Reducing ballast-generated EMI to improve energy-saving lighting
08 February 2013
An important new innovation in fluorescent ballast control ICs applies frequency dithering to simplify EMI filter design, enabling further reductions in the size and cost of ballasts for energy-saving Compact Fluorescent Lamps (CFLs)
Fluorescent lighting - including CFL and tube lamps – continues to be an extremely important energy-saving technology for domestic, industrial and commercial lighting applications. Legislation in the EU and the US, aiming to reduce average household energy consumption and CO2 emissions by preventing the sale of wasteful bulbs, is advocating CFL replacement bulbs as one of few practicable alternatives to old-style incandescent bulbs. The EU expects the switch to energy-saving bulbs to save the equivalent energy consumption of 11 million households a year, by 2020, while reducing average household electricity bills by €25 a year.
Fluorescent lighting is proven to deliver long life and high efficiency; CFLs were first commercialised in the 1980s, and are known to consume up to 80% less energy for the same light output compared to conventional incandescent light bulbs. As today’s demand for energy-saving lighting increases, however, vendors are under pressure to deliver better performing units at extremely competitive prices. Improving the design of electronic ballasts, through increased integration and use of smaller, lower-cost filter components, holds the key to meeting these goals.
A typical architecture for an electronic ballast comprises a rectifier and DC bus capacitor, feeding a half-bridge resonant output stage for lamp control. An EMI filtering circuit is also needed, to block interference from the switched-mode resonant circuit from feeding back into the AC line. Historically, a discrete control IC and driver module have been used to manage the resonant switching. A separate Power-Factor Correction (PFC) control IC would also be required, in high-power ballasts where PFC is mandatory.
The arrival in the market of highly integrated ballast controllers combining logic and driver circuitry, such as the IRS2526DS, has enabled designers to eliminate numerous individual components and simplify ballast design. A similar device, the IRS2580DS also integrates PFC control circuitry on-chip, thereby presenting a single-chip solution in a compact 8-lead SOIC Combo8 package for high-power applications.
Figure 1 illustrates one of the most popular approaches to powering fluorescent lamps up to 26W. This power range covers the full range of domestic bulb applications up to the equivalent of a conventional 100W incandescent bulb, and does not require mandatory PFC. The ballast must also be able to manage lamp filament preheating, constant ignition voltage control, and end-of-life (EOL) protection. The control circuit indicated can be implemented using the IRS2526DS, which is housed in a Mini-8 SOIC8N package.
The IRS2526DS integrates a 600 V half-bridge control circuit working at 50 percent duty-cycle with a fixed non-overlapping dead-time, and at variable frequency, for driving the resonant mode lamp output circuit. It also has a high-accuracy Voltage-Controlled Oscillator (VCO), which is controlled by a single analogue-to-frequency input pin used to set the different operating frequencies of the ballast. Complete fault protection circuitry is included for protection against such conditions as mains interrupt or brown-out, lamp non-strike, lamp filament failure and end-of-life.
The half-bridge driver produces a high-voltage square-wave that feeds the resonant tank and lamp. The frequency of the square-wave first starts at a higher frequency to preheat the lamp filaments, and is then decreased through resonance to ignite the lamp and finally selects a lower frequency for normal running. This sequence is controlled via the VCO input, which starts at a higher voltage during preheat and is then decreased smoothly to a lower voltage for ignition and running.
Frequency Dithering Technique
The controller also incorporates an innovative technique for reducing the size and cost of the EMI filter by spreading and reducing the EMI generated by the switched-mode resonant circuit. This is implemented using a frequency dithering function that varies the frequency linearly above and below a nominal level, continuously, at a given dithering rate. The frequency dither circuit, shown in figure 2, includes a current source and current sink for charging and discharging the VCO voltage above and below the nominal level. The source and sink currents are turned on and off from a second oscillator (dither oscillator) running at 50% duty-cycle and at a given frequency. The frequency of the dither oscillator directly sets both the desired rate and amount of dithering necessary for spreading the operating frequency to reduce EMI.
Figure 3 shows the schematic for a fully functional 26W ballast designed around the IRS2526DS. The circuit includes the complete control for the half-bridge resonant output stage and the lamp. The controller’s VCO pin sets the frequency of the half-bridge gate driver outputs; the HO and LO pins. A resistor voltage divider at the IC’s VCO pin programs the desired voltage levels to control the frequency of the internal VCO. The internal oscillator signal then feeds into the high-and low-side gate driver logic circuitry to generate the correct preheat, ignition, and running frequencies for the half-bridge and resonant output stage.
The EMI inductor shown in figure 3 is a single-winding differential-mode inductor (LF). This is a simpler, lower cost type of inductor, compared to the multi-winding common-mode type normally needed to provide adequate filtering of ballast-generated noise.
The waveforms shown in figure 4 have been collected during evaluation of the 26W ballast described, and indicate that the half-bridge resonant stage and lamp are both working properly. The waveforms include the half-bridge switching voltage, the AC lamp voltage and AC lamp current during running at maximum (left waveforms) and minimum (right waveforms) dithering frequencies.
Figure 5 shows the conducted EMI measurements for a wide range of measured frequencies, from 9kHz to 300MHz. The results confirm that the circuit is able to provide proper fluorescent lamp control, with conducted EMI below the allowed limits, using only a single-winding filter inductor.
As legislators mandate the use of energy-saving lighting to reduce energy consumption and CO2 emissions, CFL technology represents a benchmark in terms of performance and potential energy savings.
Research into improving the performance and simplifying the design of electronic ballasts for fluorescent lamps continues to yield valuable advances; the latest integrated ballast control ICs with on-chip frequency dithering circuitry help engineers to simplify the design of ballasts covering the full range of power ratings for domestic light bulbs.
Figure 1. Electronic ballast block diagram.
Figure 2. Frequency dither effectively reduces EMI from the half-bridge resonant circuit.
Figure 3. Schematic for 26W electronic ballast.
Figure 4. Half-bridge voltage (red trace), lamp current (green trace) and lamp voltage (yellow trace) during running with maximum dithering and minimum dithering.
Figure 5. Conducted EMI measurement results for 9kHz to 30MHz and 30MHz to 300MHz.
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