The SEPIC Converter
The Sepic Converter:
Fig: The Basic SEPIC topology Fig: The practical SEPIC topology with coupled inductors
- The SEPIC stands for “Single-Ended Primary Inductance Converter” but it can also be interpreted as “Secondary Polarity Inverted Cuk”. The basic input–output relation can be derived by considering the two inductors to have average null voltage across themselves. If the link capacitor has a voltage Vc across itself (consider it to be reasonably constant), then for the input inductor, the volt-secs during the ON and OFF periods of the switch are:
- Thus the SEPIC is also basically a BOOST-BUCK converter akin to the CUK converter. (The Boost stage comes first followed by the Buck stage and it is also I-V-I converter).
- In the practical SEPIC converter, the two inductors are coupled with the polarities. The turns ratio is and the coupling is very tight. For such a coupled-transformer SEPIC, equating the positive and negative volt-secs for the two inductors,
for the input inductor and
- Initially when the transistor is OFF, the capacitor C2 charges to the supply voltage Vin. When the transistor is switched ON, The circuits to the left and right of the transistor are identical and both the windings are induced with the supply voltages, resulting in null emfs on either side, which explains why the ideal circuit will not work.
- However, neither the coupling between the inductors nor the effective turns ratio can be unity. This results in a circuit with the features of the uncoupled circuit and the circuit performs. The second voltage source, VC, induces N.VC into the primary, where N is the turns ratio.
- For the interesting case, Vin = VC = V1, if the turns ratio, n, is increased slightly from unity, by 1/k (where k < 1 is the coupling coefficient between windings), then the voltage induced by Vin will increase the voltage at the Drain of the transistor to N. V1, thereby "bootstrapping" the leakage inductance of the input inductor.