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International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016 Highly efficient discontinuous mode interleaved dc-dc converter Praful V Nandankar Dr. (Mrs.) Jyoti P Rothe Electrical Engineering Department St.Vincent Pallotti College of Engg. & Tech. Nagpur, India [email protected] Electrical Engineering Department St.Vincent Pallotti College of Engg. & Tech. Nagpur, India [email protected] Abstract—The bidirectional dc-dc converters are inefficient due to switching loss, conduction loss and passive component loss. The ripple reduction is possible in the load current by interleaving of inductor currents in multiphase bidirectional dcdc converter. The bidirectional dc-dc converter is operated in discontinuous conduction mode in order to minimize the size of passive inductor. The snubber capacitance is designed to reduce the turn-off loss which occurs due to discontinuous mode of operation and hence it improves the efficiency. The design procedure is presented and an algorithm is devised to optimize the size of inductor and capacitor in order to achieve high efficiency operation from low to full load operating conditions. The performance of this converter is presented with single legged and three legged converter in simulation. The prototype of proposed converter is designed and its implementation is given with operational results. The efficiency curves are also plotted to validate the results. Keywords—Bidirectional power flow; zero voltage switching; snubbers. I. INTRODUCTION Bidirectional DC/DC converter has gradually gained interests in both industry and academic world of power electronics, which can perform as the transaction platform of different voltage levels and make management of the power of two voltage level. It has promising prospects in application of automation electronics, solar photovoltaic technology and wind power generation etc. Bidirectional dc-dc converters can be non-isolated [2]-[4] or isolated [5]-[7], depending on the application. Buck and Boost type dc converter are usually chosen for non-isolated power conversion systems. The high frequency transformer is an ideal candidate to achieve isolation between source side and load side. But in order to improve efficiency and to reduce size, weight and cost, non-isolated dc-dc converters are attractive. The basic non- isolated bidirectional dc-dc converter is the combination of step-up stage together with step-down stage and these two stages are connected in antiparallel. The dc-dc converter stepup stage is used to boost the battery voltage and to control the inverter input in motor drive operations. The dc-dc converter step-down stage provides a vehicle regenerative braking. The step-down stage offers a path for the braking current and it recovers vehicle energy in the battery. The converter is operated in DCM to achieve a high power density. By operating the converter in DCM, the size of an inductor can be 978-1-4673-9939-5/16/$31.00 ©2016 IEEE minimized. But DCM operation increases the high-frequency switching current ripple which can be reduced by interleaving multiple phases [8], [9]. Zero conduction loss and minimum diode reverse recovery loss are two major advantages of DCM operation. But, the DCM operation increases turn-off loss as the main switch is turned off at twice the load current or higher. This is the main drawback of reduction in inductor size. The reduction in inductor size increases inductor current parasitic ringing [8] because the inductor tends to oscillate with the device output capacitance during device turn-off period. The efficiency can be hampered due to all side-effects produced by DCM. Although the lossless capacitor snubber can be added across the switch for soft turn off, it requires certain amount of energy stored in the inductor to discharge the capacitor energy before device is turned on. Thus, both soft-switching turn on and turnoff are achieved. In this paper, the design of a high-efficiency non isolated buck and boost dc-dc converter is proposed to achieve high efficiency and high power density. Zero voltage switching, low diode reverse recovery loss and ripple current minimization are the main advantages of this proposed converter. As the converter input current can be shared among the phases, therefore the converter is most desirable for heat dissipation which improves the efficiency and reliability. An interleaved bidirectional dc-dc converter is an ideal candidate for current sharing when there is a need to handle high currents. II. CIRCUIT TOPOLOGY WITH ITS OPERATING PRINCIPLE A. Circuit Topology Non-isolated bidirectional dc-dc converters are based on a half bridge configuration where step-down and step-up stages are combined. Fig.1 shows the non-isolated single phase bidirectional dc-dc converter. With smaller inductance, the single phase converter can operate with an inductor current that flows in both directions during each switching period. The ZVS operation of single phase converter is also achieved by operating the converter in discontinuous mode. In Fig.1, Su operates as the main switch and Sd as the auxiliary switch for buck mode operation. The proposed bidirectional dc-dc converter topology is shown in Fig.2. The bidirectional dc-dc converters can be classified into buck and boost type depending on the placement of auxiliary energy storage. The energy storage is placed on high voltage side in buck type and it is placed on low voltage side in boost type. In order to have power flow in both the directions, the switch should have antiparallel diode and it should carry current in opposite direction. DTs (1-D)Ts S1u S1d Ts/2 S2u Su S2d iL L Chigh Vdc Ts S3u Sd Clow Vo S3d Fig. 3. Fig. 1. Circuit diagram bidirectional dc-dc converter S1u of S2u zero-voltage switching single phase S3u Ld1 Ld2 Vdc Chigh Ld3 S1d S2d S3d Clow Load Fig. 2. Circuit diagram of zero-voltage switching three phases interleaved bidirectional dc-dc converter. In Fig.2, MOSFET switches S1u-S3u and S1d-S3d serve as the main switches for either buck mode or boost mode. Each switch has its own antiparallel diode which is carrying current during freewheeling period. Each switch is paralleled with a lossless snubber capacitor. Three inductors Ld1-Ld3 can be used as the boost inductor under boost-mode operation or lowpass filter inductor under buck-mode operation. Capacitors Clow and Chigh serve as the smoothing energy buffer. With interleaved inductor currents, the ripple current going into these capacitors is minimized. When the top three switches are actively switching, the power is transferred from high voltage side to low voltage side in buck mode. When bottom three switches are conducting, then the power is transferred from low voltage side to high voltage side in boost mode. ZVS can be achieved by simply utilizing the existing switches and without using any extra components. Fig.3. shows the timing diagram of the 3-phase bidirectional dc–dc converter with duty cycle defined in buck mode. Each phase has 1200 phase shift. Two active switches in the same leg of half-bridge configuration are complementary to each other. Timing diagram for a duty cycle (D=0.5) in buck mode operation. B. Circuit Topology The inductor stores energy and when the current through the inductor stops, it releases its energy in another circuit. Too low of an inductance will cause the switches to go into discontinuous mode. Too high of an inductance will cause excess resistance on the core and lower the efficiency of circuit. The ripple current in the inductor is determined by the choice of inductance for the inductor. The smaller the inductance, the bigger is the ripple current. The inductor design improves the system performance by realizing ZVRT soft switching and by reducing switching loss, system size and inductor loss. The inductor value in a buck converter is usually kept at a high value. The high value of inductance reduces the inductor ripple current (ILp-p). This is done to minimize output ripple voltage and maximize output load current in a dc-dc converter. The large inductor values allow the converter to operate in discontinuous mode only at light loads. Therefore all the design considerations are required to optimize the inductance value. Typically a minimum inductance can be obtained at the boundary of CCM-DCM condition. The equations (1)-(5) provides the relationship between inductor peak current Ipeak, minimum current Imin, and inductor root-mean-square (rms) current Irms where Iload is the load current, Ts is the switching period, ∆I is the inductor current ripple and P is the load power. ΔI = 1 Vin − V o V o . . .T s 2 L V in I load = P V o I peak = I load + ΔI (1) (2) (3) I min = I load − ΔI 2 I rms = I load + (4) ΔI 2 3 calculated. The inductor value in a buck configuration is usually kept large to minimize the ripple current and to reduce the ripple in output voltage. In such cases, the discontinuous mode will be related with critical value of the resistance. (5) Rcrit = The critical value of inductance is obtained by making Imin to zero value. This critical value of inductance makes the converter to operate at the boundary condition between CCM and DCM. 1 (V − Vo ) Vo2 Lcrit = . in . .Ts P Vin 2 Lcrit R(1 − D).Ts = 2 2L (1 − D)Ts (10) START Enter the Pomax,Vo,Vlmax,Vlmin,Vlnom, fs (6) Compute maximum load current(I omax) and minimum load resistance(RLmin) (7) An optimized value of inductor should satisfy both the parameters i.e. zero voltage switching condition and lowest volume. Compute dc voltage transfer functions MVDCmin , MVDCnom and M VDCmax In case of light loads, a high value of inductor is selected for discontinuous conduction. DCM operation for light loads is always associated with large inductor values. Therefore in order to select an optimum value of inductor, a tradeoff should be established between buck and boost mode of operation. Hence, the DCM operation solely depends upon critical value of load i.e. Rcrit . Enter the desired efficiency Compute the maximum duty cycle at the CCM/DCM boundary at full load Compute the maximum inductance required for DCM operation, Lmax R>Rcrit for DCM R<Rcrit for CCM C. Snubber Capacitor Design A large capacitance decreases turn-off loss but it may increase turn-on loss. Thus, the design tradeoff is to minimize the total turn-on and turn-off losses. The design of snubber is obtained by charge balance of Cv2 and 1/2Li2. Cv 2 = 1 Li 2 2 Whether continue for multiphase No (8) where i is inductor current and v is capacitor voltage. D. Algorithm for design of inductor The algorithm is devised to compute the critical value of inductance at the boundary of CCM/DCM condition for single phase and multiphase. Lmax = RL min (1 − DB max ) 2 fs (9) After obtaining critical value of inductance from algorithm shown in Fig.4, the critical value of resistance can also be Yes For each phase inductance Ln=Lmax*n Display the result STOP Fig. 4. Flowchart of generalized algorithm III. SIMULATION RESULTS The simulation of single and three phase bidirectional dcdc converter has been performed. The simulation has been done in order to check the operating principle of single phase and three phase bidirectional dc-dc converter. MODELING OF BIDIRECTIONAL CONVERTER Value 250 µH 0.1 Ω 4700 µF 220 µF 0 Vds (V) 0 0.1502 0.1504 0.1506 0.1508 0.1502 0.1504 0.1506 0.1508 0.151 0.1512 0.1514 0.1516 0.1518 0.152 0.151 0.1512 Time (sec) 0.1514 0.1516 0.1518 0.152 0.1512 0.1514 0.1516 0.1518 0.152 0.151 0.1512 Time (sec) 0.1514 0.1516 0.1518 0.152 IL (A) 10 5 (a) Vds (V) 40 20 0 0.1502 0.1504 0.1506 0.1508 0.1502 0.1504 0.1506 0.1508 0.151 IL (A) 4 2 0 -2 0.15 (b) Vds (V) 40 20 0 0.15 0.1502 0.1504 0.1506 0.1508 0.151 0.1512 0.1514 0.1516 0.1518 0.152 4 IL (A) 0.1502 0.1503 0.1504 0.1505 0.1506 0.1507 0.1508 0.1505 0.1506 0.1507 0.1508 ZVS 2 0 -2 0.15 0.1501 0.1502 0.1503 0.1504 Time (sec) Fig. 6. Drain-source voltage and current through switch (including antiparallel diode). 20 0.15 0.1501 4 40 0 0.15 20 0.15 A. Single phase Converter Performance The simulated inductor current and drain-source voltage are shown for continuous mode, boundary of continuousdiscontinuous mode and discontinuous mode in Fig. 5(a), Fig.5 (b) and Fig.5(c) respectively. 0.15 40 Vds (V) Parameters Inductor Inductance Inductor Resistance Input Capacitance Output Capacitance Is (A) TABLE I. The single phase dc-dc converter operates at the boundary of CCM-DCM condition for the critical value of load resiatance. If the load resitance is increased above the critical value, then the circuit enters into discontinuous mode of operation. In DCM condition, the inductor current increases from positive direction to negative direction and then it again swings back to positive value. ZVS condition is achieved in single phase dc-dc converter when it operates in DCM condition . ZVS condition is shown in Fig. 6. In DCM condition, the switch current flows only when the drain-source voltage across switch is zero. Before that ZVS instant, the current was flowing through a diode connected antiparallel to the switch. This antiparallel diode current makes drain –source voltage to a zero value. When the switch is gated on under zero voltage condition, then switching losses are minimized and it improves the efficiency. Both the switches of a single leg ie. upper and lower switches are gated on at zero voltages. The current gets transferred naturally from antiparallel diode to the switch. Hence, the diode also turns off naturally without any occurrence of reverse recovery loss. When the gate-source voltage is applied to the switch, then drain-source voltage across switch becomes zero. The inductor starts to increase from zero value after a certain time interval and it is clearly indicated fin Fig.6. The switching losses become zero due to operation of single phase dc-dc converter in DCM condition. The switching losses increase with increase in switching frequency. These losses reach a high value at higher switching frequencies. ZVS condition is observed only under DCM condition and at the boundary of CCM-DCM condition. 2 0 -2 0.15 0.1502 0.1504 0.1506 0.1508 0.151 0.1512 Time (sec) 0.1514 0.1516 0.1518 0.152 (c) Fig. 5. Drain source voltage across switch and inductor current under CCM operation, (b) Drain source voltage across switch and inductor current at the boundary of CCM-DCM operation, (c) Drain source voltage across switch and inductor current under DCM operation. B. Three phase Converter Performance In Fig 7(a), the three inductor currents are 1200 separated having magnitude of 1.2 A peak to peak. The total current is averaged at 3.7A with only 0.4A peak to peak ripple, or 1/3rd of the individual phase current ripple. Similarly in Fig. 7(b), three inductor currents are having magnitude of 1.2A peak to peak with only 0.4 A peak to peak ripple and the total current is averaged at 1.8A. In Fig 7(c), the total averaged current is at 0.9A and peak to peak ripple is 1/3rd of the individual phase current ripple. iL1 iL2 3 iL3 iL (A) IL (A) 2 1 -3 0.065 0 0.15 0.1501 0.1502 0.1502 0.1502 0.1503 0.1504 0.1504 0.067 0.068 0.069 0.07 0.071 Time (sec) 0.072 0.073 0.074 0.075 0.066 0.067 0.068 0.069 0.07 0.071 Time (sec) 0.072 0.073 0.074 0.075 9 7 0.065 3.5 0.066 11 Vo (V) 0.15 4 IL-all (A) 1 -1 iL-all 3 0.15 0.15 0.1501 0.1502 0.1502 Time (sec) 0.1502 0.1503 0.1504 0.1504 Fig. 8. Transient response of the converter under buck mode operation from no load to loading condition. (a) IL (A ) 2 iL1 iL2 iL3 1 0 -1 0.15 0.15 0.1501 0.1502 0.1502 0.1502 0.1503 0.1504 0.1504 0.1502 Time (sec) 0.1502 0.1503 0.1504 0.1504 IL-all (A ) 2.5 2 1.5 C. Efficiency curve obtained through simulation results The efficiency curve of 3-Ф and 1-Ф dc-dc converter shows that 3-Ф dc-dc converter has higher efficiency as compared to 1-Ф dc-dc converter. The % improvement in efficiency is found to be 2%. The reason of higher efficiency for 3-Ф dc-dc converter is due to ripple cancellation and low switching losses. The efficiency is plotted against critical values of load resistance corresponding to their respective critical frequencies. iL-all 1 0.15 0.15 0.1501 0.1502 (b) 2 IL (A) iL1 iL2 1 iL3 0 -1 0.15 0.15 0.1501 0.1502 0.1502 0.1502 0.1503 0.1504 0.1504 IL-all (A) 1.5 Fig. 9. Efficiency curve at different values of critical resistances corresponding to respective frequencies. 1 0.5 0.15 IV. EXPERIMENTAL RESULTS iL-all 0.15 0.1501 0.1502 0.1502 Time (sec) 0.1502 0.1503 0.1504 0.1504 (c) Fig. 7. Three phase inductor currents, total current, output voltage and load current in continuous mode of operation, (b) Three phase inductor currents, total current, output voltage and load current at the boundary of continuous-discontinuous mode, (c) Three phase inductor currents, total current, output voltage and load current in discontinuous mode The output voltage and load current is shown for all the modes of operation. The converter operation from no-load to full-load is shown in Fig. 8. The output voltage is maintained constant. The inductor current flows continuously under all the loading conditions and hence the transient response of this converter is smooth. The specifications of DC-DC converter are tabulated in Table II. The performance of the DC-DC converter is evaluated with R load. TABLE II. CIRCUIT PARAMETERS FOR SINGLE PHASE CONVERTER ANALYSIS Parameters Inductor (L) Inductor Resistance Input Capacitance (Cin) Output Capacitance (Cout) Values 250 µH 0.1 Ω 4700 µF 220 µF To evaluate the performance of the converter, the efficiency of DC-DC converter under different operating conditions is evaluated. The experimental inductor current is shown in Fig. 10 (a), (b) and (c) for CCM, boundary of CCMDCM and DCM at 10 kHz switching frequency, 40 to 20V, 100W buck mode operation. The CCM and DCM mode of operation depends upon the critical value of load. (a) (b) Fig. 12. Efficiency curve of three phase bidirectional dc-dc converter. V. CONCLUSION (c) (d) Fig. 10. Experimental waveforms of inductor current under (a) continuous mode, (b) boundary of continuous-discontinuous mode, (c) discontinuous mode, (d) Experimental waveforms of gate-source voltage and inductor current indicating ZVS operation. Zero voltage switching of switches obtained experimentally is shown in Fig. 10(d) and it is observed that ZVS occurs at discontinuous mode of operation. The design algorithm of inductor and snubber capacitance is presented for bidirectional dc-dc converter which operates at the boundary of CCM-DCM condition. The design algorithm gives an optimized value of inductor at a fixed duty cycle in order to increase the efficiency. The ripples in load current are reduced by interleaving of three phase inductor currents. The micro-controller is used to provide switching pulses to bidirectional dc-dc converter. The ZVS operation is indicated in the simulation of three phase bidirectional dc-dc converter. The ZVS operation improves the efficiency of dcdc converter up to 98%. The prototype of dc-dc converter is tested and its hardware results are obtained to validate the operation of bidirectional dc-dc converter. References [1] (a) (b) Fig. 11. Three phase inductor currents at 10 kHz switching frequency, (b) Gate-source voltage, Vgs and resultant current at 10 kHz. The circuit of three phase bi-directional DC-DC converter is realized through a prototype. The prototype has been implemented for a 100W system. The three phase inductor currents at 10 kHz switching frequency is shown in Fig. 11(a). The three phase inductor currents are 1200 separated having a magnitude of 1A. The resultant current is 1/3rd of individual phase current ripple which is shown in Fig. 11(b). The comparison between single phase and three phases is plotted graphically and shown in Fig.12. It is seen that three phase is having higher efficiency because of ripple cancellation. The regulation of three phase bidirectional DCDC converter is better as compared to single phase DC-DC converter. 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