DC-Link Capacitors and Aluminium Electrolytic Capacitors - Odd Couple or a Perfect Match?
It is an obvious truth: DC-Link capacitors are an important part in the majority of power electronic converters, which contribute to cost, size and failure rate on a considerable scale.
An intermediate circuit capacitor (DC-Link) is used in the intermediate circuit of converters of different kinds where it couples different electrical grids to one DC voltage level. When selecting a DC-Link capacitor design engineers want to avoid neither under-designing nor over-designing: an under-designed DC-Link capacitor can cause a premature failure or will cause EMI interference resulting in problems with other electronic circuitry. An over-designed DC link capacitor, on the other hand, is not as cost- or size-efficient. DC-Link capacitors have a wide range of power electronics application such as in power tool devices, electrification in vehicles, motor drives and lighting systems. For applications like photovoltaic cells, inverters and wind turbines, the voltage requirements are higher and smaller capacitance is required. The right capacitor selection is extremely important to achieve the best voltage and current-carrying capability for your DC-Link capacitors. For many years, aluminium electrolytic capacitors have been a mainstay of power-supply designs, and they are still the best choice for many applications.
Aluminium electrolytic capacitors (E-Caps) assume a special position among the various types of capacitors since their principle of operation relies, in part, on electrochemical processes. E-Caps are polarized – with a positive and negative terminal similar to a battery – and is filled with a chemical liquid electrolyte. The advantage of E-Caps as your choice for a DC-Link capacitor are foremost their large capacitance values with up to one Farad capacitance, as well as aluminium electrolytic capacitors provide a high ripple current capability. This coupled with their low ESR with a high reliability offer an excellent price/performance ratio. Nonetheless, there have been also major drawbacks in choosing E-Caps: as designers opt for higher-frequency switching power circuits to reduce transformer size, the spikes generated by the power device (switching on and off) need to be more effectively decoupled. To minimize heat and best protect the power devices, DC link capacitors need to have very low equivalent series resistance (ESR) and low self-inductance. The other critical performance specification is ripple-current. All of these weren’t addressed in former E-Caps design and technology.
Although we haven't seen any revolutionary leaps in aluminium electrolytic technology, there has been a lot of technology creep over the years. This creep has made them physically smaller, more reliable and available with lower ESR- and ripple current ratings. The latter two are of particular importance for any power circuit that employs DC link circuitry. Of course, that includes switching power supplies and inverters. By taking the advantage of the progress in new dielectric materials and manufacturing process, Panasonic has been releasing with its FK- and FKS-Series new generations of Aluminium electrolytic capacitors with improved reliability and cost performance which iron out and address former disadvantages of the technology.
The Panasonic FK Series capacitors are AEC-Q200 qualified, surface mount type, polarized aluminium electrolytic capacitors and offer ESR values as low as 35 mΩ. The FK Series is ideally suited for use as DC-Link capacitor as well as for inverter circuits and rectifier circuits for a wide range of automotive and power supply applications. They have an operational life of up to 5,000 hours rated at 105degC. The capacitance value ranges from 3.3µF to 13000µF. These capacitors are the perfect solution for applications, which require ultra low ESR, very high ripple current and very long life in small mounting form. The vibration-proof product is available upon request (8 mm and larger). This design enables capacitors to withstand up to 30G acceleration.
More information on Panasonic's broad E-Caps portfolio can be found in the white paper below.
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