Q: If I size my power components for 10% ripple on L1 & C1 & L2, and 1% ripple on C2, will it work?
A: No.

Q: Why not?
A: Because a converter with those component values will not be minimum phase, and will oscillate when you close a feedback loop around it.

Q: So you're saying I have to damp.
A: That's not enough!

Q: What?! Huh!?
A: You must also meet Dr. Middlebrook's impedance requirement:

"The output impedance of the input filter must be much less than the input impedance of the output filter."

Q: Why doesn't anyone do that for the Buck, Boost, Flyback, SEPIC, etc.?
A: They do, but don't know they are doing it!

Q: How?
A: Take any of the isolated Buck topologies for instance. Since inductors are heavy, the output inductor is no larger than it has to be to keep the current ripple manageable. The output capacitors are large.

The resulting input impedance of this output filter, Ro2=SQRT(L2/C2) is relatively low, and the resonance is damped by the load resistance.


On the input side, most engineers just use a common mode choke, which has only a small series inductance (differential mode.) The input cap is huge to smooth out the square wave of current drawn by the transistor switch.

The resulting output impedence of this resonance is thus Ro1=SQRT(L1/C1) which is very small (<100 mOhm.) This resonance is damped by the series wire resistance of the choke, copper traces, and bus coming from the source voltage.

Q: With the result that...?
A: L1 is much less than L2; Ro1 is much less than Ro2; and the requirement is met automatically!

Q: Why is the Cuk Converter different?
A: In this converter, the input inductor L1 has a square wave of voltage across it just like L2, due to its position in the topology. Thus it cannot be tiny, or the current ripple would be huge!

Q: Then why the Hell should I use this stupid topology?
A: Because when it is properly designed, it outperforms all the others, as explained in the second part of this website. Also, don't use profanity, it reflects badly on your upbringing....

Q: But won't it be heavier?
A: No.

Q: Bigger?
A: No.

Q: Harder to design.
A: Not any more!

Q: So what do I do?
A: Send for my laminated Design Guide. Though all the necessary information is to be found scattered around the rest of this site, the Guide brings it all together in one place. Also, it has step by step instructions for determining D, L1, C1, L2, C2 and switch stresses. These values will give you a stable, high performance regulator!

Q: Okay okay. How much does it cost, and how do I order?
A: The Guide costs $25.00 and may be ordered by clicking on the link below. Ignore the stuff about email address and password.

Q: What's next?
A: Click on "Conversion to Simple Converter" below, and follow the links to learn how to reduce a multi output isolated power supply to the simple case, so you can determine D and the 4 reactances, switch stress, and the necessary damping networks!

Q: Aw come on, no one really does all that!
A:  http://www.jayabel.com/cuk/cuk.html