{v_0}(t)dt = } \frac{1}{T}\int_0^{{t_1}} {{v_s}dt = k{V_s}} Power Electronic Converter: Step Down (Buck) Converter
Therefore, with buck chopper, you can only obtain step-down of voltage.

Power Electronic Converter: step-down of voltage

k = \frac{{{t_1}}}{{{T_s}}} = {t_1}{f_s} 
V0(average) = kVs and V0(rms) = √k Vs
R = \frac{{{V_s}}}{{{I_{0(averaage)}}}} = \frac{{{V_s}}}{{k{V_s}/R}} = \frac{R}{k}
The dc component of the output, Vo is controlled by varying the duty ratio k (sometimes called D), which is the fraction of the period that the switch is closed,
k = D \equiv \frac{{{t_{on}}}}{{{t_{on}} + {t_{off}}}} = \frac{{{t_{on}}}}{T} = {t_{on}}f
Two operations are possible
    1. Constant frequency operation
    2. Variable frequency operation 
    3. Variable frequency operation: either on time t1 or off-time t2 is adjusted. But this approach generates wide spectrum of harmonics.
    4. PWM is more common (variable k).
    previous DC-DC CONVERTERS (DC Choppers)

    next Control of DC-DC converters by PWM
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