TDK: ALUMINIUM ELECTROLYTIC CAPACITORS FOR ON-BOARD CHARGERS

With the increasing market for electrified vehicles (EVs), the demand for on-board chargers (OBCs) is growing fast. The function of an OBC is to convert the AC voltage from an external source to a specific DC voltage that is based on the requirements of the battery management system. By this, a battery-saving and fast charging process can be reached. Due to the complexity of such systems, the OBC needs a certain bulk capacitance to stabilize the DC voltage that is charging the battery. Aluminium electrolytic capacitors are an attractive solution here since they can fulfill the key requirements, such as high voltage ratings of up to 500 V, large capacitance of up to 820 µF and high ripple current capabilities at an operating temperature range of -40 °C to 105 °C.

Challenges for aluminium electrolytic capacitors in on-board chargers
The DC link capacitor does not only have to fulfill the capacitance requirements of the system, but it also must withstand the continuously increasing ripple currents since the power densities of the OBCs are steadily rising. As a result, higher power losses appear that heat up the complete system which can cause performance degradation and reduced lifetime. Therefore, being competitive inevitably requires a cooling system that is also connected to the DC link capacitors. To cover these requirements, TDK developed the new large-size series B43652 for OBC applications that are optimized for base cooling and perfectly combine all of the above-mentioned characteristics.

The reason for thermal management
The lifetime of an aluminium electrolytic capacitor is mainly influenced by its core temperature. Generally said, high ripple currents and increased ambient temperatures heat up the capacitor significantly and therefore reduce the lifetime. With the B43652 series, TDK developed a large-size capacitor that has both a very low ESR and an improved internal thermal resistance across the full lifetime. With an external cooling system providing an efficient heat transfer between the capacitor’s can bottom and the heatsink, customers can get the maximum out of these capacitors, i.e. a high ripple current capability for a significantly increased lifetime.

Inner design in the context of capacitor cooling
The inside of an aluminium electrolytic capacitor contains a winding element that naturally has a much higher thermal conductivity in the axial than in the radial direction. To gain a base cooling option, this thermal conductivity in the axial direction was further improved for the B43652 series. A direct metal contact between the winding element and the can bottom decreases the thermal resistance from hotspot to can and the improved stability of the can bottom itself avoids bulging over a lifetime that would impair this thermal connection. As the bottom side of the capacitor usually contains a pressure relief vent that a heatsink would block, it was shifted to the side wall of the capacitor. Overall, the new B43652 series of TDK is a side-vented large-size capacitor targeting OBC applications with a base-cooling option.

TDK’s large-size capacitors of the B43652 series are optimized for OBC applications with a base cooling option and can significantly reduce the number of pieces in the DC-link capacitor bank, making this series not only technically but also economically interesting.