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.
For our example, the requirements for the DC-Link capacitor (C3) will be given as:
- Total Cap value needed: 12 μF
- Working voltage: 420 VDC at 125°C
- Total RMS current: 10.3 A at 100°C at 10 kHz
- Application: DC-Link
- Max Ambient temperature: 125°C
Step #1: Select a Candidate Based on Capacitance and Voltage
For this example, we will select a component from the catalog of KEMET’s new C4AK metalized film power box capacitors. To do that, the first step is to select a part number from the catalog that satisfies the required capacitance value and voltage.
In this case, the selected PN is C4AKJBU5125A32J, with a capacitance of 12 μF and a voltage rating of 700 VDC.
Step #2: Find Maximum Irms Value
The next step is to find the Irms capability for this part number from the graphs provided in the datasheet at 100°C ambient temperature.
As you can see, the Irms value in the catalog corresponds to an ambient temperature of 95°C. For higher temperatures, the graph above can be used. In our application, the given requirement for the current was 10.3 A at 100°C, so the graph tells us that the maximum Irms current for this component in our application is 90% of the rated current. Referring back to the catalog or datasheet, the rated Irms at 95°C is 12.5 Arms, so in our application, the Irms maximum is 11.25 A at 100°C, which meets our requirements.
Step #3: Adjust Capacitor Selection or Quantity as Necessary
If the current rating was insufficient (meaning it was less than our required 10.3 A at 100°C) the solution would be to use more parallel capacitance capable of reaching the current requirement. But in our example, one capacitor is sufficient to meet both the capacitance and current requirements.
The KEMET C4AK DC-link power box capacitor in example is a metalized film capacitor in a miniaturized plastic box design that meet the demanding AEC-Q200 qualification requirements.