The Cuk converter, invented in about 1977 by Dr. Slobodan Cuk of Caltech, is called an Optimum Topology Converter and has a zero input ripple and zero output ripple characteristic. It's a cascade of a boost converter with a buck converter and has only one gated switch (usually a MOSFET) and a diode. IT has other advantages as well. The output voltage can be either lower or higher than the input voltage. It has inherent EMI filtering of both the input and the output due to its topology, and the energy is transferred by a series cap, therefore, there's no danger of the series switch shorting the input to the output as with a buck converter (unless the cap shorts). The energy flow is continuous, minimizing EMI, and the power can flow bidirectionally. There are both non-isolated and isolated versions of this topology. The main disadvantages are that the output voltage is inverted in the non-isolated version, and that the switch current stress can be high.
Please refer to the circuit diagram, below. The first section, up to the series cap C1, is the boost converter and the second section that includes the cap, is the buck converter. The boost converter section operation is fairly straightforward. When switch S is on (closed), inductor L1 charges up with current until S opens. At that time, the voltage across L1 reverses such that it's in series aiding with the input voltage Vs and the two voltages add for the boost.
Now, for the buck section. Note that the cap takes the place of the series switch and input voltage in the Cuk's buck converter section. When switch S is on, the left-side of cap C1 is grounded, so it becomes a switched voltage source to feed inductor, L2. For the buck section, once S1 is off and the cap is no longer grounded, L2's current continues to flow thru diode D. The buck's output is across smoothing cap C2.
It's interesting that the switch S1 switches for both the boost and the buck sections at the same time when it's closed and when it's open, it allows C1 to be charged up, ready for the next cycle.
A very fascinating thing about the Cuk converter is that both the input current ripple and the output current ripple can be reduced to zero by coupling the two inductors, L1 and L2. Prototypes of this converter made by Dr. Cuk back in the 1970s had a mechanism to adjust the coupling of L1 & L2 using a crank with both the input and output currents visible on a scope. The observers could adjust the crank and find the minimum or null at a particular setting. In a production environment, the input & output ripple can be significantly lowered, but not quite to zero, but very close. This is due to the tolerances in the cores and other non-idealities when making a single magnetic unit with both coupled inductors while trying to achieve a particular coupling factor, K.
When the duty cycle, d gets changed, so does the output voltage and it can be higher or lower than the input voltage. The duty cycle is equivalent to the percent the switch S1 is on in relation to the switching period T. T is ton + toff and d= ton/T. When d= 0.5, or 50% duty cycle, then Vout will equal Vin. When d< 0.5, Vout < Vin and when d> 0.5, Vout > Vin. The formula for Vout/VIn is as follows: Vout/VIn = d/(1-d), which is the same formula as for a buck-boost converter.
The problem with Vout being negative really isn't that bad as an isolated version can be used to solve that problem as well as provide isolation. Please refer to the second circuit diagram, below the non-isolated version. TO get the isolated version from the non-isolated version, the cap, C1 gets split into two caps, C2 & C3 on either side of the isolation transformer. The turns ratio can be 1:1 for the same functionality as the non-isolated version in terms of Vout/Vin or ir can be changed to raise or lower the output voltage and this can help to minimize the stress on the main switch S1. Notice that the diode is reversed and so is the polarity on the output cap. Therefore, in the isolated version, the polarity is normal with the positive side of the output up. However, it doesn't really matter in an isolated output since it can be connected any way you want with respect to ground or not grounded at all. It can also be put in series with another voltage source. In addition to the transformer, this isolated version requires one extra cap and that's about it.
For many years, the Cuk converter was under US patent and one would need to license it from Dr. Cuk if they wanted to manufacture it. But the patent has long since run out and anyone is free to design and manufacture Cuk converters these days.
