Keywords
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| Passive auxiliary circuit, synchronous converter, zero-voltage switching (ZVS), zero-voltage transition (ZVT), feed forward control, PWM buck converters. |
INTRODUCTION
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| The field of Power Electronics is very broad and contains components from several disciplines of electrical engineering. Most Power Electronic systems can be simplified into three general components thesource, converter, and load. The source provides the input energy and the load uses that energy to perform the desired task. The converter, being central to the energy flow, can be one of the best places to reduce unwanted losses. The ideal converter does not have any losses and the power in is equal to the power out. In any real converter this is not the case of course and there are losses. Reducing this loss to a minimum is necessary to have a high level of efficiency. |
| Current trends in consumer electronics demand progressively lower voltage supplies. Portable electronic equipment, such as laptop computers, cellular phones, and future microprocessor and memory chips, require low-power circuitry to maximize the battery run time.However, higher input voltages and lower output voltages have brought about very lowduty cycles, increasing switching losses, and decreasing conversion efficiency. So earlier,asoft-switching active snubber was proposed to reduce the turn-off losses of the switches in abuck converter. The soft-switching snubber provides zero- voltage switching for the switch,thereby reducing its high turn- off losses. The proposed snubber uses an auxiliary switch to discharge the snubber capacitor. But the use of switches in the active snubber circuit againresulted in producing some switching losses during the transition period of the switches. So later on, the efficiency of the synchronous buck converter is optimized by eliminating theswitching losses using a soft switching technique with the help of a passive snubber . The voltage-mode soft-switching method that has attracted the most interest in recent years isthe zero-voltage transition (ZVT). This is because of its low additional conduction lossesand because its operation is closest to the pulse width modulated (PWM) converters. Theauxiliary circuit of the ZVT converters is activated just before the main switch is turned on and ceases after it is accomplished. |
| Passive snubbers are generally used to reduce other important losses, i.e., switching losses. Moreover, they provide tolerable voltage and current stress across semiconductor switches used in converter-mitigated electromagnetic interference. Much research has already been done using passive snubbers in rectifier and boost topologies emphasizing on the energy recovery and efficiency properties. In addition, it has been reported that the passive circuits are cheaper and more reliable and had a higher performance/cost ratio than the active ones.The proposed circuit not only serves as an energy recovery turn-on snubber, which greatly reduces the reverse-recovery peak current of the power boost diode and the turn-on loss of the boost switch, but also provides a zero-voltage or reduced voltage turn-off condition for the boost switch with an extended operating range. Since it uses fewer circuit components, the circuit layout is simple. The snubbers energy is recuperated to the load. |
| In dc-dc converters ,the dc output voltage is directly proportional to the dc input voltage. Therefore, it is difficult to achieve good line regulation. Feedforward control is accomplished by sampling the converter input voltage (instead of the output voltage, as in negative feedback control) and using this signal to control the duty cycle[4][5][9][12]. |
ZVT-PWM SYNCHRONOUS BUCK CONVERTER
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| A. Circuit Description |
| The ZVT-PWM synchronous buck converter is shown in Fig. 1. It is the combination of the conventional PWM synchronous buck converter and the proposed auxiliary snubber circuit. The auxiliary circuit consists of a resonant inductor Lr, a resonant capacitor Cr, a buffer capacitor Cb, and three auxiliary Schottky diodesDS1,DS2, and DS3. The body diodes of the main switch S and synchronous switch S1 are also utilized in this converter. |
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| B. Design of Auxillary Circuit |
| The resonant inductor and the resonant capacitor are the most important components when designing the auxiliary circuit. The proposed auxiliary resonant circuit provides soft switching conditions for the main transistor. |
| 1. Snubber inductor Lr is selected to permit its current to rise up to, at most, the maximum output current within trtime periods during the turn-on of the main transistor or the turn off of the synchronous switch. Here tris the rise time of the main transistor. |
| vi/Lr ≤ Iomax |
| 2. Snubber capacitor Cris selected to discharge from Vito zero with the maximum output current over at least the time period tfduring the turn off of the main transistor. Here, tf is the fall time of the main transistor. |
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| The value of Cbis normally larger than the value of Crto provide the condition for the energy transfer from inductor Lrto Cb. |
FEEDFORWARD CONTROL OF BUCKCONVERTER
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| In PWM dc-dc buck-derived converters operated in Continuous Conduction Mode (CCM) and without any control, the dc output voltage is almost independent of the output current, but it is directly proportional to the dc input voltage. Therefore, it is not easy to achieve a good line regulation in these converters. |
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| Fig 2 shows a buckcircuit with a peak-voltage-modulation feedforward control circuit. The feedforward control circuit consists of a sawto oth generator, a reference voltage VREF, and a comparator. The sawtooth generator in Fig 3 consists of a voltage divider, a resistor Rc, a solid-state switching device, a capacitor C, and a clock oscillator. When the MOSFET is off, the capacitor C, is charged through the combination of resistors R1, R2, and Rc..When the MOSFET is ON, the capacitor Ccis discharged rapidly through the MOSFET on-resistance. The output voltage of the sawtooth generator for the charging time interval is given by |
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| The sawtoothvoltage Vt is applied to the inverting input of the comparator and the reference voltageVREF is applied to the non inverting input. When the sawtooth voltage Vt is lower thanthe reference voltage VREF, the comparator output voltage goes high and, therefore, thegate-to-source voltage VGS of the power MOSFET in the buck converter also goes high.The crossing time of the two voltages determines the duty cycle D. |
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| Fig 4 shows the waveforms for the buck converter with peak voltage feedforward control. From the figure and geometry, duty cycle |
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| Thus, the duty cycle D decreases when the input voltage Vi increases. |
FEEDFORWARD ZVT BUCK CONVERTER
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| The proposed Feedforward ZVT Buck converter is shown in Fig 5. In this technique, the main switch S of the ZVT Buck converter is been driven by the pulses generated by the Feedforward control technique. For varying inputs, the Duty cycle gets automatically adjusted in order to maintain a constant output voltage. |
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SIMULATION RESULTS AND EXPERIMENTAL RESULTS
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| An ZVT Buck converter operating with an input voltage = 20.4V, output voltage = 8.38V with a switching frequency of 23 KHz is proposed, and is shown in Fig.11. Fig 12 gives the ZVS switching achieved across the switches. The converter is simulated using simulation software PSIM and MATLAB. The major parameters and components are given in Table I. Fig 6 shows the voltage across and the current through the switch of a conventional converter. An overlapping between current and voltage waveforms occurs because of the switching losses. |
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| Fig.7 shows the voltage across and the current through the switch of a conventional converter provided with an auxiliary resonant circuit. Fig 8 is a Buck converter without Passive snubber circuit with Vi=28V and Vo=13.76V. Fig 9 and Fig 10 gives the output waveforms for FeedforwardBuck converter with Passive snubber circuit with different input voltages. A constant output voltage was obtained in both the cases with almost zero switching losses. For Vi= 28V, the value of duty cycle ,D = 0.51,and for Vi = 35V, the value of D changed to 0.41. Thus the duty cycle got automatically adjusted inorder to get a constant output. |
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| The ZVT buck converter is provide a 20.4 V DC supply and obtained a 8.38 V output. The switching frequency is 23 KHz. It can be observed that the voltage across the switches rises only after the switching pulse fully ends. ZVS across the switches is shown in the Fig 12. |
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CONCLUSION
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| The concepts of the ZVT used in medium and high power were implemented in a synchronous buck converter, and it was shown that the switching losses in the synchronous buck were eliminated. Aside from the main switch being turned on under ZCT and turned off under ZVT, the synchronous switch is also turned on under ZCT and turned off under ZVT. Hence, the switching losses are reduced, and the newly proposed ZVT synchronous buck is highly efficient than the conventional converter. The additional voltage and current tresses on the main devices do not take place, and the auxiliary devices are subjected to allowable voltage and current values. Moreover, the converter has simple structure, low cost, and ease of control. Feedforward control with peak voltage modulation provides a good dc voltage regulation against input voltage variations in PWM buck converters. However, it cannot regulate against load current variations. Small output voltage deviations are also caused by the converter parasitic components. A combination of feedforward and negative feedback is recommended for future research. |
References
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- Tsai-Fu Wu, Yung-Chu Chen, Jeng-Gung Yang, and Chia-Ling Kuo , “Efficiency Improvement of Synchronous Buck Converter by PassiveAuxiliary Circuit,” IEEETRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 46, NO. 6, NOVEMBER/DECEMBER 2010.
- Ahmed Elasser, Student Member, IEEE, and David A. Torrey, Member, IEEE, “Soft switching active snubbers for DC/DC Converters,” IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 11, NO. 5, SEPTEMBER 1996.
- Ching-Jung Tseng and Chern-Lin Chen, Member, IEEE, “Novel ZVT-PWM Converters with Active Snubbers,” IEEE TRANSACTION S ON POWER ELECTRONICS, VOL.13, NO. 5, SEPTEMBER 1998.
- Marian K. Kazimierczuk, Senior Member, IEEE, and Anders J. Edstrm, “Open-LoopPeak Voltage Feedforward Control of PWM Buck Converter,” IEEE TR ANSACTIONS ON CIRCUITS AND SYSTEMS :FUNDAMENTAL THEORY AND APPLICATIONS, VOL. 47, NO. 5, MAY 2000.
- Marian K. Kazimierczuk, Senior Member, IEEE, Antonio Massarini, “Feedforward Control with Reference Voltage Modulation”, Department of Electrical Engineering Dayton, OH 45435, U. S. A.
- GuichaoHua, Eric X. Yang, Yimin Jiang, and Fred C. Lee, Fellow, IEEE, “Novel Zero-Current-Transition PWM Converters,” IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 9, NO. 6, NOVEMBER 1994.
- P.S.Bimbra, “Power Electronics,” Fifth Edition 2012.
- Bose, B.K, “Need a Switch,” IEEE Industrial Electronics Magazine, Volume 1 issue 4 2007.
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