DC-Link capacitors for DC filtering and energy storage are expected to operate at higher temperatures, in more extreme conditions, and for longer lifetimes, than ever before. Automotive applications are leading those demands for better performance, but most existing power box, DC-link film technologies are not suitable for these applications and struggle under those severe conditions.
Vehicles primarily use electric compressors in two ways: in air conditioning and electric turbocharger systems (E-turbo). The circuit design for each of these applications is similar, as is the harsh condition in which these systems must operate.
When designing a power converter and selecting a DC-Link capacitor, there are several key considerations designers must take into account.
Operating Ambient Temperature
Required temperature ratings for under-the-hood automotive components are higher than for other commercial or industrial applications. Automotive-rated components need to operate up to 125°C ambient. However, existing power box DC-link film technologies cannot meet those requirements. They are limited to 105°C ambient temperatures, and even then, they require derating as they approach that limit. These increased temperature ratings, and unmatched performance at high temperatures, enable applications such as E-chargers that increase the efficiency and performance of HEVs and internal combustion engine vehicles.
Derating for Temperature
Another consideration when selecting a DC-Link capacitor is the necessary derating for temperature and voltage. Every capacitor is defined by a curve that shows the relationship between operating temperature, voltage rating, and life expectancy. As an example of DC-Link film capacitor specification see KEMET’s derating charts for the C4AQ-M and C4AK series below (Figure 1. and Figure 2.).
To obtain the same life expectancy of 100,000 h at an 85°C hotspot temperature (see the arrow in the image below), C4AQ-M requires a voltage derating of more than 10% compared to C4AK, which can operate at the same hot-spot temperature without any voltage derating required.
Therefore, a key design consideration is what operational life is needed and how that will relate to the required voltage and temperature ratings.
Other Harsh Conditions
Beyond just extreme temperatures, DC-link capacitors must be able to withstand the conditions into which they will be operating – they must be able to withstand the relative humidity, vibration, or contamination as required.
KEMET’s C4AK series capacitors are smaller and more reliable than alternatives. They boast unmatched harsh condition ratings, and because of their small size, they also have excellent vibration ratings.
Power Dissipation
A DC-link and DC filter capacitor will experience internal heating, which will increase as the frequency of the ripple current of the semiconductors increases. The heating effect is caused by the current flowing through the internal series resistance of the capacitor. The formula used to calculate the max power dissipated by the capacitor is the following:
Then, the formula used to calculate the maximum power that may be dissipated by the capacitor, during thermal stability, is given by the following:
Capacitor datasheets will give the Tamb (max. ambient temperature surrounding the capacitor rating), and the graph on the following page will provide ΔTlim vs ΔTamb:
Finally, a capacitor must be selected so that the combined harmonics dissipation power in Watts does not exceed the limit conditions Pcmax < Pclim .
