Transformerless Buck-Boost Converter with Positive Output Voltage and Feedback

Transformerless Buck-Boost Converter with Positive Output Voltage and Feedback

Aleena Paul K

PG Student

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

Babu Paul

Professor

Siny Paul

Associate Professor Electrical and Electronics Engineering Mar Athanasius College of Engineering

Kerala, India

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

Abstract A transformerless buck-boost converter with simple structure is obtained by inserting an additional switched network into the traditional buck-boost converter. Compared with the traditional buck-boost converter, its voltage gain is quadratic of the traditional buck-boost converter. It can operate in a wide range of output voltage, that is, the proposed buck- boost converter can achieve high or low voltage gain without extreme duty cycle. Moreover, the output voltage of this transformerless buck-boost converter is common-ground with the input voltage, and its polarity is positive. The two power switches of the buck-boost converter operate synchronously. The operating principles of the buck-boost converter operating in continuous conduction modes are presented. A new buck- boost converter is presented by providing a feedback to the converter. By this, constant output voltage can be maintained under varying load conditions in both buck and boost operation. The PSIM(POWER SIM) simulations are provided to compare and validate the effectiveness of the buck-boost converters. A prototypecircuit is constructed. Microprocessor dsPIC30f2010 is used to generate the control pulses.

Keywords BLDC (Brushless DC), Discontinuous Inductor Current Mode (DCM), Voltage Source Inverter (VSI)

I. INTRODUCTION

Switching mode power supply is the core of modern power conversion technology, which is widely used in electric power, communication system, household appliance, indus- trial device, railway, aviation and many other fields. As the basis of switching mode power supply, converter topologies attract a great deal of attention and many converter topologies have been proposed. Buck converter and boost converter have the simple structure and high efficiency. However, due to the limited voltage gain, their applications are restricted when the low or high output voltage are needed. The voltage bucking/boosting converters, which can regulate output voltage under wider range of input voltage or load variations, are popular with the applications such as portable electronic devices, car electronic devices, etc. The traditional buck- boost converter with simple structure and high efficiency, as we all know, has the drawbacks such as limited voltage gain, negative output voltage, oating power switch, meanwhile dis-

continuous input and output currents. The other three basic non-isolated converters, Cuk converter, Sepic converter and Zeta converter which also have the peculiarity to step-up and step-down voltage, have been provided. However, the limits of the voltage gain along with other disadvantages in Cuk, Sepic, and Zeta converters are also nonignorable.

Typical PWM DC-DC converters include the well-known buck, boost, buck-boost, Cuk, Zeta, and Sepic. With proper reconfiguration, these converters can be represented in terms of either buck or boost converter and linear devices, thus, the buck and boost converters are named BCUs[2]. The PWM converters are, consequently, categorized into buck and boost families. With this categorization, the small signal models of these converters are readily derived in terms of h parameter (for buck family) and g parameter (for boost family).Using the proposed approach, not only can one find a general configuration for converters in a family, but one can yield the same small-signal models as those derived from the direct state-space averaging method. Additionally, modeling of quasi-resonant converters and multi resonant converters can be simplified by adopting this approach[2].

Interleaved non-isolated high step-up DC/DC converter consists of two basic boost cells and some diode-capacitor multiplier (DCM) cells as needed. Because of the DCM cells, the voltage conversion ratio is enlarged and the extreme large duty ratio can be avoided in the high step-up applications. Moreover, the voltage stress of all the power devices is greatly lower than the output voltage. As a result, lower- voltage-rated power devices can be employed, and higher efficiency can be expected. Since the two basic Boost cells are controlled by the interleaving method, which means the phase difference between the two pulse width modulation (PWM) signals is 180 and the input current is the sums of the two inductor currents, the input current ripple is decreased and the size of the input filter could be reduced, which make it a suitable choice in the photovoltaic power generation system and hybrid electric vehicles, etc. But their operating mode, converter structure and control strategy are complicated[4].

The transformerless buck-boost converter is obtained by inserting an additional switched network into the traditional buck-boost converter. The main merit of the proposed buck- boost converter is that its voltage gain is quadratic of the traditional buck-boost converter so that it can operate in a wide range of output voltage, that is, the proposed buck-boost converter can achieve high or low voltage gain without extreme duty cycle. Moreover, the output voltage of this new transformerless buck-boost converter is common-ground with the input voltage, and its polarity is positive[1].

This paper proposes a new transformerless buck boost converter with a feedback to obtain constant output voltage regardless of varying load conditions. And it works with simple operating modes. The complete system is simulated in PSIM and hardware section of the converter is done.

1. TRANSFORMERLESS BUCK-BOOST CONVERTER WITH POSITIVE OUTPUT VOLTAGE AND FEEDBACK

   Fig-1: Proposed converter

   A new transformerless buck-boost converter is obtained by inserting an additional switched network into the traditional buck-boost converter. The main merit of the proposed buck- boost converter is that its voltage gain is quadratic of the traditional buck-boost converter so that it can operate in a wide range of output voltage, that is, the proposed buck-boost converter can achieve high or low voltage gain without extreme duty cycle. Moreover, the output voltage of this new transformerless buck-boost converter is common-ground with the input voltage, and its polarity is positive.

   1. Converter Structure

      The circuit configuration of the new transformerless buck- boost converter is shown in fig-1. It consists of two power switches (S1 and S2), two diodes (D1 and DO), two inductors (L1 and L2), two capacitors (C1 and Co), and one resistive load

      R. Power switches S1 and S2 are controlled synchronously. According to the state of the power switches and diodes, some typical time-domain waveforms for this new transformerless buck-boost converter operating in CCM are

      displayed in fig- 2, and the possible operation states for the proposed buck-boost converter are shown in figures 3 and 4. Figure 3, it denotes that the power switches S1 and S2 are turned on whereas the diodes D1 and DO do not conduct. Consequently, both the inductor L1 and the inductor L2 are magnetized, and both the charge pump capacitor C1 and the output capacitor CO are discharged. Figure 4, it describes that the power switches S1 and S2 are turned off while the diodes D1 and DO conduct for its forward biased voltage. Hence,

      bth the inductor L1 and the inductor L2 are demagnetized, and both the charge pump capacitor C1 and the output capacitor CO are charged.

   2. Operating Principles

   As shown in fig-2, there are two modes, that is, mode 1 and mode 2, in the new transformerless buck-boost converter when it operates in CCM operation. Mode 1 between time interval (NT<t<(N+D)T). Mode 2 between time interval ((N+D)T<t<(N+1)T).

   Fig-2: Typical Time-Domain Waveforms for the Buck-Boost Converter Operating in CCM.

   • Mode 1(NT<t<(N+D)T)

     Mode 1 is during the time interval (NT<t<(N+D)T). During this time interval, the switches S1 and S2 are turned on, while D1 and DO are reverse biased. From fig-3, it is seen that L1 is magnetized from the input voltage Vin while L2 is magnetized from the input voltage Vin and the charge pump capacitor C1. Also, the output energy is supplied from the output capacitor CO. Thus, the corresponding equations can be established as,

     VL1= Vin……………………………………..(1)

     VL2= Vin + VC1…………………………….(2)

     Fig-3: Equivalent circuit of the buck-boost converter in mode 1

   • Mode 2[t1 t3] ((N+D)T<t<(N+1)T)

   State 2 is during the time interval ((N+D)T<t<(N+1)T). During this time interval, the switches S1 and S2 are turned off, while D1 and DO are forward biased. From fig- 4, it is seen that the energy stored in the inductor L1 is released to the charge pump capacitor C1 via the diode D1. At the same time, the energy stored in the inductor L2 is released to the charge pump capacitor C1, the output capacitor CO and the resistive load R via the diodes DO and D1. The equations of the state 2 are described as follows

   VL1= -VC1……………………………………………..(3)

   VL2= -(VC1+VO)……………………………………(4)

   Fig-4: Equivalent circuits of the buck-boost converter in mode 2.

   If applying the voltage-second balance principle on the inductor L1, then the voltage across the charge pump capacitor C1 is readily obtained from equations (1) and (3) as

   VC1={D/(1-D) }Vin………………………………..(5)

   Here, D is the duty cycle, which represents the proportion of the power switches turn on time to the whole switching cycle. Similarly, by using the voltage-second balance principle on the inductor L2, the voltage gain of the proposed buck-boost converter can be obtained from equations (2), (4), and (5) as

   M=VO/Vin = (D/(1-D))2……………………………..(6)

   From equation (6), it is apparent that the proposed buck-boost converter can step-up the input voltage when the duty cycle is bigger than 0.5, and step-down the input voltage when the duty cycle is smaller than 0.5.

2. SIMULATION MODEL AND RESULTS

   The circuit of the new transformerless buck-boost converter is simulated using the PSIM software to confirm the aforementioned analyses. Circuit parameters chosen are shown in the table.

   Table-1: Simulation Parameter

   Parameter	Value
   Vin	18V
   fs	20kHz
   D	0.4 – 0.6
   L1	1mH
   L2	3mH
   C1	10µF
   C2	20µF

   1. Simulation Model

      Fig-5 shows the image of simulation circuit of the new transformerless buck-boost converter. It consists of two power switches (S1 and S2), two diodes (D1 and Do), two inductors (L1 and L2), two capacitors (C1 and Co), and one resistive load R. Power switches S1 and S2 are controlled synchronously.

      Fig-5: PSIM Model of Transformerless Buck-Boost Converter with Feedback

   2. Simulation Results

   Fig-6 shows the time-domain waveforms of the output voltage VOUT, the charge pump capacitor voltage VC1 and the driving signal VSIG.

   Fig-6: PSIM simulations for the buck-boost converter operating in step-up mode

   Fig-7: PSIM simulations for the buck-boost converter operating in step-up mode

   Fig-7 shows the currents of the two inductors L1 and L2, and the driving signal VSIG for the new transformerless buck- boost converter operating in step-up mode when the duty cycle is 0.6. Since the two power switches conduct synchronously, only one driving signal VSIG is chose. From fig-7, one can obtain that the charge pump capacitor voltage VC1 is within (25.8V, 27.5V), the output voltage VO is within (40.4V, 40.1V), the inductor current IL1 is within (0.07A, 0.3A), and the inductor current IL2 is within (0.36A, 0.52A). Also, the ripples of the inductor current IL1 and the inductor current IL2 are 0.23A and 0.16A, respectively. Additionally, the ripples of the two capacitors VC1 and VCO are 1.7V and 0.3V, respectively.

   From the design equations[1] the theoretical results are VC1=27V, VOUT =40.5V, IL1=0.34A, IL2=0.68A, IL1=0.54A, IL2=0.45A, VC1=2V, VCO=0.4V, respectively.

   For the proposed buck-boost converter operating in step- down mode when the duty cycle is choosing as 0.4. Fig-8 displays the time-domain waveforms of the output voltage VOUT , the charge pump capacitor voltage VC1 and the driving signal VSIG

   Fig-9 shows the currents of the two inductors L1 and L2, and the driving signal VSIG. It is clearly seen that the charge pump capacitor voltage VC1, the output voltage VOUT, the inductor current IL1, and the inductor current IL2 are within (11.6V,12.32V), (7.77V, 8.00V), (-0.27A, 0.03A) and(0.36A,

   0.52A), respectively. Also, the ripples of the inductor current 4IL1 and the inductor current 4IL2 are 0.3A and 0.16A, respectively. And, the ripples of the two capacitors VC1 and VCO are 0.72V and 0.23V, respectively. Similarly, the theoretical calculations from the design equations are VC1=12V, VOUT=8V, IL1=-0.15A, IL2=0.44A, IL1=0.36A, IL2=0.2A, VC1=0.89V, VCO=0.27V, separately.

   Fig-8: PSIM simulations for the buck-boost converter operating in step-down mode

   Fig-9: PSIM simulations for the buck-boost converter operating in step-down mode

   Table-2: Comparison between the converters

   	Transformerless Buck-Boost Converter	Transformerless Buck-Boost Converter with Feedback
   No. of switches	2	2
   No. of diodes	2	2
   No. of inductors	2	2
   No. of capacitors	2	2
   Output voltage ripple (Buck mode)	±0.135V	±0.115V
   Output voltage ripple (Boost mode)	±0.2V	±0.15V

   Table 2 shows the comparison between the two converters, transformerless buck-boost converter[1] and transformerless buck-boost converter with feedback, output voltage ripple is decreased by 55 percentage in the boost mode and 14.8 percentage in the buck mode.

3. EXPERIMENT SETUP AND RESULTS Hardware setup is done in a Printed Circuit Board (PCB). Control circuit and power circuit are implemented in two PCBs. Here dsPIC30F2010 is used for generating a pulse of constant switching frequency and duty ratios. The components list for the hardware is given in table 3.

   Table-3: Prototype Components

   Components	Specification
   Input Voltage	12V
   Output Voltage	40V/8V
   Switching Frequency	20kHz
   Diode	Byq28e200e
   MOSFET	IRF840
   Inductors(L1 & L2)	1mH & 3mH
   Capacitor(C1)	10µF
   Output Capacitor(CO)	20µF
   Controller	dsPIC30F2010
   Driver IC	TLP250

   Hardware setup is done i.e the converter section. Experimental setup is shown in fig-10.Sections in the hardware is rounded and marked separately.

   1. Converter without feedback

      Pulse for buck operation is shown in fig-11(a). Pulse for boost operation is shown in fig-11(b). The frequency is 20kHz.

      The output voltage of the transformerless buck boost converter varies with changing load. The load is varied using rheostat. Load change from 20 to 40 ohm is provided in the buck mode. The voltage varies from 6.45V to 7.5V. The output voltage of

      Fig -10: Experimental set up

      Fig -11: (a)Pulse for buck operation D=0.4(b)Pulse for boost operation D=0.6

      the converter in buck operation is shown in fig-12. Figure 12(a),(b),(c) respectively shows the output voltage for load 30,20 and 40.

      Fig -12: Output Voltage varying with load -buck operation

      The output voltage of the converter in boost operation is shown in fig-13. Load change from 120 to 180 ohm is provided in the boost mode. And the voltage varies from 21V to 21.5V.Figure 13(a),(b),(c) respectively shows the output voltage for load 150, 120 and 180.

      Fig -13: Output Voltage varying with load -boost operation

   2. Converter with feedback

   A feedback is provided to the transformerless buck boost converter. So that the output voltage remains constant irrespective of load conditions. Rheostat is provided as the load.

   Fig -14: Output Voltage constant -buck operation

   Output voltage for the buck operation is shown in fig- 14. Figure 14(a),(b),(c) respectively shows the output voltage for load 30, 20 and 40. From the figure it is clear that the output voltage is constant irrespective of the load change. Output voltage is 7.55V

   Output voltage for the boost operation is shown in fig-15. Figure 15(a),(b),(c) respectively shows the output voltage for load 150 ,120 and 180 . From the figure it is clear that the output voltage is constant irrespective of the load change. The output voltage is 16.7V

   Fig -15: Output Voltage constant -boost operation

4. CONCLUSION

Transformerless buck-boost converter is simulated using PSIM and analyzed. It is obtained by inserting an additional switched network into the traditional buck-boost converter. Transformerless buck-boost converter possesses the merits such as high step-up and step-down voltage gain, positive output voltage, simple construction and simple control strategy. Hence, the proposed buck-boost converter is suitable for the industrial applications requiring high step-up or step-down voltage gain. The converter operate in a wide range of output voltage without using extreme duty cycles. It provides enough gain within the duty ratio 0.4-0.6. It has simple operating modes. In order to make the output voltage constant irrespective of load conditions a feedback is provided.

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