CLOSED LOOP APPROACH OF DC DC SEPIC POWER CONVERTER

CLOSED LOOP SEPIC:

                                                             
                           Figure 5: MATLAB SIMULINK closed loop circuit diagram

The basic working of closed loop SEPIC is same as the open loop SEPIC except the addition of the feedback loop. The feedback circuit enables automation of the circuit as it changes the duty cycle dynamically as the input is varied.

Feedback Loop: The output is given as input to the arithmetic comparator. The other input to the comparator is the required constant voltage. Here the error voltage is obtained which is fed as input to the PI controller. The PI controller has two parameters ‘proportional gain (Kp)’ and ‘integral gain (Ki)’. The values used are Kp = 0.045 and Ki = 0.91 which are based on Ziegler-Nichols method. The output of PI controller is the control voltage. This voltage is given to the Saturation block which ensures that the voltage level is within the amplitude of the sawtooth repeating sequence. The output of the Saturation block is fed to the gain block with gain value of 1.01. The Repeating sequence produces sawtooth wave of frequency 100KHz which is used to produce pulses. The output from the gain is compared with the sawtooth wave and based on the relational operator function a pulse is generated which is given as the input to the GATE terminal of the MOSFET. This circuit dynamically changes the duty cycle of SEPIC by varying the ON and OFF time of the MOSFET.

SIMULATION RESULTS:

Figure 8: Output of closed loop SEPIC for an input of 2V

                           Figure 9: Output of closed loop SEPIC for an input of 5V  

                             Figure 10: Output of closed loop SEPIC for an input of 12V 

Figure 11: Output ripple current for an input of 10V

  

From the graph it is observed that ripple current in open loop SEPIC is higher than that in closed loop SEPIC.

Average efficiency of open loop SEPIC is observed to be at around 93%

Average efficiency of closed loop SEPIC is observed to be at around 99%

The following table is tabulated for closed loop SEPIC:

Input Voltage(in V)	Loop Response Time(in ms)
2	4
5	2
12	4.5

CONCLUSION: As the result suggests, the open loop SEPIC has more ripple current and voltage than that is desired in high level applications. It is also not automatic as the duty cycle must be reset each time the input is varied to get the desired output. Hence the need to design an automatic SEPIC arises. The automation is brought about by the feedback loop. The main controlling component of the feedback loop is the PI Controller which also helps to reduce the steady state error voltage. With the use of PI Controller the SEPIC becomes more robust and gives good dynamic response.

SEPIC can also be designed to act as a multiple output isolated DC-DC converter. The main change is that the input to the SEPIC is given through a transformer with 1:1 turns ratio. The secondary coil is wound around the same core for all the multiple outputs. However it becomes a bulky circuit as a different circuit has to be designed for each of the multiple outputs as a different constant has to be set in the feedback network.