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886 POWER CONDITIONING FOR PHOTOVOLTAIC POWER SYSTEMS<br />

I PV<br />

S 1<br />

L<br />

i L<br />

V PV<br />

C 1 D C 2<br />

V load<br />

R load<br />

Figure 19.23<br />

Equivalent circuit diagram of a step-down converter<br />

I PV<br />

V PV<br />

S 1<br />

L<br />

C 1 D C 2<br />

V load<br />

i L<br />

Figure 19.24<br />

Step-down converter during “on” state<br />

V PV<br />

S 1<br />

L<br />

C 1 D C 2<br />

V load<br />

i L<br />

Figure 19.25<br />

Step-down converter during “off” state<br />

diode instead (Figure 19.25). Neglecting the voltage drop across the diode, the current<br />

falls down, however, due to the following equation:<br />

di L<br />

dt<br />

=− V load<br />

L<br />

The capacitor C 1 is used to support the supply voltage (V PV ). In principle, S 1 is<br />

turned on and off with a switching frequency (i.e. with “t on ”and“t off ”). With regard to<br />

Ohm’s law, the behaviour of the load voltage can be obtained from the load current (= i L ).

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