Aluminum Electrolytic Capacitors for DC-Link in On-Board Charger Applications

TDK application note discusses the use and benefits of aluminum electrolytic capacitors for DC links in onboard charger applications.

With the increasing market for electrified vehicles (EVs), the demand for onboard chargers (OBCs) is growing fast. OBCs allow charging the car at fast-charging DC stations and with AC sources in a reasonable time. Such systems are currently going up to 22 kW with operating voltages up to 800 V.

The function of the 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. Especially in remote areas without sufficient fast DC charging infrastructure, OBCs are essential to make EVs more attractive.

Due to the complexity of such systems, the OBC needs a certain bulk capacitance to stabilize the DC voltage that is charging the battery. Aluminum electrolytic capacitors are an attractive solution here since they can fulfil the key requirements, such as high voltage ratings of up to 500 V, a large capacitance of up to 820 µF and high ripple current capabilities at an operating temperature range of -40 °C to 105 °C.

To cover these requirements, TDK developed the new large-size series B43652* for OBC applications that is optimized for base cooling and perfectly combines all of the abovementioned characteristics.

The reason for thermal management

The right choice of the DC link capacitor depends on several parameters. The rated voltage (VR) results from the operating voltage of the OBC and shall cover the average plus peak ripple voltage. For systems >500 V, in-series connected capacitors can be considered. The rated ripple current IR, the required lifetime and the operating temperature range arise from the mission profile of the OBC. The operating temperature range shall cover the expected ambient temperatures across the complete service life.

While some requirements are given and can hardly be changed, some characteristics can be optimized either by the supplier or the customer. The lifetime of an aluminum electrolytic capacitor is mainly influenced by its core temperature. Generally said, high ripple currents and increased ambient temperatures significantly heat the capacitor and reduce its lifetime.

Based on the Arrhenius equation as a rule of thumb, one can consider a lifetime reduction of 50% when increasing the core temperature by 10 K. To reduce core temperatures under the same load conditions, the ESR of the component can be reduced, and the thermal management can be optimized. With the B43652* series, TDK developed a large capacitor with a very low ESR and improved internal thermal resistance throughout the lifetime.

Figure 2. Key factors influencing aluminum capacitors’ useful life; source: TDK

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. From an economic point of view, such optimizations are always to be preferred compared to using either more capacitors in parallel or capacitor designs with longer rated lifetime.

Inner design in the context of capacitor cooling

The inside of an aluminum electrolytic capacitor contains a winding element that naturally has a much higher thermal conductivity in the axial than in the radial direction. This thermal conductivity in the axial direction was further improved for the B43652 series of compact snap-in aluminium capacitors to gain a base cooling option.

Figure 3. The internal design of an aluminum electrolytic capacitor and its influence on heat dissipation; source: TDK

A direct metal contact between the wind 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.

The non-cooled version presents a gradient in the other direction; heat transfer mainly happens in the direction of the PCB. Hence, the non-cooled design shows a weak heat flux through the bottom side and, besides this, also an asymmetric heat flux for the center capacitors.

Further, while the non-cooled version displays a spread across the core temperatures, which means a thermal asymmetry with centre capacitors having higher core temperatures, the base-cooled version does not have such a spread, resulting in a significantly reduced escalation risk.

The overall improvements that can be reached in an OBC application are presented in Figure 6. Comparing two B43652 capacitors of the same size, rated voltage and rated capacitance, the base-cooled capacitor can withstand 85% more ripple current than the same design with just natural convection.

While the capacitor with natural convection reaches a maximum ripple current IAC, max of 6.11 A, the base-cooled version reaches 11.28 A. Furthermore, for the same load conditions, two base-cooled capacitors can withstand the mission profile almost two times longer than four non-cooled capacitors.

Figure 6. Comparison of natural convection and base cooling of aluminum capacitors; Source: TDK

Based on these results, it is clearly evident that TDK’s large-size capacitors of 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.

Source: TDK