# Buck converter design problem/confusion

I have problem with designing buck converter.

I have tried to design it by using instructions from this site:

Buck converter

And here are my parameters:

V input - 72V max

V output - 12 V switching frequency - 100 kHz ( LTC3703)

I max -1,5 A

However, my teacher has given me a $$\15 \mu\$$H inductor and said that I should use that one, and that is where my problems start.

When I go to design it using formula for inductor, LIR is extremely big ( somewhere around 4-5, instead between 0.2-0.5 like proposed on web site )

This is the inductor we are using

Inductor

So can someone help me to get good LIR, or should I say that we can't use this inductor?

• You should be able to get less ripple by increasing your switching frequency. – tehwalris Dec 15 '14 at 7:30

In a Buck Converter the inductor ripple current is determined by (approximately): $$\dfrac{V_o.(1-V_o/V_i)}{L.f}$$
Increase the switching frequency to about 800kHz which is very high for standard MOSFETs leading to excessive switching losses. Increase L to 133 $\mu$H (fairly large and higher DCR $I^2R$ losses); or a combination of both: i.e higher switching frequency and higher inductance. I would aim for about 300kHz and 47$\mu$H.