Propelled by its long lifetime and high efficiency there is little doubt that LED lighting will experience strong future growth. But because of the operating requirements of an LED and its drive circuit, which converts unregulated AC voltage from an electrical outlet to DC constant current output, the need for careful selection of capacitorshas become acute. LED lighting systems may have an expected life of more than 40,000 hours functioning in high temperatures (often in excess of 80°C), especially in the electronics to drive the LED light.
All of that makes the job of specifying capacitors for LED use doubly difficult. For example, the majority of high power LED drivers employ electrolytic capacitors either on the input AC stage to enable filtering of noise or on the output channel DC stage of the driver. A potential problem arises because the useful life of an electrolytic capacitor decreases exponentially as the capacitor body temperature increases. At higher temperatures dissipation of the liquid electrolyte from the capacitor reduces the cathode connection, resulting in a decrease of capacitance. The reaction rate of the dielectric materials — a combination of a liquid and filler — will double for each 10°C increase in operating temperature, meaning the life of electrolytic capacitors drops by half for every 10°C temperature increase. As the capacitor degrades, its ESR, or equivalent series resistance (the sum of electrolytic resistance, dielectric loss and electrode resistance) increases, leading to an increase in output current ripple, which affects the input current to the LED, and in turn the LED’s light output and efficacy.
The bottom line is that it is essential to specify a high temperature rated capacitor within the LED driver (high temperature electrolytic capacitors are available with ratings from 105°C up to 125°C) and to ensure that the maximum operating temperature is well below the temperature rating.
Ceramic Caps Can Have Issues, Too
Ceramic capacitors last a long time and X7R type multilayer ceramic capacitors (MLCCs) are popular in various applications including power supplies because of their stable capacitance with temperature. But ceramic capacitors are not without faults. Because of the piezoelectric nature of the dielectric material-which means that a large-capacitance MLCC will be physically distorted in a specific direction when a voltage is applied−vibration or mechanical shock can be transformed into an AC noise voltage on the capacitor. Higher K-factor MLCCs produce higher noise levels.
The common Hi-K MLCC uses powders based on barium titanate to achieve high capacitance. This material within a capacitor operating in a pulse width modulation (PWM) dimmer circuit can cause acoustic noise and light flickering, due to the piezoelectric effects within the ceramic. In some cases, the piezoelectric effect may result in the appearance of electrical noise, while in other cases an acoustic sound may be heard, emanating from the capacitor itself.
It turns out that distortion in the electric field can be reduced using dielectrics with weak ferroelectric properties as compared to traditional MLCC materials. As an example, Murata Electronics developed a ceramic material that exhibits a lower piezoelectric content (dielectric constant). This allows for an improved DC bias characteristic, higher allowable ripple current, lower temperature rise, and lower acoustic compared to its standard MLCC product. Murata’s material is said to be most appropriate as a countermeasure against noise and for smoothing in circuits. Murata reports it demonstrates comparatively little mechanical deformation and noise (sound pressure level) at the application of DC voltage.
The GR3 and RDE series of MLCCs have been designed by Murata to be incorporated in LED lighting circuits, immediately following the bridge rectification stage, to provide DC supply smoothing and EMI filtering and overcome flickering and noise problems. Compared to standard ceramic capacitors, tests by Murata showed a 10dB reduction in acoustic noise by using GR3 MLCC caps. Both families are reported to provide better DC bias and acoustic noise reduction performance than X7R series caps. The GR3 is available in 250, 450 or 630 VDC rating and in popular capacitances. Package sizes include 0805, 1206, 1210, 1812 and 2220.
Another example of how passive component suppliers are approaching the need for low noise is found in AVX’s QM series of ceramic capacitors, which employ a new internal electrode structure to minimize output noise. In its QM series AVX has changed the internal electrode structure from horizontal to vertical so vibration does not amplify and cause board ringing. By aligning MLCC electrode direction vertically to the PCB the structure reduces low frequency audio noise while providing high capacitance with a low working voltage, making it well-suited for mobile phone, hard disk drive, and LCD panel driver circuit applications. Standard MLCC construction has multilayer electrode direction horizontal to the PCB, which can allow mechanical resonance from the PCB to induce audio frequency noise.
Taiyo Yuden also has developed MLCC material that is said to reduce distortion by one half when compared to earlier products. While offering the same 10 μF capacitance as the predecessor model., in-house measurements show that the new type reduces audible ringing to between one half and one third previous levels. The company’s low noise MLCC’s are targeted for use in current smoothing, in power supply and display drive circuits of laptop computers, LCD TVs and monitors.
Different features of capacitor technologies should be considered when choosing optimal filtration capacitors. Tantalum capacitors (from AVX, Kemet, Vishay and other suppliers) deliver noise free filtering performance without any degradation of signal. In mobile phones, tantalum capacitors can provide decoupling and smoothing of the output of a current source LED backlight driver working at 1MHz switching frequency. As rated voltages increase, tantalum polymer devices may also be suitable as input capacitors, as unlike aluminum electrolytic devices they have no wear out mechanism. This would make them more appropriately match the increasing life expectancy of power supplies in LED lighting systems. Recently released 63V and 75V rated tantalum polymer devices have extended the range of tantalum polymer caps. As the voltage for constant-current mode driver topology with LEDs connected in a series is typically in the range of 28-60 VDC, these capacitors are suitable for such applications. Additionally, tantalum polymer devices do not exhibit any piezo effect and thus avoid the typical shortcomings of ceramic capacitors.
Statements of fact and or opinions expressed in MarketEYE by its contributors are the responsibility of the authors alone and do not imply an opinion of the officers or the representatives of TTI, Inc.
Murray Slovick is Editorial Director of Intelligent TechContent, an editorial services company that produces technical articles, white papers and social media posts for clients in the semiconductor/electronic design industry. Trained as an engineer, he has more than 20 years of experience as chief editor of award-winning publications covering various aspects of consumer electronics and semiconductor technology. He previously was Editorial Director at Hearst Business Media where he was responsible for the online and print content of Electronic Products, among other properties in the U.S. and China. He has also served as Executive Editor at CMP’s eeProductCenter and spent a decade as editor-in-chief of the IEEE flagship publication Spectrum.