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2 twofold mode series echoing dc dc converter for ample load


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2 twofold mode series echoing dc dc converter for ample load

  1. 1. Research Explorer January - June 20137 Vol . II : Issue. 6 ISSN:2250 - 1940 TWOFOLD MODE SERIES ECHOING DC-DC CONVERTER FOR AMPLE LOAD VARIATIONS Harine Knagaraj M.Tech Candidate, Software Engineering Institute of System Sciences, National University of Singapore (NUS), Singapore. ABSTRACT In order to satisfy demands like higher conversion efficiency and power density, many topologies and control methods are proposed. Among them, dc to dc series resonant converters with zero voltage switching features are getting more attention. This paper presents the design of a dual mode full-bridge series resonant converter (FB-SRC). It is operated in series resonant mode at normal loads or higher loads. The switching frequency is varied to regulate the output voltage. The fixed frequency phase shifted pulse width modulation , on the other hand, is used to adjust the effective duty cycle and regulate the output voltage at light loads . The proposed converter exhibits high conversion efficiency for wide range load conditions. Keywords - Conversion efficiency, phase-shifted full-bridge converter, series resonant converter (SRC), Zero volatge switching (ZVS), pulse width modulation (PWM) Introduction The switching devices in converters with a pulse width modulation (PWM) control can be gated to synthesize the desired shape of the output voltage or current. However, the devices are turned on and off at the load current with a high di/dt value. The switches are subjected to a high voltage stress and the switching power losses increases [2]. The turn on and turn off losses could be a significant portion of the total power loss. The electromagnetic interference is also produced due to the high di/dt and dv/dt in the converter waveforms. The disadvantages of the pwm control can be eliminated if the switching devices are turned on and turned off when the voltage and current are forced to pass through zero crossing by creating an LC-resonant circuit , thereby called a resonant pulse converter [4]. The primary design feature of ZVS PWM power converters is the addition of an auxiliary switch in the quasi-resonant circuit. Resonance is dominated by the auxiliary switch, which generates resonance and temporarily stops a period that can be regulated, there by overcoming the disadvantages of fixed conduction or cutoff time in a quasi-resonant power converter. Themain design feature of ZVT soft-switching power converters is the installation of resonant components that reduce conduction losses [6]. The main benefit of the converter is the extension of resonant time using two clamp diodes. The improvement in the voltage and current stress over those obtained using traditional resonant components implies in reduction of switching losses and the elimination of parasitic effect. Due to its high current gain, series resonant converters are mainly used for applications like arc welding, electronic ballast, induction heating and fluorescent lighting involving wide range load variations. Series Resonant Convertor A. Principle of operation The series resonant converter shown in fig.1 converts dc voltage into ac through full bridge inverter and then converts ac voltage again to dc. It works on the basis of resonant current oscillation. The resonating components and switching devices are placed in series with the load to form an underdamped circuit. The size of resonating components is small Available online at Research Explorer ISSN : 2250 - 1940 Vol II : Issue. 6 January - June 2013
  2. 2. Research Explorer January - June 20138 Vol . II : Issue. 6 ISSN:2250 - 1940 due to the high switching frequency. The operating frequency is generally close to the resonant frequency of the tank. Operation with switching frequency lesser than resonant frequency is called sub resonant frequency operation. The input voltage sees a net capacitive tank circuit and facilitates ZCS. When switching frequency is greater than resonant frequency, the operation is termed as super resonant frequency and the tank presents a net inductive circuit which facilitates Fig 1 Operating principle of SRC B. Dual mode condition For a series resonant converter, the output voltage is regulated by changing the switching frequency. However, it is impractical to raise the switching frequency at lighter loads due to the limitation of semiconductor switch device. Several schemes are proposed to solve this problem such as burst mode control [7] , turn off time modulation,etc. The penalty is that the ZVS feature is no longer kept. In this paper, the phase-shifted duty cycle control with ZVS at a fixed highest switching frequency is proposed to regulate the output voltage at light loads. Although the phase-shifted modulation features the constant switching frequency and ZVS function over wide input voltage and output load ranges, its efficiency at heavy load is lower than that of an SRC due to the high duty cycle loss. Therefore, the proposed control scheme adopts the frequency modulation with heavy-load efficiency and the phase- shifted modulation [5] with a better output voltage regulation and ZVS function at light loads. Through this dual mode operation higher conversion efficiency is fulfilled for wide-range load variations. C. Zero voltage switching When the PM-SRC is operated such that its switching frequency is greater than the resonant frequency of the tank, zero-voltage turn-on of the inverter devices is possible because the effective impedance offered by the resonant tank is inductive. Tank current lags the input voltage. ZVS ensures the inherent output capacitance in the switching devices is discharged prior to switch turn-on, thus prevent turn-on losses and generated EMI. D. Basic requirements of ZVS · The device should turn off with a positive current flowing through it · The delay time and turn off current have to be large enough to completely charge/discharge the snubber capacitors and subsequent turn on of the reverse diode for conduction · The delay time has to be small enough to prevent the tank current from reversing before the switch turns on. The Resonance Concept · From a circuit standpoint, a dc-to-dc resonant converter can be described by three major circuit blocks as shown in the figure 2 · The dc-to-ac input inversion circuit, the resonant energy buffer tank circuit, and the ac-to-dc output rectifying circuit · The resonant tank serves as an energy buffer between the input and the output is normally synthesized by using a lossless frequency selective network · The ac-to-dc conversion is achieved by incorporating rectifier circuits at the output section of the converter Fig 2 Typical block diagram of soft-switching dc- to-dc converter Proposed Circuit and its Operation It consists of a full bridge inverter consisting of four MOSFETS fed by a dc source. The next section is the resonant tank section formed by a resonant inductor and capacitor connected in series. Finally a diode rectifier along with filter and load circuit is used. Here Dc to Ac and again to Dc
  3. 3. Research Explorer January - June 20139 Vol . II : Issue. 6 ISSN:2250 - 1940 conversion is carried out. Figure 3 shows the circuit diagram of the proposed series resonant converter. Fig 3 Proposed series resonant converter The proposed FB-SRC has 4 MOSFET switches Q A ~ Q D with the output parasitic capacitors Coss, A ~ Coss, B. Lr and Cr forms the series resonant circuit. A centre tapped transformer of turn ratio n: 1:1 is used. Two rectifying diodes D1 and D2 are employed. The filter capacitor is Co. RL is the load resistance. The control signals of QA/QD and QB/QC are complementary. Dead times preventing the simultaneous conduction of switches are inserted to delay the turn-ons of the switches. When QA/QD or QB/QC conducts, the input power is transferred to the output load. Zero voltage switchings are achieved by the resonance of Lr and the equivalent capacitor formed by the parallel connection of Cr and the output parasitic capacitors of the switches during dead times. The gate pulses applied to the above converter is shown in fig 3. The leading leg switches are given by S1 and S1’ and lagging leg switches are given by S2 and S2’. The tank current i(t) is rectified by a diode bridge rectifier and filtered by a capacitive filter to get required output voltage. The magnitude and wave shape of the resonant current depends on fs, D and the load factor (Q) of the converter. Q is defined as the ratio of resonant tank characteristic impedance and the resistive load as seen from the resonant tank. For phase modulation full bridge inverter with fully controlled devices is required as shown in fig 3 each device is switched at 50% duty ratio with the switching of the devices on the same leg being complementary. As shown in fig 4, conduction of switches on the same leg of the inverter (S1 and S1’) is phase shifted with respect to the conduction of switches on the lagging leg (S2 and S2’) , resulting in the quasi-square input voltage. Fig 4 Gate waveforms of series resonant converter Modes of Operation There are two modes of operation in the proposed FB-SRC. They are: · Frequency Modulation keeping duty ratio constant · Phase Shift Modulation keeping switching frequency constant A. Switching frequency modulation mode The gate signals for switching frequency modulation mode is shown in fig 5. Fig 5 The gate pulses for switching frequency modulation mode This mode of operation can be explained under 3 states. They are: · First energy transfer state (t0 < t < t1) · First resonance state (t1 < t < t2)
  4. 4. Research Explorer January - June 201310 Vol . II : Issue. 6 ISSN:2250 - 1940 · First commutation state (t2 < t < t3) · First Energy Transfer State (t0 < t < t1) : In this state, QB and QC are turned on, and QA and QD are turned off. D1 conducts and energy is transferred to the secondary through the transformer. · First Resonance State (t1 < t < t2) All the switches are turned off during this state. Since the inductor current iLr must be continuous, it discharges Coss, A and Coss, D to zero voltage, and charges Coss, B and Coss, C to VI. Then zero- voltage turn-ons of QA and QD can be achieved. As long as iLr is larger than the reflected secondary load current, D1 is still conducting. The load power is supplied by Lr. · First Commutation State (t2 < t < t3) : In this state, QA and QD are turned on, and QB and QC are turned off. iLr flows through body diodes DA and DD initially. Since the energy at the primary side is insufficient, the load power is supplied by C0. B. Phase shift modulation mode The gate signals for phase shift modulation scheme are presented in fig 6.. For the PS PWM, it can be observed that dead times. During which ZVS is accomplished, are inserted before turning on switches . It can also be noticed that before ZVS takes place, there are two resonance states (t1 ~ t2 and t3 ~ t4) . Fig 6 The gate signals for phase shift modulation scheme Here there are 5 operating states They are : · Energy transfer state ( t0 < t < t1) · First resonance state (t1 < t < t2) · Linear Discharge state (t2 < t < t3) · Second resonance state (t3 < t < t4) · Commutation state (t4 < t < t5) · Energy Transfer State ( t0 < t < t1) : In this state, QB and QC are turned on, and D1 conducts. The input energy is transferred to the secondary through the transformer, and C0 is charged. · First resonance state (t1 < t < t2) : At t1, QC turns off. iLr stops increasing , then charges Coss, C to VI and discharges Coss, D to zero voltage. DD conducts at t = t2 . The equivalent resonant inductor (Lr) and the equivalent resonant capacitor ( Cr + Coss) starts resonanting. Since the primary current is larger than the reflected load current , D1 still conducts and D2 carries no current. · Linear Discharge state (t2 < t < t3) : DD conducts at the end of the last state. Therefore QD can be turned on at zero voltage . The primary voltage is zero. The energy stored in Lr is transferred through the transformer to the secondary. · Second resonance state (t3 < t < t4) : This state starts when QB is turned off. iLr charges Coss, B to VI and discharges Coss, A to zero voltage. Then DA conducts and the resonance stops. During this state, Lr is not capable to supply the required energy. The transformer is in free-wheeling state. A short circuit appears at the transformer secondary . To achieve ZVS, the energy stored in the equivalent resonant inductor must be larger than that in the equivalent resonant capacitor. · Commutation State (t4 < t < t5) : During this state, the transformer primary is short-circuited. A voltage of -VI is across the Lr – Cr combination. Therefore, iLr decreases linearly until its magnitude is larger than the reflected load current. Then, the transformer starts to transfer energy and the other half switching cycle begins. D1 is turned off, and D2 conducts. Co is also charged. Defining Terms and Assumption The resonant tank has a natural frequency determined by the resonant capacitor and resonant inductor. fs = switching frequency fr = resonant frequency Pin = Input power
  5. 5. Research Explorer January - June 201311 Vol . II : Issue. 6 ISSN:2250 - 1940 Pout = Output power D = Duty ratio = Efficiency where Lr and Cr are resonant tank elements D = Ton / Ts/2 Where Ts = switching period VI = Input voltage Vo = Output voltage short-circuited. A voltage of M = gain = Vo/ VI The parameter Zc called the characteristic impedance of the tank is defined as Capacitor Cr can be found by the following relation Cr = 1/ωrZc Inductor Lr can be given by Lr = ωr/Zc Current is given by i = VI / Zc Simulated Results VDC(v) Time (s) Fig 7 Dc supply voltage VAB (v) Time (s) Fig 8 Bridge voltage 0.4471 0.44711 0.44712 0.44713 T ime (s) 0 -500 500 VP9 Vout (v) Time (s) Fig 9 Output Voltage of both frequency modulation and phase shift modulation VDS,VGS(V) Time (s) -- Fig 10 VDS and VGS I(Lr) A Time (s) -- Fig 11 Inductor current
  6. 6. Research Explorer January - June 201312 Vol . II : Issue. 6 ISSN:2250 - 1940 VQA,VQD and VQB,VQC Time (s) Fig 12 Gate pulses for switching frequency modulation VQA,VQB,VQC,VQD Time (s) -- Fig 13 Gate pulses for Phase shift modulation Iout (A) Time (s) -- Fig 14 Output current Conclusion To avoid poor output voltage regulation and low conversion efficiency at light loads, a dual-mode control strategy is presented in this paper. The FB SRC is operated under switching frequency modulation for most of the load range to achieve ZVS and low switching noises. For the lighter loads, the FB SRC is operated under phase-shifted duty cycle modulation to regulate the output voltage and maintain the ZVS feature. The proposed two-mode control scheme for a FB SRC is especially suitablefor applications with wideinput voltage and load variations. References 1. M.K.Kazimierczuk , “Synthesis of phase modulated resonant Dc/Ac inverters and Dc/Dc converters”, Proc. Inst.elect. Eng. B – Elect. Power Appl. . vol. 139, no.4, pp. 387-394, Jul 1992. 2. M.K.Kazimierczuk and D.Czarkowski, Resonant Power Converters, New York : Wiley- Interscience, 1995 3. X.Ruan and Y.Yan,” An improved phase shifted zero-voltage and zero-current switching PWM converter”, in Proc. IEEE. Appl. Power. Electron.conf.1998, pp 811-815. 4. S.B.Zheng and D.Czarkowski, “ Modelling and digital control of a phase-controlled series-parallel resonant converter” , IEEE Trans. Ind. Electron ., vol. 54, no.2, pp. 707-715. Apr. 2007. 5. Z.M.Ye, P.K.Jain, and P.C.Sen, “ A full-bridge resonant inverter with modified phase-shift modulation for high frequency ac power distribution systems”, IEEE Trans. Ind. Electron., vol. 54, no. 5, pp. 2831-2845, Oct. 2007 6. G.B.Koo, G.W.Moon and M.J.Youn, “New zero- voltage-switching phase-shift full-bridge converter with low conduction losses”, IEEE Trans. Ind. Electron ., vol.52, no1, pp 228-235 , Feb 2005. 7.Y.K..Lo, S.C.Sen and C.Y.Lin , “ A high efficiency ac-to dc adaptor with a low standby power consumption”, IEEE Trans. Ind. Electron., vol.55, no.2, pp. 963-965, Feb 2008. 8.B.R.Lin, K.Huang, and D.Wang, “ Analysis and implementation of full-bridge converter with current doubler rectifier “, Proc. Inst. Elect. Eng. – Elect. Power Appl., vol 152, no.5, pp.1193-1202, Sep. 2005.