Isolated Bidirectional DC-DC Power Supply for Charging and Discharging Battery

Transcription

1 Iolated Bidirectional DC-DC Power Supply for Charging and Dicharging Battery Muhammed Shamveel T M Department of Electrical Engineering Indian Intitute of Science, Bangalore Bangalore hamveel7@gmail.com Dr. Vinod John Department of Electrical Engineering Indian Intitute of Science, Bangalore Bangalore vjohn@ee.iic.ernet.in Abtract Thi paper preent an iolated bidirectional DC- DC converter for ue in low to medium power application. The propoed topology i baed on a double witch forward converter. Achieving bidirectional flow of power uing the ame power component provide a imple, efficient and galvanically iolated topology that i epecially attractive for ue in battery charge/dicharge circuit uch a dc UPS. The dc main power the down tream load converter and the bidirectional converter which eentially operate in the buck mode to charge the battery to a nominal value of 24 V. On failure of the dc main the converter operation i comparable to that of a boot and the battery regulate the bu voltage and thereby provide power to the downtream converter. The deign of a laboratory prototype i included. Cloed loop control i deigned to prevent variation in the output voltage or output current, due to any diturbance. Experimental reult from the prototype, under different operating condition, i ued to validate and evaluate the converter topology. I. INTRODUCTION Bidirectional DC-DC converter allow tranfer of power between two dc ource, in either direction. Due to their ability to revere the direction of flow of current, and thereby power, while maintaining the voltage polarity at either end unchanged, they are beneficial when ue in application like dc uninterruptable power upplie, battery charger circuit, telecom power upplie and computer power ytem [1]. Generally, the voltage difference between the battery and DC bu i large. So bidirectional DC-DC converter with teep voltage converion ratio i required for the above application. Theoretically, high voltage converion ratio can be achieved by power converter in very high or very low duty ratio. But, the efficiency of power converter i reduced at very large or very mall duty ratio due to the effect of paraitic element [2]. For the different application, bidirectional DC-DC converter may be iolated type or non-iolated type. High voltage converion ratio in bidirectional DC-DC converter i able to be achieved eaily by adjuting turn ratio of the iolated tranformer. The bidirectional flyback DC-DC converter i a imple and low-cot configuration [3]. However, it ha ome drawback about high voltage tre and low efficiency due to the leakage-inductor. The bidirectional half-bridge/puh-pull DC-DC converter and the bidirectional full-bridge/puh-pull DC-DC converter are the other configuration [4]. But more component increae the cot and ize of converter. The noniolated bidirectional DC-DC converter can not achieve high voltage converion ratio eaily. Thi work wa upported by the DST IRHPA, India, under project Facility for deign, development and demontration of advanced batterie and ultracapacitor. V i C i S 1 D c1 D c2 S 2 n : 1 Fig. 1. Baic power topology for propoed bidirectional DC-DC converter. Thi paper preent an iolated bidirectional DC-DC converter topology for application a battery charger/dicharger. The propoed converter, Fig. 1, i a modification of wellknown topology, namely double witch forward converter. The propoed converter provide the deired bidirectional flow of power for battery charging and dicharging uing only one tranformer in a ingle converter, a oppoed to two independent iolated power converter in conventional cheme. Other advantage of the propoed topology include (a) reduced part count due to ue of the ame component in both direction of power flow, (b) low tree on the witche, (c) galvanic iolation, (d) low ripple in the battery charging current, and (f) minimal number of active witche. The paper alo provide detailed teady tate and mall ignal analyi for both forward and revere power flow topological mode. A generalized deign procedure i given that facilitate deign of the power converter. The paper i organized a follow: Section II preent the decription and operating mode of the converter. Hardware deign, which include tranformer and output filter deign and election of witche, i decribed in Section III. The controller deign by uing mall ignal analyi with tate pace model i provided in Section IV. Key experimental reult in Section V provide verification of the propoed topology. II. TOPOLOGY AND OPERATING MODES OF CONVERTER A. Topology of the Converter The baic power circuit topology i hown in Fig. 1. The tranformer provide galvanic iolation between the dc main and the battery. The primary ide of the converter i a tandard double witch forward configuration and i connected to the dc main. The econdary ide i connected to the battery through the output filter. The converter ha two mode of operation. In the forward/charging mode the energy from the dc main charge the S 3 S 4 L C o V b

2 battery over a pecified input voltage range while powering the down tream load converter. In thi mode of operation only the witche S 1 and S 2 are gated and the body diode of the witche S 3 and S 4 provide battery ide rectification. In the backup/current-fed mode, the witche S 3 and S 4 are gated and the body diode of the witche S 1 and S 2 provide rectification at the load ide. The ue of the double witch forward topology over other poible configuration can be jutified a follow. Switche in the off tate in thi topology are ubject to a voltage tre equal to the dc input voltage and not twice that a in the puh-pull and ingle ended forward converter. Deign of tranformer i imple a compared to other iolated topologie a there i no reet or plit econdary winding that are required. The clamp diode recover the magnetiing energy in the core that tored during on time and it i fed back to upply. B. Operating Mode of Converter 1) Forward Mode: In thi mode the dc main, V i, powering the load converter, provide the battery charging current. Thi charge the battery of the bidirectional converter at the nominal voltage. The witche S 1 and S 2 on the primary ide are gated with ame PWM ignal at duty ratio le than 0.5, while S 3 and S 4 are not witched at all. Operation of the bidirectional converter during thi mode i comparable to that of a buck converter. Output voltage of the converter under ideal condition, neglecting the effect of paraitic element like reitance of inductor, ESR of capacitor, i given by V b = V i nd (1) Fig. 2 how variou circuit waveform during operation in thi mode. Switche S 1 and S 2 are on for duration of dt time. Voltage V i appear acro primary winding. The body diode of witch S 3 i forward biaed and provide rectification on the econdary ide. It alo carrie the battery charging current. The primary current build up a it conit of the linearly increaing inductor current reflected from the econdary, and the tranformer primary magnetizing current. Switche S 1 and S 2 are off for next (1 d)t time. In thi interval for firt dt time duration the magnetizing current i fed back to main upply through the clamp diode D c1 and D c2. Voltage V i appear acro primary winding. For the ret of interval clamp diode top conducting. Zero Voltage appear acro primary winding. There i no power i tranferred to the econdary ide in thi interval. The energy tored in L o reult in the free-wheeling of the current through the body diode of S 4 to charge battery. Only half the upply voltage appear acro each witch S 1 and S 2 during thi interval. 2) Current-fed Mode: In thi mode the battery dicharge to upply the load power. The witch S 4 i driven at duty ratio greater than 0.5 and witch S 3 i driven complementary to S 3, while S 1 and S 2 are not witched at all. Operation of the bidirectional converter during thi mode i comparable to that of a boot converter. Output voltage of converter under ideal condition i given by /n V i = V b n(1 d) (2) V S3 /n V i V S4 V S1,V S2 Vi/2 I bat V DC1,V DC2 V i V i /2 V b I bat V l V b /n V l V i /n I S3 I m + ( /n) I S4 I S1,I S2 I m + ( /n) I DC1,I DC2 I m I SD1,I SD2 I DC1,I DC2 I m I DS3 V S1,V S2 /2 I DS4 V S3 V S4 V i /n V i /n t dt S 2dT S T S Fig. 2. Waveform during the forward mode. V DC1,V DC2 /2 (1 - d)t S 2(1 - d)t S T S Fig. 3. Waveform during the current-fed mode. Fig. 3 how variou circuit waveform during operation in thi mode. Switch S 3 i on and S 4 i off for duration of (1 t

3 d)t. Voltage V b /(1 d) appear acro econdary winding. The energy tored in the inductor during the previou interval i now tranferred to the load through the body diode of S 1 and S 2. Switch S 3 i off and S 4 i on for next dt time. Zero or negative voltage appear acro the econdary winding. There i no power i tranferred to the primary ide in thi interval. A the total battery voltage appear acro inductor the inductor current ramp up linearly. III. HARDWARE DESIGN The iolated bidirectional DC-DC power converter circuit deigned i a 100W, 200V to 24V double witch forward converter. Deign of the double witch forward converter include the tranformer, the filter element, choice of witching device, witch gate drive circuit, deign of nubber and controller. A. Deign of Tranformer Deign of tranformer include number of turn and cro ectional area of wire in primary and econdary, election of core baed on cro ectional area of core and window area [5]. The pecification of tranformer are Turn ratio, n = Vid = 3 Primary winding rm current = 0.982A Secondary rm current = 2.95A For the above pecification tranformer core i elected uing area product approach. The number of turn and cro ectional area of wire alo elected. The correponding equation are following. A c A w = 2V id max i 1rm BK w Jf (3) N 1 = V id max Bf A c (4) a w = I rm (5) J The tranformer parameter deigned uing (3) (5) are given in Table.I TABLE I TRANSFORMER DESIGN SUMMARY Specification V i = 200V, d = 0.35, P = 100W, f = 100 khz Core Selected ETD39 No. of turn N 1 = 33, N 2 = 11 Wire Size SWG21 (primary), SWG15 (wcondary) L m p 9 mh Leakage inductance L σp = 9.14µH, L σ = 1.02µH Reitance R p = Ω, R = 4.98 mω Lo 1.08W B. Deign of Output Filter It comprie the deign of inductor and capacitance at input and output ide. 1) Deign of Inductor: Inductance i deigned for 40% current ripple a follow δi Lop p = (1 d) (6) L o f The inductance calculated a 400µH. Inductor i deigned uing area product approach. ETD49 core i elected, Number of turn i calculated a 38 and air gap i 0.955mm. 2) Selection of capacitor: The filter capacitor are choen depending on the voltage ripple which i choen to be le than 0.1%. The input and output capacitance are calculated to be 100µF and 1000µF. Electrolytic capacitor SAMWHA SG M1A and A9A are ued. C. Selection of Switche Primary witche S 1 and S 2 need to have blocking voltage capability of more than V i and i rm rating more than i S1rm (max), clamp diode D C1 and D C2 need to have blocking voltage of more than V i and i rm rating more than maximum rm of magnetizing current. Secondary witche S 3 and S 4 need to have blocking voltage of more than Vi n and i rm rating more than i Lrm 1 d for S 3 and i Lrm d for S 4. Table Iit the elected device for Bidirectional DSFC along with their voltage and current rating. TABLE II SEMICONDUCTOR DEVICE VOLTAGE AND CURRENT RATING Device Selected Blocking Voltage Current Device Rating Rating Primary Switche S 1, S 2 IRF V 8A Clamp Diode MUR V 4A Secondary Switche S 3, S 4 IRF V 43A D. Deign of Snubber Ideal witching voltage wave form of econdary witche are hown in Fig.3 under the aumption that leakage inductance i zero. But practically due to leakage inductance of tranformer tranient over voltage i appeared acro witche. To reduce thi overvoltage, RC nubber i ued. Snubber capacitor and reitor i elected a C nub = i max t f 2 (7) R nub = t on min 5C nub (8) where t f i the fall time and t on i the on time of the device. It i calculated that C nub = 200 pf and R nub = 100Ω in primary ide and C nub = 2 nf and R nub = 10Ω in econdary ide. Baed on the above circuit component deign and waveform power lo analyi wa done. Table III how the different lo in the power circuit during revere power flow at rated load. TABLE III POWER LOSS BREAK UP OF THE CONVERTER AT RATED LOAD Tranformer Cu lo 0.12W Tranformer core lo 0.97 W inductor Cu lo 0.18 W inductor core lo 2.02W loe in capacitor 0.39W witching lo 10.69W Total lo 12.35W Efficiency 87.7%

4 V ref V fb PI I ref PI I()/d() V()/I() I fb Fig. 4. Cloed loop control tructure of the iolated bi-directional DC-DC converter IV. CONTROLLER DESIGN The control tructure adopted i the normal two loop hierarchical control: outer voltage loop and the inner current loop a hown in Fig.4. The controller are deigned baed on the mall-ignal analyi uing tate pace [6]. A. Forward Direction Control with Reitive Load 1) Inner Current Loop: The mall ignal tranfer function of current to duty ratio i ĝ() = îl() ˆd() = V i nr (1 + C o R) (1 + L R + 2 LC o ) For V i = 200V, n = 3, C = 1000µF, L = 400µH, R = 5.76 Ω, bode plot i given in Fig.5(a) ) ĝ() = îl() ˆd() = 2.78 (1 + ( ) The bandwidth of the inner current loop i choen a 10krad/ec.The PI controller ued to achieve the control objective i ĥ() = (10) 100 The bode plot of the loop gain GH i given in Fig. 5(b). 2) Outer Voltage Loop: The mall ignal tranfer function of voltage to current i (9) ĝ() = ˆv o() î l () = R (11) 1 + RC o ĝ() = ˆv o() î l () = Bode plot of thi tranfer function i given in Fig.5(c) The voltage loop PI controller i choen for a bandwidth of 1000 rad/ec. The tranfer function of the PI controller ued i ĥ() = The bode plot of the loop gain GH i given in Fig. 5(d). (12) B. Revere Direction Control with Reitive Load 1) Inner Current Loop: The mall ignal tranfer function of current to duty ratio i ĝ() = îl() ˆd() = n 2 V i (2 + C i R) R(1 D) 3 n (1 + 2 L R(1 D) + 2 n 2 LC i 2 (1 D) ) 2 (13) For V i = 24V, n = 3, C = 100µF, L = 400µH, R = 400 Ω, bode plot i given in Fig.6(a). ĝ() = îl() ˆd() = 12.6 ( ) ( ) The bandwidth of the inner current loop i choen a 10krad/ec.The PI controller ued to achieve the control objective i ĥ() = (14) The bode plot of the loop gain GH i given in Fig. 6(b). 2) Outer Voltage Loop: The mall ignal tranfer function of voltage to current i ĝ() = ˆv o() î l () 50 = nr(1 D) 1 + C i R (15) ĝ() = ˆv o() î l () = Bode plot of thi tranfer function i given in Fig.6(c) The voltage loop PI controller i choen for a bandwidth of 1000 rad/ec. The tranfer function of the PI controller ued i ĥ() = (16) 25 The bode plot of the loop gain GH i given in Fig. 6(d). V. RESULTS The converter i controlled uing dpic30f2023 microcontroller. The current and voltage i ened are given to ADC pin of micro-controller. The witching gate ignal generated from the PWM module of micro-controller i given to the gate drive card. A. Reult for Forward Direction 1) Efficiency of Converter: Table IV how the efficiency of the double witch bidirectional forward converter during forward direction of power flow at different power level. It can be oberved that the experimental and analytical efficiencie matche well. TABLE IV EFFICIENCY DURING FORWARD POWER FLOW AT DIFFERENT POWER LEVEL. Power (W) Experimental Analytical Efficiency (%) Efficiency (%) ) Open Loop Reult: Fig. 7 how the experimental waveform of output voltage and voltage acro witch S 1 in the forward power flow direction. The Input voltage i kept at 200V and witche S 1 and S 2 are witched with duty ratio d = 0.35 for a reitive load of 25W. The output i oberved a 22.8V with voltage ripple le than 1%. 3) Cloed Loop Reult: Cloed loop operation of the power converter i oberved by giving a tep change in load current. Fig. 8 how the repone of current controller mode to a tep change of load current reference from 0.4A to 1A in forward direction of power flow with reitive load of 24Ω for an input voltage of 200V. Settling time i oberved a 60m. Fig. 9 how the repone of voltage controller mode to a tep change of load current from 2A to 1A in forward direction of power flow by changing reitive load for an input voltage of 200V. Settling time i oberved a 75m.

5 (a) (b) (c) (d) Fig. 5. Bode plot ued for controller deign for forward power flow direction. (a) Current to duty ratio tranfer function îl(). (b) Current control loop gain ˆd() tranfer function. (c) Voltage to current tranfer function vo() ˆ. (d) Voltage control loop gain tranfer function. î l () Fig. 7. Open loop waveform during forward power flow direction. Channel1: Voltage acro S 1 (50V/div), Channel2: Output voltage (10V/div), Time: 5µ/div Fig. 8. Step change in load current command during forward power flow direction under current control. Channel1: 50V/div, Channel2: 500mA/div, Time: 25m/div B. Reult for Revere Direction 1) Efficiency of Converter: Table V how the efficiency of the double witch bidirectional forward converter during revere direction of power flow at different power level. TABLE V EFFICIENCY DURING REVERSE POWER FLOW AT DIFFERENT POWER LEVEL Power (W) Experimental Analytical Efficiency (%) Efficiency (%) ) Open Loop Reult: Fig.10 how the experimental waveform of inductor current and voltage acro witch S 4 in the revere power flow direction. The Input voltage i kept at 24V and witche S 3 and S 4 are witched in complementary mode with duty ratio of witch S 4 d = 0.65 for a reitive load of 100W. The output i oberved a 181V with voltage ripple le than 1%. VI. CONCLUSION Thi paper preent theoretical analyi and experimental reult for a iolated bidirectional DC-DC converter. The hardware i deigned and built for 100W power. Current and voltage controller i deigned for both forward and revere direction of power flow. The hardware i validated in open

6 (a) (b) (c) (d) Fig. 6. Bode plot ued for controller deign for revere power flow direction. (a) Current to duty ratio tranfer function îl(). (b) Current control loop gain ˆd() tranfer function. (c) Voltage to current tranfer function vo() ˆ. (d) Voltage control loop gain tranfer function. î l () Fig. 9. Step change in load current and output voltage repone during forward power flow direction under voltage control. Channel1: 10V/div, Channel2: 1A/div, Time: 25m/div. loop for both mode and controller i evaluated for the forward mode. Efficiency of the converter i evaluated in both mode and the experimental and analytical value matche cloely. REFERENCES [1] H. G. Langer and H.-Ch. Skudelny, DC to DC converter with bidirectional power flow and controllable voltage ratio, in Proc. IEE EPE Conf., JuneJuly 1989, pp [2] N. Mohan, T. M. Undeland, and W. P. Robbin, Power Electronic: Converter, Application and Deign, Third Edition, John Wiley & Son, Inc., [3] K. Ventatean, Current mode controlled bidirectional flyback converter, Proc. IEEE PESC Conf., vol. 2, pp , Jun [4] Chuanhong Zhao, Simon D. Round, Johann W. Kolar, An Iolated Three-Port Bidirectional DC-DC Converter With Decoupled Power Flow Management IEEE Tran. Power Electron., Vol. 23, no. 5, pp , Set Fig. 10. Open loop waveform during revere power flow direction. Channel1: Voltage acro S 4 (50V/div), Channel2: Inductor current (2A/div), Time: 5µ/div [5] Dataheet of core from Section/ProductCatalog/Ferrite. [6] V. Ramanarayanan, Coure Material on Switched Mode Power Converion.