Voltage and Current Ripple Considerations for Improving Lifetime of Ultra-Capacitors used for Energy Buffer Applications at Converter Inputs

Transcription

1 Voltage and Current Ripple Considerations for Improving Lifetime of Ultra-Capacitors used for Energy Buffer Applications at Converter Inputs Supratim Basu Bose Research Pvt. Ltd., 34 2 nd Main Cholanagar Bangalore , INDIA Tore M.Undeland Department Of Power Electronics, Norwegian University Of Science And Technology, Trondheim, Norway Abstract-When Ultra-Capacitors are used for providing energy to power downstream switch mode converters used for power conversion, the significant change in their capacitance with frequency can result in significantly higher voltage ripple and current ripple in these capacitors. This results in over voltage and temperature rise in these capacitors resulting in their consequent lifetime reduction. Based on extensive measurements of these voltage ripple and current ripple at different converter switching frequencies, design considerations that can significantly reduce these voltage and current ripple is proposed in this paper. I. INTRODUCTION Ultra-capacitors or electric double layer capacitors (ELDC) are electrical energy storage devices, which offer high power density, extremely high cycling capability and mechanical robustness [1]. Due to these features, Ultra-Capacitors have a high potential of being used in industrial applications. To improve their performance, reliability and lifetime, efficient charge balancing circuits [2], power circuits that do not overcharge or overheat these capacitors due to high ripple voltage or ripple current, are very important. When using Ultra-Capacitors as an energy storage buffer for downstream PWM converters, the resulting voltage and current ripple can cause temperature rise and lifetime reduction of the Ultra-Capacitor. Based on extensive measurements of their voltage and current ripple for different downstream PWM converter switching frequencies, design considerations that significantly improves the lifetime of the Ultra- Capacitors that can be used in energy storage buffer applications, is proposed. II. THE PROBLEM IN SPECIFIC Either over voltage or capacitor temperature rise due to charging or discharging ripple current, reduce the lifetime of Ultra-Capacitors. Though the standard temperature rating for Ultra-Capacitors is 25 C to 70 C, ambient temperature rise in combination with over voltage reduce their lifetime too. In general, raising the ambient temperature by 10 C will decrease their lifetime by at least a factor of two [3]. Thus the maximum operating voltage of an Ultra- Capacitor should be reduced with increasing ambient temperature. Overheating of Ultra-Capacitors occur due to either high charging ripple current or high discharging ripple current or over voltage during charging, leading to increased gas generation, resulting in decreased lifetime, leakage, venting or rupture. Though for highest energy storage the Ultra-Capacitor must be operated to its maximum rated nominal working voltage, care needs to be taken that the charging voltage ripple or reflected voltage ripple does not cause over voltage to the capacitor. Moreover, as Ultra-Capacitors have a higher ESR compared to aluminum electrolytic capacitors, they are more susceptible to internal heat generation when exposed to higher ripple current. R 5 R 4 R 3 R 2 R 1 i SC R P C 5 C 4 C 3 C 2 C 1 U S C Figure 1a Model of a Ultra-Capacitor

2 Capacitance(F) Frequency(Hz) Figure 1b. Capacitance vs. Frequency Plot of Ultra-Capacitor In order to ensure a long lifetime, it is [3] thus recommended that the maximum ripple current should not increase the surface temperature of the Ultra- Capacitor by more than 3 C above ambient and also to have the lowest possible operating ripple current and ripple voltage on these capacitors. In addition to the above issues, the nominal capacitance of an Ultra-Capacitor is applicable only at dc with its capacitance dropping rapidly to near zero at higher frequency [4]. Figure1a shows a typical Ultra- Capacitor model where the leakage resistance is represented as R P, the highest and lowest capacitances are respectively represented by C5 and C1 and the capacitor s ESRs are represented as R1 to R5. As apparent from the frequency (F SC ) response given in Figure 1b, an Ultra-Capacitor cannot significantly attenuate the high frequency voltage ripple generated by a switching PWM converter since its large capacitance (C SC ) at dc rapidly reduces to a very low value beyond just 100 Hz. Ultra-capacitors can be simultaneously charged and also used as an energy storage buffer by paralleling them to an energy source, i.e. battery, fuel cell, DC-DC converter [5], etc. The voltage and current ripple caused by the charging converter or the converter that these capacitors intend to provide power back up to, can also cause over charging or temperature rise of these capacitors. III. STRATEGIES FOR REDUCING RIPPLE VOLTAGE AND RIPPLE CURRENT As discussed so far, reduction of both operating voltage ripple and ripple current in Ultra-Capacitors is necessary for increasing their lifetime. Higher voltage ripple could cause over voltage in these capacitors while higher ripple current could overheat them. Though one solution would be to increase the filter inductance and or reduce the switching frequency of the buck derived DC-DC converter that is usually used for power conversion downstream, this will significantly increase converter size and cost. This is even more significant as these downstream converters are usually high power rated since Ultra-Capacitor applications are becoming increasingly common for high power applications like electric vehicle and solar converters. Moreover increasing inductance requires higher turns and this increases both the radiated fields from the inductor and conducted noise due to the increased interwinding capacitance of the inductor. Thus these radiated fields and the feed through noise through the inter-winding capacitance [6] from the inductor mainly couple to surrounding circuits and increase EMI. Thus a better solution would be to use such type of additional filter circuits that attenuates both the voltage ripple and ripple current and also allows higher frequency operation of the converter that causes this ripple. Design strategies that significantly improve the energy storage performance and lifetime of these capacitors without being overstressed by the voltage and current ripple issues described above, are analyzed and evaluated below in the next section. IV. EXPERIMENTAL RESULTS To develop a better understanding about the effect of high ripple current through Ultra-Capacitors, measurements were made for a synchronous rectified buck converter at various switching frequencies. A 58 F Ultra-Capacitor and a current limited 12 V dc source was connected at its input. The block schematics of the converter s circuit model is shown in Figure2. The 58 F Ultra-Capacitor comprised of six 350 F (C1-C6) Ultra- Capacitors connected in series with charge balancing resistors (R1-R6) across each capacitor. All measurements were made when the input current reached its minimum steady state value indicating that the Ultra-Capacitors were almost fully charged. R7 kept the converter in continuous conduction mode (CCM) and also provided a discharge path for the Ultra- Capacitors. The charging converter s switching frequency was varied from 30 khz to 133 khz with the output being regulated to 6 V by the PWM circuit inside the control circuit. V + C1 in= 12V C1...C6 350F - I SC IEL SW1 C6 IIN R1 1K 6X R6 1K SW2 C7 1000uF IRF3709Z Q1 C8 CONTROL 1uF CIRCUIT Q2 C9 0.1uF C uF R7 V o ~ 6V 2.7E Figure 2. Buck converter circuit model with Ultra-Capacitor at the input The voltage feedback loop was closed from the output and since the converter operated in CCM mode, the converter s operating duty cycle was about 50%. IRF3709Z L1 + -

3 The converter s output was loaded to about 5 A. By controlling switches SW1 and SW2, the effect of the voltage ripple and current ripple on the Ultra- Capacitors, with and without an electrolytic capacitor in parallel, was investigated at different switching frequencies of the synchronous rectified buck converter described above. Measurements were also made with both the Ultra-Capacitors and the electrolytic capacitor in parallel. These are presented in the oscillograms given in Figure 3 through Figure 5. Figure 3 shows measurements made at converter frequency of 30 khz, Figure 4 shows these measurements made at converter frequency of 90 khz while Figure 5 shows these measurements made at converter frequency of 133 khz. In all these oscillograms, Channel 1 shows the voltage across Q2, Channel 2 shows the Ultra-Capacitor s ripple current I SC or the electrolytic capacitor s ripple current I EL, while Channel 3 shows the corresponding capacitor voltage ripple. From the below measurements, it was observed that with only the Ultra-Capacitors connected, the ripple current reduced with increasing switching frequency while the voltage ripple was reasonably constant for all frequencies. Clearly the large capacitance of the Ultra-Capacitor could not attenuate the switching frequency voltage ripple generated by the downstream converter, though the inductor ripple current reduced with increasing frequency. The drastic reduction of Ultra-Capacitors capacitance [4] with increasing frequency explains this observation. On connecting a single low ESR 1000 µf/50 V electrolytic capacitor (C10) in parallel to the Ultra- Capacitor, both the voltage and current ripple reduced significantly and now as it should be expected, the voltage ripple also reduced with increasing frequency. A reasonably constant capacitance value performance of electrolytic capacitors at high frequency, when compared to Ultra-Capacitors, explains this observation. On making ripple measurements with only the electrolytic capacitor connected, it was observed that the ripple current and the voltage ripple were reasonably constant for all frequencies. In this case a reasonably constant capacitance value and ESR value performance of electrolytic capacitors at high frequency, when compared to Ultra-Capacitors, explains this observation. Figure 3a. I SC with only Ultra-Capacitor at 30 Khz Figure 3b. I SC with electrolytic & Ultra-Cap. at 30 Khz Figure 3c. I EL with only Electrolytic-Capacitor at 30 Khz

4 Figure 4a. I SC with only Ultra-Capacitor at 90 Khz Figure 5a. I SC with only Ultra-Capacitor at 133 Khz Figure 4b. I SC with electrolytic & Ultra-Cap. at 90 Khz Figure 5b. I SC with electrolytic & Ultra-Cap. at 133 Khz Figure 4c. I EL with only Electrolytic-Capacitor at 90 Khz Figure 5c. I EL with only Electrolytic-Capacitor at 133Khz

5 Moreover since with only the Ultra-Capacitors connected the ripple current reduced with increasing switching frequency, the above measurement with only the electrolytic capacitor connected demonstrated that the Ultra-Capacitor s ESR reduced with increasing frequency. Thus with only the Ultra-Capacitor connected, the high voltage and current ripple generated by the downstream converter could cause over voltage and heating of the Ultra-Capacitor. On connecting the electrolytic capacitor (C10) in parallel to the Ultra- Capacitor, not only could the voltage ripple be reduced significantly but the converter s ripple current could also be steered away from the Ultra-Capacitor. In fact by selecting an electrolytic capacitor with an even lower ESR than what was used in the above experiment, it would be possible to further reduce the ripple current through the Ultra-Capacitor. Since a 105 C grade low ESR electrolytic capacitor designed for a smaller lifetime multiplier will easily guarantee a lifetime beyond ten years, reliability issues of using an electrolytic capacitors is not a concern. Moreover without the electrolytic capacitor, the lifetime of the Ultra-Capacitor would anyway be reduced. Thus this way by connecting a small electrolytic capacitor in parallel to the large Ultra- Capacitors used mainly for energy storage buffer, both over voltage due to high voltage ripple and capacitor heating due to high ripple current, can be significantly reduced in Ultra-Capacitors. V. CONCLUSION This paper discusses how downstream PWM converters can generate high voltage ripple and high current ripple in Ultra-Capacitors used as an energy storage buffer and thus result in their heating and lifetime reduction. Based on extensive experimental measurements, a simple novel design scheme is proposed by which both the voltage ripple and ripple current of an Ultra-Capacitor could be reduced significantly by more than 40%, resulting in significant increase in capacitor lifetime. The change in capacitance and ESR of an Ultra-Capacitor with frequency is also highlighted. REFERENCES [1] B.E. Conway, Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications. London: Kluwer Academic/Plenum Publishers, [2] Linzen, D.; Buller, S.; Karden, E.; De Doncker, R.W.; Analysis and evaluation of charge-balancing circuits on performance, reliability, and lifetime of supercapacitor systems Industry Applications, IEEE Transactions on Industry Applications Volume 41, Issue 5, Sept.-Oct Page(s): [3] Cooper-Bussmann Application Guidelines, Available at: 20 Guidelines.pdf [4] O. Garcia, DC/DC-Wandler für die Leistungsverteilung in einem Elektrofahrzeug mit Brennstoffzellen und Superkondensatoren, D.Tech. Dissertation, Prof. Leistungselektronik & Messtechnik, ETH, Zürich, [5] Carl Klaes Maximum Charging of an Ultra-Capacitor Using Switch Mode Rectifiers in a Regeneration Cycle Vehicle Power and Propulsion, 2005 IEEE Conference 7-9 Sept Page(s):5 pp [6] Basu. S; Undeland. T. M., A Novel EMI Reduction Design Scheme for Continuous Mode PFC Converters, (NORPIE 2006), Lund, Sweden, June 2006.