Single-Stage Reconfigurable DC/DC Converter for Wide Input Voltage Range Operation in HEVs

Transcription

1 The 014 nternatonal Power Electroncs Conference Sngle-Stage Reconfgurable DC/DC Converter for Wde nput oltage Range Operaton n HEs Sandra Zeljovc, Tomas Reter nfneon Technologes AG Neubberg, Germany Deter Gerlng Unversty of Federal Defense Munch Neubberg, Germany Abstract Ths paper proposes a sngle-stage reconfgurable topology [1] for an solated DC/DC converter that supples the 1 power net from a H-battery pac n hybrd and electrc vehcles. The proposed topology solves the problem of reduced converter effcency n the upper range of H-battery voltages. The converter s based on the ZT phase shft full brdge DC/DC converter that s dependng on the nstantaneous value of battery voltage reconfgured nto the push-pull converter. The ZT phase shft confguraton covers upper range of nput voltages whereas t s reconfgured nto the less effcent, hardswtchng, push-pull confguraton to cover the lower range. The pont of reconfguraton s chosen to maxmze the average effcency accordng to the hstogram of H-battery voltage durng typcal drvng cycle. Expermental valdaton of the proposed converter and the effcency mprovement s also presented. Keywords automotve, DC/DC converter, drvng cycle, (H)Es, phase shft converter, push-pull converter, reconfgurable, wde nput voltage range, ZT.. NTRODUCTON n the applcaton of the hgh voltage (H) to low voltage (L) DC/DC converter n hybrd and electrc vehcles (HEs), a zero voltage transton (ZT) phase shft (PS) full brdge DC/DC converter (Fg. 1) s commonly used. Ths topology presents a good compromse between effcency and power densty. One of ts advantages s a soft swtchng behavor durng turnon of the H sde swtches, acheved by parastc components n the crcut: the transformer leaage nductance and the put capactance of swtches [], [3]. n Table from [4], typcal requrements gven by the automotve ndustry for such a converter are lsted. When operatng n the wde range of nput voltages and load currents, the converter cannot mantan the hgh effcency n all operatng ponts. Snce the transformer turns rato s chosen accordng to the mnmal specfed nput voltage n,mn, the converter wll have reduced effcency at hgher nput voltages. Ths topology exhbts the hghest effcency when the converter s desgned for a tght nput voltage range, when t operates wth the phase shft of nearly 180 (close to the unty duty cycle). Methods for effcency mprovement of solated DC/DC converters n wde nput voltage range applcaton proposed n the lterature can be roughly classfed nto two man groups: 1) two-stage converters (a non-solated voltage regulator as nput stage) ) sngle-stage modfcatons of topology or control method n commonly used topologes. Fg. 1. Conventonal ZT phase shft full brdge DC/DC converter from []. TABLE CHARACTERSTCS OF H TO L DC/DC CONERTER N (H)ES [4] Parameter energy flow,mn...,max,rpple n,mn n,max P,max alue undrectonal < 0. pp (PH)E 40 / HE 180 (PH)E 40 / HE 310 3W 14 t has been shown that n certan carefully optmzed desgns the reduced amount of total es and the reducton of the converter volume can be acheved wth a two-stage topology n automotve applcatons wth wde nput and put voltages [5]. Stll, an addtonal hardswtchng converter n seres that s requred n such cases s usually undesrable due to addtonal components and power es [6]. Furthermore, wth a H-battery pac at the nput of the converter, there s no need for a power factor correcton stage (PFC) typcal for front-end converters, what s addtonal reason aganst the use of pre-regulatng stage. Sngle-stage topologes adapted for wde nput voltage range n the lterature often need multple power transformers and/or rectfers [7] [11]. n addton, ther control strategy may requre modfcatons as well (e.g. [8], [1], [13]). Another approach to ncrease the effcency of wde nput voltage range sngle-stage converters s the attempt to mprove dfferent resonant topologes (e.g. seresparallel, LLC descrbed n [14]). Resonant converters, when desgned for the narrow range of operatng ponts, exhbt extremely low swtchng es and consequently hgh effcency at the prce of an addtonal passve resonant tan. The desgn of the resonant tan becomes more complcated when the converter operates n a wde 014 EEJ

2 The 014 nternatonal Power Electroncs Conference range of operatng condtons. Several adaptatons of resonant converters for mproved operaton at wde nput voltages have been recently proposed n the lterature [15] [17]. Stll, the applcatons n focus are ntended for consderably lower load currents compared to the specfcaton from Table. Ths fact maes offered solutons less effcent when used as H to L DC/DC converters n (H)Es. Most of the methods revewed here are ether focused on fuel cells [10], [18], [19] or photovoltac applcatons [1], [17], [0] as typcal examples of applcatons wth wde nput voltage range, or optmzed based on the holdup requrements for front-end converters [8], [1]. They do not exactly match the applcaton requrements n (H)Es, where the dstrbuton of nput voltages s determned by the dscharge behavor of a H-battery pac, usually the Lthum on (L-on) or the Ncel- Metal Hydrde (NMH) battery type. Methods to optmze electrc machnes [] or power electronc components n (H)Es (e.g. a man nverter [3] or a boost stage [4]) accordng to standard drvng cycles have been proposed. Operaton of a DC/DC converter that supples 1 system from the H-battery s not drectly affected by the vehcle s msson profle n the sense of put power that has to be delvered. t s affected rather ndrectly, through the varatons of the H-battery voltage durng ts dscharge cycles. n ths paper, a sngle-stage reconfgurable converter topology based on the ZT PS converter s proposed for use n (H)Es. t requres no addtonal passve elements, but apples only two addtonal swtches that do not exhbt swtchng es. The control strategy for both operaton s of the converter s unque, and the dfference comes only at the level of gate drvng patterns. The converter desgn procedure taes nto account specfcs of the dscharge curve of automotve battery (Fg. ) n order to optmze the converter s voltage-weghted average effcency. The remander of the paper s organzed as follows: Secton gves the bacground of the battery voltage behavor durng a typcal dscharge cycle n Es, Secton presents the proposed sngle-stage reconfgurable topology, Secton deals wth the proposed desgn procedure that maxmze the voltage-weghted average effcency of the reconfgurable converter accordng to the hstogram of battery voltages. Secton gves the expermental valdaton of the proposed soluton and dscusses the effcency mprovement. Secton concludes the wor.. OPERATNG CONDTONS OF H TO L DC/DC CONERTER The voltage of the H-battery pac at the nput of a DC/DC converter n changes durng a drvng cycle, dependng on the state of charge (SoC), but s also affected by the temperature and the dscharge current. Snce Es operate n the charge-depleton, they are characterzed by the deep dscharge cycle wth ts SoC droppng down durng the full cycle almost lnearly. On the contrary, HEs operate n the charge-sustanng, where the battery SoC may ncrease or decrease over a drvng profle, but on average remans at ts ntal level. Accordng to ths, wde ranges for n are specfed n Table for both HEs and Es. Furthermore, from the typcal dscharge curve of today s automotve batteres (Fg. (a), the example of the L-on battery obtaned usng the smplfed battery l from [5]), t can be concluded that for Es, the battery voltage remans close to a certan value for most of the drvng cycle, whle n a sgnfcantly shorter part ts voltage vares more notably (up to the maxmum or down to the mnmum specfed battery voltage). Thus, the hghest converter effcency would be desrable n the range of mostly present battery voltages, and not, as usual, at n,mn. Fg.. Hstogram of H-battery voltages (d) durng one dscharge cycle of the L-on battery n E derved from (a) the battery voltage (b) the SoC and (c) the battery dscharge current obtaned usng the FTP75 drvng cycle data. The hstogram of H-battery voltages (Fg. (d)), used n ths paper for the desgn of the proposed reconfgurable converter, s derved from the example of the E L-on battery wth the nomnal voltage of 365, operatng range from 40 to 40, ampacty of 66. Ah and the pea power of electrc motor of 80W. The data descrbng the battery voltage (Fg. (a)) and the dscharge current (Fg. (c)) are used to obtan the hstogram. The US cty drvng cycle FTP75 s assumed for the analyss. To quantfy the relatonshp between the battery voltage hstogram and the average converter effcency, weghtng factors n, can be defned from the hstogram n Fg. (d) for dfferent values of n n the range from n,mn to the maxmal specfed nput voltage n,max. Coeffcents n are tme percentage of the presence of a certan battery voltage over the complete battery dscharge cycle.. PROPOSED TOPOLOGY Fg. 3. The proposed reconfgurable topology based on the ZT PS converter (S add and D add are addtonal components compared to the conventonal ZT PS topology that do not exhbt repettve swtchng es, but only conducton es).

3 The 014 nternatonal Power Electroncs Conference Fg. 3 presents the proposed reconfgurable topology [1] based on the ZT PS converter. The confguraton of the proposed converter, dependng on the value of n, can be ether a ZT PS or a push-pull. n the upper range of n, the conventonal ZT PS converter s extended by addton of a dode D add (see Fg. 4(a)). The dode D add wll conduct the nput current n the upper range of n. n the lower range of n, the converter operates as a pushpull converter (see Fg. 4(b)). Fg. 4. Two dfferent confguratons of the proposed topology: (a) the ZT PS confguraton n the upper range of n (on the left) (b) the push-pull confguraton n the lower range of n (on the left) wth correspondng prmary wndng currents and gate sgnals on the rght. Gate drve sgnals for the push-pull operatng can be derved from those for the ZT PS confguraton usng two AND logc gates. n ths confguraton, the transformer operates wth a center-tapped prmary wndng, and ts mddle pont s connected to a voltage source (battery) through an addtonal actve swtch S add. Ths swtch s contnuously on and no swtchng es occur durng the operaton n the push-pull confguraton. n the lower range of n, D add needs to dsable the conducton of the freewheelng dodes parallel to the GB S 3 and S 4, not used n the push-pull confguraton. The swtch S add n the ZT PS confguraton has to be able to bloc specfed n,max. n both operatng s, the proposed converter operates wth the duty cycle longer than 50%, allowng ncreased duty cycle utlzaton. The prncples of the operaton of a ZT PS and a push-pull converter are explaned n detal n [3] and [14] respectvely. The phase shft modulaton strategy [3] s used to generate the gate drvng sgnals for the ZT PS converter. The modulaton strategy requred for the pushpull confguraton s consderably smpler and can be easly derved from the drvng sgnals for the ZT PS confguraton. n phase shft modulaton, durng the power transfer perod, two dagonal swtches n the H sde H-brdge (e.g S 1 and S 3, or S and S 4 ) are always conductng smultaneously, so that they can be used to generate the gate drvng sgnal for the correspondng push-pull swtches S 1 and S. For the purpose, n e.g. analog mplementaton of the controller, two addtonal AND gates can be actvated when the converter starts ts operaton n the push-pull (see Fg. 4(b)).. DESGN PROCEDURE FOR PROPOSED RECONFGURABLE CONERTER The am of the proposed procedure s to fnd the value of the nput voltage reconf at whch the converter wll be reconfgured from one topology to another n order to acheve the optmzed average converter effcency ave, tang nto account the typcal battery voltage hstogram (Fg. (d)). The procedure conssts of followng steps (Fg. 5): Step 1 - Specfcaton of operatng condtons. The desgn procedure of the proposed converter taes nto account requrements gven n Table. From the typcal dscharge curve of the L-on battery n Fg. t can be concluded that the voltage range around the voltage plateau s the most present at converter s nput. n ths range of nput voltages, t s benefcal that converter operates wth more effcent ZT PS confguraton. Durng the search for maxmum ave, reconf s ncremented by the voltage step n untl maxmum nput voltage s reached. n ths specfc case, an ncrement of 0 s used n the analyss. Fg. 5. Flowchart of the desgn procedure for proposed reconfgurable converter. Step - Calculaton of ave for dfferent values of reconf. The es and the effcency for both converter confguraton are calculated n ths step: n the range from n,mn to reconf for the push-pull confguraton and n the range from reconf to n,max for the ZT PS confguraton. reconf s beng ncreased wth n defned n step 1. For that purpose, the transformer turns rato and the sze of the put flter nductor L are calculated for each consdered value of reconf. The parameter calculaton based on reconf s addressed n Secton -A n detal. Based on the calculated effcency of the reconfgurable converter for the complete range from n,mn to n,max, the average converter effcency (1) s calculated usng voltage weghtng factors n,m defned n Secton.

4 The 014 nternatonal Power Electroncs Conference Loss ls for both converters are addressed n Secton -B. Fg. 6 presents results of ave calculaton over dfferent values of reconf. η ave = n n,. (1) n,, 1 n, m m Fg. 6. (a) oltage-weghted average effcency of the proposed converter as a functon of chosen reconf (b) mprovement of voltageweghted average effcency acheved by the proposed converter wth dfferent reconf up to 300 compared to the conventonal ZT PS converter. Step 3 - Converter desgn based on chosen value of reconfguraton voltage reconf,fnal. Fg. 6(a) can be used to decde on reconf,fnal and fnal desgn parameters can be calculated accordngly. Certan practcal lmtatons n the choce of reconf,fnal are dscussed n Subsecton -A together wth results of the proposed desgn procedure. A. Desgn Parameters and Results of Desgn Procedure The transformer turns rato used n the proposed desgn procedure n s chosen accordng to reconf consderng the ZT PS confguraton as the man one, whle the proper operaton at n,mn of the converter operatng n push-pull must be guaranteed (). n push-pull, the same transformer wll be used wth the turns rato n/ : n/ : 1. ( n,mn drop, prm ) D max, pp,max + drop,sec () n mn, ( reconf drop, prm ) D max, zvt,max + drop,sec The tghter range of n for both consdered confguratons wll affect the sze of L (3), leadng to the lower value compared to the conventonal ZT PS converter when the same put current rpple,,rpple, s specfed, whle the requred value of put capactor C (4) wll not change. 1 (1 Dmn, pp ), rpple (3) L max 1 (1 Dmn, zvt ), rpple, rpple C (4) 4 8, rpple Results from Fg. 6(a) show the ncrease of the voltage-weghted average effcency when reconf ncreases, wth the H sde swtches assumed to have the same blocng voltage capablty over the whole range of n. However, ths analyss does not consder the constrant for the choce of blocng voltage of the H sde swtches mposed by operaton n the push-pull confguraton. n the push-pull confguraton, the H sde swtches must be able to bloc two tmes reconf (plus margn for the voltage overshoot caused by nteracton of the transformer L lea,prm and C oss of the H sde swtches). n the case of the GBT swtches (used for the study n ths paper), ths leads to the need for another class of swtches regardng blocng voltage after a certan value of n chosen for reconf. Thus, the practcal lmtaton for the effcency mprovement gven by the blocng voltage capablty of the GB s mared at Fg. 6(a) at reconf of 300. Furthermore, Fg. 6(b) shows the mprovement of ave when reconf s ncreased from 60 to 300 as well as compared to the conventonal ZT PS converter desgned for n,mn = 40. The transformer desgn n the reconfgurable converter dffer from the one of the conventonal converter as follows: the transformer turns rato, n : 1, whereas the sze of transformer core s not consderably affected. Furthermore, thans to the longer duty cycle that leads to lower maxmum rpple of the put current, L n the reconfgurable converter s decreased. Fg. 7. Waveforms of transformer currents for the ZT PS confguraton n: (a) the prmary wndng and (b) the secondary wndng and for the push-pull confguraton n: (c) the prmary wndng and (d) the secondary wndng. Slope coeffcents used for lng of current waveforms are mared here and defned n Appendx B. The choce of the rectfer topology for the converter proposed n ths paper s made accordng to the l gven n [6], whch also taes nto account a voltage overshoot caused by the transformer L lea,sec and C oss of swtches. Thus, a brdge rectfer s adopted for remanng analyss, although other possble rectfer topologes can be used as well. t has to be assured at ths pont that blocng voltage ratng of L sde swtches are chosen accordng to the worst case of two possble converter confguratons. n the presented analyss, the L sde MOSFE wth the same blocng voltage capablty are used n the conventonal as well as n the reconfgurable converter. n case of reconfgurable converter, there s stll a room for reducton of the L sde MOSFE blocng voltage by approxmately 0%. The beneft of MOSFE wth lower voltage class s reduced r ds,on and consequently the conducton reducton. B. Loss Modelng for Desgn Procedure An overvew of segments of a swtchng perod T s that are sgnfcant for the lng of prmary wndng

5 The 014 nternatonal Power Electroncs Conference currents and currents of L sde swtches are gven n Table and respectvely, for both the ZT PS and the push-pull confguraton. At the H sde of the converter, one half of T s can be dvded nto the followng subperods: a of duty cycle (d T/), a power transfer (from d T/ to D T/, duty cycle determned by controller) and a freewheelng perod (Fg. 7(a)). Two transtons, n the leadng (consstng of S and S 3 ) and n the laggng leg (consstng of S 1 and S 4 ) of H brdge, occur durng dead-tmes t d,lead and t d,lagg. On the other hand, n the push-pull confguraton, there s no of duty cycle and no current n the prmary wndng durng the freewheelng perod (Fg. 7(b)). Tme-doman pece-wse defned waveforms of the prmary tr,prm as well as the secondary wndng current tr,sec are defne for each of consdered parts of T s n Appendx B. Due to the sgnfcantly hgh magnetzng nductance of the transformer, the magnetzng current s neglected n ths analyss. The mechansms consdered n the presented desgn procedure are as follows: conducton es of H sde swtches (GB and dodes): P = + r, (5) P cond, GBT ave, GBT ce 0 RMS, GBT ce = + r, (6) cond, dode ave, dode d 0 RMS, dode d where ce,0 and d,0 are threshold voltages of the H sde GBT and dode, r ce and r d dfferental resstances and ave and RMS are average and RMS currents of each swtch calculated usng tme-doman pece-wse defned swtch currents over the swtchng perod T s ; TABLE CONDUCTON PERODS AND SWTCHNG TRANSTONS OF H-SDE SWTCHES DURNG TS (ZT PS FB S. PUSH-PULL) Part of T s Conductn g swtches/ Swtchng events n ZT PS ntervals n ZT PS t,prm n ZT PS Conductn g swtches/ Swtchng events n pushpull ntervals n push-pull t,prm n push-pull Loss of Duty Cycle S 1,S 3. half: S,S 4 0 to d T s/ Power Transfe r S 1, S 3. half: S, S 4 d T s/ to D T s Leadng Leg Transto n S 3 off S on. half: S off S 3 on Freewheeln g Perod S 1, S. half: S 3, S 4 Laggn g Leg Trans. S 1 off S 4 on. half: S 4 off S 1 on t d,lead D T s/ to T s t d,lagg Eq. 13 Eq. 14 Eq. 15 / S 1. half: S 0 to D T s/ S 1 off. half: S off / D T s/ to T s/ Eq S on. half: S 1 on P conducton es of the L sde synchronous rectfers: = r, (7) cond, SR RMS, SR ds, on where r ds,on s channel resstance of the L sde MOSFE and bd,0 voltage drop of a conductng body dode; the body dode (BD) conducton es durng dead-tmes: P =, (8) cond, BD ave, BD bd 0 where ave,bd s the current through the body dode of the L sde MOSFET durng the dead-tme perod (that has to be estmated) that s averaged over complete T s ; copper es of the power transformer: P = r, (9) P copp, prm RMS, prm prm =, (10) cond, sec RMS,sec rsec where r prm and r sec are prmary and secondary wndng resstances, whereas RMS,prm and RMS,sec are RMS currents n prmary and secondary transformer wndngs; copper es of the put flter nductor (1): P = r, (11) copp, L RMS, L L where r L s resstance of the put nductor; equvalent seres resstance (ESR) es of the put capactors (13): PESR = RMS, rpple ESR, (1) where ESR s nternal resstance of the put capactor; es due to trace resstances of a prnted crcut board (PCB) on the L sde; swtchng es on the H sde swtches (GB and dodes); core es of the power transformer and reverse recovery of the dode (for push-pull operatng ). TABLE CONDUCTON PERODS AND SWTCHNG TRANSTONS OF L-SDE SWTCHES DURNG TS (ZT PS FB S. PUSH-PULL) Part of T s Conductng sw. ntervals sw Loss of Duty Cycle 1.half: all SRs.half: all SRs 0 to d T s only ZT PS 1.half: Eq.0.half: Eq.17 Power Transfer SR 1,SR 3. half: SR,SR 4 d T s/ to D T s ZT PS and pushpull : Eq. 16 Dead Tme SR 1, SR 3 BD, BD 4. half: BD 1, BD 3 SR, SR 4 t d,lead,s Freewheelng Perod all SRs. half: all SRs D T s/ to T s / ZT: Eq. 19. half: Eq. 18 Pushpull : Eq. 1 Dead Tme BD 1, BD 3 SR, SR 4. half: SR 1, SR 3 BD, BD 4 t d,lagg,s /

6 The 014 nternatonal Power Electroncs Conference Swtchng es at the H sde, transformer core es and the dode reverse recovery are based on the characterzaton data of chosen components scaled to the operatng pont of nterest. f dead tmes are chosen properly, the turn-off of the body dodes s always soft and the voltage rses after the body dode current has already fallen to zero. Thus, swtchng es n the rectfer are neglected n ths analyss. Also, the body dode turns on before the gate sgnal occurs and the MOSFET channel only taes over the dode s current causng no turn-on es.. EXPERMENTAL ALDATON Fg. 8. Famly of effcency curves (prototype measurements) of (a) the conventonal ZT PS FB wth the transformer turns rato of 10:1 (b) the proposed reconfgurable converter wth the transformer turns rato of 10:10:1 (0:1) and reconf = 300. Detals of the prototype used for expermental valdaton of the proposed converter and ts desgn procedure can be found n Appendx A. The measured effcency of the conventonal ZT PS converter for several values of n s compared n Fg. 8 to the effcency of the converter proposed n ths wor. The power consumpton of the auxlary supply (for a controller, gate drvers etc.) s not ncluded n presented effcency results but does not dffer n the reconfgurable converter operatng n the ZT PS confguraton compared to the standard ZT PS. The swtchng frequency n both converters s set to 100 Hz. t s confrmed that n the upper range of n, wth values close to the nomnal battery pac voltage, the proposed reconfgurable topology operates wth ncreased effcency (curves from 300 to 400). n the lower range of voltages, e.g. at 50, the reconfgurable topology operatng n the hard-swtchng push-pull confguraton ndeed cannot reach the effcency of the conventonal converter. Loss dstrbuton s roughly determned usng the effcency measured for both the proposed and the conventonal converter from the example of n = 350 n Fg. 9(b)-(d). At ths value of n, the reconfgurable converter operates as well n the ZT PS confguraton wth D add conductng. t can be concluded that the man contrbutor to ths effcency mprovement acheved wth the proposed converter are reduced H sde currents and shorter freewheelng perod. Ths leads to sgnfcant reducton of both swtchng and conducton es at the H sde (Fg. 9(b)). Furthermore, n both reconfgurable and conventonal converter prototypes, the same L sde swtches are used, and thus no consderable dfference n es can be observed (Fg. 9(b)). Stll, under the consdered operatng condtons the proposed reconfgurable converter allows use of L sde swtches wth 5% lower blocng voltage capablty the addtonal reducton on the L sde of the proposed converter can be assured. Nevertheless, the overall converter operaton s barely affected by ths fact nowng that ths operatng voltage s present at the converter nput for a sgnfcantly shorter perod of tme compared to hgher voltage values. To quantfy the effcency mprovement obtaned wth the proposed reconfgurable converter durng one battery dscharge cycle, the voltage-weghted average effcency of both converters s calculated based on (1) and the data measured from Fg.8. Fg. 9(a) shows that when the converter effcency s quantfed n ths way, the proposed reconfgurable converter offers mproved voltage-weghted average effcency compared to the conventonal ZT PS n the complete range of load currents. n ths way, up to 50% of es that occur at the H sde swtches can be saved n the consdered example, wth further postve mpact on the chp temperature swng and lfetme. From the converter cost optmzaton pont of vew, the ncrease of the effectve converter s effcency up to % presents the potental to use a smaller chp sze. Fg. 9. (a) oltage-weghted average effcency of the proposed converter compared to the conventonal one calculated based on prototype measurements from Fg. 8. On the rght, the comparson of es dstrbuton n the reconfgurable and the standard ZT PS converter at n = 350 at (b) H sde swtches (c) L sde swtches and (d) passve elements (transformer, put flter and es n PCB).

7 The 014 nternatonal Power Electroncs Conference. CONCLUSONS Ths paper ntroduces an adaptaton of the conventonal ZT PS converter to mprove ts operaton over a wde range of nput voltages. As typcal operatng voltages are far away from the worst case desgn values n the consdered (H)E applcaton, the target s to ncrease the effcency n the range of the most common (typcal) operatng voltages. The optmzaton s acheved n a way that the transformer turns rato of the ZT PS converter s desgned for the hgher, more common range of nput voltages. As the ZT PS confguraton wth a gven turns rato s desgned for mnmum nput voltage hgher than the one requred n the specfcaton, t s not able to transfer the complete requred power n the lower range of nput voltages. From that reason converter s reconfgured to push-pull, that wll have no problem delverng requred power to the put wth the same, gven turns rato down to specfed n,mn. The reconfguraton happens at runtme, when H-battery voltage falls below specfed value of reconf. t s mplemented usng an addtonal dode and a swtch, whch add only nsgnfcant conductng es. Requred gate drvng patterns for the push-pull confguraton are derved from the ZT PS controller wth two logc AND gates. The concept of the proposed reconfgurable converter and the desgn methodology are valdated usng the prototype converter. Despte slghtly lower effcency under the less common nput voltage condtons (lower voltage range), the effectve converter effcency (the hstogram-weghted average over an applcaton-based voltage profle) s ncreased by ab %, whch s a sgnfcant mprovement nowng the fact that the conventonal converter topology already has an effcency greater than 90%. The acheved mprovement leads to slower agng of swtches due to the lower juncton temperature transents and can be utlzed n future wor for further reducton of chp szes resultng n more cost-effectve and compact desgns. APPENDX A: DESCRPTON OF PROTOTYPE CONERTER Components of the prototype converter used for the expermental verfcaton n Secton are as follows: Component Type Ratngs H GB/ dodes F4-50R07W1H3 B11A bl = 650 L MOSFE PB019N08N3 G (x per swtch) bl = 80 Transformer ColCraft NA637-AL n = 10:10:1 L TDK T H C 3x TDK B4179B7158Q 3x 1500 F APPENDX B: MODELNG OF SWTCH AND TRANSFORMER CURRENTS d = t (13) d T t) = + D ( t d ) (14) n mn s D ( frw mn = + D (( D d ) ) frw t (15) n ( t + t (16) D, sec ) = mn D, sec d,sec T s d = frw non cond (1 D) + t (17), n ( t t (18) frw1,sec ) = frw, non cond frw,sec = mn + D,sec ( D d ) frw, non cond t (19) d t D d,sec ( ) = mn + D,sec ( ) (0) T s d frw, cond (1 D) t n frw, pp = D,sec ( D ) frw, pp t (1) Coeffcents used n equatons (13) - (1) are defned as follows: ( 1 D ) rpple = () L d mn rpple =, ave (3) mn Llea = (4) n T n n d Llea, prm D s = (5) n = n (6) L n drop, prm frw = (7) Llea, prm frw, nc frw, c D,sec frw, PP n drop, prm + Llea, prm L = (8) n drop, prm Llea, prm L = (9) n = n L (30) = L (31) where:,ave s average value of put nductor current; rpple s value of rpple of put nductor current; L lea,prm s leaage nductance of transformer prmary wndng; D s a duty cycle determned by the controller; drop,prm s voltage drop over transformer prmary wndng durng freewheelng perod. REFERENCES

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