How to improve OBC performance of film capacitors?

DC film capacitors are widely used in many parts of new energy vehicles such as on-board chargers (OBC), DC/DC, main motor drives, auxiliary drives, etc.

Key components of new energy vehicles - film capacitors

Film capacitors, as a good substitute for electrolytic capacitors, are now used in large quantities in new energy vehicles and have attracted much attention. Film capacitors essentially use metal foil as an electrode, which is overlapped with a plastic film such as polyethylene, polypropylene, polystyrene or polycarbonate from both ends, and then wound into a cylindrical structure.

Compared with other common capacitors, film capacitors are between aluminum electrolytic and ceramic capacitors. The upper limit of capacitance is lower than that of aluminum electrolytic capacitors, and the upper limit of working voltage is lower than that of ceramic capacitors, but they combine the advantages of high capacitance and high voltage level. Especially compared with aluminum electrolytic capacitors, film capacitors have better ripple tolerance, and thanks to their self-healing ability, they have an extremely long service life.

At the same time, film capacitors exhibit the advantages of higher withstand voltage, lower ESR, and non-polarity, which simplifies the design of film capacitor-based systems and enables them to function reliably in harsh environments.

New energy vehicles need to take into account many requirements such as earthquake resistance, high temperature, high voltage and high current, and high reliability. Although the capacitance of electrolytic capacitors is sufficient, other aspects can no longer be met. These characteristics of film capacitors just fit the scenarios of electric vehicles, especially the needs of OBC and DC/DC. Therefore, film capacitors have become ideal devices in the process of automotive electrification upgrades.

A representative application is OBC, which requires the use of film capacitors from EMI filtering to PFC, to DC-link, to LLC resonance stage, and to output.

Film capacitors and OBC

There are many applications of film capacitors in new energy vehicles, among which the largest ones are on-board chargers (OBC), DC/DC, main motor drives, and auxiliary drives. Its representative application is in OBC. In these applications, film capacitors mainly realize three functions: EMI, bypass coupling/decoupling, filtering, and energy storage functions are also used but not much.

In OBC applications, film capacitors need to pay special attention to the three characteristics of self-healing, failure open circuit, and low dissipation coefficient. Especially the self-healing property, good self-healing ability means that the device has strong overvoltage resistance and good high frequency. Self-healing property means that the dielectric of the film capacitor can adjust itself to restore normal working state after being broken down.

The self-healing characteristic gives film capacitors a large application space. Its overvoltage resistance can reach several thousand volts. Coupled with its good high-frequency characteristics, it can significantly improve the performance of electric vehicles and extend the use of batteries, thus solving new problems. Energy vehicles have low vehicle load and short cruising range. Currently, the industry is dominated by polypropylene film capacitors with excellent characteristics.

As mentioned above, film capacitors are needed from EMI filtering to PFC, DC-link, LLC resonance stage, and output. In OBC EMI filtering, film capacitors are used to assist OBC in developing in the direction of small size and high power. Now, they are made as small as possible on the premise of passing the double 85 test. Moreover, manufacturers now provide options with a wider range of capacitance values in small size, so that the overall design can be more compact.

The application of film capacitors in DC-link is very mature. Its function is to do current support filtering between the OBC rectifier circuit and the DC-DC circuit, absorb the high pulse current at the DC-LINK DC bus end, and prevent high pulse voltage from being generated on the impedance to affect the load end. The film capacitors here require large current and large capacity design.

The LLC resonance stage places more emphasis on the stable pulse strength and ripple current performance of the film capacitor, while the output end requires large capacitance and low ESR in order to improve the transient response characteristics of the OBC DC output.

Summary

In some applications, film capacitors can handle EMI and filtering, and also use their energy storage function. The application of these film capacitors has greatly improved the performance level of OBC and the entire new energy vehicle electrical system, and accelerated the upgrade of the vehicle electrification architecture.