Zeta Converter with Improved Voltage Conversion Gain

Zeta Converter with Improved Voltage Conversion Gain

Anson K A

UG Student

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

Fidha Hashmi K P

UG Student

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

Harikrishna Jothis Jacob

UG Student UG Student

Electrical and Electronics Engineering Electrical and Electronics Engineering

Mar Athanasius College of Engineering Mar Athanasius College of Engineering Kerala, India Kerala, India

Sera Mathew

Assistant Professor Electrical and Electronics Engineering

Mar Athanasius College of Engineering Kerala, India

AbstractDemand for electricity is increasing day by day. However, due to the shortage of fossil fuels as well as their huge burden of environmental pollution, the proliferation of renewable energy systems is increasing. DC-DC converters are very popular and important because of the need for step up of the low output voltage of most of these renewable energy systems. The traditional Zeta converter among other DC-DC converters has many benefits like low output voltage ripple and can be used to step up as well as step down the input voltage. It is difficult to apply the traditional Zeta converter in renewable energy systems as the voltage step up capacity of the traditional converter is limited at high duty cycle due to the negative influence of parasitic parameters from the components. A single switch zeta converter with improved voltage conversion ratio is used to address the issue. The converter not only achieves high voltage gain based on our requirement but also has considerably lower voltage stress across the switch. The voltage multiplier circuit is composed of passive components which ensures a continuous output current like the traditional Zeta converter. The drive and control circuits are simple and remain the same as the converter has only one active switch. The performance study is carried out with MATLAB/SIMULINK and simulation results are obtained.

KeywordsZeta converter, switched capacitor, improved voltage conversion gain, DC-DC converter

1. INTRODUCTION

   The use of electricity is increasing day by day with an ever- increasing population. This energy demand will be co- accelerated by the rapid industrial growth. This energy crisis, depletion of fossil fuels, an alarmingly rising level of environmental pollution and ever-increasing population are the primary reasons for stressing to switch over to renewable

   energy sources [1]. About 75% of the total CO2 emissions in the world is contributed by the power systems energy production sector. Thus, the UN is urging all nations to go green [2]. The non-conventional energy resources like photovoltaic (PV) modules, small wind turbines and fuel cells generate low dc voltage and need to be stepped up to the grid requirement voltage.

   A DC-DC converter is an electronic circuit that converts a source of DC from one voltage to another voltage. There are different types of traditional DC-DC converters like buck, boost, buck-boost, Cuk, SEPIC and Zeta converters. These are widely used for application in both industrial and commercial sectors. The increase in the use of renewable energy sources has increased the demand for high gain DC-DC power converters that do not compromise efficiency [3].

   Zeta converter is a fourth-order DC-DC converter that works as a buck-boost converter with a non-inverted output. It is also known as an inverting SEPIC converter which is made up of two inductors and two capacitors capable of operating in either step-up or step-down mode. However, unlike the buck-boost converter, it does not give an inverted output and has the same polarity as the input. The benefits of the Zeta converter over the SEPIC converter includes low output voltage ripple as well as easier compensation [3].

   In ideal conditions, by adjusting the duty cycle of the switch, any voltage conversion gain can be achieved. However, in actual working conditions, the boost capacity of these traditional converters is greatly limited by parasitic elements of the circuit and voltage drops across the switch and diodes. The conduction losses across the switch increase with higher duty

   Fig. 2. Equivalent circuit of converter in Mode 1

   Fig. 2. Equivalent circuit of converter in Mode 1

   cycles. This makes the application of the converters difficult in situations where a large voltage conversion ratio is required, especially in renewable energy systems. To achieve a high voltage conversion ratio, a family of generalized passive voltage multiplier circuits termed coat circuits are employed [4]. These circuits are termed coat circuits for DC-DC converters because such circuits surround the DC-DC converters like a coat. In addition to increasing voltage gain, these coat circuits also reduce the voltage stress across the switch comparatively [8].

   A higher voltage gain can be achieved by employing a switched capacitor circuit in the traditional zeta converter. This switched capacitor is formed by splitting the energy transfer capacitor in the traditional zeta converter into two capacitors [5]. The advantage of this topology is that there is only one active switch [6]. The voltage boosting techniques presented in [6]-[7] either use transformers or coupled inductors which increase the losses in the converter.

2. PROPOSED CONVERTER

   The proposed zeta converter is shown in Figure 1. The converter consists of a single switch S1, four diodes Dc, D1, D11, D12, four inductors L1, L2, L11, L21, seven capacitors C1, Cc, C11, C21, C2, C12 and output capacitor C22. Capacitors C1 and

   Fig. 1. Proposed converter

   Cc and diodes D1 and Dc together constitute a switched capacitor combination. Capacitors C11 and C12, inductor L11 together with diode D11 constitute a single unit of the voltage multiplier circuit. To perform analysis of the converter, a converter with two units of the voltage multiplier circuit is considered.

   Some assumptions are considered during the analysis of the converter. They are as follows:

   1. The capacitances of all capacitors are assumed to be large enough that the effects of voltage ripple across them are negligible.

   2. All devices are assumed to be ideal, i.e , the influence of parasitic parameters is negligible

3. MODES OF OPERATION

   Since the proposed converter has only one active switch, there are only two operating modes in continuous conduction mode. These modes of operation are shown in Figure 2. Mode 1 is between the time interval (t0 < t < t1).

   Fig. 2. Theoretical Waveforms

   Mode 2 is between the time interval (t1 < t < t2). The modes of operation are as follows:

   Mode 1:

   Fig. 3. Equivalent circuit of converter in Mode 1

   In this mode, switch S1 is turned ON and all diodes work in the OFF state. During this stage, the input voltage source charges the inductor L1. The capacitor C1 and Cc discharge into the inductor L2 and the capacitor C2. The capacitor C11 and C1 discharge into the inductor L11 and the capacitor C12. Similarly, the capacitor C21, C11 and C1 discharge into the inductor L21 and the capacitor C22. Therefore, all inductor currents are found to increase linearly. In this interval, C2, C12 and C22 are charging while, C1, CC, C11 and C21 are discharging. The equivalent circuit of Mode 1 is presented in Figure 3.

   The equations of Mode 1 are as follows:

   VL1 = Vin

   VL2 = Vin + VC1 + VCc VC2

   VL11 = Vin + VC11 + VCc VC12

   VL21 = Vn + VC21 + VCc VC22

   Mode 2:

   Fig. 4. Equivalent circuit of converter in Mode 2

   From equation (21) it is evident that the converter can be designed as per the voltage requirement.

4. DESIGN CONSIDERATIONS

   The input voltage is taken as 48V. The pulses are switched at a frequency of 100 kHz. The output power is taken to be 350W.

   Voltage conversion gain, M = Vo/Vin = 4D/(1-D)

   where D is the duty ratio of the switch S1. D is taken to be 0.74.

   1. Inductor Design

      The ripple current for the inductor L1,

      L1 < 204% of Io = 1.35 A

      Similarly, the ripple currents of the inductors L2, L11, L21

      In this mode, switch S1 is turned OFF and all diodes work in the ON state. During this stage, the inductor L1 discharges into

      L2 = L11

      = L21

      < 52% of Io = 0.35 A

      capacitor C1 and Cc. The inductors L2 and L11 discharge into

      capacitors C11 and C21 through D11 and D21 respectively. The inductor L21 discharges through the output stage (C22/RL). In this interval, C1, Cc, C11 and C21 are charging while C2, C21 and C22 are discharging. The equivalent circuit of Mode 2 is presented in Figure 4.

      The equations of Mode 2 are as follows:

      VL1 = -VC1 = -VCc

      VL2 = -VC11 = -VC2 – VCc VL11 = -VC21 = VC11 – VC12 = VC2 + VCc – VC12 VL21 = VC21 – VC22 = VC12 VC11 – VC22

      VL21= VC12 VC2 VCc – VC22

      On applying the volt-second balance principle on the inductors L1, L2, L11 and L21 and considering the above analysis of the operation of the converter, we get the following equations.

      Vin D = VC1 (1-D) = VCc (1-D) (Vin VC2 + VC1 + VCc) D = VC11(1-D) (Vin VC2 + VC1 + VCc) D = (VC2 + VCc)(1-D) (Vin + VC11 + VCc VC12) D = VC21(1-D) (Vin + VC11 + VCc VC12) D = (VC12 – VC11)(1-D)

      (Vin + VC11 + VCc VC12) D = (VC12 – VC2 – VCc )(1-D)

      Vin + VC21 + VCc VC22) D = (VC22 – VC21)(1-D)

      Vin + VC21 + VCc VC22) D = (VC22 + VC11 – VC12)(1-D) (18)

      Vin + VC21 + VCc VC22) D = (VC22 + VCc + VC2 – VC12)(1-D) (19)

      The voltage conversion gain can be obtained from the above equations. It is shown in equation (20)

      M = Vo/Vin = 4D/(1-D) (20)

      This converter can be generalized to contain n basic units of the voltage multiplier circuit. The generalized equation is as follows.

      L1 = VinDT L1 263.11 µH (22) L2,11,21 = VinDT L2,11,21 896.96 µH (23)

      To keep the ripple currents well within range, the value of the inductors is taken as L1 = 300 µH and L2,11,21 = 950 µH.

   2. Capacitor Design

   The voltage ripple for the capacitors C1, Cc, C2, C11, C21

   VC1,Cc,C2,C11,C21 < 5% of VC1,Cc,C2,C11,C21 = 6.83 V

   The voltage ripple for the capacitor C12

   VC12 < 5 % of VC12 = 20.49 V

   The voltage ripple for the output capacitor C22

   VC12 < 0.5 % of VC12 = 2.645 V

   C1 = Cc = 3IoDT/VC1,Cc 2.157 µF (24)

   C11 = 2IoDT/VC11 1.483 µF (25)

   C2 = C21 = IoDT/VC2,C21 0.719 µF (26)

   C12 = IoDT/VC12 0.283 µF (27)

   C12 = IoDT/VC22 1.849 µF (28)

   Considering the uniformity of devices and to keep the voltage ripples well within limits, the value of the capacitors.

   C1 = C2 = Cc = C11 = C12 = C21 = C22 = 4 µF

5. SIMULATION RESULTS TABLE I. SIMULATION PARAMETERS

   Parameters	Specification
   Input voltage Vin	48 V
   Switching frequency fs	100 kHz
   Output voltage Vo	529 V
   Inductor L1	300 µH
   Inductors L2, L11, L21	950 µH
   Capacitors Cc,C1,C2,C11,C12,C21,C22	4 µF
   Load resistance RL	800

   Parameters	Specification
   Input voltage Vin	48 V
   Switching frequency fs	100 kHz
   Output voltage Vo	529 V
   Inductor L1	300 µH
   Inductors L2, L11, L21	950 µH
   Capacitors Cc,C1,C2,C11,C12,C21,C22	4 µF
   Load resistance RL	800

   M = Vo/Vin = (n+2)D/(1-D) (21)

   Fig. 5. Simulink Model

   The simulation parameters of the converter are shown in Table 1. A converter with two basic units of the voltage multiplier circuit is simulated. An input voltage Vin of 48 V gives an output voltage Vo of 529 V for an output power Po of 350W. The pulses are switched at a frequency of 100 kHz . The duty ratio is taken as 0.74. The converter is simulated in MATLAB/SIMULINK by using the mentioned parameters. The Simulink model is shown in Figure 5.

   The simulation results of the converter are shown in the following figures. It can be seen from Figure 6. (a) and 6. (b) that the input voltage Vin = 48V and the output voltage is about 529 V. The output voltage has a ripple of 0.017% which is very low and one of the characteristics of the zeta converter.

   Fig. 6. (a) Input voltage Vin (b) Output voltage Vo

   Fig. 7. (a) Input current Iin (b) Output current Io

   Fig. 8. Inductor currents (a) IL1 (b) IL2 (c) IL11 (d) IL21

   Figures 7. (a) and 7. (b) represent the input and output current waveforms. The peak input current passing through the switch is equal to 12.67 A and the peak output current is equal to 0.661 A. Figures 8. (a), 8. (b), 8. (c) and 8. (d) are the inductor current waveforms. It can be observed that the current ripples are well within the designed limits. The voltage waveforms of the capacitors C2, C12 and C22 are shown in Figure 9.

   Fig. 9. Capacitor voltages (a) VC2 (b) VC12 (c) VC22

   Fig. 10. Capacitor voltages (a) VC1 (b) VCc (c) VC11 (d) VC21 Fig. 12. Diode voltages (a) VD1 (b) VDc (c) VD11 (d) VD21

   The voltages across capacitors C1, Cc, C11 and C21 are -131.6 V,

   -131.6 V, -130.38 V and -130.32 V respectively and are shown in Figure 10. It can be observed that the voltage ripples of the capacitors are well within the designed limits.

   The voltage across the switch S1 is equal to 183.68 V and is shown in Figure 11. (a). This voltage stress across the switch is low considering the large increase in output voltage. Figure.

   1. (b) shows waveform of the switching pulses.

      Figure 12. shows the voltages across the diodes D1, Dc, D11 and D21. They are 181.8 V, 181.8 V, 182.9 V and 182.7 V

      respectively.

      Fig. 11. (a)Voltage across switch VS1 (b) Switching pulse Vgs

6. ANALYSIS

   The analysis of the proposed converter is performed by considering various parameters like efficiency, duty ratio and output voltage ripple.

   From the plot of efficiency vs output power, the converter is found to have a maximum efficiency of 88.5% at a power of 864 W.

   Fig. 13. Efficiency Vs Output Power

   On performing the analysis of voltage gain vs duty ratio, the plot in Figure 14 is obtained. The voltage gain is found to

   Fig. 14. Voltage gain Vs Duty ratio

   increase with increase in duty ratio. However large duty ratios can result in larger conduction losses, thus reducing the efficiency of the converter. Therefore, large duty ratios are avoided.

   From Figure 15, it is observed that the output voltage ripple decreases with increase in the duty ratio. However, increasing the duty ratio to very high values is not practical as conduction losses increase.

   Fig. 15. Voltage ripple vs Duty ratio

   Based on these and other parameters, the proposed converter is compared to the traditional zeta converter as well as the zeta converter with only voltage multiplier circuits. The comparison is given in Table II.

   Parameters	Conventional	Coat circuit	Proposed
   Efficiency	82.9 %	85.15 %	88.5 %
   Output voltage ripple	0.22 (0.165%)	0.1 (0.025%)	0.09 (0.017%)
   Gain	2.77	8.33	11.02
   Input current	11.604 A	11.825 A	8.592 A
   Voltage stress of switch	184 V	184 V	183 V

   Parameters	Conventional	Coat circuit	Proposed
   Efficiency	82.9 %	85.15 %	88.5 %
   Output voltage ripple	0.22 (0.165%)	0.1 (0.025%)	0.09 (0.017%)
   Gain	2.77	8.33	11.02
   Input current	11.604 A	11.825 A	8.592 A
   Voltage stress of switch	184 V	184 V	183 V

   TABLE II. COMPARISON

7. CONCLUSION

   The proposed converter offers improved voltage conversion gain and low output voltage ripple. This is achieved by the addition of a switched capacitor circuit as well as voltage multiplier circuits. On analysis and simulation, the proposed converter is found to have high voltage conversion gain. The number of basic cells of the voltage multiplier circuit can be adjusted based on different applications. Since the proposed converter has only a single active switch, the driver and control circuit are uncomplicated to design and cost-effective to implement. The converter helps avoid extreme duty cycles. These characteristics show that the converter is suitable for applications where a wide voltage conversion ratio is required.

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