www.newnespress.com

55DC-DC Converter Design and Magnetics

the MAX of the current limit speciﬁ

cation. So, in this case, a “ loose ” current limit

speciﬁ cation effectively amounts to requiring bigger components (transformer) for the

same maximum power-handling capability.

Note: Some makers of off-line integrated switcher ICs (e.g., the “ Topswitch ” from

Power Integrations) often tout their “ precise ” current limit, thus suggesting that

we get the best power-to-size ratio (i.e., converter power density) when using their

products. However, we should remember that in most cases, their product families

have a discrete set of ﬁ xed current limits. And that is a problem! For example, we

may have devices available with current limits in steps of 2 A, 3 A, 4 A, and so on.

So yes, we may indeed get a higher power density when operating at the maximum rated

output power of a particular IC. But when operating at a power level between available

current limits, we are not going to get an optimum solution. For example, in an

application where the peak current is 2.2 A, then we would need to select the 3 A

current limit part, and we will need to design our magnetics to avoid core saturation

at 3 A. So in effect, we have a very imprecise current limit now! The best solution is

to look for a part (integrated switcher or controller plus MOSFET solution) where

we can precisely set the current limit externally , depending upon our application.

With all these subtle considerations in mind, a designer can hopefully pick a more

appropriate inductor current rating for his or her application. Clearly, there are no hard

and fast rules. Engineering judgment needs to be applied as usual, and perhaps some

further bench-testing may also be needed to validate the ﬁ nal choice of inductor.

In the worked examples that follow, the general approach and design procedure will

become clearer.

3.15 Worked Example (1)

A boost converter has an input range of 12 V to 15 V, a regulated output of 24 V, and a

maximum load current of 2 A. What would be a reasonable goal for its inductance, if the

switching frequency is (a) 100 kHz, (b) 200 kHz, and (c) 1 MHz? What is the peak current

in each case? And what is the energy-handling requirement?

The ﬁ rst thing we have to remember is that, for this topology (as for the buck-boost), the

worst-case is the lowest end of the input range, since that corresponds to the highest duty

cycle and thus the highest average current I

L

I

O

/(1 D ). So for all practical purposes,

we can completely disregard V

INMAX

here—in fact it was a red herring to start with, for

this particular analysis!

Get *Power Sources and Supplies: World Class Designs* now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.