Principle of Step Down Operation

The principle of step down operation of DC-DC converter is explained using the circuit shown in Figure 3a . When the switch S₁ is closed for time duration T₁, the input voltage V_in appears across the load. For the time duration T₂ is switch S₁ remains open and the voltage across the load is zero. The waveforms of the output voltage across the load are shown in Figure 3b .

 

Figure 3a: Step down operation	Figure 3b: Voltage across the load resistance

The average output voltage is given by

                                                                                  (1)

The average load current is given by

                                                                                                                (2)

Where

T is the chopping period

is the duty cycle

f is the chopping frequency

The rms value of the output voltage is given by

                                                                                                  (3)

In case the converter is assumed to be lossless, the input power to the converter will be equal to the output power. Hence, the input power ( P_in) is given by

                                                                                 (4)

The effective resistance seen by the source is (using equation 2)

                                                                                                                         (5)

The duty cycle D can be varied from 0 to 1 by varying T₁, T or f. Thus, the output voltage V_oarg can be varied from 0 to V_in by controlling D and eventually the power flow can be controlled.