High Performance Axial Capacitors for Automotive Electronics

Rong Xu1, rongxu@kemet.com Thomas Newton1, thomasnewton@kemet.com Christer Larsson2, christerlarsson@kemet.com Victor Andoralov2, victorandoralov@kemet.com

1 Electrolytic Innovation Centre

KEMET Electronics Limited Units 16-20 Oxford Court, Cambridge Road Granby Industrial Estate. Weymouth, Dorset, DT4 9GH, UK Tel: (+44) - 1305-830 757

2 KEMET Electronics AB,

Skiftesvägen 16, Gränna, 56331, Sweden Tel: (+46) - 390-124 35

Abstract A new type of electrolyte, with optimized formulation, has been developed. The performance of high voltage aluminium electrolytic capacitors has been significantly improved by using the electrolyte. The development allows the achievement of enhanced characteristics of the capacitors in conjunction with relatively low cost. Capacitors display a life time of at least 3500 hours at 125°C, low ESR and a maximum drift of 10% for the main characteristics (ESR, tan(δ) and C) when using the electrolyte. Special capacitor designs using the electrolyte give high ripple current capabilities of around 14A at 105°C for a 150uF, 200V, Ø20xL35 mm, heat-sink mounted capacitors. These properties and characteristics are perspective for the use in a broad range of real applications within the automotive industry. Introduction

1. Theory

The aluminium electrolytic capacitor is a complex system consisting of several capacitances (anode barrier layer capacitance, anode electrical double layer capacitance, cathode electrical double layer capacitance, cathode protective layer capacitance) and conductors (electrolyte, and electrodes) connected in series. Actual working capacitance corresponds to the capacitance across the aluminium oxide layer, also known as the barrier layer. The barrier layer plays the role of the insulator and provides voltage capability for the system that allows high voltage capacitors to exist (up to 800 or even 1000V); thus, the barrier layer is a critical component of the aluminium electrolytic capacitor. Another important behaviour of aluminium oxide is its ability for aluminium and oxygen ion migration within the layer under certain conditions; this

makes electrochemical growth of the layer possible. Mostly the barrier layer grows by field-assisted transportation of Al3+ and O2-/OH- ions across the layer with new layer material produced at the metal/oxide and oxide/electrolyte interfaces [1]. In this process the aluminium foil is a source of aluminium ions and the electrolyte provides oxygen species from different components depending on conditions like temperature, current and electrolyte composition. Normally water is the main source of oxygen and the process can be illustrated by following scheme: 2Al3++3H2O=Al2O3+6H+ (1)

However, in the case of very low water content (nearly anhydrous conditions) oxygen of other compounds, such as organic and inorganic acids or solvent molecules, can be used for the barrier layer formation [2]. In the conditions of real applications, the barrier layer growth is a mixture of different processes where one can distinguish some by-processes like electrolyte species incorporation, field-assisted dissolution of oxide, different redox reactions at the metal/oxide and oxide/electrolyte interfaces etc. [3-5]. Thus the barrier layer composition and properties depend on both the electrolyte formulation and the conditions of its formation. Pre-etched and pre-formed anode foil is usually used for capacitor manufacturing which helps to eliminate most of the possible by-processes. The ability to form the barrier layer directly within an application, allows for a capacitor self-healing feature. On the other hand, interaction between electrolyte and the oxide layer adjusts system sensitivity to different parameters such as chemical nature of the electrolyte and the layer, concentration of ions, electrolyte conductivity, temperature etc. There are certain limitations that come with certain combinations of the parameters; one of the most known limiting phenomena is the breakdown voltage (Vbd). Actually, Vbd is the voltage level when “normal” oxide growth is terminated by breakdowns or sparks. The phenomenon demonstrates a natural relation between all parameters in the electrode/oxide/electrolyte system. The Vbd depends on number of characteristics such as the nature of the barrier layer correlating with the band gap of the oxide, the electrolyte conductivity and the electrolyte composition. The empirical equation (2) of Vbd dependence on conductivity has been known for long time [6, 7]. !"# = %& − (& lg + (2) Where aB and bB – empirical constants, + – electrical conductivity of electrolyte. Ikonopisov’s theory of electrical breakdown during formation of anodic barrier films is one of the first approaches to describe the fundamental nature of the breakdown phenomenon [8]. Assuming electronic current as major factor in the breakdown process, Ikonopisov derives equation (3) denoted as the “general equation for the breakdown voltage”.

!"# = -

./0ln (3&) − ln (3/5)6 (3)

Where 7 – the difference between the mean energy of an electron when it is able to ionize and the mean energy of the electrons emerging from the ionization event; 8 – the recombination constant; 9 – the electron charge; 3& – the electronic current at breakdown and 3/5 – the primary current density injected at the electrolyte/oxide interface, in fact it is the charge transfer current. Using the general equation one can derive other dependencies, for example !"#(+). 03/56:,< = %/+"= (4) Where ae and bB – constants.

0!"#6:,< =-

./0ln (3&) − ln (%/)6 −

>.@-"=

./lg (+) (5)

The expression (5) shows that for certain oxide and electrolyte natures, at constant temperature (T) and surface potential (E), breakdown voltage drops if electrolyte conductivity increases; this causes a limitation for the conductivity of electrolytes in high voltage capacitors. For some applications the limitation is not critical. For instance, if the projective surface area of the electrodes is big enough (as it is for screw-terminal capacitors), a low conductivity electrolyte is normally able to achieve the required parameters such as relatively low equivalent series resistance (ESR); this allows high voltage capability whilst retaining low conductivity. As for small case size capacitors the limitation becomes more crucial. To develop a small case aluminium electrolytic capacitor characterized by low ESR and high rated voltage is a challenge. One can conclude that the special electrochemical behaviour of aluminium in addition to it’s relatively low cost, make this material optimal for the electrolytic capacitor industry. Thus to achieve better performance for small case aluminium electrolytic capacitors, electrolyte improvement is one of perspective directions of the development. Enhanced electrolytic systems must provide lower ESR and greater ripple current capability at high rated voltages. Traditionally conventional electrolytes are classified into two groups: Low voltage (gamma-butyrolactone based) and High voltage (ethylene glycol based). To provide high capacitor performance, electrolyte solutions must have the following behaviours: good thermal stability, wide operation temperature range, high barrier layer self-healing ability, good compatibility with all capacitor materials, high breakdown voltage and high ionic conductivity. The last two parameters are directly responsible for the main capacitor characteristics: ESR and voltage capability. The importance of high voltage capability is clear; it allows the capacitor to withstand high voltage conditions in high voltage applications. The importance of low ESR (or high electrolyte conductivity) requires special discussion. For a specific capacitor design, electrolyte conductivity and ESR will be coupled by following equation:

ABC = D

E(FGH) (6)

Where ESR - equivalent series resistance, l– distance between electrodes,A - projective area of electrodes, σ - ionic conductivity, LM. – spacer impact constant. Usually, small aluminium electrolytic capacitors are used as DC links in automotive electronics. Ripple current (IAC) capability is one of the most important parameters, since many automotive applications require high AC current. Which electrolyte or capacitor parameter is responsible for high ripple current capability? First and foremost is the stability of the capacitor components at high temperatures. While AC current is applied internal heat is generated, equal to power loss (Ploss). Equations 2 – 4 show relationships between Ploss, ESR, IRAC, thermal resistance (Rth), hot-spot temperature – temperature of the hottest point in an AC loaded capacitor (Th) and ambient temperature (Ta). NDOPP = QRES

> ABC (7) ∆U = NDOPPCVW (8) UW = UM + ∆U (9) The capacitor components begin to degrade fast when Th reaches a critical level. This determines maximum Th as well as rated temperature (TR). On the other hand thermal resistance can be reduced by capacitor design optimization. That allows achieving additional IRAC capability at a given Ta. For instance, KEMET has been using the so-called extended cathode approach [9] for many years. In this design, cathode foil is extended from the winding. It offers direct contact between the foil and the aluminium can bottom. This is resulting in a very low internal thermal resistance (CVW YZV.) – thermal resistance between winding and can. For such a design, CVW YZV. is around 2-3 oC/W vs. 7-8 oC/W for regular designs. Further reduction of the CVW YZV. is limited. To improve the system further by optimisation of thermal parameters, significant changes of design are obligatory. That will most likely lead to dramatic increase in cost. On the other hand, development of the electrolyte has a much larger field of potential enhancements. Moreover electrolyte properties directly determine key parameters such as maximum Th and ESR. Thus by optimizing electrolyte formulation one can achieve higher ripple current capabilities.

2. Aluminium electrolytic capacitors in automotive electronics

The automotive industry traditionally puts strict requirements on aluminium electrolytic capacitors. These include: small case size (to reduce space and weight of final application), long operational life (Lop), high temperature and ripple current capability and high vibration resistance. Today the automotive industry demonstrates a new trend - increasing voltage of the main electrical system of cars [10]. This entails new requirements for electrolytic capacitors, particularly in having higher voltage capabilities.

The development trend from low voltage, to hybrid (today) and high voltage (in the future) car electrical systems is undeniable today [11]. There are at least two reasons for this: The first is the ecological benefit, the use of high voltage system can decrease carbon dioxide emission. The second is the design advantage, the same power of a device might be achieved at much lower current using high voltage system, and thus powerful units can be much smaller. It is clear that to support a high voltage system, high voltage capacitors are required.

On the topic of current applications for small case size electrolytic capacitors rated for high voltages, the fuel injection system can be mentioned. Since 2003 a modern type of fuel injector has been available on the market known as the piezo actuated injection system [12]. Nowadays the most effective engines are built based on the piezo injection technology. The piezo actuator is the heart of this innovation and it is driven by high voltage pulses (150V – 300V) [12] [13]. The high efficiency can be obtained by using high power pulses, so for this purpose an electric capacitor of the same voltage range is required.

Experiments

1. Materials and Methods

Electrolytes were prepared using high purity conductive components and additives. Gamma-butyrolactone ≥ 99.9% (GBL) was from BASF (Ludwigshafen, Germany). The electrolytes were tested in both laboratory conditions and in actual working capacitors. A thermostatically controlled cell was used for conductivity (+), pH and breakdown voltage (Vbd) measurements. Conductivity was measured at 25 oC with conductivity meters a FE-30 from Mettler-Toledo AB and a CDM210 from MeterLab, pH with an Orion 720A from Scandinovata AB and a PHM220 from MeterLab pH-meters. Vbd was measured at 90 oC and for this a constant current of 3.0 mA was applied between 20 cm2 pre-formed anode aluminium foils immersed in the test electrolyte. In-house-created dielectric breakdown voltage equipment was used for Vbd measurements. The water content was measured using a Karl Fisher titration instrument AQUAPAL III. PEG124 and PEG226 capacitors were designed and manufactured by KEMET Electronics AB in Sweden. Endurance tests were carried out at capacitor’s rated temperature and voltage. All electrical measurements on capacitors were performed at 20 oC and rated voltage. Mean values of main electrical parameters have been calculated using at least eight capacitors for each tested electrolyte.

2. Results 2.1 Electrolyte composition optimization

A new GBL based electrolyte (GRN) has been developed. The electrolyte consists of GBL, conductive components and water. The contents of water and the conductive components have been varied to find the optimal electrolyte parameters (Table 1). Dependencies of Vbd and conductivity on water content are plotted in Fig. 1 for different

concentrations of the conductive components. Conductivity is linearly increasing with increasing water concentration. Similar behaviour takes place for dependencies of conductivity on concentration of the conductive components at constant water content. However in this case the dependencies are not linear and the change of concentration from 7% to 8% does not give as significant an increase in conductivity as it does in 6%-7% range (Fig. 1A). Vbd of the electrolyte is sensitive to water content in the range below 4%. This phenomenon indicates that the conductivity is of an ionic dependency, which mainly relies on the dissociation of the conductive components in electrolyte solution. As one can see from Fig. 1B, the water content decrease from 4% down to 3% results in a Vbd drop around 100V - 150V. There is no significant difference in Vbd between GRN-A-2, GRN-B-5 and GRN-C-7. However GRN-B-5 electrolyte variant has been chosen for further tests as the most optimal in terms of use efficiency of the conductive component.

Preparation of the electrolyte requires high temperature treatment to allow reactions between compounds to complete. Duration of the treatment impacts important parameters such as thermal stability. Thermal stability has been tested for GRN-B-5 samples prepared using a slightly different method (Table 2). Table 1. GRN electrolyte variations with different conductive component and H2O.

Electrolyte Conductive components

/ wt.% H2O / wt.% + / mS*cm-1 Vbd / V pH

GRN-A-1 6.0

2.5 1.78 394 8.2 GRN-A-2 4.2 1.89 466 7.2 GRN-A-3 5.7 2.11 464 7.2 GRN-B-4

7.0 3.0 2.07 301 8.0

GRN-B-5 4.4 2.19 456 7.2 GRN-B-6 5.8 2.35 454 7.1 GRN-C-7

8.0 3.3 2.13 307 8.0

GRN-C-8 4.5 2.28 454 7.2 GRN-C-9 6.3 2.46 451 7.1

The most stable electrolyte GRN-B-51 was prepared with the shortest time of high temperature treatment. Electrolyte GRN-B-52 demonstrated medium stability and GRN-B-53 had the worst stability of all the tested variations.

A B

Fig. 1. Dependencies of conductivity (A) and breakdown voltage (B) on water content

for different conductive component concentrations. The main parameters of the GRN-B-51 electrolyte are compared with two references in

Table 3. Ref-1 and Ref-2 are examples of typical electrolytes which are based on GBL

and ethylene glycol (EG), respectively. These can be found in commercial capacitors.

Table 2. Stability test results for GRN-B-5 prepared applying different conditions.

Vbd (V, 90oC) Fresh

GRN-B-51 GRN-B-52 GRN-B-53 Vbd test 1 463 461 410 Vbd test 2 454 461 411 Vbd test 3 - - 389

Average (Vbd) 458 461 403 After 1week at 125°C GRN-B-51 GRN-B-52 GRN-B-53

Vbd test 1 451 422 268 Vbd test 2 451 365 249 Vbd test 3 - 351 244

Average (Vbd) 451 377 253 Stability Stable Medium Not Stable

Treatment conditions

120min at 35°C; 30min 35oC to 125°C;

30min at 125°C

120min at 35°C; 30min 35oC to 125°C;

60min at 125°C

50min 20oC to 100°C; 140min from 100 to 130°C

Table 3. Physicochemical parameters of high voltage electrolytes.

Electrolyte +/ mS*cm-1 Vbd / V pH w (H2O) / wt. % GRN-B-51 2.2 460 7.2 4.4

Ref-1 1.5 500 7.5 12.0 Ref-2 1.1 450 6.0 ˂ 1.0

2.2 Characteristics of capacitors with GRN-B-51 PEG124 capacitors were filled up with GRN-B-51 electrolyte. Rated voltage of the

capacitors was 200 V at rated temperature 125 oC. Overloading tests were carried out

for the capacitors. Average results are presented in Fig. 2.

A B

Fig. 2. Overloading tests for PEG124RJ316 with GRN-B-51 electrolyte. Conditions: (A)

differing temperature at 200V, (B) differing voltage at 125 oC. Endurance test results are important to evaluate ripple current (IRAC) capability at a given

temperature. The obtained results were plotted in Fig. 3 - 4. The rate of capacitor weight

loss increases with temperature (Fig. 3A). Fig. 3B shows how vapour pressure depends

on temperature. The curves were plotted based on literature data available for individual

solvents (GBL and H2O). For a mixture of the solvents one can apply Raoult's law as a

first approximation. At a water content of 4 wt.% vapour pressure acting on the mixture

is higher in comparison with pure GBL.

A B

Fig. 3. (A) GRN-B-51 electrolyte loss dependences obtained for PEG124RJ316 at

different temperatures: (1) 85oC, (2) 105oC, (3) 125oC, (4) 135oC. (B) Temperature

dependences of vapour pressure for different liquids.

GRN-B-51 offers high capacitor stability, the ESR dependence for capacitors with this

electrolyte was flat while the same parameter for Ref-1 and Ref-2 was rising. Capacitor

end of life, caused by high internal pressure, was detected around 2500 hours with

Ref-1. ESR of Ref-2 capacitors was much higher than for GRN-B-51 (Fig. 4A).

Capacitance was quite stable and leakage current was stable and low (around 3 µA) for

all samples (Fig. 4B). Several capacitors were taken out during endurance test, opened and analysed. No

corrosion marks or damages were detected for GRN-B-51 capacitors after 4000 hrs. In

the case of Ref-2 minor corrosion marks appeared along the edges of cathode foil after

3000 hrs. As for Ref-1 strong corrosion of cathode foil caused end of life at around 2500

hours. For all samples the electrolyte had a characteristic brown colour and a Vbd

decrease after 2000 – 3000 hours.

A B

C D Fig. 4. (A-C) Endurance test for PEG124RJ316 with different electrolytes. Conditions:

125oC, 200V. (D) ESR dependences for PEG124RJ316 with GRN-B-51 at 200V and

differing temperatures. Maximum IRAC was calculated for two designs (PEG124 and PEG226) and two types of

thermal path: first - natural air convection, second – capacitor mounted with low thermal

resistance path - heat-sink (Table 4) [9, 14].

Table 4. Initial ESR and maximum specified IRAC at 10 kHz

Cap type Ø20mm L35mm

Electrolyte ESR -40oC / mOhm

ESR 125oC / mOhm

IRAC 105oC natural convection

/ A

IRAC 105oC heat-sink

/ A

PEG124 GRN-B-51 500 30 5.2 8.9

Ref-1 1300 30 2.0 3.4 Ref-2 1500 33 3.5 6.0

PEG226 GRN-B-51 420 25 6.0 14.0 Discussion A new type of electrolyte for high voltage applications has been developed. This type of electrolyte has several advantages that, conjunctively within the same formulation, have not been achieved before. These advantages include relatively high conductivity, high voltage capability and high chemical stability across a wide range of temperatures. Capacitors produced with this electrolyte demonstrate enhanced stability. Low corrosion activity can be achieved in typical capacitor interfaces: electrolyte/alumina, electrolyte/aluminium, electrolyte/rubber, electrolyte/paper. The electrolyte has a good ability to form high voltage barrier layer which is an important feature for the capacitor’s ageing process during production as well for reliability through operational life. The developed electrolyte is a GBL based system; GBL has a wide operational temperature range from -55 oC up to 150 oC. GBL has been actively studied and used as a main solvent for low voltage (up to 100 V) electrolytes; long life capability has been shown for some low voltage GBL based systems but not for high voltage ones [9, 14, 15]. The conductivity of the electrolyte directly influences the ESR value. High conductivity electrolyte provides lower ESR compared to a low conductivity one in the same design of capacitor. Normally it is possible to increase conductivity by using a higher concentration of salts. However, as was suggested by the Ikonopisov’s model and experimentally supported, Vbd decreases while conductive component concentration is rising [8]. This is one of the major reasons to optimize electrolyte composition in a way to get a conductive component concentration as low as possible, whilst keeping Vbd at a sufficiently high voltage level. In some cases electrolytes can have different concentrations of main components what is resulting in different Vbd levels even at the same conductivities. Such a situation was described by Song et al. [16] and experimentally shown by Dou et al. [17]. Table 3 shows that GRN-B-51 electrolyte has higher conductivity than for the references. Vbdis in the range between 400V and 500V for all of the electrolytes. The GRN-B-51 electrolyte demonstrates high stability at conditions of temperature (Fig. 2A) and voltage (Fig. 2B) overloading. The stable results provide a safety margin and resistance to intermitted overload. This property is important for real operating

conditions encountered in a broad range of automotive applications like electric gear box, power steering and electric cooling units, fuel injection system. Xu et al. showed that capacitor weight loss with increasing temperature mainly relates to the diffusion of electrolyte [18]. In fact, the diffusion is related to the vapour pressure of the solvent used in the electrolyte. At 125°C, if GBL solvent contains water, the vapour pressure will be increased and this is the main reason why GBL-based high-temperature electrolytes with low water contents are preferred. Electrolyte loss dependences are presented in Fig. 3A. The electrolyte loss is usually considered as the main criteria for Lop evaluation at different temperatures. However in the case of GRN-B-51 electrolyte this criterion is not the only factor defining Lop. Water content is a second important factor which has to be taken into account for Lop calculations. Water increases the vapour pressure significantly in the capacitor when compared with pure GBL or low water content electrolytes. The difference between vapour pressures of low and high water content electrolytes can be very pronounced at high temperatures. It is reasonable to expect a higher rate of water loss at high temperatures and a lower rate of water loss at low temperatures. This statement is in a good agreement with experimental data; e.g. the experimental values of Lop obtained at high temperature were 2600 hours and 4400 hours for experiments carried out at 135oC and 125oC, respectively (Fig. 4D). Thus actual operational lifetime at 125oC is 15% longer than the value calculated based only on the electrolyte loss factor. Taking into account both factors (electrolyte and water losses) one can describe temperature dependence of Lop with following equation: lg2(Lop)=23.5-T/10.9 (10) The equation shows that using this approximation approach, one can conclude that Lop doubles every eleven degrees of temperature reduction. Endurance test results are presented in Fig. 4. ESR dramatically increases with time for the reference electrolytes. Capacitors based on GRN-B-51 electrolyte demonstrate very good ESR stability as well as other parameters. Life time of the capacitors is more than 4000 h at 125 oC and 200 V. Ref-1 electrolyte is not suitable for 125oC, probably due to the higher water content (more than 11 %, Table 3). Ref-2 electrolyte degrades at high temperature in the specified capacitor design. Endurance test results help in deciding what type of strategy should be used for the IRAC capability evaluation [9, 14]. Maximum possible hot-spot temperature can be used for IRAC calculations, but only in the case of very small ESR drifting during operational life. This case is typical for GRN-B-51 based prototypes. If ESR drift is significant it should be compensated by hot-spot temperature limitations. Table 4 shows that despite the close initial ESR, the IRAC capability significantly differs between the tested capacitors. This is obvious since an average ESR(instead of initial value) or lover Th must be used for IRAC calculations because of significant ESR drift in cases of Ref-1 and Ref-2 during carried out endurance tests.

In Table 5, parameters for aluminium electrolytic capacitors from different companies are listed. The electrolyte presented in this article offers much better characteristics in comparison to high voltage capacitors which are available on the market.

Table.5 Electrolytic capacitor with rated voltage 200V from different manufactures

Company Size

/DxL

C /uF

ESR /mOhm

10kHz

20oC

IRAC /Arms

10kHz

T /oC

LOP /hrs.

KEMET current development

20x35 150 90 6.0 (105oC)

(max 14)

125 3500

A 18x39 150 160 2.3 (105oC)

(max 5.6)

125 2500

B 36x53 220 - 1.9 (125oC) 125 3500

C 22x35 390 191 1.8 (105oC) 105 3000

D 16x25 68 - 0.4(150oC) 150 2000

E 18x35 150 - 0.4 (105oC) 125 1000 Conclusion Performance of high voltage aluminium electrolytic capacitors is significantly improved by using a new type of electrolyte. This development makes it possible to design and produce new high voltage, high temperature aluminium capacitors with excellent performance characteristics which are currently in high demanded from the modern automotive industry. Acknowledgements The authors thank Mr. Mark Wright, Mr. Leif Eliasson and Mr. Jorge Vacas for their helpful discussions and participation in the preparation of this article. References

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