Low ESR Aluminium Electrolytic Capacitors for … ESR Aluminium Electrolytic Capacitors for Medium...

©2011 ECA (Electronics Components, Assemblies & Materials Association), Arlington, VA CARTS USA 2011 Proceedings, March 28-31, Jacksonville, FL

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Low ESR Aluminium Electrolytic Capacitors for Medium to High Voltage Applications

Alfonso Berduque, Juliet Martin, Zongli Dou, Rong Xu

KEMET Electronics Ltd.

20 Cumberland Drive, Granby Industrial Estate Weymouth, Dorset DT4 9TE, United Kingdom

E-mail: [email protected]

Abstract Low ESR (Equivalent-Series-Resistance) aluminium electrolytic capacitors for medium to high voltage applications have been developed, with a typical ESR reduction of up to 30%, in comparison with our current aluminium electrolytic capacitor products. The effects on ESR have been considered by a combination of modelling and experimentation, evaluating the electrolyte, paper separators, cathode foils, anode foils and the number of tab connections. This evaluation indicated that the electrolyte and paper combination make a significant contribution to the ESR under the application conditions. In order to achieve the target ESR reduction, a new electrolyte of increased conductivity had to be developed. The new electrolyte, namely NE-400, with a working temperature range of –40 to 105˚C, is suitable for voltage applications up to 400 V at 85°C and 350 V at 105°C. Capacitor endurance tests at 400 V and 85˚C, showed that the NE-400 electrolyte improves the capacitor performance significantly, when compared with the current CE-450 electrolyte. NE-400 results in low and stable ESR values over a testing period of 6000 hours. NE-400 was also compared to our current CE-350 electrolyte at 350 V and 105˚C, showing significant improvement in the capacitor ESR and stability by using the new electrolyte. Suitable surge voltage capability and excellent characteristics at both low and high temperatures were also achieved.

1. Introduction

There is an increasing demand for aluminium electrolytic capacitors with low Equivalent-Series-Resistance (ESR) and for medium to high voltage applications. High ESR values result in power loss and increase of internal temperature [1], which in turn affects the capacitor performance and reduces the capacitor life. The capacitor ESR is generally considered as the most important parameter to achieve a high ripple current capability, i.e. low ESR values leads to greater ripple current capacity [1]. Equation 1 shows that the power loss (P) in a capacitor is directly proportional to the capacitor ripple current (i) and the ESR:

ESRiP ∗= 2 (1) The internal capacitor temperature (hot-spot temperature) does not only depend on the power loss but also the thermal parameters of the capacitor. The thermal parameters of the capacitor have a significant impact on the hot-spot temperature and capacitor life: a low thermal resistance path reduces the hot-spot temperature and either increases the ripple current capability or extends the capacitor life. In an electrolytic capacitor the aluminium anode and cathode foils (the surface area of the foils and the oxide layer on the anode), paper separators (paper thickness and density), tabbing (number of tabs) and electrolyte (conductivity of the electrolyte) will contribute to the capacitor’s ESR. Therefore, all these different parts must be carefully taken into account when a capacitor is designed. The contribution of all these parts to the overall capacitor ESR depends on the temperature and frequency, e.g. the electrolyte tends to play a more important role than the other capacitor parts at high temperatures and high frequencies (85°C and 10 kHz); whereas the aluminium oxide is the main contributor to the overall ESR at lower temperatures and lower frequencies (20°C and 100 Hz). Hence, the selection of suitable foils is crucial to reduce the capacitor ESR, as well as the paper separators used (thicker and denser paper separators will increase the ESR); the number of tabs used (more tabs will decrease the ESR); and the electrolyte’s conductivity (higher conductivity will decrease the ESR). The contribution of the foils, paper separators and tabs to the capacitor ESR can be easily tailored. Therefore, these parts can be designed to have a specific ESR contribution. In this paper a screw-terminal capacitor design, namely Current


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design, was investigated. This has been specifically designed for a maximum of 400 V applications at 85˚C. This design has a typical ESR of 17.8 mΩ at 20°C 100 Hz, and 3.2 mΩ at 85°C 10 kHz. Due to customer requirements, the capacitor ESR had to be reduced by a minimum of 30%. To achieve this ESR reduction, while maintaining the same capacitor size and capacitance, the effects on ESR were considered theoretically by evaluating the paper separators, cathode and anode foils, as well as the number of tab connections. The ESR was calculated to see the contribution of the paper separators, foils and tabs to the final capacitor ESR. In order to reduce the ESR, the number of tab connections was increased from 5 to 6; the number of paper separators was reduced from 3 to 2; and an anode foil of lower capacitance was used (0.44 µF cm-2 foil instead of the current 0.59 µF cm-2 foil). Table 1 summarises a series of capacitor modifications considered, where the ESR values are calculated at different temperatures and frequencies.

Table 1. Current design and material modifications considered to reduce the ESR.

Design Anode Foil Paper Separators Number of Tabs

ESR at 100 Hz and 20 ˚C / mΩ

ESR at 10 kHz and 85 ˚C / mΩ

Current Design

Anode 1 (0.59 µF cm-2)

Paper 1: 30 µm thickness, duplex

5 17.8 3.2 Paper 2: 30 µm thickness, simplex


Modification 1 Paper 1: 30 µm thickness, duplex

5 16.3 3.1 Paper 1: 30 µm thickness, duplex





Modification 4

Anode 2 (0.44 µF cm-2)

Paper 1: 30 µm thickness, duplex 5 12.7 2.8






Target ESR: 12.5 2.2

The ESR calculations in Table 1 show that: a) Reducing the number of paper separators from 3 (Current Design) to 2 papers (Modifications 1 and 2) reduces the

capacitor ESR. b) The use of thinner and simplex paper separators reduces the ESR (see the comparison between Modifications 1, 2,

and 3; and Modifications 4 and 6 in Table 1). c) The use of an anode foil of lower capacitance (Modification 4 vs. Current Design) results in a significant ESR

reduction. This is because lower anode capacitance requires an increase in the surface area, hence lowering the ESR.

d) The ESR is reduced when the number of connecting tabs is increased from 5 to 6 (Modification 4 vs. Modification 5).

e) Modification 5 leads to the greatest ESR reduction. The calculated ESR is within the target ESR at 20˚C and 100 Hz. However, the ESR reduction is not sufficient at high temperatures and high frequencies (e.g. 85˚C and 10 kHz).

Therefore, a new modification needs to be introduced to achieve the target ESR reduction: a new electrolyte of increased conductivity has to be developed. This new electrolyte, namely NE-400, must have higher conductivity than our current CE-450 electrolyte and it must be suitable for 150 to 400 V applications. The electrolyte conductivity needs to be increased from ca. 1.35 mS cm-1 (CE-450) to ca. 2.0 mS cm-1 (NE-400). One of our current electrolytes, CE-350, having a typical


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conductivity of ca. 1.8 mS cm-1, may be enough to achieve such a low ESR in some capacitor designs. However, the latter electrolyte is not designed for voltages above 350 V at 85 °C or above 315 V at 105 °C. To date only a few works have been focused on the development of electrolytes with high conductivity (i.e. low resistivity) for low ESR capacitors and working for low voltage applications [2]. Nevertheless, there is a demand for the development of electrolytes with high conductivity and for medium to high voltage applications. These electrolytes must have high ionic conductivity; wide operation temperature range (at least from –40 to 85°C); thermal stability and good compatibility with the capacitor parts. They must also have anodizing ability to form the edges of the anode and the tab foils and allow self-healing of the aluminium oxide layer; excellent wet ability and swell ability to the paper separators in the capacitor. Additionally they must have reduced vapour pressure to minimise the capacitor internal pressure at the working temperature; and specific breakdown voltage, suitable water content and pH values [3-7]. Table 2 summarises the capacitor modifications proposed to achieve the target ESR reduction over the entire temperature and frequency ranges, comparing the Current design vs the New design. Figures 1 and 2 show the contribution of the different capacitor parts to the final capacitor ESR, at 20˚C 100 Hz; and at 85˚C 10 kHz. These figures also show that the calculated ESR values using the New design meet the target 30% ESR reduction, using the new electrolyte. The oxide contributes the most to the capacitor ESR at 20˚C and 100 Hz (Figure 1), whereas the electrolyte and paper separators contribute the most at 85˚C and 10 kHz (Figure 2). The main aim of this work was the development and test of a new electrolyte, NE-400, of higher conductivity than our current electrolytes to achieve low capacitor ESR, and working up to 400 V at 85°C and up to 350 V at 105°C.

Table 2. Current design vs. New design.

Capacitor part Current Design New Design Comments

Anode foil Anode 1 Anode 2 Lower the capacitance

of the foil to increase the surface area and hence lower the ESR (0.59 µF cm-2) (0.44 µF cm-2)

Cathode foil Cathode 1 (50 µm thickness) Cathode 1 (50 µm thickness) No change required

Paper separators

3 papers: 2 papers:

Reduce the number of paper layers to

improve the ESR

1) Paper 1: 30 µm thickness, duplex


2) Paper 2: 30 µm thickness, simplex


3) Paper 1: 30 µm thickness, duplex -

Number of tabs 5 6 Increase the number of tabs to reduce the

ESR

Electrolyte CE-450 NE-400 New electrolyte of increased conductivity


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0

2

4

6

8

10

12

14

16

18

20

Current design (Typical ESR 17.8mΩ)

New design(Typical ESR 11.7 mΩ)

mΩ

ESR (20°C, 100 Hz) OXIDE

FOIL

PAPER + ELECTROLYTE

TABBING

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Current design (Typical ESR 3.2mΩ)

New design(Typical ESR 2.2 mΩ)

mΩ

ESR (85°C, 10 kHz) OXIDE

FOIL

PAPER + ELECTROLYTE

TABBING

Figure 1. ESR comparison at 20°C and 100 Hz. Current design vs. New design.

Figure 2. ESR comparison at 85°C and 10 kHz. Current design vs. New design.


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2. Experimental 2.1. Electrolyte preparation and characterisation 2.1.1. Electrolyte preparation and electrolyte parameters The electrolytes tested in this work were based on ethylene glycol (EG) solvent and contained different conductive salts, acids and bases, and also some additives such as corrosion inhibitors and hydrogen absorbers. These electrolytes were prepared and tested as described elsewhere [5]. Two electrolytes, designed for 350 V and 400/450 V capacitor applications (i.e. CE-350 and CE-450), were used as reference electrolytes. These two current production electrolytes provide excellent capacitor performance at 85°C and 105°C. CE-350 is designed for 350 V maximum at 85°C and 315 V maximum at 105°C. CE-450 is used at 85°C and 105°C for voltage applications up to 450 V. A new electrolyte, NE-400, has been developed for both 85 °C (up to 400 V) and 105 °C (up to 350 V) in order to meet the customer requirements. This electrolyte has a higher conductivity than CE-350 and CE-450, which will result in lower capacitor ESR. The aim of this work was to develop an electrolyte resulting in improved capacitor performance: lower ESR and impedance (Z); and longer capacitor life (more stable capacitor). Table 3 summarises the electrolyte parameters: water content, pH, conductivity (κ) and breakdown voltage. Table 3. Electrolyte parameters: water content; pH; conductivity (κ), and breakdown voltage. Two current electrolytes (CE-350 and CE-450) vs the NE-400 electrolyte.

PARAMETER CE-350 NE-400 CE-450

Water content (%) 1.5 – 2.5 2 – 3 2 – 3

pH (at 25°C) 5.8 – 6.2 6.2 – 6.6 5.5 – 6.0

κ (mS cm-1, at 25°C) 1.7 – 1.9 2.1 – 2.3 1.25 – 1.45

Breakdown voltage (V, at 90°C) 425 minimum 440 minimum 460 minimum 2.1.2. Viscosity measurements The viscosity of the electrolytes was measured at different temperatures. A Brookfield Digital Viscometer instrument (model DV-E), from Brookfield Engineering Laboratories Inc. was used to measure the viscosity of the electrolytes. The viscosity of an electrolyte is an important parameter to take into account, as it will affect the capacitor impregnation efficiency. Very high viscosities make it more difficult for the electrolyte to be absorbed into the capacitor paper separators. 2.1.3. Electrolyte conductivity as a function of the temperature The conductivity of the electrolytes was also measured between 20 and 60 °C, obtaining conductivity – temperature curves, which can be used to estimate the conductivity at different temperatures. The NE-400 electrolyte was compared to CE-350 and CE-450. 2.2. Capacitor preparation and test using the New design A series of capacitor samples using the New design were prepared with the three electrolytes (CE-350, NE-400 and CE-450) and for different capacitor tests. Therefore, the anode and cathode foils, paper separators and tabs described in Table 2 for the New design, were used in these studies. The capacitors were prepared and measured as described previously [5]. Table 4 summarises the basic design information.


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Table 4. Basic capacitor design information, using the New design.

Capacitance (μF, 100 Hz), 20°C 4100 (-10 + 30%)

Capacitor diameter (mm) 90

Capacitor length (mm) 99

Working Voltage (V) 400 maximum

Surge Voltage (V) 440

Temperature Range (ºC ) –40 to +85

Test temperature (ºC ) 85 and 105 The capacitor electrical parameters, i.e. capacitance (C); ESR; and impedance (Z); were measured at 20°C, using a Thermotron’s SE-300 Environmental Test Chamber. An Agilent 4263B LCR Meter was used to measure the capacitance and the ESR at 100 Hz, 0.5 V; and the impedance at 10 KHz, 0.5 V. 2.2.1. Endurance tests a) Endurance tests at 400 V and 85°C. NE-400 vs CE-450 Eight capacitors using NE-400 electrolyte and eight capacitors with CE-450, were tested at 400 VDC, 85°C for 6000 hours. Six capacitors of each electrolyte were also tested at 385 VDC + 13.5 A ripple current, 85 °C for 3000 hours. The constant voltage was provided by a Nemic–Lambda GenesysTM GENH600–1.3 DC power supply and the calculated maximum allowable ripple current by an in-house constructed variable transformer. An Agilent 34970A Data Acquisition System was used to monitor and record the leakage current and the time continuously. Unless stated otherwise, the capacitors were discharged every 1000 hours of endurance test, then cooled down to room temperature, disconnected and removed from the oven for capacitor electrical measurements (C, ESR and Z) at 20ºC. The purpose of these measurements was to study the capacitor stability over time. After the electrical measurements, the capacitor endurance tests were resumed until the next 1000 hours interval. b) Endurance tests at 350 V and 105°C. CE-350 vs NE-400 Four capacitors using CE-350 electrolyte and four capacitors with NE-400, were tested at 350 VDC, 105°C for 3000 hours. The endurance tests were carried out as described in section a). CE-350 is a suitable electrolyte at 105 °C for voltage applications up to 315 V. However, this electrolyte was tested at 350 V for an accelerated comparison with NE-400 electrolyte. 2.2.2. Surge tests. NE-400 vs. CE-450 This test was conducted with the purpose of determining the effects on the electrical characteristics of a capacitor by overcharging the capacitor for a specific period of time. This test consisted of applying 1.1 times the maximum capacitor working voltage. In this case, the voltage applied was 440 VDC. This test was carried out for 1000 cycles, where each cycle is based on an On-period (30 seconds applying 440 VDC) and an Off-period (330 seconds with no voltage applied). At the end of the test, the capacitor electrical parameters were measured and compared to the initial values. 2.2.3. Storage at low and high temperatures. NE-400 vs CE-450 a) Storage at –40°C This test was carried out with the purpose of determining the effects on the electrical characteristics of a capacitor, resulting from the exposure of the capacitor to low temperatures for a specified period of time. In this case, the capacitors were subjected to 16 hours at –40°C in order to investigate the electrolyte stability at low temperatures. After this test, the capacitor electrical parameters were measured at 20 °C. Both CE-450 and the NE-400 electrolytes were compared.


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b) Storage at 85°C This test was conducted with the purpose of determining the effects on the electrical characteristics of a capacitor, resulting from the exposure of the capacitor to an elevated ambient temperature for a specified period of time. In this case, the capacitors were subjected to 85°C for 96 hours. After this test, the capacitor electrical parameters were measured at 20°C. The capacitor measurements, using CE-450 and NE-400 electrolytes, were compared. 2.2.4. Temperature vs. Frequency scans at –40, 20 and 85°C. NE-400 vs CE-450 The capacitor electrical parameters were measured at –40°C at different frequencies, ranging from 20 Hz to 100 KHz. The same frequency scans were also carried out at 20 and 85°C for comparison. The frequency scan results using NE-400 were compared to the results using CE-450.

3. Results and discussions

3.1. Electrolyte characterisation 3.1.1. Viscosity measurements The viscosity of NE-400 was measured at different temperatures and compared to the viscosities of CE-350 and CE-450 electrolytes. Figure 3 shows the viscosity vs. temperature curves for the three electrolytes.

Figure 3. Viscosity measurements as a function of the temperature.

CE-350 ( ); NE-400 (); and CE-450 (). Figure 3 shows that the viscosity of NE-400 is lower than the viscosity of the other two electrolytes. This is an advantage because it is easier to impregnate capacitors with electrolytes having low viscosities. 3.1.2. Electrolyte conductivity as a function of the temperature The conductivity of the electrolytes was measured at different temperatures (from 20 to 60°C). Figure 4 shows the higher conductivity of NE-400, over the 20 to 60°C temperature range, compared to CE-350 and CE-450 electrolytes. The higher conductivity of NE-400 over the entire temperature interval, compared to the other two electrolytes, will result in lower capacitor ESR and hence improved capacitor performance.


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Figure 4. Electrolyte conductivity as a function of the temperature. CE-350 ( ); NE-400 (); and CE-450 ().

3.2. Capacitor test using the New design 3.2.1. Endurance tests a) Endurance tests at 400 V and 85°C. NE-400 vs CE-450 Eight capacitors using NE-400 and eight capacitors with CE-450, were tested at 400 VDC, 85°C for 6000 hours. Figure 5 shows the capacitor leakage current as a function of time, where each curve is the average of eight capacitors for each electrolyte. The results show very little differences in leakage current during capacitor operation for 6000 hours and using the two electrolytes.

Figure 5. Endurance tests at 400 VDC, 85°C. Capacitors using NE-400 electrolyte () and CE-450 electrolyte ().

Throughout the endurance tests, the capacitance, ESR and impedance were measured and the results are summarised in Figure 6. These results show very little differences in the capacitance using the two electrolytes. This was expected because electrolytes do not affect the capacitance in a great extent (unless they have corrosive effects or are very unstable). However, Figure 6 shows significant differences in ESR and impedance using the two electrolytes. At the beginning of the test, the initial average ESR using CE-450 electrolyte was 12.78 mΩ, above the target 12.5 mΩ, whereas the ESR of the capacitors using NE-400 was 10.61 mΩ (measured at 20 ˚C and 100 Hz). Between 0 and 4000 hours of endurance test, the ESR increased approximately at the same rate using the two electrolytes. However, after 4000 hours the ESR of the capacitors using NE-400 was still much lower than the ESR of the capacitors with CE-450. Furthermore, the ESR using CE-450 increased very fast after 4000 hours. In contrast, the ESR using NE-400 increased very slowly. After 6000 hours, the ESR of the capacitors using CE-450 was 22.60 mΩ, which is 77% higher than the initial value; and the ESR of the capacitors using NE-400 was 13.85 mΩ, only 31% higher than its initial value.


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The same trendline described for the ESR changes with time was observed in the impedance vs time curves, i.e. at the end of the test, the impedance using NE-400 was much more stable (32% increase) than using CE-450 (97% increase); and the impedance using NE-400 was also much lower (8.27 mΩ) than the impedance using CE-450 (16.96 mΩ). These results show that NE-400 is a more stable electrolyte than CE-450, resulting in an increased capacitor useful life. Also, the ESR and impedance using NE-400 were significantly lower than the ESR and impedance using the current CE-450, making NE-400 a better electrolyte for 400 V applications at 85°C. The target 30% ESR reduction was only achieved using the New design in conjunction with NE-400 (and not with CE-450).

Figure 6. Capacitor electrical parameters during capacitor endurance tests at 400 VDC, 85°C. NE-400: ; CE-450: . Capacitance and ESR measured at 100 Hz; Impedance measured at 10 kHz.

Six capacitors using NE-400 and six capacitors with CE-450, were tested at 385 VDC + 13.5 A ripple current, 85°C for 3000 hours. The capacitors where connected and tested in pairs. Figure 7 shows the capacitor leakage current as a function of


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time, where each curve is the average of three pairs of capacitors for each electrolyte (the current response is due to two capacitors instead of one). Figure 7 shows similar leakage currents during capacitor operation for 3000 hours and using the two electrolytes.

Figure 7. Endurance tests at 385 VDC + 13.5 A ripple current, 85°C. Capacitors using NE-400 electrolyte () and CE-450 electrolyte ().

Figure 8 shows the changes (average of six samples with each electrolyte) in capacitance, ESR and impedance with time, during capacitor test at 385 VDC + 13.5 A ripple current, at 85°C. As expected, very little difference in capacitance, using the two electrolytes, was observed. The ESR and impedance increased slowly during this test. At the end of the test, the ESR of the capacitors using CE-450 was 16.63 mΩ, which is 31% higher than the initial value; and the ESR of the capacitors using NE-400 electrolyte was 11.90 mΩ, only 11% higher than its initial value. After the 3000 hours of test, the ESR using NE-400 was still below the target ESR reduction (12.5 mΩ at 20 ˚C 100 Hz). The same trendline described for the ESR changes with time, was observed in the impedance vs time curves, with less impedance changes using the NE-400 than with CE-450 electrolyte. These results show that the NE-400 electrolyte leads to lower capacitor ESR and impedance; more stable and longer capacitor life, at 85 °C; working both in DC only applications and in DC and Ripple applications.


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Figure 8. Capacitor electrical parameters during capacitor endurance tests at 385 VDC + 13.5 A ripple current, 85°C. NE-400: ; CE-450: . Capacitance and ESR measured at 100 Hz; Impedance measured at 10 kHz.

b) Endurance tests at 350 V and 105°C. CE-350 vs NE-400 CE-350 and NE-400 were tested at 105°C. These electrolytes were tested at 350 VDC for 3000 hours. Figure 9 shows a stable leakage current curve using NE-400 (red curve); and an unstable leakage current using CE-350 (green curve). Such an unstable current may be a consequence of the rapid ESR and impedance increase, caused by internal heat in the capacitor (capacitor dry-out). Therefore, NE-400 electrolyte results in improved capacitor performance at 105°C and at voltages up to 350 V. When CE-350 electrolyte is used in a current product at 105 °C, its maximum voltage rating is 315 V, instead of 350 V. These tests were carried out at 350 V to accelerate the comparison between electrolytes. NE-400 electrolyte, not only has a longer life at 105°C, but also leads to lower capacitor ESR and impedance. Figure 10 shows the rapid ESR and impedance increase after 2000 hours of test, using CE-350. In contrast, the ESR and the impedance using NE-400 were very stable after 3000 hours of endurance test. The differences in capacitance, using the two electrolytes, are insignificant.


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Figure 9. Capacitor endurance tests at 350 VDC, 105°C. Capacitors using CE-350 electrolyte ( ) and NE-400 electrolyte ().

Figure 10. Capacitor electrical parameters during capacitor endurance tests at 350 VDC, 105°C. CE-350: ♦; NE-400: . Capacitance and ESR measured at 100 Hz; Impedance measured at 10 kHz.


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3.2.2. Surge tests. NE-400 vs CE-450 After demonstrating the improved capacitor performance using NE-400, surge tests were carried out using this electrolyte. This test shows the capacitor stability after applying 440 V surge voltage. Table 5 shows that the capacitor electrical parameters are very stable after this test: very little changes in capacitance, ESR, impedance and leakage current, were observed.

Table 5. Capacitor electrical parameters before and after the surge tests. NE-400 vs CE-450.

Capacitance (μF, 100 Hz), 20 °C

ESR (mΩ, 100 Hz), 20 °C

Z (mΩ, 10 kHz), 20 °C

Leakage current (mA), at 400 V

Initial Final Initial Final Initial Final Initial Final

NE-400 3980 3946 10.28 10.38 6.28 6.02 0.450 0.410

CE-450 3957 3914 13.20 13.47 8.25 8.13 0.395 0.336

3.2.3. Storage at low and high temperatures. NE-400 vs CE-450 a) Storage at –40°C A series of samples were stored at –40°C for 16 hours in order to study the electrolyte stability at low temperatures. Table 6 summarises the results, showing very little changes in the capacitor parameters after storage at –40°C. Table 6. Capacitor electrical parameters before and after storage at –40°C for 16 hours, using NE-400 and CE-450 electrolytes. Average results of six capacitors for each electrolyte.

Capacitance (μF, 100 Hz), 20°C

ESR (mΩ, 100 Hz), 20°C

Z (mΩ, 10 kHz), 20°C


Initial Final (after 16 hours at

–40°C) Initial

Final (after 16 hours at

–40°C) Initial


–40°C) Initial


–40°C)

NE-400 4029 4030 10.59 11.05 5.64 6.07 0.348 0.340

CE-450 4081 4080 12.61 12.78 7.89 8.05 0.445 0.374

b) Storage at 85°C Another set of capacitor samples (six samples for each electrolyte), were stored at 85°C for 96 hours in order to study the electrolyte/capacitor stability at high temperatures. Table 7 shows very little changes in the capacitor parameters after storage at 85°C.


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Table 7. Capacitor electrical parameters before and after storage at 85 °C for 96 hours, using NE-400 and CE-450 electrolytes. Average results of six capacitors for each electrolyte.

Capacitance (μF, 100 Hz), 20°C

ESR (mΩ, 100 Hz), 20°C

Z (mΩ, 10 kHz), 20°C


Initial Final (after 96 hours at

85°C) Initial


85°C) Initial


85°C) Initial


85 °C)

NE-400 4026 3965 10.23 9.83 5.58 5.48 0.402 0.519

CE-450 4049 3999 12.51 11.55 8.31 7.38 0.451 0.586

3.2.4. Temperature vs. Frequency scans at –40, 20 and 85°C. NE-400 vs. CE-450 Six capacitors using NE-400 and six capacitors using CE-450 were used for frequency scans at –40, 20 and 85 °C. The capacitance, ESR and impedance of the capacitors were measured at these temperatures from 20 Hz to 100 kHz. Figure 11 summarises the results. At –40°C, huge differences in ESR can be observed, using the two electrolytes, mainly between 20 Hz and 1 kHz. From 2 kHz to 40 KHz, the ESR differences become smaller. At 100 kHz, the ESR values using CE-450 and NE-400 are very similar. At this temperature, the capacitor impedance was lower using NE-400, predominantly between 100 Hz and 10 kHz. In terms of capacitance, there is no difference between the electrolytes, in the frequency ranges of 20 Hz to 100 Hz; and from 10 kHz to 100 kHz. Nevertheless, the capacitance is considerably higher using NE-400, between 100 Hz and 10 kHz. At 20°C, the ESR of the capacitors using NE-400 is significantly lower than the ESR of the capacitors using CE-450, over the entire frequency range scanned. This difference in ESR becomes more apparent from approximately 100 Hz to 100 kHz. The only differences in impedance between the two electrolytes are between 4 kHz and 100 kHz, and mainly between 10 kHz and 40 kHz. In the latter frequency range, the impedance using NE-400 is significantly lower than the impedance using CE-450 electrolyte. At this temperature, the capacitance using the two electrolytes is very similar over the entire frequency range. Note: the capacitance values above 1 kHz are not included due to electrical resonance in the measurements at such frequencies. At 85°C, the main difference between the two electrolytes is the lower ESR using NE-400, mainly at frequencies above 200 Hz. At frequencies above 4 kHz, the ESR of the capacitors using NE-400 is ca. 20 – 25 % lower than the ESR using CE-450. At 85˚C and 10 kHz, the ESR using NE-400 is 1.96 mΩ, which is lower than the target 2.2 mΩ. The impedance using NE-400 is significantly lower than CE-450, at frequencies above 10 kHz. At 85˚C the capacitance using the two electrolytes is very similar over the entire frequency range. Note: the capacitance values above 1 kHz are not included due to electrical resonance in the measurements at such frequencies. Using the New design with NE-400, the target 30% ESR reduction was achieved throughout the entire frequency range at –40, 20 and 85˚C.


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Figure 11. Capacitor characteristics at low (–40°C), medium (20°C) and high temperatures (85°C). Frequency scans from 20 Hz to 100 kHz.

NE-400: ; CE-450: . Capacitance and ESR measured at 100 Hz; Impedance measured at 10 kHz.


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4. Conclusions

Low ESR aluminium electrolytic capacitors for medium to high voltage applications have been developed, by examining the effects on ESR using a combination of modelling and experimentation to evaluate the electrolyte, paper separators, cathode foils, anode foils and the number of tab connections. These capacitors had a target ESR reduction of up to 30% in comparison with our current aluminium electrolytic capacitors and a working voltage of up to 400 V. The new electrolyte, NE-400, in conjunction with the New design, has been developed for medium (e.g. 150 V) to high voltage applications (400 V). Using NE-400, the ESR value of the New design is 10.61 mΩ at 20 °C 100 Hz and 1.96 mΩ at 85 °C 10 kHz, in comparison with the Current design of 17.8 mΩ at 20 °C 100 Hz, and 3.2 mΩ at 85 °C 10 kHz. Using NE-400, the resulting ESR exceeds the target ESR reduction. Capacitor endurance tests up to 6000 hours at 85 ˚C and 400 V compared NE-400 with CE-450. The results showed that NE-400 electrolyte produced a 31% ESR changes in comparison with the 77% ESR changes using CE-450 under the same testing conditions. Thus, NE-400 extended the capacitor life compared with CE-450. Therefore, the improved capacitor performance using NE-400 at 400 V (lower ESR and impedance, and longer capacitor life) was demonstrated. NE-400 was also compared to our current CE-350 electrolyte at 350 V and 105 ˚C, showing significant improvement in the capacitor ESR and stability by using the new electrolyte. The endurance tests were followed by surge tests at 440 V, showing good capacitor stability using NE-400. On the other hand, the capacitance, ESR, impedance and leakage current were measured after storing the capacitors at –40 and 85 ˚C, showing similar results for both CE-450 and NE-400. Finally, frequency scans were carried out at –40, 20 and 85 ˚C, observing improved performance using NE-400. Acknowledgements The authors would like to thank Mr. Mark Wright for his helpful suggestions and discussions and all other colleagues from KEMET Electronics Ltd. in Weymouth, especially to Gavin Candy, Mike Andrews and Nick Mason.

References [1]. L. Eliasson. CARTS USA 2008 Proceedings. Newport Beach, California. [2]. S.D. Ross and M. Finkelstein. US Patent 5006964, 1991. [3]. A. Berduque, Z. Dou and R. Xu. CARTS Europe 2009 Virtual Conference Proceedings. [4]. M.-L. Tsai, Y.-F. Lu and J.-S. Do. J. Power Sources, 2002, 112, 643-648. [5]. Z. Dou, R. Xu, A. Berduque. CARTS Europe 2008, 79-92. [6]. R. Xu, A. Berduque, Z. Dou. CARTS USA 2009, 83-94. [7]. R. Takaoka, K. Honda, Y. Tsubaki, Y. Sugihara. US Patent 7163643, 2007.

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