Linear Transformer based Sepic Converter with Ripple Free Output for Wide Input Range Applications

Linear Transformer based Sepic Converter with Ripple Free Output for Wide Input Range Applications

Karthik Sitapati Professor, EEE department

Dayananda Sagar college of Engineering Bangalore, India

Kirthi.C.S

Dayananda Sagar college of Engineering,

Mtech, EEE department Bangalore, India

Abstract The conventional power factor correction(PFC) single ended primary inductor converter(SEPIC) topologies proposed so far have achieved low conduction losses and reduced input ripple current but they have failed to achieve optimized magnetics. A linear transformer based SEPIC converter with ripple free output is proposed in this paper. The topology utilizes a linear transformer in order to obtain optimized magnetics . In addition input current ripple and the voltage stresses across the switches is reduced by utilizing a clamping capacitor across the switch. A detailed analysis is carried out on MATLAB/SIMULINK platform and the various results are tabulated and analyzed. The hardware prototype model of proposed topology is designed and implemented for the input voltage of 24/55 v DC for which the expected output voltage 150v DC with the circuit efficiency of 94% is achieved.

Index Terms–Power electronics, power factor correction (PFC), bridgeless converter, single ended primary inductor converter (SEPIC)

1. INTRODUCTION

   There has been an increasing demand for high power factor and low total harmonic distortion (THD) in the current drawn from the utility. With the precise requirement of the power quality, power factor correction significant efforts have been made in the development of PFC converters. These kinds of converters generally have full bridge diode rectifier on an input side so the conduction losses occur at the input side where the full bridge diode is present. In order to overcome this problem the full bridge diode is eliminated and bridgeless converters are introduced.

   The boost converter topology has been widely used as a PFC converter because of its simplicity and high power capability. It can be used with the universal input voltage range. The boost converter always uses three semiconductor voltage drops in the current flow path. To increase the converter efficiency bridgeless boos rectifiers were introduced. The conduction losses are reduced by reducing the number of semiconductor devices that conduct current from the source to the load. However, the output diode operated in high voltage has severe reverse recovery problems due to high diode forward current and high output voltage. As the switching frequency increases,, the large reverse recovery currents of the output diodes effect the switches in the form of additional turn on losses and also produce electromagnetic interference (EMI) noises. The major disadvantage of using boost converter is that its output voltage should be higher than its peak input voltage. Relatively low output voltage of

   PFC converter is required in many applications such as switched mode power supplies.

   Therefore in order to overcome the disadvantages of the PFC boost converter, PFC buck converters are introduced. These converters are more suitable for many applications due to their low output voltage range. Moreover buck converters perform better at light load owing to a wide range of choices of lower voltage rated semiconductor devices and reduction of losses and sizes of isolation transformers. However, since the input current of the PFC buck converter has dead angles during the time intervals when the input voltage, there is a strong tradeoff between the power factor and output voltage selection.

   Therefore in order to overcome the disadvantage of the PFC boost and buck converter, SEPIC converters is proposed in this paper. It is used to obtain high power factor regardless of its output voltage which can be stepped down or up accordingly. SEPIC converters have been adopted for many applications such as high power factor correction, photovoltaic system and LED lightening. However it has several drawbacks such as

   • High voltage stresses of power semiconductor devices

   • Low efficiency due to hard switching operation of the power switches

   • Electromagnetic interference noises are significant in high-frequency operation

   • A bulk inductor should be used to minimize the current ripple.

   In order to overcome the above mentioned drawbacks a Linear transformer based SEPIC converter with ripple free output for variable input application is developed in this project. It mainly consists of a linear transformer which will overcome the disadvantage of using three magnetic components in order to reduce the current ripple and the high voltage stresses of the switches can be reduced by using a clamping capacitor across the switch. Therefore due to the reduced ripple and voltage stresses of the power semiconductor devices the efficiency is improved.

2. PROPOSED TOPOLOGY

   The proposed linear transformer based SEPIC converter topology consists of a linear transformer; it is another excellent technique applied in high voltage step up

   applications by adjusting the turns ratio of the transformer. The capacitor C2 is added across the switch to reduce the switching losses. Diodes D1 and D2 are the input rectifiers and operate like a conventional SEPIC PFC converter. The other components C1 ,L1 ,Do and Co are similar to those of conventional SEPIC converter. It is assumed that the converter operates in discontinuous conduction mode (DCM). The converter operation is analyzed during one switching period in the positive half line cycle of the input voltage. The circuit arrangement is shown in fig 1.

   Figure 1: A linear transformer based SEPIC converter

   The advantages of using this topology are-

   1. Closest to UPF is achieved.

   2. Conversion efficiency is better compared to others.

   3. Low voltage stress across the main switch is achieved by using effective clamping technique.

   4. The ripple is further reduced due to ZVS operation The proposed linear transformer based SEPIC converter operates in three modes, in one switching period. Before to, the switch S1and diode Do are turned off and the switch S2 is conducting. The theoretical waveform of the proposed converter is shown in fig 2.

3. OPERATION OF A PROPOSED CIRCUIT

   1. Region of operation

      The converter operates in three modes namely,

      1) Mode 1(t0, t1):

      During this mode the switch S1 is turned ON, the tertiary winding of the transformer is charged and the current is induced in the primary and the secondary winding of the transformer. Therefore the diode D1 is forward biased and the capacitor C1 and the inductor L1 is charged. Hence the

      Fig. 2. Key waveforms of the proposed converter

      2) Mode 2(t1, t2):

      During this mode the switch S1 is turned OFF and the switch S2 is turned on. The inductor L1 which was charged previously in mode 1 will now start to discharge thus forward biasing the diode Do. The capacitor Co is fully charged and supplies the load. The capacitor C2 and diode D3 are connected across the switch S2 in order to reduce the stress across the switch S2 by performing ZVS operation. The capacitor C2 also opposes the change in voltage. Therefore a ripple free current is fed into the inductor L1. The current is decreases from its maximum value linearly as follows

      (t) = I – 1 ( ) ..(2)

      inductor L1 is fully charged. The capacitor Co supplies to the s

      load. The current is increases from its minimum value Is2

      1

      linearly as follows.

      3) Mode 3 (t

      t ):

      s(t) = I2 +

      1

      ( 0) ………. (1)

      2, 0

      This is a transition mode where switch S2 is about to turn off and switch S1 to turn on. By the end of this mode the switch S2 will be turned off and switch S1 will be turned on. The output capacitor Co supplies to the load.

      In all the modes of operation the diode D2 is reverse biased in order to ensure a continuous flow of current into the linear transformer and also to avoid the reverse flow of current into the switch S2 which may cause damage to the switch

   2. Rating of switching devices

      In this proposed circuit, the voltage stresses of all the switching devices are equal to the sum of the maximum input voltage and output voltage as follows

      VD,max = VSW,max= Vin,peak +Vo …(3)

      The current stresses across the diode is given as follows

      I = I = I = 2n 4Po ls (4)

      D,max D1 D2 ls

      fsw

      3(a)

      3(b)

   3. Clamping capacitor

      As the switches conduct, they develop voltage stresses across them, thus a clamping capacitor is used across the main switch in order to reduce the stress across the switch and is given as follows.

      C2 = .. (5)

      The voltage across the switch S1 is same as that of the diode voltage but the voltage across the S2 should be less when compared switch S1 because of the clamping effect, the voltage stress across the switch S1 is reduced

      D .Conditions to operate in DCM

      In order to guarantee that the proposed converter operate in DCM mode the inequality of 1 < 1 – D must be satisfied. The voltage gain can be extended greatly without an extreme duty cycle as turns ratio of the transformer increase which makes the converter suitable for high step up and high power conversion. Duty cycle is obtained as following inequality.

      = 1 (1)

      . (6)

      E. Average inductor current

      IL1 = ..(7)

      The average current flowing through the inductor is given as follows

      3(c)

      Figure 3: shows the mode of operation of proposed circuit

      1. Mode1, (b) Mode 2, (c) Mode 3

         1. DESIGN PROCEDURE

            1. Design Specification:

               Design specifications of the proposed converter are as follows:

               1. Main voltage Vin = 55Vdc;

               2. DC output voltage: Vo = 150V

               3. Maximum output power: Pout = 250W

               4. Switching frequency: fSW = 100KHz

            2

            L1

            = Vin D.(8)

            L1 f

            F. Output capacitor

            As the output ripple voltage is two times the input line frequency the output capacitor Co should be large enough to minimize the output voltage ripple Vo. Therefore, Co can be obtained from the following equation

            o

            C = PO (9)

            4fVO Vo

            1. SIMULATION

               1. Circuit Arrangement:

                  The main aim of this project is to improve the voltage gain and efficiency and to reduce the ripple by using a linear transformer. Active clamp technique is adopted to reduce the voltage stresses. The simulation work is done in MATALB SIMULINK. The output voltage of 150v is obtained and simulation diagram is shown in fig 4

                  Figure 4: Circuit Arrangement in MATLAB Simulation Package

                  1. The fig 5 shows the proposed linear transformer based SEPIC converter for ripple free input current for a wide input range.

                  2. A clamping capacitor and a diode are used a cross switch S2 for ZVS operation in order to reduce the stress

                  3. The value of output capacitor is reduced further so that the hardware is no more bulky

                  4. The waveforms of output voltage, output current and output power are shown below

               2. Circuit Specifications:

                  The specific values used in the simulation as tabulated as below in table 2,

                  Table1: specified values used in simulation

                  Parameters	Values
                  Vin	55v
                  fs	100KHz
                  Ls	127µH
                  L1	25µH
                  C0	41.6µF
                  RL	90
                  C1 , C2	0.4µF
                  Cs	0.3µF

               3. Simulation Results:

               The figure 5 shows the simulation results of input voltage, output current, output power for the proposed linear transformer based SEPIC converter respectively as shown below.

               (a)

   Fig 5: waveform of output voltage and output current of proposed converter.

   (a) Waveform for 55V input (b) Waveform for output current

   (c) Waveform for 150V output voltage (d) Waveform for 250W output power

   Figure 5(a) shows the simulation results obtained for a input voltage of 55v.And the figure 5(b) shows the simulation results for output current of 1.6amps.And figure 5(c) shows the simulation results for output voltage 150V and figure 5(d) shows the simulation results for output power of 250W which is greater than the conventional converter respectively

   1. MEASUREMENT RESULTS

      6(a)

   2. CONCLUSION

A linear transformer based SEPIC with ripple free output for variable input application is presented in this project.ZVS soft switching is achieved for the switches during the whole switching transition. The linear transformer reduces the reverse recovery losses. By employing linear transformer the voltage gain can be greatly extended and the switch voltage stresses are far lower than the output voltage and high performance MOSFETS available to reduce the conduction losses in high input application. Finally the converter is designed to show the converter performance and experimental result. The advantage of the proposed topology

It produces considerably more current and voltage. High weighted efficiency than conventional converter. Wide input voltage range. In future more advanced control techniques can be used to improve the performance and efficiency of the system.

ACKNOWLEDGEMENT

I would like to thank my teachers, parents and my friends for helping me out with this paper. It wouldn't have been possible without them

REFERENCES

6(b)

Figure 6: Shows the measurement results of proposed circuit

1. Power Factor, (b) Efficiency

In fig 6 (a), the power factor is measured more than 0.995 in the converter as shown in Fig.6(a). In addition, Fig.6(b) shows the measured efficiency of the bridgeless SEPIC PFC in and the proposed converter. When the same gate signals in are applied to the switches, the efficiency of the proposed converter is similar to that of the conventional converter. When the proposed gate signals a are applied to the switches, the efficiency is improved compared with the conventional converter

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