New KEMET DC-Link Power and Resonant Film Capacitors for High Temperature in Industrial and Automotive Applications

DC-LINK POWER BOX FILM CAPACITORS

The DC-Link capacitor is present in all power converters, in the DC part of the circuit between the input and output stages. This capacitor is critical to filter the DC voltage and to store energy to provide instantaneous current to downstream circuits. The DC-Link capacitor must be able to withstand high power, high ripple currents, and a large amount of charge/discharge cycles. Furthermore, in this specific application, they must also withstand high temperatures and harsh conditions in the engine compartment of a vehicle. [1]

Traditionally, electrolytic capacitors have been the choice for power conversion applications due to their low cost per capacitance and high energy storage (i.e. capacitance) per volume. But the trend of higher ripple current and higher voltages is forcing designers to consider more film technology. The DC-Link application requires capacitor that can be able to work at high frequency with a range of working voltage up to 220V/μm able to reach temperatures up to 135°C.

Main targets for a DC-Link Film Capacitor:

1. High Vrms/ Irms values in a frequency range up to resonance frequency. This characteristic is allowed through the usage of base film with very low Tgδ and a proper design of the film metallization with the purpose to have a low ESR (Equivalent Series Resistance).
2. High reliability. Film capacitors are mainly used for this characteristic in application where catastrophic failure as short circuit must be avoided. As for Resonant capacitors this important property is given by the two important factors of film capacitors as flexibility and self-healing.
3. High capacitance and Tgδ stability. These capacitors are used as DC link to stabilize the voltage and reducing the ripple voltage for a quite constant DC output. The decreasing of the capacitance value in the lifetime would reduce the filtering effectiveness of the circuit design. The designed stability is obtained by a proper design of the metallization and of the process parameters for the components manufacturing. The metallization profile of each film layer is tailored to maintain the right compromise between a high self-healing property, low ESR and high capacitance and Tgδ stability. One of key technical choices is the design of the metallization thickness of the active area of the film layers inside the capacitors. Increasing the metal thickness, the ESR reduces, increasing the Vrms/Irms withstanding, increasing the capacitor stability also reducing the sensitivity of the internal element to be oxidized. On the contrary the metal thickness reduction increases the self-healing property and therefore its reliability versus transient or spike voltages.
4. High volume efficiency increasing the Capacitance density (with a cap density range from 0.14 nF/mm3 @ 1500Vdc/70°C to 1.37 nF/mm3 @ 500Vdc/70°C). The miniaturization of DC-Link Film capacitor is usually made increasing the Voltage for every μm of film thickness used. Such DC Links are working @ 70°C up to 220V/μm. In this way a 1500Vdc rated voltage is using a 7um instead of a 500Vdc that is using a 2,7um. The V/μm increasing is usually obtained also decreasing the metal thickness of the film (self-healing improving)
5. High Peak current measured as dV/dt value (in a range of 15-100V/μs). The current is easily calculated by the following formula Ip=C*dV/dt. Usually these high peak current values are obtained with optimized process parameters especially during the building of the “Schoopage” layer and of the element compacting phase
6. High humidity robustness grade. Especially in solar market but also in the Automotive the 85°C/85%R.H. with Voltage applied is nowadays became a usual requirement. In such conditions the metallization oxidation of the film is the main root cause for capacitance drop and Tgδ increasing. The corrective action adopted by KEMET are mainly focused to reduce the humidity absorption rate by internal elements with a deep analysis and relative adoption of all properly enclosure materials and film. The production process has been also mainly reviewed to optimize the compactness level of the internal film elements. The main phases form winding up to the thermal treatment have been meaningfully reviewed for such purpose.

As per the abstract of this paper, the market is currently requiring DC-Link Power Box capable to work at 125°C and for enhanced applications up to 135°C with high voltage and longer lifetime at 125°C.

The main failure mechanisms at 125°C and at 135°C are:

1. IR (Insulation Resistance) decreasing of the BOPP film dielectric. IR decreasing is equivalent to the leakage current increasing, that for BOPP film starts to be meaningfully at 105°C. Above 125°C such film is not used for electronic market application due to this high leakage current and its starting of fast degradation mechanism. At about 140°C it starts melting. For applications with a maximum temperature of 125°C KEMET has selected an improved BOPP film because it assures great electrical performances and good reliability. Proper film base material is selected to maintain good BDV (Break Down Voltage) and self-healing up to 125°C from one side and maintaining good stability up to 125°C in terms of ΔC/C% and Δtgδ. The standard Polypropylene essentially loses most of its strength above 105°C therefore for applications with enhanced voltage at 125°C and a working temperature up to 135°C KEMET started to use a High Temperature Dielectric Film (HTDF). The selected HTDF increases the mechanical strength and reduces the insulation resistance in a wider temperature range up to 135°C.
2. High oxidation level of the metallization. The DC-Link capacitors to work up to 220V/μm have been designed using a very thin metal level in the range of 15-40nm. Above 105°C the thin metallization starts to become sensitive to the oxidation and for this reason KEMET has worked to reduce such failure mechanism. Here below it’s possible to see some pictures of film with different kind of oxidation after an endurance test performed @ 125°C – 0,78Vrdc. As for THB Grade achievement, the main technical activities have been focused to optimize the compactness level of internal film elements, protecting the thin metallization and reducing the humidity absorption through a proper design of all enclosure materials and capacitor assembly process. See Figure 14.

3. High level of TD (Transversal Direction) shrinkage of the film. Increasing the working temperature, the film shrinkages in transversal direction with the risk of decreasing the mechanical contact versus the spraying layer. This mechanical stress could increase the Tgδ and reducing the dV/dt withstanding of the capacitors. Also the Tgδ increasing is an effect really sensitive for DC-Link power box capacitors that have to bring high Irms/ Vrms values. To reduce the natural shrinkage of BOPP or HTDF film at high temperature the process parameter is proper designed. The main tuning is related to the thermal treatment and to the compactness variables of the film capacitors.

KEMET to meet the Customer requirements of high working temperature for DC-Link power box capacitors has developed two products’ series based on two different type of dielectric film:

• C4AQ-P in BOPP (standard Biaxially Oriented Poly Propylene) with enhanced Performances at 115°C & 125°C with longer lifetime
• C4AK with HTDF able to work up to 135°C with increased V/μm at 115 & 125°C

Here below the main characteristics and performances of the new 2 series:

C4AQ-P & C4AK – Characteristics and Performances
In this chapter the characteristics and the performances of the two new series will be compared with the standard C4AQ.
The comparison is structured considering the following schema:

• Characterization of C4AK vs C4AQ-P of ΔC/C [%] vs t [°C], Tgδ vs t [°C] and IR [Mohm] vs t [°C]
• Electrical performances of the DC-Link power box series (C4AQ vs C4AQ-P & C4AK), in particular:
  • Operating voltage vs T for the different series
  • Typical ΔC/C behavior in the endurance test performed at 85°C, 105°C, 125°C and the typical behavior in the endurance test of C4AK at 135°C
  • ΔT vs Tamb for the different series
  • Irms at 85°C & 105°C for the different series for a 14μF/900Vdc at 85°C
• Lifetime comparison of the different series considering the hot-spot temperature at 85°C, 105°C, 115°C, 125°C and 135°C only for C4AK

Here below the different graphs for the characterization of the series:

The capacitance trend versus T is showing meaningful difference of C4AK series made with the new dielectric HTDF vs the series C4AQ & C4AQ-P made with PP base @ T > 125°C. The graph on Fig 16. is showing a dissipation factor trend @ 1kHz of the C4AQ & C4AQ-P series slightly better than C4AK one. However, considering the dependence of this value vs the Rmet value of the metallization and its tolerance, these values are considered not meaningful different.

In the graph on Figure 17. is clearly visible the improvement in terms of IR vs T of the C4AK series made with the new HTDF vs the C4AQ & C4AQ-P series made with PP film. C4AK measured of IR at 900Vdc with 5um film is showing an IR higher than PP 5um at 500Vdc. The curves are related to the IR*μF [s] to plot values independently to the capacitance considered.

Such improvement it’s really a key factor of the working of C4AK at high voltage level at 125°C and of the working extension up to 135°C.

Here Below the operating voltage versus temperature of the series C4AK & C4AQ-P. The C4AQ-P has the same operating voltage than C4AQ but with an increased lifetime at high temperature conditions. For instance, @ 125°C C4AQ-P can work at the operating voltage for 4000h instead of 200h of the std C4AQ.

As for above graph on Fig.18 and above table Tab1. is possible to note that the C4AK can work at higher orating voltage than C4AQ-P series. In particular C4AK @ 700Vdc has an operating temperature @ 125°C (500Vdc) higher than the C4AQ-P @ 900Vdc (480Vdc). Moreover, the C4AK @ 700Vdc can be used safely in the 450Vdc automotive bus up to 125°C.

Here below the different graphs for the typical behavior of the series in the endurance tests.

All DC-Link series show a high stability level up to 3000h @ 85°C. Both C4AQ & C4AQ-P show an increasing of the capacitance value due to the increasing of the compactness level during the endurance test for the simultaneous presence of DC high electric field and temperature as explained in the resonant capacitor session. C4AK is instead showing a decreasing of the cap value because such capacitors are more compact and more thermally stabilized than the C4AQ & C4AQ-P series.

All DC-Link series show a high stability level up to 3000h @ 105°C. In these conditions also C4AK starts showing an increasing of the capacitance value due to the increasing of the compactness level.

It’s important to underline as C4AK has been tested at a voltage level 20% higher than C4AQ & C4AQ-P series.

In the graph on Fig.21 it’s important to underline that C4AK has been tested @ 125°C at a voltage 40% higher than C4AQ & C4AQ-P. The endurance @ 125°C highlight the great achievements of the development of the series C4AQ-P & C4AK versus the std C4AQ. C4AQ-P with PP film increase the lifetime of 200h @ 125° of the C4AQ up to 4000h at 0,6Vrdc. C4AK can work at 125°C 0,85Vrdc for 4000h thanks to the KEMET design and the usage of the new HTDF film.

Very good stability is showed by this endurance test @ 135°C – 0,7Vrdc by C4AK – see Fig.22. Thanks to high DC electric field and 135°C applying compactness increasing is visible. In this test any meaningful oxidation or meaningful clearing is not showed.

Up to a TAMB = 70°C the ΔTLIM of all 3-power box DC-Link series is = 30°C and therefore the max Irms allowed by the 3 ones is almost similar. For a TAMB > 70°C C4AQ-P & C4AK start showing higher Irms capability versus C4AQ as per following examples.

C4AQ-P is allowing almost 22% a higher [email protected]°C than std C4AQ. C4AK can withstand almost 10% higher Irms than C4AQ-P and therefore almost 35% higher Irms than C4AQ.

C4AQ-P is allowing almost 11 times a higher [email protected]°C than std C4AQ. C4AK can withstand almost 18% higher Irms than C4AQ-P and therefore almost 12 times higher Irms than C4AQ.

C4AQ-P & C4AK – Product Lifetime

The development activity to increase the working performances at high temperature from 115 to 135°C and also increasing the operating voltage as for C4AK series at high temperature have positively contributed to increase the maximum Irms of the series C4AQ-P & C4AK than std C4AQ at temperatures > 85°C.
Here below the different graphs for the Lifetime comparison for the different power box DC-Link series.

From the graph on Fig. 26 left it’s possible to underline the following improvements and characteristics of the series C4AQ-P vs C4AQ:

• @70°C the lifetime is almost the same between the 2 series except for the increasing up to 220kh for C4AQ-P at Vrdc
• @85°C the lifetime is almost the same between the 2 series except for the increasing up to 220kh for C4AQ-P at about 0,85Vrdc
• @95°C & 105°C C4AQ-P has increased meaningfully the lifetime versus C4AQ
• @115°C the lifetime of C4AQ-P at 0,7Vrdc has been increased from 500h to 4000h
• @125°C the lifetime of C4AQ-P at 0,6Vrdc has been increased from 200h to 4000h

From the graph on Fig.27 right it’s possible to underline the following improvements and characteristics of the series C4AK vs C4AQ-P:

• @70°C & 85°C the lifetime is the same for C4AQ-P & C4AK
• @95 & 105°C C4AK has increased meaningfully the lifetime and operating voltage versus C4AQ-P
• @125°C the lifetime of C4AK at 0,6Vrdc has been increased from 4000h to 15000h. Moreover, the maximum voltage at 125°C has been increased from 0,6Vrdc to 0,8Vrdc

All lifetime showed in the previous 2 graphs are a result of focused research and design activity and long endurance test with the aim to maximize its for the power box DC-Link series. Such activity can not consider just the Arrhenius and voltage formula, but it should consider all the failure root causes as clearing effects, oxidation at high temperature and aging with the combination of voltage and temperature. The sum of all different failures considered explains the fact that the curves are not simply a line in a semi-logarithmic graph, but they are a more complex curve. Moreover, all the lifetime curves are saturated at 220kh only because there is not available data to support higher number of hours.