Single-ended primary-inductor converter
A SEPIC converter ( Abbreviation for single-ended primary inductance converter ) is in the form of an electronic DC-DC converter. The positive input voltage UE of the SEPIC converter can be either greater or less than the positive output voltage UA. Was presented to the circuit in 1977 by Massey and Snyder on the PESC ( Power Electronics Specialist Conference).
Circuit
As an essential element of the circuit serve three energy store and a switch (S). The energy accumulator, two coils (L1 and L2) and a capacitor (C2). These may be designed as a double coil with two oppositely wound coils on the same core or two independent coils. In the second case, the coil L2 can be constructed as a transformer. Thus, a galvanic separation is possible.
SEPIC converter to a similar structure in the topology have the Cuk converter and the Zeta converters.
Continuous operation
In the study of the SEPIC are basically two modes of operation to distinguish. The SEPIC can operate in continuous and discontinuous operation. At the start of continuous operation (continuous current mode ) will be described. The term refers to the current in the coil L1. The continuous operation can be broken down into two periods. The first section corresponds to the time in which the switch S is closed.
The second portion represents the remaining time in which the switch is open.
The examination is carried out on the analysis of the flows within the circuit. As an assumption is that the capacitor C2 is large enough so that the voltage across the capacitor does not change. One can also determine that the capacitor charges to the exact value of the input voltage. When the switch is closed, are obtained for L1 and L2 following current increases:
Taking into account both coils, results
The peak value of the current at the end of Section 1 is then calculated as
Where D is the duty cycle represents. Section 2 begins with this peak. Thereafter, the current decreases as a function of the output voltage.
For the peak value is valid:
Since at the boundary between Section 1 and 2, the values must be the same, the equations (3) and (5) can be equated.
Solving for the ratio of the resulting output to the input voltage
Discontinuous operation
The discontinuous mode ( discontinuous current mode ) begins with the two sections of continuous operation. The same equivalent circuit models, such as for continuous operation. Joining them will be a third section, while the lead the two inductors is no electricity, but the load energy to the output capacitor C4 extracts, as shown in the adjacent equivalent circuit.
Since the currents of the inductors start at zero and end at zero, eliminating from consideration I0. The equation ( 3) changes to:
And equation (5) becomes:
D2 corresponds to the duration of the second period of time. Equated results
Now we need the consideration of the third section.
The output voltage is calculated from the current and the load resistance.
The current corresponds to the average current through the diode D, which it is still to be calculated. Power is transferred from the input through the diode into the output capacitor Only in Section 2. About the mean value of the current based on the total switching cycle results
Using equation (10 ) and equation ( 12) are two equations are available, both of which depend on Vin, Vout, D, and D2. If you both equations for D2 and puts them equal, one can determine the ratio of the output to the input voltage:
And thus
In discontinuous operation, that is, the output voltage of the SEPIC not only on the duty cycle of the switch is dependent.