DC DC SEPIC POWER CONVERTER: BASIC DESIGN AND ITS WORKING

INTRODUCTION:

They are many DC-DC conversion techniques available. The most basic technique is linear regulation. However the efficiency of this technique is very low. Hence switching regulation is used when higher efficiency is required. The design and working of switching regulators is complex as they contain non-linear components. Buck-Boost converter performs same operation as SEPIC but its efficiency is lower when compared to SEPIC. Hence SEPIC is preferred in high efficiency applications such as spacecrafts where power losses are critical.

SEPIC gives a constant output when the inputs are lower or higher or in the range of the constant output. The SEPIC operates at high efficiency in the range of 90%-95%. Open loop SEPIC has a lower efficiency when compared to the closed loop SEPIC. However due to some limitations the open loop SEPIC is not widely used in practical applications.It is best to use the SEPIC converter with feedback to hold a single output without the need for control when using a SEPIC as part of a large circuit.  

WORKING:

SEPIC configuration provides both buck and boost action in a single device depending on duty cycle. If the duty is higher than 50%, then it acts as a boost converter. If the duty cycle is less than 50%, then it works as a buck converter. If the duty cycle is exactly 50%, then output is same as the input.

Equations:

Duty cycle, D=Vo+Vd/Vin+Vo+Vd

Output voltage, D*Vin/(1-D)   

 Open loop SEPIC:

   

                                                    Figure 1: Basic SEPIC circuit diagram

This is the circuit diagram of the basic SEPIC topology. There are two stages of working i.e. when the MOSFET switch S1 is closed and when it is open.

When the MOSFET switch is closed:

                                          Figure 2: Open loop SEPIC when the switch is closed

When the switch S1 is closed, energy is stored in the input inductor L1 and the current through it increases. The voltage drop across this inductor is equal to the input voltage as the diode D1 is in open position during this mode. Inductor L2 is charged by capacitor C1. Capacitor C2 discharges through the load, thus giving an output. The equations in this mode of operation:

        

                              

When the MOSFET switch is open:

                                         Figure 3: Open loop SEPIC when the switch is open

When switch S1 is open, the diode D1 comes in to play. As the diode D1 conducts, the input inductor current decreases, charging capacitor C1. The current through the second inductor L2, decreases linearly to charge the capacitor C2. The equations in this mode of operation:

                                                                    

MATLAB SIMULINK Circuit:

                                Figure 4: MATLAB SIMULINK open loop SEPIC circuit diagram

Circuit components:

Component Name	Value
L1	150 microH
L2	150 microH
C1	100 microF
C2	100 microF

The resistances R1, R2, R3 and R4 are parasitic resistances in the order of milli ohms.

The input and load resistance can be varied as the circuit follows line and load regulation respectively.

Drawbacks: In open loop SEPIC, there is some ripple current and voltage at the output which is undesirable. Also, the average efficiency is low when compared to closed loop SEPIC. However, the main drawback is that the duty cycle should be varied manually which is not suitable for most of the practical applications.

Figure 5: shows output voltage and current when input is 10V 

Figure 6: shows spike in the output volatge and current

Figure 7: shows ripple in ouput voltage and current