TECHNICAL INFORMATION INVESTIGATION INTO THE EFFECTS OF CONNECTING TANTALUM CAPACITORS IN SERIES by J.A. Gill AVX-Kyocera Group Company Paignton England TQ4 7ER. Abstract: This paper demonstrates how high voltage capacitors can be made by connecting lower voltage rated parts in series. How to create large banks of capacitance by parallel and series combinations of capacitors without sacrificing reliability is also discussed. INVESTIGATION INTO THE EFFECTS OF CONNECTING TANTALUM CAPACITORS IN SERIES by J.A. Gill AVX-Kyocera Group Company Paignton England TQ4 7ER. Introduction Tantalum capacitors are being used as input filter capacitors in dc/dc converters and other types of power supply. The capacitors are thus likely to be subjected to both voltage and current transients. An example application would be a 12 volt battery driving a portable PC. A 12 volt battery can vary in voltage from 9 volts to 18 volts depending on its state of charge. Thus if we apply the 50% derating rule, as recommended by tantalum capacitor manufacturers to increase dynamic reliability, the minimum input capacitor allowed would be a 35 volt part. A range of such capacitors is available. But if the input is say 24 volts, no part is available which will keep the derate rule. Discussion of results 20 TAJD22M25 parts were characterized for capacitance at 120 Hertz, DF at 120 Hertz, ESR at 100 KHz and leakage current at 25 volts. The parts were then connected in series with parts 1 and 2 forming one unit, 3 and 4 another and so on. These units were then characterized for the same parameters together with 10 pieces of TAJD10M35. The results are shown in table 1. Table 1 (a) 20 pieces of TAJD22M25 Sample no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Cap@ 120Hz (µF) 21.98 21.40 21.30 21.84 22.91 22.51 22.14 21.70 22.32 21.13 22.35 23.32 22.58 22.10 22.31 21.35 21.93 22.91 23.15 22.03 DF @ 120Hz 1.89 1.77 0.59 1.76 2.28 2.15 1.79 1.81 2.14 0.62 2.35 2.09 1.62 1.75 1.77 0.59 1.97 1.81 1.95 1.79 ESR @ 100KHz(Ohms) 0.105 0.105 0.109 0.107 0.124 0.129 0.114 0.108 0.124 0.110 0.157 0.118 0.106 0.105 0.100 0.105 0.108 0.104 0.121 0.105 DCL @ 25V (µA) 0.062 0.054 0.270 0.104 0.062 0.060 0.056 0.064 0.063 0.040 0.064 0.126 0.192 0.077 0.064 0.039 0.060 0.059 0.176 0.113 (b) Parts in series Sample no. Cap @ 120Hz(µF) 10.87 10.81 11.40 11.00 10.88 11.46 11.22 10.94 11.24 11.33 1 (1+2) 2 (3+4) 3 (5+6) 4 (7+8) 5 (9+10) 6 (11+12) 7 (13+14) 8 (15+16) 9 (17+18) 10 (19+20) DF@ 120Hz 1.60 0.97 2.02 1.67 1.13 2.07 1.62 0.96 1.76 1.69 ESR@ 100KHz(Ohms) 0.197 0.199 0.237 0.203 0.214 0.240 0.191 0.187 0.246 0.212 DCL@25V (µA) 0.021 0.034 0.028 0.026 0.020 0.030 0.041 0.022 0.028 0.037 (c) TAJD10M35 parts Sample no. 1 2 3 4 5 6 7 8 9 10 Cap @ 120Hz (µF) 10.14 10.23 10.09 10.21 10.06 10.08 10.22 10.30 10.25 10.27 Adding the 22µF 25V parts 1 and 2 together would give a theoretical ESR of ESR = ESR1 + ESR2 = 0.105 + 0.105 = 0.210 Ohms which when compared with the measured result gives good correlation. The capacitance of the part should be => 1/Cap = 1/Cap + 1/Cap2 1/Cap = 1/21.98µ +1/21.40µ Cap = 10.84µF Again this compares very well with the measured value. The leakage of the series part is the best leakage of the two 22µF parts measured at a voltage of 12.5 Volts, i.e. half the input voltage. DF @ 120Hz 1.69 1.51 1.47 1.46 1.70 1.50 1.65 1.55 1.57 1.46 ESR @ 100KHz (Ohms) 0.245 0.226 0.251 0.231 0.238 0.234 0.252 0.251 0.292 0.263 DCL @ 25V (µA) 0.024 0.023 0.024 0.024 0.026 0.024 0.026 0.024 0.027 0.024 The dynamic performance of a series combination and a 10µF part was then compared by causing a large transient current to flow. As can be seen from the results, figures 1 and 2, there is no discernible difference in the waveforms observed. The final test performed was a voltage step stress test (VSST) style test where increasing voltage was applied to the capacitor under test until it had a leakage greater than 2000µA. The results are shown in table 2 and graphically in figure 2. VSST results are proportional to the rated voltage of a tantalum capacitor. The results show, as expected, that the series parts have a higher breakdown voltage than the 10µF 35 volt part. Thus, they will perform better in an input capacitor application. Figure 1(a) Series Combination Main Menu Mem C 20 ms 10 V X CH1 2 V = CH2 50 mV T/div 20 ms 10 CH1 14.8 V DC Figure 1 (a) Figure 1(a) Series Combination Main Menu Mem C 20 ms 10 V X CH1 2 V = CH2 50 mV T/div 20 ms 10 CH1 14.8 V DC Figure 1 (b) TAJD10M35 5 Frequency 4 3 2 1 160 170 180 190 160 170 180 190 200 150 140 150 130 120 110 100 90 80 70 60 50 40 30 20 10 0 0 Bin Series TAJD22M25s 5 Frequency 4 3 2 1 200 140 130 120 110 90 100 80 70 60 50 40 30 20 10 0 0 Bin Figures 2(a) and 2(b) Sample number Series combination 10µF 35V part 1 140 v 129 v 2 143 v 93 v 3 155 v 102 v 4 174 v 80 v 5 100 v 130 v 6 142 v 104 v 7 134 v 116 v 8 175 v 129 v 9 126 v 123 v 10 174 v 92 v One important fact which should be noted is that the power ripple rating for two series parts is twice that for a single unit. Thus for example if a 10µF 35 volt part were specified as less than 300 milliohms, its maximum allowable ripple current at 25°C would be P = I2R i.e. thus I = 冑 P R Ripple current = SQRT (150 mW / 0.300 Ohms) = 707mA for the series part P = 300 mW, thus Ripple current = SQRT (300 mW/0.300 Ohms) = 1 Amp Creating large capacitor banks An attempt was made to produce a 66 µF 48V capacitor by using series and parallel combinations of capacitors. The capacitor used was a TPSE100M16. The first attempt schematic is shown in Figure 3. The voltage across C1-C6 were investigated in order to determine what effect the differences in voltage may have on each capacitor’s reliability. The results are shown in Table 3, together with the leakage current and capacitance at 120Hz of the capacitor in each location. Table 3 Capacitor C1 C2 C3 C4 C5 C6 Voltage (Volts) 9.78 10.07 10.15 9.73 10.02 10.25 DCL (µA) 0.72 0.56 0.65 0.60 0.44 0.72 Capacitance (µF) 99.18 96.76 96.47 98.79 97.76 95.58 Table 4 Capacitance Pair C1//C4 C2//C5 C3//C6 Voltage (Volts) 9.70 9.98 10.30 The differences between the voltages across each of the capacitor pairs will adversely affect the reliability of the pair with the highest voltage (see Catalogue or AVX publication “Tantalum Capacitors Technical Summary” for further details). The introduction of a resistor ladder as shown in Figure 5, forces the voltage across each pair to be one third the supply voltage, thus each capacitor will have the same reliability figure. A 66 µF 48V part can thus be made by using the configuration shown in Figure 5. This unit will have a high reliability factor. +30 • • C1 C4 C2 C5 C3 C6 As can be seen from the results in Table 3, the voltage drop across each capacitor appears to be independent of the leakage current, but inversely proportional to the capacitance. For example C4 has the largest capacitance in that series leg and V4 is the smallest, and C6 is the smallest capacitance but the largest voltage. +30 • This is because of Coulomb’s Law. If we consider the capacitors to be pure parallel plates the charge on the negative plate of C1 must have been drawn from the positive plate of C2, thus the charge on C1 is the same as C2, and so on through the chain. Coulomb’s Law states • Figure 3 • • • • • + If rearranged this shows V to be inversely proportional to C, which is as was found. Connection between capacitor nodes as shown in Figure 4, helps the capacitors current share when transient loads are switched, or the ripple current caused by filtering a voltage signal. This current sharing improves reliability by reducing the heating seen by each individual capacitor. The effective parallel capacitance of each pair and the measured voltages are shown in Table 4, which again demonstrates the inverse capacitance relationship. Figure 4 • 100K • •• • • •• • 100K Q = CV where Q = charge in Coulombs C = capacitance in Farads and V = voltage across capacitor in Volts Capacitance (µF) 197.97 194.52 192.05 Figure 5 100K Conclusion A 50V part can be made by connecting two 25 volt parts in series. In the example chosen the series combination of two 22 µF 25V parts has the following characteristics when compared to 10 µF 35V parts. a) lower ESR levels b) similar leakage levels c) Higher breakdown voltage levels d) Higher capacitance levels e) Similar dynamic circuit performance Very large capacitances can be successfully made by a combination of parallel and series connections. Acknowledgements AVX wishes to thank Milt Wilcox of Linear Technology Corporation for his technical contributions to this paper. 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