Switching Power Supplies A - Z

DC-DC Converter Design and Magnetics

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So we conclude that the peak current for the boost also decreases at high input

voltages (see relevant plot in Figure 2-4).

Therefore, for a boost, we should always start the inductor design at V

INMIN

(i.e. at D

MAX

).

The Current Ripple Ratio ‘r’

In Figure 2-2 we first introduced the most basic, yet far-reaching design parameter of the

power supply itself — its current ripple ratio ‘r.’ This is a geometrical ratio that compares

and connects the ac value of the inductor current to its associated dc value. So

r =

∆I

I

L

≡ 2 ×

I

AC

I

DC

Here we have used ∆I = 2 × I

AC

, as defined earlier in Figure 2-2. Once r is set by the

designer ...