Aluminum electrolytic capacitors have been a mainstay in electronic power circuits for their ability to provide large bulk storage, high voltage and high energy density. However, in a growing number of applications, their on-board height has become a key limiting factor. Historically speaking, capacitors have typically been among the tallest components on the circuit board. The advent of surface-mount (SMT) capacitors, across multiple dielectric technologies, has helped to shrink the height of a loaded board. In order to achieve high capacitance or voltage using SMTs, it is often necessary to bank multiple capacitors on the PCB. Unfortunately, this approach consumes a lot of valuable board real estate, especially in applications that require high capacitance for holdup.
The fairly recent development of flat, rectangular (prismatic), aluminum electrolytic capacitors is helping circuit designers achieve low profiles, space and weight savings, especially at high application voltages.
The challenge of low profile and high energy density
While most dielectric technologies employ multilayer technology to achieve low-profile SMT chip packages, conventional aluminum electrolytics use wound capacitor elements within their SMT package types. These cylindrical windings are placed in cylindrical metal containers and mounted vertically to a rectangular mounting pad. For this reason SMT aluminum electrolytics have come to be known as V-chips, short for vertical chips.
The cylindrical packages also contain a liquid electrolyte, which must be sealed into the device. Rubber or composite seals prevent this electrolyte from escaping. This design tends to be inefficient for SMT devices, as the percentage of overall capacitor volume taken up by the seals increases as total package volume decreases. This is a key point. For typical SMT aluminum electrolytics, up to 60% of the capacitor’s volume may be space-wasting end-seal gaskets and related materials!
Another consideration is the lifespan of the individual components. Even with quality end seals, cylindrical electrolytics gradually experience a long-term loss of electrolyte, a condition known as dry-out. Loss of electrolyte results in a corresponding loss of capacitance and increased equivalent series resistances (ESR). For that reason, conventionally sealed aluminum electrolytic capacitors have typically been limited to consumer, industrial and non-critical military and aerospace applications. For mission-critical military, aerospace and down-hole applications, circuit designers have historically specified higher-priced hermetically sealed wet-tantalum capacitors.
The solution to the historical limitations of electrolytics is to approach component construction with an entirely different device design. Prismatic aluminum electrolytic capacitors can be designed to deliver high bulk capacitance, in low profile, high energy-density packages that eliminate dry-out. This advancement is accomplished by replacing those space-wasting flexible seals with robotically laser-welded seams. The laser welds also do a far better job of preventing electrolyte loss, thus greatly extending component life. These new aluminum electrolytic package configurations allow engineers to improve reliability while saving space, weight and even loaded-board cost.
