Polymer Film Capacitors for LED Drivers

Author:
Ian W. Clelland and Rick A. Price, ITW Paktron, Lynchburg, VA

Date
05/01/2010

 PDF
Applications demanding long life

As is typically found with most types of high-end equipment, LED lighting systems require a minimum of 5 to 10 years of working life with 100% up?time in order to justify the user's cost and maintenance burden. It is simply impractical to try to drive long life LEDs with power components that can fail in only two years. Multi-layer ceramic (MLC), aluminum electrolytic and tantalum capacitors were originally intended for use in commodity applications for functions such as by-passing (decoupling), coupling, filtering, frequency discrimination, DC blocking and voltage transient suppression, but designers have erroneously been trying to extend their use into high performance applications (such as Military, Flight, high-end Telecom, medical, high-end consumer electronics). In these types of applications the performance and stability, as opposed to size and cost, are the critical criteria. Designing with commodity grade capacitors (i.e. MLC, aluminum electrolytic and tantalum capacitors) is applicable for use in commercial based, commodity applications, where all that is required is just having a capacitance be present, but in high performance applications, stability, and the hit that performance takes because of the lack of it, cannot be ignored. In critical applications where just ‘getting by' is not acceptable design criteria, the selection of the proper capacitor technology is paramount to a product's success.

A specification listing 2000 hour product life for a component means only 83.3 days of continuous 24/7 run time operation making the extrapolation to 10 years life a far reach for most capacitor systems that are subject to electrical degradation with time. Instead of using capacitors that simply get by, critical applications require units with an established track record of both durability and reliability. Because of the need for high reliability and long life, high tech industries such as Telecom learned decades ago that while the commodity grade capacitor technologies (i.e. aluminum electrolytic, ceramic, tantalum, etc.) have their viable uses, in pivotal applications only metallized polymer film capacitors have the inherent performance, stability and reliability needed.

There are two basic types of fixed capacitor technologies: electrolytic and electrostatic (Figure 1). Electrostatic capacitors use a bulk dielectric made from an intrinsically insulating material while electrolytic capacitors have a dielectric that depends on the formation and maintenance of a microscopic oxide layer. The electrolytic capacitor differs from the electrostatic in that only one of its conducting surfaces is a metallic plate with the other conducting surface being a chemical compound or electrolyte. The dielectric in an electrolytic capacitor is a very thin film of oxide of the metal which constitutes one of the metallic plates used in the structure.

Metallized polymer film capacitors are electrostatic capacitors that consist of thin film layers of polymer based material upon which a metal has been vapor deposited to act as electrode plates. Polymer film capacitors have always been considered to be the premier capacitors for high performance applications. The latest form of this technology is called multilayer polymer (MLP) and is a "stacked" capacitor technology that takes two offset lengths of film (or more) and winds the layers together on a large wheel to form a mother capacitor. The mother capacitor has its layers laminated together and is sawed into individual capacitors. Ceramic capacitors are electrostatic with the most common form being the multilayer ceramic capacitor (MLC). The multiple layers and high dielectric constant ceramic allows for the production of relatively high capacitance values per unit size. These types of capacitors are easily surface mountable and have found wide acceptance in signal level applications.

Electrolytic capacitors (aluminum electrolytic, tantalum, etc.) like batteries have their functionality based on a chemical reaction. Because of entropy, time will eventually slow, stop or reverse that reaction and the capacitors will become non-functioning. Electrostatic capacitors (ceramic, polymer film, etc.) do not function due to chemical reactions, but ceramic capacitors contain certain base elements and dopants that can radically affect their longevity.

The most commonly used ceramic capacitors are based on a barium-titanate dielectric that exhibits "aging" which produces decreases in capacitance values over decade-hours of time. Decade-hours are industry time periods of 1-10, 10-100, 100-1000 hours etc. For example, an X7R ceramic capacitor loses between 6.0-7.5% of its capacitance value in its initial 1000 hours while a Z5U capacitor can lose almost 20%.

The capacitance, dissipation factor and ESR of capacitors can be affected by: Temperature, Time (Aging), Frequency and Voltage (AC or DC). MLC capacitors are affected by these variables to a much greater degree than other electrostatic capacitors.

MLC capacitors have such wide variances in properties due to having not a natural but rather an "engineered" dielectric constant. Capacitance is directly proportional to the polarization capability of the dielectric material. Rather than being a simple set of molecules switching back and forth as in a polymer dielectric, the capacitance value of MLC capacitors is affected by material purity, grain size, sintering, grain boundaries, porosity, internal stress, the freedom of domain wall movement, reorientation, etc. Because of the nuances (tolerances) of each of the dielectric material variables at both the atomic and macro-material level, engineering the dielectric constant makes it susceptible to change from almost any outside influence. Although the basic attributes of ceramic dielectrics are well-publicized, every manufacturer's approach employs subtle details that can make a critical difference. While nearly all MLC manufacturers start with a dielectric based on barium titanate (BaTiO3), they then add small amounts of other materials, such as rare-earth and ferroelectric oxides, to tailor the parameters of the dielectric. They also use these oxides as additives to select a set of trade-offs among factors such as temperature coefficient, tolerance, stability, loss, distortion, breakdown voltage, and even micro-phonics. The key to the lack of stability for MLCs is the term "trade-offs". The trade-offs that have been instituted have made the ceramic capacitor manufacturers victims of their own success. By succeeding to such a degree in maximizing the amount of capacitance available per unit volume, they have decreased the stability of their class II products to the point of limiting them to commercial/commodity products. Basically, modern MLC capacitors have morphed from electrostatic to becoming pseudo-electrolytic capacitors; enjoying new capabilities but now also taking on some electrolytic capacitor weaknesses while keeping their own.

In critical, long life applications manufacturers require the use of capacitors with established track records in both durability and reliability. For these applications, only capacitors with the necessary inherent performance, stability and reliability should be used. No matter how much circuit redundancy or accelerated screening testing is done there always exists a golden nexus that can produce a single point of failure (SPoF) requiring circuit designers to spend a great deal of time trying to minimize the probability of the occurrence of such failures. The one proven design approach over the last five decades is the use of polymer film capacitors. Multilayer polymer (MLP) capacitors do not suffer from the same parametric instability as commodity grade capacitors and have been the preferred capacitor technology for high performance applications. Trying to use commodity grade capacitors in high performance/critical applications without taking "special" measures to compensate for their in-stability can result in design failure. The commodity grade capacitor industry's drive to lower cost and reduce component size has generally removed some of the safety margin in their designs or caused increased parasitic losses resulting in decreased capacitor stability. These non-ideal characteristics are detrimental to performance and are difficult to compensate for, especially when you don't realize exactly how they are impacting your circuit's operation. Commodity grade capacitors do an excellent job in commodity/consumer applications, but their use is application specific and they have their limitations. This is not an unknown problem, with the information on their instability published in most capacitor manufacturers' data sheets. By comparison, due to their low mass, outstanding performance capabilities and unmatched inherent reliability, polymer film capacitors have long ago been established as the choice in high performance, long life applications. www.paktron.com

RELATED

 


-->