Q: Remind me once more of the six beneficial characteristics that make the Cuk Converter the Optimum Topology.
A: 1) Non-pulsating input and output currents.
2) High efficiency capacitive energy transfer.
3) Most general D/D' transfer function.
4) No DC in the isolation transformer.
5) Coupled inductor and integrated magnetics
for true zero ripple at input and/or output.
6) Grounded switch.
7) No longer patented....

Q: What is the Energy Transfer Capacitor?
A: It is the (usually) film cap that connects the input and output inductors. The ETC charges from the source, and discharges into the load to transfer energy from input to output in a controlled manner. Usually, about 10% of the total charge on the part is involved in this cycle by cycle transfer. The rest stands off the algebraic difference between the input and output voltages, also serving as a reserve in case of sudden line or load changes.

Q: I thought the stress in the switch of this topology was too high. Isn't it 4X Pout?
A: It is the same (Ig + I) x (Vg + V) on both transistor Q and diode P. This is the same as the stress on the switch of ANY isolated converter.

Q: What if one uses an unisolated topology?
A: The only two that exhibit lower stress are the Buck and the Boost. These two must always be used in cascade connection as explained earlier. In that case, the sum of the stresses on the 4 switch elements is (Ig + I) x (Vg + V) as before.

Q: How does DC current get past the series capacitor?
A: The ouput is inverting. I.e. the ouput voltage of the unisolated topology is negative. Average current passes down through Q, along the ground wire to the ouput, up through the load, down through P, and back along the ground wire to Vg!

Q: What is a regulator?
A: A converter supplied with a feedback loop to maintain a fixed output voltage.

Two factors necessitate regulation:

A) The line voltage Vg may change, producing a first order variation via V=VgD/D'.

B) The load current I may change, producing a perturbation in V, due to I-squared-R losses in the converter. Note that these are only second order variations.

Also, minor shifts in the converter's circuitry due to temperature, age, etc. are reduced as in any feedback loop!

Q: What is a power supply?
A: A regulator augmented by all the necessary startup, shut down, undervoltage, overcurrent, remote sense, and other bells and whistles. Soldiers in WWII termed these bells and whistles "blivits."

Q: Why is it called the Cuk Converter?
A: It was invented by Slobodan M. Cuk of Caltech, and is named in his honor. Dr. Cuk now designs his converter through his private company TESLAco.