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# Cuk (boost buck) converter problems

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#### kripacharya

##### Banned
What is required is --

DC power Input is +12v capable of upto ~1amp.
DC output of -12v (upto -12.8v) (not -10v or -11v or anything else) with atleast 60mA.

A boost-buck Cuk topology seems ideal. Anyone know how to design one ?

I know that L1 > (1-D^2) x R / 2 x D x Fs
and also that Vo/ Vin = -D/ (1-D) where Vo is negative.

But is this the complete design criteria ? What other parameters to calculate for a good design ? How to choose L2, C1 C2, Fs etc ?

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I think the first step you should do is to model your boost buck converter , the input variables are your voltage and the control variable is the converter duty cycle

Unfortunately there isn't a great deal of practical literature on Cuk converters, since they're not often used. But most design techniques applied to SEPIC converters can be effectively applied for component selection (with some adjustments, of course). For your low power level, you don't have to be extremely optimal in your choices.

kripacharya

### kripacharya

Points: 2
Frequency is often chosen to be 20 kHz and above, which is beyond the ability of humans to hear.

To suit this frequency range, coil values are usually less than 1 mH.

* peak currents can be less than when operating at a low frequency
* components can be smaller and less expensive than those for a low freq.

... But most design techniques applied to SEPIC converters can be effectively applied for component selection (with some adjustments, of course). ...

Good point. Thanks for the tip

Also I would recommend taking a look at **broken link removed** It's made by an engineer who is basically obsessed with Cuk converters, enough that he made an entire website explaining the pros and cons and pitfalls. However, he apparently has no talent for website design, and the site is terribly difficult to navigate. In fact for all I know the author is literally insane, but even so a lot of what he writes is very thought provoking, such as **broken link removed**.

Hello there, a Cuk will work fine, suggest 50-70kHz operation to get the choke size down, separate input and output chokes to help with control stability, post your circuit so far and helpful suggestions can be made to give you proper control and power stage.

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