Ćuk converter

A Cuk converter (English Ćuk converter ) is referred to in the electronics, a discrete electronic circuit capable of transforming an electrical direct voltage into another DC electric voltage. The magnitude of the output voltage can be both smaller and larger than the amount of the original input voltage. In contrast to the SEPIC converter, the output voltage as in the inverse conversion with respect to the input voltage to a negative sign, the output voltage is inverted. Due to this fact the Ćuk converter is included in the group of inverting DC-DC converter.

The term Ćuk converter is due to the name of its inventor Slobodan Ćuk, in 1976 introduced the idea of ​​circuit together with Robert D. Middlebrook.

Design and motivation

The Cuk converter is used for the voltage converting three active energy store, two coils and a capacitor, which are loaded by means of semiconductor switches continuously with energy and is discharged. In addition, both the input and at the output, as with any power converter, a charging capacitor is required to reduce the voltage ripple.

At ordinary voltage transformers, such as the up-converter or down-converter, either at the input or appears at the output of the circuit to a discontinuous current profile, which influences the respective negative voltage ripple. So must in boost converter, the output capacitor be particularly large because the converter supplies only in the off time of the switch current to the output. In contrast, down-converter again particularly attention must be paid to the input capacitor, since only in the time of the switch current from the input to the circuit flows.

Exactly these negative characteristics do not occur in the Ćuk converter topology, as are continuous here, both the input current and the output current. Thus, both charging capacitors can be significantly smaller without degrading the voltage ripple.

Function

The transistor T is turned on, the inductor L1 is connected in parallel to the supply voltage, and the current IL1 starts to rise. After switching off of the transistor, this current will continue to flow due to the Lenz's law, and charges the capacitor C via the diode D. The current in the inductor L1 decreases while the voltage at the capacitor to. After re- switching on the transistor, the inductor L1 is reloaded, and the current IL1 increases. Simultaneously then a negative current IL2 starts to flow, which, starting from the capacitor flows through the inductor L2 and the load (including the storage capacitor ), and cut off the capacitor energy. On charging the output capacitor thus a negative voltage builds up. After again turning off the transistor, the capacitor is recharged from the current IL1. At the same time flows are also caused by the Lenz's law, the induced current IL2 of the inductor L2 by means of the load, and the diode on.

Thus, both the input current and the output current to a continuous curve.

Mathematical Description

The following considerations, it is assumed that all capacitors are very large, and the converter has settled at a constant duty cycle.

The current in the two inductors varies continuously around two values ​​. The average value of these currents is constant, and the offset in the charge on the capacitor means.

It is believed further that the current waveform is triangular in the inductances, the integrals can be simplified.

Replacing the times by the duty cycle ( d), we obtain the following expression.

The voltage across an inductor is given by:

Accordingly, the mean values ​​of the voltages at the two inductors have zero result.

The voltage across the inductor L1 is therefore given as:

The voltage across the inductor L2 is given by:

By equating the two equations to obtain an equation representing the output voltage of the Cuk converter in dependence on the input voltage and the duty ratio:

Execution

The Cuk converter can be carried out both non-insulated and insulated using a transformer. In the latter, there is also the possibility of the output voltage to increase the transmission ratio of the transformer or decrease.

The two inductors are magnetically coupled, thereby reducing the voltage ripple at the input and output. The two inductors use the same core as the magnetic coupling.