A Commercial High Temperature Capacitor Dielectric for Power Applications


Lucien Schosseler at DuPont Teijin Films, Luxembourg

Teonex HV is a new dielectric predestined for DC link and snubber power capacitors operating at high temperatures with highest energy densities.

Applications for power electronics require miniaturization more and more, reliability and efficiency improvements and therefore operation at increased power densities. As a consequence, high-temperature exigencies are set on the different elements of the power conversion inverter modules such as passive components, more precisely DC-link and snubber film capacitors. Si and SiC-based semiconductor switches operation temperatures are increased in the range of 175oC and above and film DC-link and snubber capacitors are required to function at temperatures above 105oC [1,2,3,4].

Common film dielectric used in power capacitors is based on the Polypropylene (BOPP Biaxial Oriented Polypropylene) technology. This dielectric material is known as a cheap and reliable reference solution for power capacitors as it operates at a high specific voltage in the range of 150 to 200 V/mm in the capacitors [5]. The drawback of BOPP film technology is the limited temperature range up to 115oC continuous operation [9], as a result of the polymer’s low melting temperature (at 165oC).

Polyester film technology, based on PET (Polyethylene Terephthalate) and PEN (Polyethylene Naphthalate), operates at 125oC resp. at 150oC but is limited to low specific operating voltage in the capacitors [5] and therefore has only limited access to power applications as the capacitors will not cope with the power electronics trend of miniaturization.

Other high-temperature film options, operating continuously above 125oC, such as PPS or PI, suffer from low voltage strength and poor self-healing capability which rules them out from power applications. Indeed the aromatic structures giving the high-temperature capabilities of the polymers are responsible for the limited intrinsic electrical strengths [6].

DuPont Teijin Films has overcome these physical limitations with the development of the film dielectric Teonex HV combining the high-temperature capability of a polyester PEN with excellent electrical breakdown strength performance.

 

Properties of Teonex HV

Teonex HV is a high-temperature material with a melting temperature Tm at 265oC and a glass transition Tg at 145oC making a continuous operation in a temperature range up to 175oC possible.

The electrical breakdown strength of the Teonex HV film is comparable to BOPP and function of the film thickness:  a voltage strength on film sheet over 500 V/mm at room temperature is achieved as indicated in Figure 1. Electrical performance is improved relative to polyester films (PEN, PET) and far superior (2.0 x) than other high-temperature materials such as PPS, PEEK, and PI.

 

Figure 1: Electrical breakdown strength of Teonex®  HV compared to BOPP and other dielectrics

Figure 1: Electrical breakdown strength of Teonex  HV compared to BOPP and other dielectrics [13].

 

Teonex HV break-down strength shows on film sample, as presented in Figure 2, a voltage derating of 12% at 125oC and 19 % at 150oC.

 

Figure 2: Electrical breakdown strength of Teonex®  HV in function of temperature as per method

Figure 2: Electrical breakdown strength of Teonex  HV in function of temperature as per method [14].

 

The dielectric constant is 3.05 at 25oC and increases to 3.73 at 175oC. The permittivity increases with temperature showing an improved charge storage capacity with temperature (Figure 3).

 

Figure 3: Permittivity at 1 kHz of Teonex® HV

Figure 3: Permittivity at 1 kHz of Teonex HV.

 

The film dissipation factor is stable up to 125oC and inferior to 0.45%  and less than 1.0% for 150oC (for frequencies ranging from 1-10 kHz). The total dissipation factor of the capacitor is the result of the dissipation of the leakage current in the dielectric film and the current flow through the contacts and the metal electrode. In the DC link capacitor, to assure the dielectrics optimum self-healing, high resistivity metal electrodes are of common use and the resistance of the metal electrode becomes then the predominant factor in the total equivalent series resistance (ESR) and the dissipation factor of the capacitor.

The Teonex HV film has an outstanding energy density, outcome of the material’s dielectric constant and the high electrical voltage strength in the capacitor. Especially above 105oC operation, Teonex HV shows the highest energy storage density of all dielectric materials (Figure 4).

 

Figure 4, Theoretical Energy density of Teonex ® HV compared to other common dielectrics

Figure 4, Theoretical Energy density of Teonex HV compared to other common dielectrics.

 

In power capacitors, the ability of dielectric to self-heal is of key importance for safe capacitor operation and end of life behavior of the passive component [8]. In case of an electrical breakdown related to a weakness in the metalized film, the discharge energy evaporates the polymer and metal electrode and creates a hole in the polymeric film surrounded by a de-metalized area and so the defect is insulated.

To assure the optimum self-healing capability, the dielectric must operate in a real condition below a certain interlayer pressure limit or with an equivalent minimum air layer. Above this threshold, the film loses its self-healing function: the dielectric is damaged and the clearing energies are too low to adequately evaporate the dielectric and the adjacent metal electrode and consequently result in the capacitor short circuit [7].

Manufacturing of Teonex HV capacitor requires soft winding and flattening process to reduce the processing interlayer pressures and to withstand the electrostatic pressure component resulting from operation at nominal voltage. Support by modeling, the impact of the winding and flattening conditioning on the capacitors electrical performance, is given on request for the Teonex ® HV dielectric film.

In the optimum scenario, the dielectric Teonex HV can operate at the specific voltage up to 150 V/mm at 125oC, pending on employed metallization and manufacturing technology; above this temperature a derating should be applied.

The film can be used with state-of-art, Zn or Al or Zn/Al metallization (as known from BOPP capacitor) in the wound or stacked power capacitor technology as well as in film foil or impregnated capacitor device.

The film is commercially available in 3,4,5,6 and 8 mm and so a large voltage range from 350 to 1000 V can be covered with single metallization pattern or above (> 1kV) with serial pattern metallization or serial/parallel capacitor design.

 

Power Converter Applications

The power converter or inverter use in their intermediate circuit DC link capacitors for ripple filtration and electrical energy storage and delivery [5,8,11].

Inverters are used in automotives (EV/HEV/PHEV), renewable energies (solar and wind), industrial applications (drives and building facilities), traction as well as energy conversion (HVDC). Use of high-temperature capacitor in these inverters will improve their reliability, reduce their cost and size by cooling arrangement simplification and improve their maintenance [8, 9].

The introduction of high-frequency switching device such as GaN/SiC in inverter technologies will further reduce the size of DC link capacitors and consequently result in larger power densities and therefore temperature constraints on the passive components [12].

 

Summary of Modern Power Electronics

Modern power electronics trends toward compact, reliable and power efficient designs and will require new advanced high-temperature DC-link capacitor and snubber technologies.

Today’s major dielectric film technology, based on Polypropylene and Polyesters can only insufficiently satisfy these needs: BOPP operates at high voltage strength and low temperatures while PET is only adequate for low voltage and medium temperature utilization.

The new commercialized DuPont Teijin’s Film’s dielectric technology Teonex HV is a possible answer to the future specifications of inverter module requirements.

 

[1]
H. Kerstin et al , “Where is the Journey Headed ? The Future of High-Power Semiconductors”,Bodo’s Power Systems , November 2014 pp.30-33.
[2]
K.Vogel et al.,”IGBT with higher operation temperature - Power density, lifetime and impact on inverter design”. PCIM 2011.
[3]
R.Nagarajan et al. ,”Aspects of increased power density with the 5th generation IGBT demonstrated by application relevant measurements”,PCIM 2015,VDE Verlag
[4]
A.Ciliox et al. “Next step towards higher power density with new IGBT and diode generation and influence on inverter design”,PCIM Europe 2013.
[5]
D.Montanri et al., “Film capacitors for automotive and industrial applications”, CARTS Europe 2008.
[6]
P.Phillips,”From insulator to superconductor”, Nature Vol. 406, August 2000.
[7]
I.Rytöluoto et al. ,”Effect of the interlayer pressure on dielectric breakdown characteristics of metallized polymer films for capacitor applications”, Proceedings of IEEE ICSD, pp.682-687,2013.
[8]
J.Luthin , “Film Capacitors for DC link applications”, Power Electronics 4/2007.
[9]
T.Ebel , “ Researching Film capacitors for converters in wind power plants”,Bodo’s Power Systems, September 2015.
[10]
Ducati Energia Product Catalogue, p.26 & p.38, December 2015.
[11]
H.Wang, “Reliability of capacitors for DC-link applications in Power Electronic Converters”, IEEE Transactions on Industry Applications, Vol.50, no.5, Sept/Oct 2014.
[12]
C.Buttay et al., “State of art of High Temperature Power Electronics”, Microtherm, June 2009,Lodz,Poland.
[13]
DTF method - electrode Al. film sheet 25 cm2, dV/dt = 350 V/s, I max = 10 uA.
[14]
M. Ritamäki et al.,”Temperature Effect on Breakdown Performance of Insulating Polymer Thin Films”, method - 4 Teonex ® HV, Weibull scale, film sheet surface 139 cm2.

More information: DuPont    Source: Bodo's Power Systems, April 2016