Energy Conversion and Management

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1 Energy Conversion and Management 49 (2008) Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: A bidirectional soft switched ultracapacitor interface circuit for hybrid electric vehicles Hosein Farzanehfard *, Dawood Shekari Beyragh, Ehsan Adib Electrical and Computer Engineering Department, Isfahan University of Technology, Isfahan 8456, Iran article info abstract Article history: Received 9 December 2007 Accepted 2 July 2008 Available online 3 August 2008 Keywords: Hybrid electric vehicle capacitor Soft switching Buck-and-boost converter capacitors are used as auxiliary elements beside batteries to increase peak power capability and battery life in hybrid electric vehicles. In such a configuration, a bidirectional high efficiency converter is required as an interface between ultracapacitors and batteries. Since the voltage level of ultracapacitors and batteries are different, the interface must be able to increase or decrease the voltage level in each power flow direction while limiting the current. This paper presents a zero voltage transition (ZVT) buck-and-boost converter for ultracapacitors interface. All the switches in the proposed converter are soft switched to reduce switching losses and increase efficiency. The converter operational modes are analyzed and its performance is discussed. Finally, the experimental results from a 50 W laboratory prototype are presented which justify the theoretical analysis. Ó 2008 Elsevier td. All rights reserved.. Introduction * Corresponding author. Tel.: ; Mobile: ; fax: addresses: hosein@cc.iut.ac.ir (H. Farzanehfard), shekari@ec.iut.ac.ir (D.S. Beyragh), adib.ehsan@gmail.com (E. Adib). A hybrid electric vehicle (HEV) combines a conventional propulsion system with an on-board rechargeable energy storage system (RESS) to achieve better fuel economy than a conventional vehicle without having range limitation like an electric vehicle. Most commonly, HEVs use an internal combustion engine (ICE) and electric batteries to power electric motors. Modern mass produced HEVs, prolong the charge on their batteries by capturing kinetic energy via regenerative braking or use the combustion engine to generate electricity by spinning an electrical generator. This contrasts electric vehicles which use batteries charged by an external source. Many HEVs reduce idle emissions by shutting down the ICE at idle and restarting it when needed. An HEV engine is smaller and may run at various speeds, providing better efficiency. capacitors are new family of energy storage devices with many applications in power electronics. capacitors (UCs) have 20 times more energy storage capacity than electrolytic capacitors and in comparison to batteries; UCs can provide much higher power pulses []. However, they can store less energy than batteries. Furthermore, they can be charged and discharged thousands of times without performance deterioration. This characteristic can be used in combination with normal electrochemical batteries to improve the transient performance of an electric vehicle and to increase the useful life of batteries [ 6]. Fast and sudden battery discharge during acceleration, or fast charge during regenerative braking can be avoided with the help of ultracapacitors. Furthermore when ultracapacitors are not used, the battery in an electric vehicle must be sized to provide both the energy storage capacity (kw h) required to attain the vehicle design range and the peak power (kw) needed to satisfy acceleration and grade ability requirements. The periods of peak power demand during vehicle acceleration are relatively short, generally less than 30 s. Thus ultracapacitors can be easily used at these periods to provide the peak power demand. The ultracapacitor bank can be charged from the main storage battery during periods when the vehicle power demand is low. The ultracapacitor voltage rating is usually low. Therefore, in joint use of ultracapacitors and batteries, an interface circuit is required to match the low voltage of ultracapacitors with higher batteries voltage while limiting the charging and discharging current of ultracapacitors. Several interface converters have been previously proposed but they are hard switched and thus their efficiencies are relatively low [3,4,7,8]. In[9] a soft switched isolated interface circuit is introduced, but its efficiency is low because of transformer losses. The transformer in this converter is used for isolation and to provide soft switching. Since in HEV there is no need for isolation, this converter is not a proper solution. A high efficiency interface circuit is essential to reduce the number of ultracapacitors and to reduce the overall volume of energy storage elements and the interface circuit. The basic circuit usually used as interface between ultracapacitors and batteries is a buck-and-boost converter shown in Fig.. This converter operates in bucking mode while charging the /$ - see front matter Ó 2008 Elsevier td. All rights reserved. doi:0.06/j.enconman

2 H. Farzanehfard et al. / Energy Conversion and Management 49 (2008) S 2 version would considerably decrease the converter efficiency. Therefore, regular pulse width modulation (PWM) converters have low efficiency at high frequencies because of switching losses. Zero voltage transition (ZVT) technique is a soft switching technique that incorporates soft switching function to PWM converters. In this paper a new soft switched buck-and-boost interface circuit is developed for ultracapacitors, using ZVT technique. Fig.. Buck-and-boost converter. 2. Derivation of ZVT switch cell capacitors and in boosting mode while discharging the capacitors. In switching instances, the voltage across the switch and the current through the switch are both high and thus, the switching losses are significant. To reduce the volume of the converter, high switching frequency is indispensable. High frequency power con- S a S Fig. 2. ZVT switch cell. In ZVT technique to reduce switching losses a snubber capacitor is added across the switch. Thus the switch is turned off under zero voltage condition, which decreases switching losses at switch turn off time. But, it is necessary to discharge the capacitor before turning the switch on. It is possible to completely discharge a capacitor charged to voltage V through an inductor and an auxiliary switch in a voltage source with V/2 voltage. Therefore, to discharge the snubber capacitor when the switch is off we need a voltage source that its voltage is V/2. To realize this voltage source a capacitive voltage divider can be used. In this manner two equal capacitors are connected in series between positive line and ground. The ZVT switch cell obtained by this method is shown in Fig. 2. Fig. 3a shows the buck converter with ZVT switch cell and Fig. 3b shows the boost converter with ZVT switch cell. In these figures switch S is the converter main switch and S a is the auxiliary switch used to discharge the snubber capacitor through the auxiliary inductor. By combining the ZVT buck converter and ZVT boost converter, the ZVT buck-and-boost converter as shown in Fig. 3c is created. This converter can be used as an ultracapacitor interface circuit. S S a a Sa S Sa Sa2 S 2 Fig. 3. (a) ZVT buck converter (b) ZVT boost converter (c) ZVT buck-and-boost converter.

3 3580 H. Farzanehfard et al. / Energy Conversion and Management 49 (2008) Circuit description and operation The proposed converter shown in Fig. 3c is composed of two main switches, and S 2, two auxiliary switches, S a and S a2, a main inductor,, an auxiliary inductor,, two capacitors C i and C i2 as capacitive divider of input voltage, and a snubber capacitor C s.to analyze the converter operation, it is assumed that the voltages of the DC bus and ultracapacitor are constant in a switching cycle. Furthermore it is assumed that the change in the voltage of the capacitive divider is negligible when charge pulses are drawn and injected in every switching cycle. Operation of the auxiliary circuit in buck and boost modes are similar and thus, only the buck mode operation is discussed. In buck mode, the circuit has eight distinct intervals in a switching cycle. Important waveforms of this converter operation in the buck mode are shown in Fig. 4 where the transition times are exaggerated to provide better illustration. Also the equivalent circuit for each operating interval is shown in Fig. 5. Before the first interval, diode D 2 is conducting the output current I 0 and the main switch is off. Detail description of each interval is as follows: Interval [t 0 t ]: this interval starts by turning S a on. By turning this switch on, the current of auxiliary inductor,, starts to increase linearly, until it reaches the output current I 0 and prepares the condition for diode D 2 to turn off under zero current (ZC) condition. The equation of auxiliary inductor current in this interval is i a ðtþ ¼ 2 ðt t 0 Þ Also the duration of this interval is t t 0 ¼ 2I 0 According to Eqs. () and (2), for design purposes, the charge drawn from capacitive voltage divider in the first interval can be obtained as: Q ¼ I 2 0 Interval 2 [t t 2 ]: When current becomes equal to output current, diode D 2 turns off under ZC condition and a resonance starts between and C s. During this resonance C s is charged from zero to and provides zero voltage (ZV) condition for turn on. The important equations of this interval are i a ðtþ ¼I o þ 2Z sinðx ðt t ÞÞ v CS ðtþ ¼ 2 ð cosðx ðt t ÞÞÞ ð5þ ðþ ð2þ ð3þ ð4þ S a Switchstate ON Don t care OFF S a2 V 2 in V Sa V 2 in V Sa I a I 0 I S V S t 0 t t 2 t 3 t 4 t 5 t 6 t 7 Fig. 4. Main theoretical waveforms.

4 H. Farzanehfard et al. / Energy Conversion and Management 49 (2008) Fig. 5. Equivalent circuit for each operating interval. where x ¼ pffiffiffiffiffiffiffiffiffi C sffiffiffiffiffi s Z ¼ C s Duration of second interval is half a resonance period as follows: t 2 t ¼ p x Using (4) and (6), the charge drawn from the capacitive voltage divider is pffiffiffiffiffiffiffiffiffi Q 2 ¼ C s þ pi 0 C s ð9þ Interval 3 [t 2 t 3 ]: the main switch is turned on at the beginning of this interval under zero voltage zero current (ZVZC) condition, thus the inductor current starts to descend from I 0 to zero and current rises from zero to I 0. At the end of this interval, S a current becomes zero and S a can be turned off under ZC condition. The current of auxiliary inductor in this interval is i a ðtþ ¼I o ðt t 2 Þ 2 Also, the duration of this interval is ð6þ ð7þ ð8þ ð0þ Using Eqs. (0) and (), the charge drawn from capacitive voltage divider is as follows: Q 3 ¼ I 2 0 ð2þ This charge is exactly equal to the charge drawn in the first interval. Interval 4 [t 3 t 4 ]: at this interval the converter operates like a regular buck converter and the main switch current is almost constant and equal to I 0 (the output inductor is assumed large). Interval 5 [t 4 t 5 ]: to replenish the charge drawn from the capacitive voltage divider in the first through the third interval and to stabilize its voltage at /2, an equivalent amount of charge must be injected back to the capacitors in the fifth through the seventh interval. For this purpose, this interval starts by turning S a2 on which linearly increases the current through auxiliary inductor in the opposite direction with respect to the previous intervals. The auxiliary inductor current in this interval can be obtained as below i a ðtþ ¼ 2 ðt t 4 Þ ð3þ This interval ends by turning off at t 5. Thus the amount of charge injected to the capacitive voltage divider during this interval is equals to t 3 t 2 ¼ 2I o ðþ Q 5 ¼ 4 ðt 5 t 4 Þ 2 ð4þ

5 3582 H. Farzanehfard et al. / Energy Conversion and Management 49 (2008) Interval 6 [t 5 t 6 ]: this interval begins with turning the main switch off. Due to snubber capacitor, the voltage across the main switch changes slowly and is turned off under almost zero voltage (ZV) condition. At the end of the interval, C s discharges to zero. The auxiliary inductor current, i a, in this interval can be obtained as follows: i a ðtþ ¼ A sinðx ðt t 5 ÞþhÞþI o ð5þ Where the amplitude and phase constant of this equation are sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 A ¼ þði i aðt 5 ÞÞ 2 ð6þ 2Z h ¼ tan 2Z ½I 0 i a ðt 5 ÞŠ ð7þ Also, the voltage of snubber capacitor, C s,is b Sa S a2 S 2 Boost Buck Fig. 6. Modified interface circuit. Switch state S a ON Don t care OFF S a2 S 2 I a I S t 0 t t 2 t 3 t 4 t 5 t 6 t 7 Fig. 7. Main theoretical waveforms of modified interface circuit. S a S a2 S 2 Super Fig. 8. Simplified auxiliary circuit.

6 H. Farzanehfard et al. / Energy Conversion and Management 49 (2008) v CS ðtþ ¼AZ cosðx ðt t 5 ÞþhÞþ 2 Duration of this interval is t 6 t 5 ¼ x p 2 tan 2Z ½I 0 i a ðt 5 ÞŠ ð8þ ð9þ The charge injected to the capacitive voltage divider in this interval can be obtained as ð20þ Q 6 ¼ C s I 0 x p 2 tan 2Z ½I 0 i a ðt 5 ÞŠ Since the average value of voltage across the auxiliary inductor during this interval is zero, its current at the end of this interval equals to its starting value. Interval 7 [t 6 t 7 ]: D 2 begins to conduct under ZVZC condition and conducts the output and auxiliary inductor currents. Because of the positive voltage across the auxiliary inductor, its negative current starts to decrease and becomes zero. Thus, S a2 can be turned off under ZC condition at the end of the interval. The current through the auxiliary inductor can be obtained as the auxiliary circuit is equal to ; but for compensating current pulses, parallel diode of b blocks the current and the effective inductance of the branch rises up to the sum of the inductances and b. In this manner, by turning S a2 on at the start of the fifth interval, the current slope decreases by a factor of /( + b ); so i a ðtþ ¼i a ðt 5 Þþ 2 ðt t 6 Þ ð2þ The duration of the interval is: t 7 t 6 ¼ 2 i a ðt 5 Þ ð22þ This duration is precisely equal to the duration of the fifth interval since the voltage across the auxiliary inductor in this interval is exactly equal and opposite to the fifth interval and thus the charge injected to capacitive voltage divider in this interval can be obtained as Q 7 ¼ Q 5 ¼ 4 ðt 5 t 4 Þ 2 ð23þ Interval 8 [t 7 t 8 ]: converter operation in this interval is like a regular buck converter and the output current runs through D 2. As described, all switching actions in the proposed converter occurs under ZV, ZC or ZVZC condition, which results in low switching losses and high efficiency. 4. The modified topology for current stress suppression As mentioned in the buck mode operation, for compensating the charge drawn from the capacitive divider in the first through the third interval, S a2 must be turned on at a specific time before turning off. This time can be calculated from Q þ Q 2 þ Q 3 ¼ Q 5 þ Q 6 þ Q 7 ð24þ Turning S a2 on injects a negative current pulse to the capacitive divider and rises its voltage toward the nominal value. In the same manner, in the boost mode of operation, S a must be turned on at a specific time before S 2 is turned off. This would drain the charge which was injected to the capacitive divider in the first through the third interval. Since is used for charge compensation and its value is small, a high amplitude pulse current is applied to the main switch in the fifth interval as shown in Fig. 4. To obviate this problem, the duration of compensation mode can be increased to reduce the amplitude. The scheme used for implementation of this method is shown in Fig. 6. In this method, an inductor, b, which is paralleled with a diode is inserted in series with the auxiliary inductor,. The direction of the paralleled diode with b is controlled by a relay which connects appropriate diode in each mode of operation. In the buck mode, the positive current pulses is bypassed through the parallel diode of b, thus the effective inductance of Fig. 9. Experimental results voltage (top waveform) and current (bottom waveform) of (a) main switch (b) auxiliary switch (c) main diode (vertical scale is 0 V/ div or 0 A/div and time scale is ls/div).

7 3584 H. Farzanehfard et al. / Energy Conversion and Management 49 (2008) Table Comparison of losses in regular buck-and-boost converter and the proposed ZVT buck-and-boost converter Type of loss Formula Regular buck-and-boost ZVT buck-and-boost S switching losses 2 V in I 0 ðt on þ t off Þþ ð00 þ 00Þ0 9 þ Zero due to soft switching ði 0 þ I rrþt rrþf SW 24 ð0 þ 2Þ Þ0 0 3 Sa switching losses 2 I 0 ðt on þ t off ÞF sw N.A Zero due to soft switching S parasitic capacitance losses 2 Cout V 2 in Fsw Zero due to soft switching Sa parasitic capacitance losses 2 Cout ðv 2 Sa ÞFsw N.A S conduction losses R ds F sw R T I2 S 0.07 * 0 * 0 3 * 5 * * 0 * 0 3 * 5 * 0 4 Sa conduction losses R ds F sw R T I2 N.A 0.07 Sa * 0 * 0 3 * 3.6 * 0 5 D2 conduction losses I ave D V F 4.4 * * 0.8 Body diode of Sa2 conduction losses I ave Da V F N.A 0.47 * 0.8 Total converter losses 2.7 W 7. W the amount of charge required for compensating the charge drawn from capacitive voltage divider is achieved by a current pulse which is wider in time and shorter in amplitude. The waveforms corresponding to modified circuit in buck mode of operation is shown in Fig. 7. The equations obtained for the circuit operation in the last section are also valid for the modified topology except for the fifth through the seventh intervals (Eqs. (3) (23)). To validate these equations for the modified topology, must be replaced by + b. In the modified topology, due to the short length of the sixth interval in relation to the fifth and seventh intervals, the charge contained in compensating current pulse can be estimated as the sum of charges in the fifth and seventh intervals as below: Q cmp 2ð þ b Þ ðt 5 t 4 Þ 2 ð25þ The maximum value for b in order to reduce the current stress of the main switch can be obtained from the above equation by substituting the time length of the fifth interval with the minimum of DT or ( D)T. Also, the compensating charge should be replaced by the maximum current drawn from capacitive divider in the first through the third intervals. Thus, the maximum of b depends on the minimum and maximum values of duty cycle where any further increase in b requires a time length bigger than DT or ( D)T for the fifth interval. 5. Simplified interface circuit and experimental results Since more than one battery cell is usually available in the hybrid electric vehicle, the converter can be simplified much more. In this case, C i and C i2 can be replaced by batteries and thus there is no need for charge compensation any more and the interface can be simplified as shown in Fig. 8. In this case, the fifth and seventh operating intervals will not occur any more and the converter would be controlled like a regular buck-boost converter. A 50 W prototype of this converter switched at 0 KHz is implemented. For all switches IRF540 is used. A 47 nf capacitor is used for C s,a 2.5 lh inductor is used for a and a 00 lh inductor is used for the output inductor. Two 2V lead acid batteries are used. The experimental results shown in Fig. 9 justify the theoretical analysis. The converter losses are analyzed in Table and the converter is compared to a 50 W hard switched counterpart. The efficiency of the proposed converter is almost 4% higher than its hard switching counterpart. Since the converter is bidirectional, thus, the total power conversion efficiency is increased by almost 8%. This means that the volume of energy storage elements can decreased by almost 8%. 6. Conclusions capacitors are used as auxiliary elements beside batteries in hybrid electric vehicle. In such a configuration a bidirectional, high efficiency converter is required as an interface between ultracapacitors and batteries. In this paper, a ZVT buck-and-boost converter is introduced and analyzed for ultracapacitors interface that guarantees soft switching condition for all semiconductor devices. In this converter there is no additional voltage stress on the main switches and diodes and the additional current stress on the main switches is very small. Also, the voltage rating of the auxiliary switches is low and soft switching condition is achieved for full load range. 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