Even for its tried and tested product groups TDK-EPC continues to offer new technologically superior solutions. Significantly improved aluminum electrolytic capacitors from EPCOS satisfy the toughest requirements in cars with respect to vibration and temperature stress resistance. They consequently open up beneficial design possibilities and extend their range of applications.
Thanks to the ongoing intensive development work, EPCOS aluminum electrolytic capacitors are indispensable for automotive electronics. These high-performance components are now becoming increasingly established in this highly demanding application field, in drive control as well as in safety and convenience applications.
Multi-tab terminals reduce ESR and ESL
As a rule, these newly developed EPCOS capacitors are based on axial designs that offer significant advantages in comparison with other types, in particular, allowing multi-tab terminals to be implemented. In these, the anode and cathode films are contacted internally via many connections rather than just a single one. This significantly reduces the electrical resistance (ESR) between the films and the external contacts. The same applies to the equivalent series inductance (ESL).
Current capability boosted by up to 50 percent
Due to the ESR, electrical losses occur in aluminum electrolytic capacitors every time they are charged and discharged, leading in turn to heating of the component. The smaller the ESR can be kept, the lower the self-heating effect. In other words, for the same level of permissible self-heating, the current capability of the multi-tab variant is up to 50 percent higher than for single-ended types with comparable dimensions. This is augmented by a new and special casing design that improves the heat dissipation of the component. Moreover, because the tabs are not riveted but cold-welded to a large surface area, the capacitor can handle high current peaks without damage.
Replacing up to three conventional capacitors
Accordingly, a single large axial capacitor with these improved properties can replace up to three conventional single-ended types. In addition to the savings in space and weight, this leads to lower costs and higher reliability of the design. The new capacitors with multi-tab terminals include the types of the B41691* (axial) and B41791* (axial with solder star) series with a diameter of 21 mm (Figure 1), which are designed for rated voltages of between 25 and 63 V DC with capacitance values from 100 to 4000 µF and temperatures of up to 150 °C. Typical applications of these rugged capacitors are engine and transmission control, electric power steering systems, controllers for fluid pumps and fans, start-stop systems, or hybrid drives.
|Figure 1: Rugged space-saving version for automotive electronics|
|The ripple current capability of the multi-tab axial capacitors of the EPCOS B41791* series is up to 50 percent higher than that of single-ended types. This enables space savings and an optimization of the layout on the PCB.|
Corrugated casing increases vibration strength
One of the most important prerequisites for the use of aluminum electrolytic capacitors in automotive electronics is high vibration strength. Adhesives, resins, or additionally soldered metal brackets are often used to fix the capacitor onto the circuit board, especially in single-ended versions. All these measures are time- and cost-intensive and usually do not offer any protection against polarity reversal (PAPR) during the placement process.
In order to improve the mechanical stability, completely new patented EPCOS case designs were developed for axial types. The aim was first to fix the capacitor winding rigidly in the case, and second, to relieve the mechanical stress on the terminal wires. The fixing was achieved by a specially designed corrugation. The axial principle, in which the negative pole (cathode) is usually connected to the aluminum case of the capacitor, offers further possibilities, for example, a soldering star with three pins connected to the case (see also Figure 1). After placement and soldering, it is flush with the circuit board, thus creating a rigid connection between the component and circuit board. Consequently, vibration forces can no longer be transferred to the central anode terminal. Another new construction principle for axial designs consists of welding contact plates that are equipped with pins for placement on the circuit board, onto both sides of the case (Figure 2). These two innovations result in vibration strengths of up to 40 g.
|Figure 2: Vibration-resistant case form|
|The cathode contacting (blue) of the aluminum electrolytic capacitor from EPCOS is set up via contact plates welded onto both sides of the case. This relieves the mechanical stress on the anode wire.|
Preventing whisker formation
In the new developments, special attention was paid to avoiding the formation of whiskers. These are needle-shaped monocrystals that occur primarily on tin-plated surfaces. Whiskers grow under specific mechanical or thermal conditions and can – as broken-off parts – lead to short circuits on components or the circuit board. This could have fatal consequences, especially in safety-relevant systems in automotive applications such as electric power steering, airbag controllers, or electronic wedge brakes. In the axial types, whisker formation is prevented by a nickel-gold coating, while in single-ended types this is achieved with additional rubber seals at the terminals.
Strong R&D team
TDK-EPC is working intensively on further innovations in the field of aluminum electrolytic capacitors. The ongoing R&D work leads to ever more compact, rugged, and lightweight capacitors that simultaneously offer ever higher performance and reliability. Design engineers and product developers are currently applying their expertise at production facilities with leading-edge technology in Germany, Hungary, China, and Brazil.