New Interleaved Current-Fed Resonant Converter with Significantly Reduced High Current Output Filter for EV and HEV application

Transcription

1 New Interleaved Current-Fed Resonant Converter with Significantly Reduced High Current Output Filter for EV and HEV application Dongok Moon Junsung Park Sewan Choi IEEE Senior Member Department of Electrical and Information Engineering Seoul National University of Science and Technology Seoul Korea Abstract- This paper proposes a new interleaved current-fed resonant converter with significantly reduced high current output filter which is suitable for EV and HEV applications The proposed interleaved converter has theoretically zero output filter capacitance low input current ripple ZCS turn on and off for all switches and diodes and zero di/dt at turn off of diodes when operated at load independent points A two-stage power conversion technique is applied for wide input and output voltage range operation of EV and HEV A 2kW prototype of the proposed converter has been built and tested to verify the validity of the proposed operation Keywords- Interleaved Current fed resonant converter Soft-switched Electric vehicle I INTRODUCTION Recently eco-friendly cars such as Electric Vehicles(EV) Hybrid Electric Vehicles(HEV) or Plug In Hybrid Electric Vehicles(PHEV) are attracting increasing attention as a solution of environmental pollution global warming and exhaustion of fossil fuels Configurations of several types of EV and HEV power train systems are described in [I] The low voltage dc/dc converter(ldc) provides power to 12V loads such as the head lamps wiper blade motor electronic power steering (EPS) radio system etc by charging a 12V auxiliary battery from a high voltage battery( V)[2] This application requires an efficient(more than 90%) compact and light weight dc-dc converter Also due to safety and high step down conversion ratio galvanic isolation is generally required The phase shift full-bridge PWM converter is widely used in the dc/dc conversion stage because of its small RMS current and inherent ZVS[3] Disadvantages of the phase shift full-bridge PWM converter is that turn off current of switches is large and turn off loss associated with the diode reverse recovery is considerable In order to reduce the turn off losses of the switches and diodes resonant converters with ZVS or ZCS capabilities such as SRC and LLC can be considered as candidates for the low voltage dc/dc converter[4]-[5] In general the resonant converter requires output capacitor for suppression of output ripple current while the phase shift full-bridge PWM converter requires output inductor for D1 r-= Fig! Proposed current-fed resonant converter suppression of output ripple voltage The volume of the output filter inductor or capacitor is considerable especially in low output voltage and high output current application[6]-[7] In order to reduce the volume of the output filter interleaved techniques can be applied to the resonant or PWM converters[8] However volume reduction of the output filter resulting from interleaving of the conventional resonant or PWM converters may not be satisfactory in the low voltage high current application such as LDC This paper proposes a new interleaved current-fed resonant converter with significantly reduced high current output filter which is suitable for EV and HEV applications The proposed interleaved resonant converter has the following features when operated at 1; = 051;: 1) theoretically zero output filter capacitance 2) low input current ripple 3) ZCS turn on and off for all switches and diodes 4) zero dj/dt at turn off of diodes resulting in negligible turn off losses associated with the diode reverse recovery A two-stage power conversion technique is applied for wide input and output voltage range operation of EV and HEY A 2kW prototype of the proposed converter has been built and tested to verify the validity of the proposed operation I PROPOSED CURRENT-FED RESONANT CONVERTER Fig 1 shows the circuit diagram of the proposed current-fed resonant converter The proposed converter consists of an input filter inductor four switches a resonant tanl<- a transformer a diode rectifier and an output filter capacitor The output voltage of the proposed converter is regulated by fixed duty and variable switching frequency 1 D2 D3 D4 Co /14/$ IEEE 1394

2 : T I hr I-_ _ ;;':"' _-+ -+: Ii : VSI t i I ;:: : / 'Iij; Loud Independent points \ i / 4j; ':---ib!---;:'-:-i=i--::' 0-:: : i J 2 j; :/ Qincrease 0::'- 4:---!0!J:S- "'" 06::-- 0"-= 7:-- -::' 0""'8 ----:0""9::---'10 II/ Fig5 Voltage gain curve of the proposed converter The resonant voltage and current are determined respectively as follow: irat)=i (l-cosmt) (1) VCr(t) =Z1 sino)t+nv; (2) Fig2 Key waveforms ofthe proposed converter atf O5f where Z2 = L/C)s characteristic impedance and w2= lil; c; is angular resonant frequency Input current 1; is determined as follow: (3) I 0 C J }} ] Lr D\ D3 - U - S\ S3 n:l J D2 D4 S2 S4 - CO where RL is load resistance and this mode ends when switches SI and S4 are turned off at tl Mode II [t1 tzl: During this mode the power is not transferred to the load and the output filter capacitor supplies the load The voltage of the resonant capacitor increases linearly The other half of a cycle is repeated in the same fashion Note that all switch and diode are turned on and off under ZCS condition E Voltage Gain Expression Assuming that Mode II the dead-time is neglected the average voltage across the resonant capacitor that is equal to the input voltage can be expressed as: Fig 3 Operating states ofthe proposed converter atf O5f (4) c L N pv! N 0 From eqn (4) the voltage gam of the proposed converter is determined by: Fig 4 Equivalent resonant circuit of Mode I A Operating Principle The key waveforms and operating states of the proposed converter at switching frequency f; = 051: are shown in Figs 2 and 3 respectively Mode I [to tj: This mode begins with Lr-Cr resonance when switches SI and S4 are turned on at to The equivalent circuit of this mode is shown in Fig 4 where Q is the quality factor which determines the slope of voltage gain curve and determined as follow: (5) (6) 1395

3 V; f iot io Co Vo Phase 1 I; * Vo io/ io Phase 2 J (a) + i02 Phase * (a) io2 II II to (b) Fig6 The conventional two-phase interleaved series resonant converter (a) circuit diagram (b) key waveforms atls = j (b) Fig7 The proposed two-phase interleaved current-fed resonant converter (a) circuit diagram (b) key waveforms atls=o5j The higher Q is the higher slope of the gain curve is The voltage gain curves of the proposed converter are as shown in Fig 5 Note that the load independent points of the proposed converter are multiple in the below resonance region and are determined respectively as follow: J = 2k;; (7) where k is the natural number It should also be noted that the voltage gain of the proposed converter converges to I while that of the SRC converges to 0 as the switching frequency decreases in the below resonance region In the above resonance region the slopes of the voltage gain curves according to Q of the proposed converter have similar values with those of the SRC C Proposed Interleaved Resonant Converter The circuit diagram and key waveforms of the conventional two-phase interleaved SRC are shown in Fig 6 The two SRC are operated at f = t; and phase shifted by n12 The output current of each phase is the secondary winding current rectified by the diode bridge and can be expressed as using Fourier series I= ( ( n) 2 (1+2m)(1-2m) 2 ) 111=1 I =-+ cos 2m wt-- The output current of the two-phase interleaved SRC is obtained by = = I + "\' 0 (8) (9) II (10) o :t(1 +4m)(1-4m) cos(4mwt) It is seen from eqn (10) that there exist multiples of fourth harmonic component in the output current In general the output current of the conventional n-phase interleaved SRC has ripple contents of multiples of 2n-th harmonic component as shown in eqn (II) = f() + II 0 N = 21 2( I) (II) cos(2awt m- n) =1 1It=1 n(l + 2m)(1-2m) N 1396

4 Non-isolat ed convert er Conventional resonant convert er Two-st age convert er Fig 6 Concept of the conventional two-stage converter :'ff;h:!i-$bldit + Non-isolat ed convert er Proposed resonant converter llrrti Cf eliminat e Proposed two-st age convert er Fig 7 Concept of the proposed two-stage converter It means that the output ripple current of the conventional interleaved SRC cannot be completely eliminated Figs 7 (a) and (b) show the circuit diagram and key waveforms of the proposed two-phase interleaved converter Note that the two current-fed resonant converters are operated at f = 05f and phase shifted by n12 The output current of each phase is the secondary winding current rectified by the diode bridge and can be obtained by iol = 10 (1-cosmt) (12) io2 = I() (1-cos(mt -n)) (13) The output current of the proposed interleaved converter is obtained by 1 I = -10(1-cosmt) +-10(1 + cosmt) = ( 14) It should be noted that ac ripple components of the output current are completely eliminated and the output current is ripple-free meaning that required output capacitance C) is theoretically zero The proposed interleaving technique is very effective especially in the low voltage and high current applications where the output filter significantly affects the efficiency and size of the whole system The concept of interleaving of the proposed current-fed resonant converter can be extended to n-phase system The output current of the n-phase interleaved current-fed resonant converter can be obtained by Fig 8 The PIPO interleaved converter Fig 9 The SIPO interleaved converter i o = i ol + i i on _ '" N 10 (15) ( 1 ( 2(m - - L - I)1t )) _ - cos rot m=] N N 0 A two-stage power conversion technique is employed for wide input and output voltage range operation of EV and REV Figs 6 and 7 show the concept of the conventional and proposed two-stage converter using a buck converter as a non-isolated converter stage respectively [t is seen from Fig 6 that the output capacitor Cf of the nonisolated converter and the input capacitor C of the conventional resonant converter are combined into one capacitor Cdc in the two-stage converter that is comparatively large since the voltage ripple of the capacitor should be within some specified limit In the meanwhile it is seen from Fig 7 that the output capacitor Ct of the non-isolated converter is eliminated and the filter inductor Lt of the non-isolated converter and the input inductor Li of the proposed resonant converter are combined into one inductor Lf in the twostage converter Note that the capacitor Cr in the proposed two-stage converter does not need to be large since it is used only as a resonant capacitor The two types of the proposed interleaved two-stage converter are shown in Fig 8 and 9: parallel input and parallel output(pipo) converter and series input and parallel output(sipo) converter The PIPO interleaved converter is better suited to relatively low input voltage application while S[PO interleaved converter is better suited to relatively high input voltage application The PIPO converter is more vulnerable to current unbalance caused by resonant component tolerances parasitic component of each converter Instead the current unbalance can be alleviated by controlling each of the 1397

5 h_in+rleuving [51/i:liv] i - : r I I I I V\'hl [loov/div] (a) C ' [!lojlsvdivl t :J"tt i i I l l t!! i! lo sydiv] iol OA/diV] (b) r iio21 OA/ ivl - - L i---i SOAIdiv 1 " i i i 01 i i t ! ! ! t t ' : ' : : : : 12Jls/divl (e) T------r T------r T I I I I I : r ] T F r : : : I I I I I I I I I I I I I I I I I I I I I I I I I I I I i +: ' Fr El:I : L :- : L_ : L_ : _ : (f) ;d"l Fig 10 Experimental waveforms at full load (a) switch voltage VShl and diode voltagevtjhl and inductor current iu (b) inductor current iu in and interleaving current ir interlem'in!: (c) switch voltage VSl and resonant current ijrl (d) diodes voltage V TJ12 and rectified current iol (e) rectified current ioh io2 and interleaved output current io (f) ripple current of output capacitor 95 to inherent charging balance of two resonant capacitors[9] III EXPERIMENTAL RESULT A 2-kW prototype of the proposed interleaved converter shown in Fig 14 was built under the following specification : 85 o Output Power [WI 2000 Po = 2 kw Vi = 400 V Va = 12 V Is = 20 khz Ir = 60 khz Np: N = 5 : 1 Lrlf2 = 560 fir Cr= 330 fif Lrlr2 = 374 fir C'Ir2= 047 ur DdT = 100 ns Fig II Measured efficiency as a function of the output power at Vi = 400V non-isolated stage of the P[PO converter On the other hand the S[PO converter is immune to current unbalance caused by resonant component tolerances due Fig 10 shows key experimental waveforms of the prop osed converter at full load Figs lo(a) and (b) show nonisolated stage switch voltage and inductor current Fig 1 O(c) and (d) show isolated stage switch voltage and CUfre nt It is seen that all switches are turned on and off under ZCS condition Fig I O( e) and (0 show interleaved outpu t current and ripple current of the output capacitor [t can 1398

6 Fig 12 Photograph of the proposed interleaved resonant converter prototype be seen that output ripple current is significantly reduced to 42Arms Fig 11 is the measured efficiency using Yokogawa WT3000 The maximum efficiency is 934% at O8kW and full load efficiency is 9l3% Fig 12 is the prototype of proposed interleaved converter [7] U Badstuebner T Biela D Christen and J W Kolar "Optimization of a 5-kw telecom phase-shift dc-dc converter with magnetically integrated current doubler" IEEE Trans Ind Electron vol 58 no 10 pp [8] K Yi and G Moon "Novel two-phase interleaved LLC series-resonant converter using a phase of the resonant capacitor" IEEE Trans Ind Electron vol 56 no 5 pp [9] B C Kim K B Park C E Kim and G W Moon "Load sharing characteristic of two-phase interleaved LLC resonant converter with parallel and series input structure" in Proc IEEE ECCE 2009 pp IV CONCLUSIONS This paper proposes a new interleaved current-fed resonant converter with significantly reduced high current output filter which is suitable for EV and HEV applications The proposed interleaved converter has theoretically zero output filter capacitance low input current ripple ZCS turn on and off for all switches and diodes and zero dildt at turn off of diodes when operated at f = 05f A two-stage power conversion technique is applied for wide input and output voltage range operation of EV and HEY A 2kW prototype of the proposed converter has been built and tested to verify the validity of the proposed operation The maximum efficiency is 934% at O8kW and full load efficiency is 913% The proposed converter could be a possible option for low voltage high current application such as LOC REFERENCES [1] A Emadi L 100 and K Rajashekara "Power electronics and motor drives in electric hybrid electric and plug-in hybrid electric vehicles" IEEE Trans Ind Electron vol 55 no 6 pp [2] Z Amjadi and S S Williamson "Power-electronicsbased solutions for plug-in hybrid electric vehicle energy storage and management systems" IEEE Trans Ind Electron vol 57 no 2 pp [3] O Hamza M Pahlevaninezhad and P lain "Implementation of a novel digital active EMI technique in a OSP-based DC-DC digital controller used in electric vehicle (EV) battery charger" IEEE Trans Power Electron vol 28 no 7 pp [4] 1 Lee and G Moon "Analysis and design of a three-level LLC series resonant converter for high-and wide-inputvoltage applications" IEEE trans Power Electron vol 27 no 6 pp [5] T Biela U Badstuebner and T W Kolar "Design of a 5- kw I-U 10-kW/dm3 resonant DC-DC converter for telecom applications" IEEE trans Power Electron vol 24no 7pp [6] S Cho 1 Lee 1 Kim and G Moon "A new standby structure based on a forward converter integrated with a phase-shift full bridge converter for server power supplies" IEEE Trans Power Electronvol 28 no I pp