Burn out: Weak links affect HB-LED lifetime
By Margery Conner, Technical Editor -- 2/18/2010
The trendy nature of consumer electronics and the rapid advances in IC features and capacity have led to a culture of products that need not outlast the next wave of devices: When an iPod or a cell phone fails after a year or two, many consumers are willing to dispose of it and buy the next version with its cool new features (Reference 1). However, electronics are finding their way into not only solid-state lighting but also automobiles, home appliances, and energy management and monitoring, and for these applications customers expect lifetimes of 10 years and beyond. Not all electronic components are unreliable. It’s difficult to think that a microprocessor would simply wear out, for example. Nevertheless, a designer’s poor choice or poor layout of the other components that surround a microprocessor, including capacitors and the PCB (printed-circuit board), can cause these components to overheat and ultimately fail.
Lighting products have historically been reliable, but as electronics-rich CFLs (compact fluorescent lights) began to replace incandescent bulbs, consumers began seeing the products’ early failures. In some cases, these failures resulted from poor product selections. CFLs are not a good choice for lights that users frequently turn on and off, such as those in a closet. CFLs also require proper airflow, which they may not get in a downward-facing light fixture. Other failures are due to low-quality lights in the product design, the components in the design, or the units’ assembly methods. The electronics components surrounding the fluorescent tubes rather than the tubes themselves are often the culprits that cause these failures. As residential, commercial, and industrial lighting begins to incorporate HB-LED (high-brightness-light-emitting-diode)-based SSL (solid-state lighting), will SSL be similarly prone to short lifetimes and reliability problems?
Product lifetime and product reliability are different things. “Lifetime” refers to the length of time an end user can expect a product to work, whereas “reliability” refers to how many products per thousand a user can expect to fail in normal use during their expected lifetime. HB-LED-device manufacturers often quote lifetimes of 50,000 hours or more for the devices (Reference 2). However, specifying lifetimes for HB-LED-based lights is more complicated than using the lifetime for an HB LED because the lighting unit comprises an LED driver—a power supply whose lifetime and reliability vary based on its internal components. Capacitors usually have shorter specified lifetimes than the other components in the driver circuit.
“People see capacitors as being the Achilles’ heel of SSL,” says Geof Potter, a power technologist at Texas Instruments. “But a nonsolid electrolytic capacitor can have the same lifetime as the components it’s supporting if the designer chooses the right capacitor.” According to Potter, the most important factor affecting the lifetime of electronic products in general and capacitors specifically is heat, including both the temperature extremes the product will experience during its life and the device’s operating temperature.
The main wear-out mechanisms for drivers are the aging of the electrolytic, the solder joints, and the optional optoisolator. The electrolytic in the electrolytic capacitor ages due to the chemical reaction that takes place as the capacitor charges and discharges. This aging accelerates with heat; however, it’s a misconception that electrolytic capacitors dry out, which would happen only if you opened the capacitor’s vent. Although the chemical reaction in a driver differs from that of a battery, their aging processes are similar.
Electrolytic capacitors, which in LED drivers are usually nonsolid aluminum electrolytic capacitors, don’t have fixed lifetimes. Spec sheets for aluminum electrolytic capacitors typically quote temperatures of, for example, 85, 105, and 125°C. However, electrolytic capacitors experience internal heat that you must also factor in when selecting their temperature range. Ambient temperature and internal power dissipation cause internal heating in aluminum electrolytic. The heat from the application’s environment and radiated heat from the other components surrounding the capacitor affect ambient temperature. Ripple current causes internal power dissipation, according to the equation PD=IR²×ESR (equivalent series resistance), where PD is dissipated power and IR is the ripple current. Ripple current contributes to the temperature rise in the capacitor’s core. The size of the capacitor and inductor on the output of the LED driver determines the ripple current. Make sure that the capacitor can support the ripple current and still maintain its rated internal temperature.
Cost is also a factor in selecting the temperature range for aluminum electrolytics. Although 85°C is a standard temperature, it can’t support a long life for SSLs in any but the most benign temperature applications, such as small indoor lamps with sizable heat sinks. Devices that can withstand temperatures of 100 and 125°C are usually necessary for outdoor applications, such as streetlights.
Aluminum electrolytics’ high CV (capacitance-voltage) number usually dictates the choice of aluminum electrolytic capacitors rather than, say, ceramic or film capacitors. Electrolytic devices are often the best choice for designs that need high capacitance and high voltage. Ceramic capacitors are making progress, however, due to recently increased voltage ratings. As a result, some ceramic units can work in LED-lighting designs (Reference 3). Note that you must derate the voltage by half with these units. If the highest voltage you anticipate is 50V, use at least a 100V capacitor. Many SSL applications require high-voltage capacitors, and most ac/dc drivers require an aluminum electrolytic capacitor on the input ac side. For example, parking-lot lighting and streetlights usually run at higher voltages, such as three-phase 308V ac or single-phase 240V ac, requiring 400 to 600V capacitors. However, ceramic capacitors sometimes work well on the output side of the driver, in which the number of LEDs in series in an array determines the output (see sidebar “LED-array size determines drivers’ output voltage”).
MLCCs (multilayer ceramic capacitors) have excellent high-frequency noise characteristics and do well on the output-dc filtering, in which they filter out the high-frequency PWM (pulse-width-modulated) noise. However, ceramic capacitors typically fail due to mechanical stress, according to Jerry Zheng, vice president of technology marketing at iWatt. “I tell customers not to go smaller than the 0805 [the package size of a surface-mount device],” he says. “The mechanical stress of a solder joint will easily cause damage. Most ceramic capacitors fail due to handling and soldering.”
Several manufacturers in the capacitor industry have introduced compliant lead packages that resist cracking. Ron Demcko, application engineering manager at AVX, describes the package as a tin-finished termination with a sublayer comprising a conductive epoxy that allows for some degree of compressibility or compliance in thermal expansion or physical force. These parts cost about 10% more than those in other packages. No matter what size case you use, though, failure due to cracking always increases if you use hand-assembled parts.
Heat again is a culprit in decreasing lifetimes of solder joints, and the most common causes of their failure are heat excursions. TI’s Potter says that HB-LED luminaires should contain as few solder joints as possible to maintain reliability and lengthen their lifetimes. Using fewer solder joints requires more integration of functions and components, and high-end, expensive luminaires often require this level of integration. However, for SSL to catch on in high volume, consumer-grade lighting, such as replacements for 60W incandescent bulbs, the lights must be inexpensive. The cost issue probably will require Chinese manufacturing. Chinese manufacturers produce most of today’s incandescent and CFL bulbs, and they usually choose hand assembly, which produces lower-quality devices than the manufacturers can produce if they choose mechanized assembly (Figure 1).
LED drivers can also use film capacitors, which are expensive but reliable. Consumers have been complaining about EMI (electromagnetic interference) from CFLs, so designers must be vigilant about suppressing EMI noise in the PWM section of LED drivers. Film capacitors, with their excellent high-frequency response, are a good—albeit expensive—choice in high-frequency noise filters.
Optional optoisolators are other wearout mechanisms. These components provide an economical form of isolation but are subject to aging, which heat accelerates. According to iWatt’s Zheng, the US Department of Energy’s Energy Star program does not mandate the use of either isolated or nonisolated LED drivers for offline LED lamps. However, LED-lamp manufacturers often use a metal heat sink for SSLs, which requires an isolated LED driver or an insulated heat sink to protect the user from coming into contact with the mains voltage through the heat sink. Adding insulation can reduce the effectiveness of the heat sink because the heat sink does not mate directly to the heat-generating LEDs. Moreover, if the heat sink is floating—that is, not electrically grounded—then it can radiate RF noise, resulting in high EMI. As such, nonisolated-LED-driver design can complicate safety and thermal management and EMI control.
One alternative to using an optoisolator is to perform the power conversion on the primary side so that there’s no need for secondary-side feedback to the primary side. The heat sink can thermally mate to the LED’s substrate on the low-voltage secondary side, and you can also ground it to reduce EMI. “Isolated LED drivers can in effect improve thermal efficiency, reduce EMI, and reduce the cost and complexity of LED-lamp designs that use heat sinks,” says Zheng.
If you use a transformer to isolate the LED driver, then the heat sink can thermally mate to the LED’s substrate on the low-voltage secondary side, and you can also ground it to reduce EMI. “Isolated LED drivers can in effect improve thermal efficiency, reduce EMI, and reduce the cost and complexity of LED-lamp designs that use heat sink,” says Zheng (Reference 4).
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|LED-array size determines drivers’ output voltage|
HB LEDs (high-brightness light-emitting diodes) usually have a forward-voltage drop of approximately 3.5V, so the number of LEDs in series determines the LED driver’s voltage, according to Geof Potter, power technologist for Texas Instruments. The simplest way to drive multiple LEDs is to put them in one long string and have one power supply drive the string. However, bright outdoor lamps, such as streetlights, may have 100 or more HB LEDs, requiring a dc output driver with more than 350V and an output-filter capacitor with a voltage of 800V, which is usually impractical.
An alternative arrangement has an array of 10 strings of 10 HB LEDs, requiring a main power supply that provides 35V dc of regulated voltage (Figure A). The 35V-dc power probably doesn’t reside on the aluminum panel that supports the LEDs and does double duty as a heat sink. Each 10-LED string has its own constant-current supply that can probably use a lower-voltage ceramic capacitor on the input to the LED string.
|LED Workshop covers LED reliability, driver design|
At the workshop, Geof Potter (photo, top), a power technologist at Texas Instruments, will present “The useful lifetime of HB LED lighting systems,” and Jerry Zheng (photo, bottom), vice president of technology marketing at iWatt, will present “The top 3 design challenges for solid-state lights.” A panel of speakers from Cree, Philips Lumileds, and Seoul Semiconductor will present their viewpoints on the challenges that can face the next generation of HB LEDs and what their companies are doing to overcome these challenges.
The workshop will also feature hands-on demos of HB-LED devices, drivers, and cooling devices. Cary Eskow, director of LightSpeed, the SSL (solid-state-lighting) and LED business unit of Avnet Electronics Marketing, will give the keynote presentation on the new products and environments that HB LEDs will enable. Click here to register now for this free workshop.
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