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Posted on 01 August 2019

HEV Converters Based on PCC Technology

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Innovative Miniaturized System Solutions

The solution can be realized on the basis of the innovative and integration-friendly Power Capacitor Chip (PCC) technology. This technology was mainly developed for the DC link capacitor, one of the key components of an HEV converter.

By Harald Vetter, Epcos AG, PM Power Electronic Capacitors

 

The PCC, a newly developed power capacitor, will set a new standard for DC link capacitors for HEV applications. The challenge for PCCs in this field is the considerably higher requirements profile in comparison with well-known industrial loads. This means that most standard PEC solutions cannot be successfully transferred to these applications e.g. the total circuit inductance Lσ must be extremely small, approximately 10 to 30nH, to avoid oscillations at the switching frequency. Also, the thermal current carrying capability Ith must be up to 250Arms/mF and the high peak current Is rating at least twice the value usual in industry. These requirements are promoting a switch to self-healing solutions such as PCC in the domain previously covered by aluminum electrolytic capacitors. The capacitor should withstand harsh environments, various geometrical dimensions lead to space and weight restrictions depending on the current HEV development status and a strong design to cost management during the design-in phase is a must. The PCCs delivers outstanding performance and support innovative and cost-effective converter designs based on the goal of miniaturization and modularity.

Polymer selection guide

PP (Polypropylene) This dielectric, and particularly its high temperature grade, is competitive with the other common dielectric films within a wide range because of its superior electrical properties, such as breakdown voltage level, insulating resistance and self-healing. PP is semi-crystalline, consisting of crystalline regions in an amorphous matrix. By optimizing the matrix structure, using ultra-pure homopolymer with extra-high isotactic content with a tailor-made molecular weight distribution (MWD), modified preparation of the catalyst and extreme process purity during film production, the BDV strength vs. temperature has been increased consistently in the past. In future, a limit will be reached at about BDV=700VDC/µm and T=120°C.

The development trend for the BDV field strength vs. temperature (for miniaturizing of VC one of the key parameters) is shown in Figure 1. In fact, the temperature limitation rose from Tmax ≈ 120°C in 1990 up to Tmax  125°C in 2005. New Laboratory results indicate increase of this value in the future up to Tmax ≈ 135°C at a BDV ≈ 670VDC/µm with optimised thermal shrinkage characteristics. The line for 2010 is a theoretical estimate, so this target remains uncertain in practice.

Breakdown voltage BDV vs. temperature history and outlook carried out to DIN 53481 (sheet test, ball-plate).

The development trend for film thickness (the next key parameter for miniaturization) is described in Figure 2. Thanks largely to the tailor-made molecular weight distribution (MWD), the polymer can be converted to films with a thickness less than 3µm. The film thickness has been gradually reduced in the past but will also reach a limit in future, which is expected at about d=2µm.

Thanks mainly to the MWD, the PP polymer can be converted to an ultra-thin capacitor grade film

The miniaturizing effect for VC due to down-gauging is shown in Table 1.

Miniaturizing effect for VC

The PP parameter collection based on the outlook estimate described above can be summarized as:

- BDV=660VDC/Vµm at T=125°C satisfies the current HEV requirements for the DC link.
- The expected outlook of BDV ≈ 670VDC/µm at T ≈ 135°C is encouraging for the future.
- The expected down-gauging to 2µm [5] is helpful for future APE applications.

Design comments on alternative films PPS (Polyphenylenesulphide)

Because of its high thermal resistance, a larger and more expensive PPS design is attractive as long as VC is not at the focus and if Tmax <=170°C is a requirement (e.g. with fully integrated HEV designs of the 3rd generation. This means an solution within the gear bell and without a second liquid cooling system). This expensive dielectric material seems to be a competitive alternative to the visionary solution discussed for this application: the ceramic capacitor.

PEN (Polyethylenenaphtalate)

This dielectric material is used in a small number of applications, such as automotive HID lamps, where the high heat resistance of Tmax <=150°C offers a technical advantage over PET and is less expensive than PPS.

PET (Polyethyleneterephthalate)

For applications that operate at elevated ambient temperatures such as Tmax <=135°C, a PET dielectric will often be selected. PET will be mainly "designed-in" at low to medium VR values. An interesting example is the mild hybrid: metallized 1µm to 2µm PEN capacitors operate in converter for integrated starter generator at VR  ≈ 42VDC. The design of a DC link capacitor with a medium voltage of VR >100VDC and a capacitance CR >100µF will be more difficult than small capacitors because of its more complex casing requirements designed to avoid overheating in large capacitor winding blocks or assemblies (e.g. with round or flat MKT windings). Special care must be taken to guarantee sufficiently high Rins values and avoid effects such as hydrolytic degradation of the dielectric system.

Modern metallizing technology

The MKK-Technology for PCCs in HEV is working with a toolbox of metallization features like: Al, ZnAl, heavy edge, single or double side, flat or CSP, with or without structure, Figure 3.

Metallization features for PCC, left to right - Smooth cut, Wavy cut, Free stripe

Advanced coating techniques

An Acrylate coating on the metallized film is now available in production quantities. The thickness of the Acrylate layer ranges in between 0,1 to 0,3µm and delivers a high temperature protective effect for the dielectric substrate. The DC-BDV strength of PPS, PEN and PET has been improved by between +20 to 30% and superior self-healing properties of PPS have also been confirmed.

An Oil-coating technique is now also available. The thickness of this layer is in the range between 0.1 to 0.3µm. In addition to improved DC-BDV strength, greater insensitivity to climate can be implemented. This makes it significantly easier to work inside the converter casing with naked and cost effective PCCs.

These coating technologies will create new development targets, especially for innovative automotive applications. The most interesting substrate for coating apart from OPP will be high temperature grade films such as PPS, PEN and PET. The improved DC-BDV voltage strength and self-healing properties will increase the ER level and thus will minimize the physical capacitor volume VC as already described.

Volume fill factor

The effect of the volume fill factor (VFF, the ratio VCphysical / VCtechnical) is often underestimated because if the existing black box is filled with a sub-optimized winding shape, not all possible dielectric materials are activated inside the box. The necessary result is a reduced design-film thickness and reduced expected lifetime for the capacitor function (Figure 4) compared with a VFF ≈1 solution with PCC.

Lifetime expectation vs. field strength ratio E1- E2

A new winding technology for PCC

This newly developed winding technology can be used to implement absolutely flat and wrinkle-free stacked windings in "power cap dimensions" for PCC using metallized polymer films, starting with PP down to 2µm and PET 1.5 µm, Figure 5.

Design of flat and wrinkle free stack winding in “power cap dimensions” for PCCs

The result is an outstandingly high pulsecurrent- handling capability without the contact edge problem (a well-known and dangerous constriction effect at the film edges of low-cost MKP windings).

Figure 6 and Table 2 show the possible dimension range from a flat stack winding in PC dimension up to large diameter e.g. D  ≈ 260mm.

Examples for the dimensional flexibility

Possible dimension range from a flat stack winding

Requirements profile

The requirements profile in automotive APE engineering is characterized by the absence of a standardized mission profile. One first approach that may be used as a basis for the capacitor layout is filed in Table 3. The requirements for under-the-hood and on-the-engine installations are well known and extremely high, up to 125°C. Also, the upcoming short-term / full operation requirements of up to 140° to 150°C differs significantly from the standard specifications for industrial applications.

Basis for the capacitor layout

Thermal design

A thermal design for the overall system by taking into account the requirement on reliability and expected lifetime is therefore of central significance in these applications to be able to minimize the capacitor volume needed. The thermal loads on the capacitor can be classified into slow temperature changes and temperature shock. Thermal shock tests are typically performed over 100 up to some 1000 cycles between e.g. -40 and +105 / 125°C. The applicable standards for road vehicles define no limits for the voltage disturbance on the traction battery system. However, a lower ripple voltage will represent an additional challenge for PCC R&D and will mean trying to increase the capacitance density in future.

Advanced design aspects

If a PCC "design in" is taken into consideration at an early stage of the converter development, optimized system solutions can be implemented for maximum customer benefit. EPCOS is the only manufacturer to master all key winding technologies: round, flat, and stacked windings in "ower cap dimensions" The rated voltage band-width extends from VR=100VDC up to VR=1000VDC and the rated capacitance range from CR=50μF up to CR=3000μF depending on the system requirements. Custom-design solutions which generate additional benefits such as an integrated busbar will give the system designers additional scope to optimize their inverter layout in order to achieve: hgh volume fill factor (Vphysical/Vtechnical ≈ 1),mniaturizing due to system integration,hgh design flexibility, easy to integrate into a converter casing, extremely low inductance, high permissible ambient temperature, low functional weight, high over voltage strength, high pulse-current-handling capability, long expected service life, high mechanical strength, and low fire hazard due to oil-free technology.

Application example

The application benefits of the PCC concept will now be illustrated by some examples.

PCC, CR=2mF, UR=450VDC, LxWxH= 270x125x56mm³,

The PCC capacitor in a aluminum casing provides additional excellent EMC behavior together with the hermetic sealed converter.

PCC with integrated bus bar in a plastic box ready for assembly up side down into the HEV converter casing

Image 1

PCC designs with screw holes radial through the winding e.g. for mounting on cooling plate

Measured current and voltage waveforms

First motor-integrated circular converter for HEV, mounted in the stator and PCC

HEV converter 2nd generation used a circular shaped PCC

For utility of the power semiconductors the stray inductance between the power semiconductors and the PCCs has to be minimized. Hence the DC-link capacitor is distributed between the power electronic modules around the circular converter assembly. To make full use of the available space the PCC were built in a circular form by winding the foils with a diameter corresponding to the diameter of the circular converter.

Trends

HEV motor output P rose from the level of 20 to 30kW in 2001 up to 120kW in 2006 and the rated voltage UR started at 100 to 300VDC and has reached now UR 700VDC. For film capacitor this trend is a good chance for future design in wins. The development trend characterized by the extreme requirements of fully integrated next generation HEV converters will be supported by improvements in plain film performance data, the use of with new coating processed films, the upcoming down gauging of film thickness on the PP, and the reduction of power losses due to optimized metallization profiles. This requires a high-quality, cost-effective and complete production process with ultra-thin film (UTF) handling capability for boosting PCCs in industrial and mainly automotive.

Conclusion

The introduction and development of the process and production resources of PCC allow the DC link application of HEV converters to be optimized. This technology can be used also to replace electrolytic capacitors and it is evident that this ultra compact capacitor - often made in a single block - is a best-case design for an integrated and miniaturized system solution. This dimensional flexibility allows customized designs and satisfies hitherto unfeasible size requirements and PCCs delivers outstanding performance data like very low ESR and ESL values and should basically be able to operate up to 150°C.

Glossary

 

 

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