Aluminum Electrolytic Capacitors for LED Drivers

­The global LED lighting market was valued at $71.59 billion in 2023 and is projected to grow at a compound annual growth rate (CAGR) of 11.20% from 2024–2032, at which point it’s predicted to have grown to a value of $186.12 billion. Many of the reasons behind this steady growth trend have to do with the inherent benefits of LED technology, which include:

· Higher energy efficiency than competing lighting technologies. These improvements help users reduce operating costs and contribute to sustainability goals.

· Long lifespans. The proper selection of LEDs, LED driving circuits, and heat control mechanisms yields reliable, long-lifetime light sources that signficiantly reduce maintenance and replacement costs compared to competing lighting technologies.

· Broad application suitability. Solid-state LED lights are insensitive to shock, vibration, and external pressure, making them well-suited for a wide variety of application environments.  

· Custom light configurations. LEDs support custom light intensity and color configurations designed to satisfy specific end-user requirements.

· Compact and custom sizes. Solid-state LED lighting offers compact footprints compatible with downsizing trends in addition to package sizes engineered to optimize space.  

The widespread adoption of LED luminaires has been a long time in the making, but making the the switch isn’t always as simple as replacing an incandescent bulb or swapping it out an for a compact fluorescent (CFL) bulb. LEDs must be driven at a specific DC current and, to complicate things further, many LEDs are typically used to achieve the equivalent luminous output of the lighting technology being replaced (e.g., an incandescent bulb). LEDs and their driver circuits also require simultaneous cooling to prevent thermal damage, and the supporting drive circuitry must be designed to operate in environmental extremes for long periods of time to coincide with the average LED die life. In addition, once an LED driver is designed and qualified, both it and the LEDs it’s designed to power must be packaged in a form factor that appeals to end customers, is easy to install, exhibits reliable performance in a variety of application environments, and supports a long service life.

LED Drivers

LED drivers and control circuitry are critically important to LED quality, intensity, and capabilities. LED drivers must be small, energy-efficient, user-friendly, and reliable to deliver the inherent benefits of solid-state lighting. So, it’s worth investigating the basics of a common LED drivers and putting its subsystems into perspective.

LED drivers designed for use with AC mains power often utilize flyback converter power supply topologies (See Figure 1). Flyback converter LED drivers tend to be low-cost and have relatively simple designs, and they are easily modified to allow for power factor correction (PFC) capabilities that can further improve energy efficiency. They also tend to offer a broad dimmability range with few consequences.

Flyback converter power supply topologies are centered on two MOSFETs that are used to control the energy transfer via a transformer that provides isolation from the mains and drives the LED on its output. Low-loss diodes are utilized for rectification, and capacitors are selected for smoothing and integrating the voltage to acceptable levels in order to avoid flicker.

Capacitor Selection

The reliability of LED drivers typically determines the long-term reliability of LED fixtures. As such, the component selection process for driver designs is a very important job. This article concentrates on the selection of capacitors deployed in LED driver circuits.

Many LED driver designs use aluminum electrolytic (AlEl) capacitors since the spectrum of available aluminum electrolytics can address a wide range of circuit requirements. LED driver designers need small-form-factor capacitors with moderate to large capacitance values, reasonable stability, low equivalent series resistance (ESR), low costs, and long lifetime performance ratings. Higher voltage ratings are commonly sought after as well, since rated-to-applied-voltage deratings can significantly increase capacitor reliability. Aluminum electrolytics sufficiently address all these needs and are available in three different technologies:

· Standard (wet) aluminum electrolytics

· Hybrid aluminum electrolytics

· Polymer aluminum electrolytics

The relative strengths and weaknesses of which are described in Table 1 below.

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Table 1: A side-by-side comparison of aluminum electrolytic capacitor technologies.

Reliability

The relative complexity of LED lighting compared to competing lighting technologies is reflected in its costs, but the high-reliability and long lifetime performance of LED lighting technologies more than compensates for that initial disparity. As such, when considering luminaires as a whole, it’s critical that the lifetime of every component in the system — including flyback drivers and enabling electrolytic capacitors — is as good or better than that of LEDs.

The wear-out mechanism for electrolytic capacitors follows an Arrhenius model (Figure 2), as is standard practice for most organic systems. Historically, the relationship for organic-based electrolytic capacitors is to double the lifetime for every 10°C reduction in operating temperature. Although, polymer aluminum capacitor lifetimes are significantly better than those of standard aluminum capacitors, which is also illustrated in Figure 2.

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Figure 2: The expected lifetime equations of standard aluminum electrolytic capacitors (top) and polymer aluminum electrolytic capacitors (bottom)

One of the reasons that polymer aluminum capacitors last so long and have such graceful wear-out characteristics is the self-healing mechanism inherent to their construction. When this dielectric breaks down at any point within the capacitor, the resulting current flow from anode to cathode creates a momentary but significant amount of heat, which reforms the polymer layer, creates a new insulating barrier, cuts off the flow of current, and enables the capacitor to continue operating with only minuscule reductions in performance.

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Figure 3: The self-healing mechanism for polymer aluminum capacitors

Standard, wet, non-polymer aluminum electrolytics also offer some degree of self-healing. Because of the liquid electrolyte, oxygen is generated during electrolysis, and this can regrow a fault site due to dielectric impurities or micro cracks, reform the dielectric layer, and reduce the leakage current. However, over time, the liquid electrolyte will be depleted by this process and accelerate the normal wear-out process.

Emerging Aluminum Electrolytic Capacitor Options

Conductive Polymer Electrolytics: The replacement of a wet electrolyte with a conductive polymer electrolyte eliminates the possibility of liquid electrolyte leakage between the seal case and leads as well as long-term aging (i.e., evaporation) concerns, which makes conductive polymer aluminum capacitors more reliable than standard wet aluminum capacitors. Conductive polymer aluminum electrolytics also exhibit nearly two times less ESR, two to three times greater RMS current capabilities, and about three times more stability with temperature compared to standard wet aluminum electrolytics, and they’re available with similar case sizes, capacitance values, and voltage ratings. In addition, a 20°C derating of each technology reveals that the life expectancy of a wet aluminum capacitor increases by a factor of four, while a conductive polymer aluminum capacitor increases by a factor of roughly 10.

The limitations of this material technology include increased DC leakage, a higher price point, and sensitivity to high shock and vibration.The DC leakage of polymer aluminum electrolyticsis approximately 0.2CV or 300–500µA compared to about 0.01CV or 3µA for wet aluminum electrolytics. But it’s a manageable increase early in the design process — and especially so given that conductive polymer material systems more than double the reliability of wet electrolytics while simultaneously reducing ESR and increasing RMS current.

Hybrid Electrolytics: Hybrid electrolytics were developed to reduce the DC leakage effects of conductive polymer electrolytics and reduce the ESR of wet electrolytics, thereby improving the reliability and performance characteristics of both materials. They also perform exceptionally well in high-humidity environments and, as such, have a higher price point than wet and conductive polymer material systems. And while they do have some CV limitations, additional materials research and process improvements are actively working to overcome those challenges.

Summary

While LED have become the de facto standard for high-quality, efficiency and -reliability consumer and industrial lighting, much of this success has been built upon the thoughtful design and development of supporting circuitry and hardware. LED drivers have stringent requirements for environmental ruggedness and wide voltage and current tolerances, and satisfying these requirements depends on selecting the right electrolytic capacitors. Standard, polymer, and hybrid aluminum electrolytics can be combined to balance their strengths and weaknesses in different aspects of LED driver designs and achieve an optimal lighting product.

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