Implementing Buck Converter for Battery Charger Using ... - IEEE Xplore

2013 IEEE 7th International Power Engineering and Optimization Conference (PEOC02013). Langkawi. Malaysia. 3-4 June 2013

Implementing Buck Converter for Battery Charger Using Soft Switching Techniques 1 M. Salem, 2A. Jusoh, 3N. Rumzi N. Idris Department of power engineering Faculty of Electrical Engineering University Technology Malaysia Johor, Malaysia

[email protected] , [email protected], 3 [email protected]

Abstract- this paper describes the performance of the resonant

ON and OFF switching, and both can work at a high switching frequency compared to a PWM hard switching converter [6]. This paper describes the principle and operation of the buck converter using soft switching in battery charger applications.

buck converter battery charger using soft switching, which has been seen as a good and convincing solution for switched mode electronic power converter. The switching frequency of the power semiconductor devices has been increased in order to decrease the losses and the stress of the switching. As a result of that, the

II.

efficiency and the reliability of the buck converter obviously

AND PRINCIPLE OF THE BUCK

CONVERTER

increased. The proposed charger circuits with both zero voltage

The step-down voltage regulator or buck converter can be defined as a switch-mode dc-dc converter that uses switches and a low pass filter to reduce the voltage value of a DC supply. The simplicity and low cost are the advantages of using the buck converter, which is shown in figure 1.

switching and zero current switching were analyzed and tested using MATLAB-SIMULINK software for a 12V 4 Ah lead acid battery. The structure of the proposed circuits does not require complex control mechanisms and a large number of components.

Keywords- zero current switching, zero voltage switching,

L

Switch

battery charging, buck converter, soft switching.

I.

THE OPREATION

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INTRODUCTION D

Rechargeable batteries are an essential component of all electrical equipment and appliances. Since each appliance requires electric energy permanently, there is a need to design and develop charging circuits for batteries. In order to design an efficient battery charger, the charger circuit should satisfy the demands of fast charging, high power density, high discharge capability, long cycle life, low maintenance and cost [1]. It is well known that the value of charging time is a function of the electric power (KW) that the charger circuit can deliver to the battery and the battery size. Moreover, all efficient chargers' demands depend on the properties of their circuits. Conventional battery chargers are not suitable for all applications because their linear regulators can work only at low power levels. Therefore, they provide low power density and efficiency, which is the opposite of modern batteries that require high power density, lightweight and high quality [2]. Switching mode power converters for battery charging applications have become the main topic of many studies in recent years [3-5]. The advantage of using the switch-mode is the possibility of minimizing the conduction and switching losses by increasing the switching frequency. There are many switching topologies that can achieve higher power flow. These topologies can provide highly efficiencies by minimizing the switching losses and the overall size of the converter. Zero voltage switching (ZVS) and zero current switching (ZCS) are able to limit the switching losses during

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Fig. 1. The Buck converter circuit

Each component of the circuit performs a specific function. The main task of the inductor is to regulate the switching current, but it also reduces the ripple current value; higher inductor values produce less ripple current in order to obtain the maximum output current. Due to the fact that the current through the inductor does not change instantaneously, the current value will never fall to a zero value (continuous mode). A.

Operating Frequency

The performance of the switch is determined by the operating frequency value. On the other hand, the efficiency requirement is a typical way to decide the value of the switching frequency. In recent researches, there has been an obvious increase in the switching frequencies. To get a smaller physical size and component value, the switching frequency must be set to a higher level. The reason for the need to reduce the total size of the power supply is the miniaturization of electronic systems [7].

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2013 IEEE 7th International Power Engineering and Optimization Conference (PEOC02013), Langkawi, Malaysia. 3-4 June 2013

2 B.

Switching Device

makes the transition losses approximately zero. As a result of that, the efficiency will be apparently improved and so a ZVS would be appropriate for high frequency buck converter applications. Furthermore the ZVS buck converter operates in a single switching cycle through the following modes:

Making the right choice of device is an important issue in order to minimize losses and to guarantee the system works. The BJT is not currently popular in electronic systems; it is preferred in current amplifiers, and the difficult choice is between MOSFET and IGBT. The choice depends on the switching frequency, voltage and power level. According to the specifications of the most commonly available switches, the largest power capabilities are in the diode and thyristor, while in terms of speed the MOSFET has the fastest switching frequency [8]. At a switching frequency of 20 kHz, the IGBT can be seen as a good device, but the MOSFET can work at this frequency as well. The IGBT has low conduction losses, but the switching time is high compared to the MOSFET. The MOSFET has replaced others in many applications where high switching frequency is needed; at voltages (> 600-1000 V) the IGBT still preferred, but at high frequency (> 20-100 kHz) the MOSFET is the only device that can be used. C.

• • • •

B.

Mode I: Capacitor Charging Stage:(from to to tl) Mode II: Resonant Stage: (from t1 to t2) Mode III: Inductor Charging Stage: (from t2 to t3) Mode IV: Power Transfer Stage: (from t3 to t4 )

Zero Current Switching

The soft switching technique was used in 1980 in order to decrease the losses and stress of switching [10]. Consequently, there will be a significant increase in the efficiency and reliability of the converters, which leads to the performance of the battery charger being improved. The concept of a zero switching current can be sununarized as the moment that the switches tum on and off while the current is zero. The ZCS is able to overcome the current and voltage overlap by ensuring that the switch current is zero before the voltage rises, which makes the ZCS more effective and useful than the ZVS. As shown in Figure 3, the output L2, C2 filter is needed to produce a constant output voltage with fewer ripples. In the ZCS performance, the resonant components Lf, CI will form a series-resonant circuit while the switch is on, and the switch will act as if it is in on mode until the current falls to zero, which turns the switch and leads to the power losses being reduced [11]. However, backing the current to zero again can be guaranteed by ensuring the condition of Vin/ Z >10. Where the impedance is defined by Z v'LI/CI. On the other hand, the ZCS buck converter operates in a single switching cycle in the following modes:

Operation Modes

The circuit shown in the figure 1 operates in two conditions depending on whether the switch is on or off. In the on mode, the diode is reverse biased, which works as an open circuit. That will cause a voltage drop across the inductor which leads to the output voltage being the difference between the source voltage and the inductor voltage. In contrast, during the off mode, the current will flow from the stored energy in the inductor across the diode in a forward biased manner. In this case, the output voltage is the negative value of the inductor voltage until the switch is on again.

=

III. AN OVERVIEW OF THE SOFT SWITCHING TECHNIUE Many modifications have been suggested for the purpose of raising the value of the switching frequency. The most commonly-used method is to add a resonant circuit component LI,C!, as shown in figures 2 and 3. Furthermore, the resonant circuit will allow the electromagnetic interference resulting from di/dt and dv/dt to be limited [9-10]. Moreover, the resonant transient components will increase the possibility of having a Zero Current Switching (ZCS) or even a Zero Voltage Switching (ZVS). A.

• • •

•

Mode I: Current linear rising Stage:(from to to tl) Mode II: Resonant Stage: (from tl to t2) Mode III: Discharging the inductor current: (from t2 to t3) Mode IV: Capacitor Charging Stage: (from t3 to t4 ) IV.

A.

Zero Voltage Frequency

THE PROPOSED BUCK CONVERTER CHARGER

For Zero Voltage Frequency

In the ZVS technique the effective duty cycle must be adjusted which is performed by changing the conversion frequency so the output voltage can be regulated. This varies the effective on-time in a ZVS design. The basis of this conversion is the volt second product equating of the input and the output. The implementation of the zero voltage switching technique for the buck converter battery charger using Matlab/Simulink is shown in figure 2.

Theoretically, the concept of zero voltage switching can be considered, such as the performance of the fixed frequency conversion, which uses an adjustable duty cycle due to the fact that it uses a constant control which varies the conversion frequency during the off-state, and maintains the regulation of the output during the ON time with the resonant switching transition. This means that it is possible to change the effective ON-time in ZVS by adjusting the duty cycle, which can be performed by varying the conversion frequency [11]. The switching voltage reaches a peak value and drops to zero again during the off time. At this time, the switch will be reactivated because of the discharging caused by the output of the MOSFET switch Coss by the resonant tank. This does not contribute to the power loss or dissipation in the switch, which

189


3 Conliluous

Fig. 3. The configuration of the battery charger using ZCS Buck converter circuit So> ,.,

The filter components L, C were made high compared to the resonant components values in order to reduce the charging current ripple and output voltage ripple. Whereas the resonant inductance and capacitance can be calculated using the following formulas: 1 Wo (4) Zo L1

Fig. 2. The configuration of the ZVS Buck converter circuit

The condition that guarantees the resonant battery charger operates under ZVS is Zolo> Vin, which leads to 10LrWo> Vin. However, the output voltage can be calculated if the switching time modes are known as well as the value of the 10. Where: Impedance Zo

L = C

Resonant angular frequency Wo 1 Resonant frequency i1 = fT7< yLC

=

1

woZo

fT7< yLC

J

t2

t1

. [LTdt

i:

=

LT12 0[1- COS2Wo(t - t1)] 2 --2Vin

]

(5)

C1

(6) (2)

(7)

(3)

The normalized load resistor is r=Ro/Zo, and the relationship between the input voltage and the output voltage isX=Vo/Vin

If we assume that the system operates in a steady state, there is no dissipated power. This means that the relationship between the input and output voltages can be found by equality between the released and stored energy equations. Nonetheless, the output voltage depends on the output current, which means any variation in the output current will cause a variation in the switch voltage. However, the regulated voltage can be controlled by varying the angle based on the variation in the output current or input voltage. B.

1

Similarly, we assumed that the dissipation power is neglected as in the ZVS situation. Therefore, the relationship between the output voltage and input voltage can be determined by the equality the formulas of the input energy and the energy released.

The stored energy in the resonant inductor and the energy released to the battery can be calculated by the following equations, respectively: 2 Wi = Vin iLTdt + 3 iLT + 10 (Ts - t3 - t1) (1)

[i:

=

Vout vin

=

is {x -2ITi1 2r

-+

IT +

[]

. -1 X r

sm

+

[ R1) ]}

X - 1+ r

X 2 1- r

(8)

V. THE SIMULAnON RESULTS AND DISCUSSION The implemented models of the battery charger using resonant buck converter with zero voltage switching and zero current switching were simulated using Matlab/Simulink, and the charger is applied to a 12V, 4 Ah lead-acid battery. The circuit parameters are: • Input voltage = 24 V • Switching frequency = 18 kHz • Duty cycle = 25% • Resonant inductor = 50 f.lH • Resonant capacitor = 0.2 f.lF • Resonant frequency = 50 kHz

For Zero Current Switching

As we noted in previous sections, it is necessary to set the condition which is Vin/ Z >10 in the battery charger to ensure that the charger operates under the ZCS technique. Figure 3 shows the implementation of a battery charger using the ZCS buck converter.

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A.

The Results OjZVS Battery Charger 0.12 "9

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1

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(10).

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Fig. 6. The output voltage (Vo). B.

THE RESULTS OF ZCS BATTERY CHARGER vg

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Vg.

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Fig. 4-b. The wave [onns of the switch and voltage. o. 8

The signal in Figure 4-a, describe the gate signal of the switch, while figure 4-b shows the process of ZVS. The voltage switch rises to its peak value during the off-time, and falls to zero while the activation signal is on, unlike the state of the resonant current. As a result, the switch will be reactivated every moment the voltage drops to zero. Obviously, this makes the switching losses go to zero and increases the efficiency. Because of the switching frequency is much lower than the resonant frequency, which is hard to obtain the clear ZVS in this range. The signals of the charging current and voltage are shown in Figures 5 and 6. The state of charging and discharging of the battery depends on the capacity and voltage of the battery. If the full charge voltage was lower than the battery voltage, the charger would be disconnected.

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Fig. 7. Waveforms of the activation signals of the both switches.

The achievement of the ZCS switching for the charger circuit is illustrated in Figures 7 and 8. In addition, the current waveform becomes zero before the voltage starts to rise, which leads to a reduction in the current and voltage overlap, and eliminates the switching losses. This means that the ZCS is more effective than the ZVS in battery charger applications.

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components. For this reason, these chargers are considered cheaper and much simpler than others available circuitry. The results of using the chargers with lead acid battery were simulated using Matlab/Simulink software. The results demonstrated that both ZVS and ZCS are suitable for achieving good performance of active power switches for battery charger applications.

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References

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13.492

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Fig. 8. The wave forms of the resonant current (iL),resonant voltage (Ve).

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The results of the output filter that provide the charging current and output voltage are described in Figures 9 and 10. Whereas, the dc charging current in the ZCS is larger than the current in the ZVS case, it is clear that the ZCS technique is more suitable to increase the efficiency and the reliability of the battery chargers. 4.61

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-" -" cc :::C 8 :::C 96o----=b:19�97 -:c :----=-: c-: . ,7: � '9:C: C:: --: :-C .' 2 1 9:-C 0 ."" 19 93 0 .:7 9 4997 5 0.' 9 0.:7 97:---:0 0 99 99::-9 0. ' =-= Fig. 9.The charging current (10).

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---:' 0.2

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13.095 13.094 13.093 13.092 13.091 13.09

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Fig. 10. The output voltage (Vo). VI.

CONCLUSION

A comparison between buck converter battery chargers using ZCS and ZVS has been presented in this paper. In brief, the implemented circuits proved the possibility of increasing the efficiency and decreasing the losses, which means these circuits' performances are matched on the theoretical side. The structures of neither circuit require complex control mechanisms or a large number of

192