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INVERTERS 887<br />

V PV<br />

V load<br />

t 0 t 1 t on t off<br />

t<br />

Figure 19.26<br />

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

I PV<br />

V PV<br />

L<br />

D<br />

i L<br />

S 1 C 1<br />

V load<br />

R load<br />

Figure 19.27<br />

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

As shown in Figure 19.26, the resulting load voltage obviously has a ripple, which can<br />

be smoothed by the additional capacitor C 2 . Anyway, its average value (V load )islower<br />

than V PV . In case the switching frequency is increased, for example, up to the kilohertz<br />

range, then the necessary inductance can be reduced considerably.<br />

The resulting voltage transformation can be described by the relation of the switching<br />

time as follows:<br />

V load<br />

V PV<br />

=<br />

19.2.4.3.2 Step-up converter (Boost converter)<br />

t on<br />

t off + t on<br />

By rearrangement of the components of the step-down converter, a step-up converter can<br />

be obtained (Figure 19.27). Contrarily, here V PV is stepped up. At a steady state as S 1 is<br />

still “off”, V load is equal to the V PV , neglecting the voltage across diode.<br />

As shown in Figure 19.28, during “on” state, without C 1 the load voltage drops<br />

immediately to zero. The circuit current (= i L ) flows through the inductor L and S 1 and<br />

rises according to the following equation:<br />

di L<br />

dt<br />

= V PV<br />

L

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