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888 POWER CONDITIONING FOR PHOTOVOLTAIC POWER SYSTEMS<br />

I PV<br />

L<br />

D<br />

i L<br />

V PV<br />

S 1 C 1<br />

V load<br />

R load<br />

Figure 19.28<br />

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

I PV<br />

V PV<br />

L<br />

D<br />

S 1 C 1<br />

V load<br />

i L<br />

Figure 19.29<br />

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

V load<br />

V PV<br />

t 0<br />

t on<br />

t off<br />

t<br />

Figure 19.30<br />

Behaviour of the load voltage of a step-up converter<br />

After S 1 is switched off (Figure 19.29), the induced voltage in the inductor adds<br />

itself to V PV , which then lies across the load. i L flows through the inductor and further to<br />

the load. Thereby, it falls down gradually because V load >V PV :<br />

di L<br />

dt<br />

= V PV − V load<br />

L<br />

The proceeding of the load voltage is illustrated in Figure 19.30. The diode D<br />

protects against a short circuit (i.e. discharge) of the charged capacitor C 1 ,whichis<br />

assumed to be so big that it can smooth the load voltage completely.

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