Voltage Controlled Non Isolated Bidirectional DC-DC Converter with High Voltage Gain

Voltage Controlled Non Isolated Bidirectional DC-DC Converter with High Voltage Gain

Fathima Anooda M P

PG Student

Electrical and Electronics Engineering Mar Athanasius College of Engineering Kerala, India

Jeena Joy

Assistant Professor

Elizabeth Paul

Assistant Professor Electrical and Electronics Engineering

Mar Athanasius College of Engineering Kerala, India

Electrical and Electronics Engineering Mar Athanasius College of Engineering Kerala, India

Abstract The renewable systems cannot provide a stable power for user, the renewable energy systems and battery can be utilized for the hybrid power systems. When the renewable energy systems cannot supply enough power for the load, the battery must replenish insucient power. Whereas the whole power of the renewable energy systems cannot be used completely by the load, the surplus energy can be used to charge the battery. Because the bidirectional DC-DC converters can transfer the power between two DC sources in either direction, these converters are widely used for renewable energy hybrid power systems. Here a voltage controlled non-isolated bidirectional DC-DC converter with high voltage gain is presented. The converter consists of two boost converters to enhance the voltage gain. Four power switches are employed in the converter with their body diodes. Two inductors and a capacitor are also employed as passive components. The input current is divided to the inductor which causes the eciency to be high and the size of them to become smaller. The voltage gain of the converter is higher than the Conventional Cascaded Bidirectional buck/boost Converter (CCBC) in step up mode. Besides, the voltage gain in step-down mode is lower than CCBC. The converter is implemented in the laboratory with high and low side voltages 25V and 2.5V, respectively. The dsPIC30f2010 microprocessor is used to generate the control pulses. The eciency of the converter is more than CCBC while the total stresses on active switches are same. Converter is simulated using MATLAB/SIMULINK.

KeywordsDC-DC converter, High voltage gain converter, Non-isolated bidirectional converter, Voltage Stress

1. INTRODUCTION

   Since the usage of the fossil fuel results in environmental pollution, the clean energies become very important in the world. In recent years, the renewable energy systems, including photo-voltaic systems, fuel-cell systems, wind- power generating systems, are developed rapidly. Because the renewable systems cannot provide a stable power for user, the renewable energy systems and battery can be utilized for the hybrid power systems. When the renewable energy systems cannot supply enough power for the load, the battery must replenish insucient power. Whereas the whole power of the renewable energy systems cannot be used completely by the load, the surplus energy can be used to charge the

   battery. Because the bidirectional DC-DC converters can transfer the power between two DC sources in either direction, these converters are widely used for renewable energy hybrid power systems, hybrid electric vehicle energy systems and uninterrupted power supplies [3].

   The isolated boost bidirectional converter size is large more component is presented in the circuit, core saturation problem, more switching losses, less eciency as compared to the non-isolated bidirectional DC-DC converter. This project deals with the non-isolated bidirectional DC-DC converter because non-isolated converter does not use a transformer and has one less output rectier. Without the transformer the overall size of the converter can be reduced, not only by the absence of a bulky component, but wasted heat from switching and copper losses is minimized.

   Depending on the application, isolated and non-isolated bidirectional converters are applied. The yback converters forward-yback converters, half-bridge converters and full- bridge converters are isolated types of the bidirectional DC- DC converters. These types of converters have large voltage gain in both step-up and step-down operations by adjusting the turn ratio of the transformers. However, the yback converters have simple structure and easy control, the leakage-inductor energy cannot be recycled, the power switches of these converters suer from high-voltage stresses and the diodes at the secondary side of the converters have reverse recovery problem. The voltage clamp technique in these converters is used to reduce voltage stresses on the switches and recycle the leakage-inductor energy in order to increase eciency. The non-isolated types of these converters have been researched, which include the conventional boost/buck type, multilevel type, three-level type, sepic/zeta type, switched capacitor type and coupled inductor types. The multilevel types are magnetic less converters, but more switches are used in this converter. If voltage gain is needed to be higher in step-up mode and lower in step-down mode, more switches are required and also the control circuit of this converter would be more complicated. In the three-level type, step-up and step-down voltage gains are low. The converters with coupled inductors

   have also complicated configuration. This paper proposes a voltage controlled non isolated bidirectional DC-DC converter, which is having a high voltage gain. The closed loop systems are performed over open loop system for maintaining the operating conditions at desired values in presence of normal disturbances.

2. NON-ISOLATED BIDIRECTIONAL DC-DC CONVERTER

   Fig-1: Circuit Diagram

   A non-isolated bidirectional DC-DC converter is studied, which has simple structure and large voltage gain. It consists of two conventional boost converters. Four power switches are employed with their body diodes. In each direction, two of the switches are used as power switches and the others are used as the synchronous rectifiers. The input current is divided to the inductors which cause the size of them to become smaller. The operation principle of the converter is also discussed.

   Fig.1 shows the system conguration of the bidirectional converter, which has a capacitor, two inductors and four switch-diodes. Two of the switches work as power switches and the remainders are applied for the synchronous rectiers. The steady-state analysis of the bidirectional converter in step-up and step-down modes is discussed as follows. In order to analyze the steady-state characteristics of the bidirectional converter, the ON-state resistance (ON) of the switches and the equivalent series resistances of the inductors and capacitors are ignored and the voltages of the capacitors are constant.

3. OPERATING MODES

   1. Step-Down Mode Of The Converter

      The non-isolated bidirectional DC-DC converter in step- down mode is shown in g.2. In this operation mode, and work as power switches and and are the synchronous rectiers.

      Fig-2: Equivalent circuit in the step-down mode

      1. Mode 1[t0 t1]

         During this time interval , ], and are turned on and and are turned o. The current-ow paths of the proposed converter are shown in g.3. As seen in this gure, the energy of the DC source is transferred to inductor . Capacitor C is discharged to inductor and capacitor . The characteristic waveforms of the proposed converter in continuous conduction mode (CCM) are depicted in g.4. The following equations can be written in this mode:

         ….(1)

         =

         …(2)

         Fig-3: Mode 1 operation in step-down mode

         Fig-4: Theoretical waveforms

      2. Mode 2[t1 t2]

      During tis time interval , ], and are turned on and and are turned o. The current-ow paths of the suggested converter are shown in g.5. Inductor is demagnetized in this mode to capacitors C and . Inductor is discharged to capacitor and provides energy to the load. The characteristic waveforms of the proposed converter in continuous conduction mode (CCM) are depicted in g.4. Therefore, the voltages of and can be written as:

      .(8)

      = ………………………………………..(9)

      …………………………….(3)

      = ………………………………….(4)

      By applying volt-second balance principle on the inductor and , and then simplifying we get the following equations:

      .(5) (6)

      Substituting Eqn(5) into Eqn(6), the voltage gain of the proposed converter in step-down mode can be obtained as:

      (7)

      Fig-5: Mode 2 operation in step-down mode

   2. Step-Up Mode Of The Converter

   The non-isolated bidirectional DC-DC converter in step-up mode is shown in g.6. In this operation mode, and work as power switches and switches and are the synchronous rectiers.

   1. Mode 1[t0 t1]

      During the interval , ], and are turned on and and are turned o. As shown in g.7, in this interval the energy of the DC source is transferred to inductor . Inductor is magnetized by the DC source and the energy stored in capacitor C. Capacitor is also discharged to the load. The following equations can be obtained in this mode. The characteristic waveforms of the proposed converter in continuous conduction mode (CCM) are depicted in g.8. The following equations can be written in this mode.

      Fig-6: Equivalent circuit in the step-up mode

      Fig-7: Mode 1 operation in step-up mode

      Fig-8: Theoretical waveforms

   2. Mode 2[t1 t2]

   During the interval , ], and are turned o and and are turned on. As shown in g.9, capacitor C is charged by the input source and the energy stored in inductor . Capacitor is also charged by the input source . and the energy stored in inductor . Therefore, the voltages across the inductors can be written as:

   .(10)

   = ……(11)

   By applying volt-second balance principle on the inductor and , and then simplifying we get the following equations:

   (12)

   (13)

   Substituting Eqn(12) into Eqn(13), the voltage gain of the proposed converter in step-down mode can be obtained as:

   (14)

   Fig-9: Mode 2 operation in step-up mode

4. SIMULATION RESULT

   In order to justify the validity of the steady-state analysis, the simulation results of both step-up and step-down modes of voltage controlled non isolated bidirectional converter is included in this section. The specications of the circuit are given in table-1

   Table-1: Simulation Parameter

   Parameters	Specifications
   	25 V
   	250 V
   Switching Frequency	30 KHz
   	1.3 mH
   	500 mH
   ,	47µF, 220µF, 220µF
   	160 W,160 W

   1. Simulink Model

      The Simulink model of feedback control and gatepulse generation circuit for step-down mode is shown in fig-10. Simulink model of converter is shown in fig-11. Simulink models in step-up mode is shown in fig-12 and fig-13.

      Fig -10: Simulink model of feedback control and gate pulse generation

      Fig -11: Simulink model of converter in step-down mode

      In the step-down mode, the output voltage is xed at 2.5V and in step-up mode it is 25V. It is obtained by the closed loop control. The output value is compared with reference or set value. This dierence is called as an error. This error value is given to PI Controller. This controller is used to regulate the output voltage. The PI Controller output is given to the gate pulse generating circuit.

      .

      Fig -12: Simulink model of feedback control and gate pulse generation

      Fig -13: Simulink model of converter in step-up mode

   2. Simulation Results

   The voltage and current waveforms of electrical components of the converter in stepdown operation mode is shown below. Here the output voltage is xed at 2.5V, that is even if we vary the input voltage the output voltage does not changes.

   Fig -14: Switching pulse

   Fig-14 shows the switching pulses for the four switches. Fig- 15 shows the input voltage in step-down mode and it is 25V. Output voltage is shown in g-16. For an input voltage of 25V, output voltage is obtained as 2.5V. Fig-17 shows the current through the inductors and in step-down mode. The voltage waveforms of switches in step-down mode are shown in g-18.

   Fig -15: Input voltage

   Fig -16: Output voltage

   Fig -17: Inductor current

   As shown in the g-17, the current of inductors and are about 0.2A and 0.45A, respectively, in step-dwn mode. From the waveform of voltage stress == 25V, = equal to square root of

   and which is equal to 7.9V and is sum of and is equal to 32.9V are obtained.

   Fig -18: Voltage stress

   Fig-19 shows the input voltage in step-up mode and it is 2.5V. Output voltage is shown in g-20. For an input voltage of 2.5V, output voltage is obtained as 25V. Fig-21 shows the current through the inductors and in step-down mode.

   The voltage waveforms of switches in step-down mode are shown in g-22.

   Fig -19:Input voltage

   Fig -20:Output voltage

   Fig -21: Inductor current

   Fig -22: Voltage stress

5. EXPERIMENT SETUP AND RESULTS

   Fig -23: Experiment setup

   The experiment setup is shown in fig-22, IRFP260 and IRFP460 are used as switches. The controller used in the prototype is dsPIC30F2010. The hardware results are shown below, Output pulse from driver IC which is of 12V is shown in fig-24. Fig-25 shows the input and output voltage in the step-down mode. In the step-down mode a gain of 0.1 is obtained. Fig-26 shows the input and output voltage in the step-up mode and in this mode a gain of 5 is obtained.

   Fig-24: Output of TLP250

   Fig-27 shows the input and output voltage pairs for the voltage controlled converter in step-down mode. Here for dierent input voltages output voltage is constant. Fig-28 shows the voltage across the four switches in the order S1 to S4. Here voltage stress of Switches S2 and S3 are equal and it is less compared to voltage stress of S1 and S4. Due to this two types of switches are used in the hardware setup..

   Fig-25: Input and output voltage of step-down mode

   Fig-26: Input and output voltage of step-up mode

   Fig-27: Input and output voltage of voltage controlled converter in step-down mode with different input voltages

   .

   Fig-28: Voltage stress of switches

6. CONCLUSION

In this paper, a voltage controlled non isolated bidirectional DC-DC converter with high voltage gain is presented. Bidirectional converter is used for the battery charging purposes. The output of the converter is made xed by the method of feedback. The complete system is simulated in MATLAB/SIMULINK R2016 and hardware section of complete system is done with high and low side voltages 25V and 2.5V respectively. From the analysis, it is found that the proposed converter has higher eciency because input current is divided to the inductors. Voltage gain of the proposed converter in both step-down and step-up mode is more proper than the conventional bidirectional buck boost converter. But the stresses on the active switches are same. So this converter can be implemented in systems where a storage element is required. In order to charge and discharge the battery bidirectional converter is needed.

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