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Select the best topology for WLED drive electronics

Posted: 18 Jun 2007 ?? ?Print Version ?Bookmark and Share

Keywords:white LEDs? LED driver? drive electronics?

By Silvestro Fimiani
Power Integrations Inc.

The incandescent light bulb's days are numbered. Over the last century Edison's traditional filament-based design has stood the test of time and become the standard for general illumination applications. But new lighting technologies, in particular LEDs, promise to eventually displace glass-encased incandescent bulbs as well as fluorescent lights.

In a world where escalating costs have every user closely examining power budgets, incandescent technology finds itself on the wrong side of the power efficiency curve (see Table 1). Approximately 97 percent of the energy consumed by an incandescent bulb is wasted in heat. Fluorescent bulbs offer a slightly better option, but still waste up to 85 percent of power consumed. Moreover, both technologies last on average only about 5,000hrs before needing replacement. Fluorescent technology also uses toxic Mercury and emits a harsh-colored light. Neither technology can compare to white LEDs which offer a lifecycle ten times as long, do not use toxic materials and can support virtually any color light. Most importantly, LEDs support light conversion efficiencies that rival fluorescents.

As a result, a move to LED technology for general illumination applications could greatly reduce energy consumption. A recent study by the U.S. Department of Energy estimated that widespread adoption of white LEDs by 2025 could cut electricity consumption by 10 percent worldwide, and slash up to $100 billion from electric bills. Those energy savings could reduce carbon dioxide emissions from power plants by up to 350Mtons a year worldwide, according to Sandia National Laboratories. Government leaders have begun to take notice. Recently, for example, Australia announced legislation to phase out the use of inefficient incandescent light bulbs as part of a plan to reduce the generation of greenhouse gas emissions and cut household power bills.

Table 1. Competing Light Technologies

While white LEDs offer an attractive alternative to current high-volume lighting technologies, designers attempting to collapse the electronics required to drive LEDs into the base of a light bulb face major challenges. Space constraints demand a small, efficient footprint. At the same time thermal considerations, which can have a major impact on device reliability, place restraints on design density. Finally, designers must also carefully consider the impact of EMI on their design.

Since the drive electronics are not user accessible, designers can use non-isolated, commercial-off-the-shelf inductor-based buck and buck-boost switched mode power supply converters in low power (3W) lighting applications. Both of these circuits eliminate the bulk and expense of a transformer and offer a number of other advantages. This article will compare these two topologies and discuss the tradeoffs associated with each one.

Two Topologies
Figure 1 shows a LinkSwitch-TN device configured as a basic buck converter (1a) and a basic buck-boost converter (1b). These devices simplify converter stage design and reduce parts count by integrating a power MOSFET, oscillator, simple on/off control scheme, a high-voltage switched current source, frequency jittering, cycle-by-cycle current limit and thermal shutdown circuitry onto a single monolithic IC. The LinkSwitch-TN device is self-powered from the DRAIN pin, eliminating the need for a bias supply and its associated circuitry. Designed as a cost-effective replacement for linear and capacitor-fed non-isolated supplies in the under 360 mA range, the device offers excellent line and load regulation, higher efficiency than passive solutions and higher power factor than capacitor-fed solutions.

Figure 1. Basic configuration of a LinkSwitch-TN as a) a buck converter and b) a buck-boost converter

The buck converter shown in Figure 1a offers a number of advantages. First, it maximizes the available output power for a selected LinkSwitch-TN device and inductor value. It also reduces the voltage stress on the power switch and freewheeling diode. In addition, the average current through the output inductor is slightly lower than in that of an equivalent buck-boost converter.

The buck-boost configuration has one main advantage over the buck converter: its output diode is in series with the load. In a buck converter, the input is connected directly to the output if the MOSFET fails shorted. If the same condition occurs in a buck-boost converter, the reverse biased output diode will block the path between the input and the output.

In both converters, the AC input is rectified and filtered by D1, D2, C1, C2, RF1 and RF2. Two diodes enhance line surge withstand and conducted EMI. Designers should use a fusible flameproof resistor for RF1, but may use a flameproof only type for RF2. The on/off control in the Linkswitch-TN device is used to regulate the output current. Once current into the feedback (FB) pin exceeds 49?A, MOSFET switching is disabled for the next switching cycle.

Minimizing Heat
Thermal management is a major challenge for LED drive electronics designers. Even with the technology's higher efficiency relative to incandescent, at 3W the circuit will approach temperature levels that can endanger the integrity of the device. Moreover, integrating the drive electronics into the tight confines of a standard GU10 lamp base presents severe challenges for heat dissipation. The only way the designer can dissipate heat is by conductively passing it to the screw-in base of the bulb. In the topologies illustrated above, the LinkSwitch-TN device helps protect the LED from potential damage by adding a thermal shutdown circuit which disables the power MOSFET when the die temperature exceeds 142C. Once the die temperature falls by 75C, the MOSFET is automatically re-enabled.

The buck-boost topology offers slightly less efficiency than the buck topology because power is not transferred to the output each time the MOSFET switch turns on. Therefore, it generates more heat than the buck topology. But the difference is not significant.

Table 2: SOURCE pin temperature as a function of Vin

To ensure the circuit topologies met thermal regulation requirements, designers at Power Integrations installed a power supply assembly into a lap socket base and then measured the temperature of the SOURCE pin on a LNK306DN, a member of the Linkswitch-TN product family. The circuit was designed to regulate the load current to 330mA to drive three LEDs connected in series. The circuit operated over a universal input range of 85 " 265Vac.

Ideally the SOURCE pin temperature should not exceed 100C. As the table above illustrates, however, SOURCE pin temperature measurements, conducted at 25C ambient room temperature, rose steadily as Vin climbed and exceeded 100C as Vin reached 265Vac. Given these results, designers determined that some additional heatsinking, such as placing the LED heatsink in contact with the top of the U1 SO-8C package, was required to meet thermal constraints.

Controlling EMI
LED drive electronics circuits must meet stringent EN55022B/CISPR22B requirements for conducted EMI. Given the high switching frequency of the switcher IC and the limited size of the GU10 lamp base, these requirements present another major challenge for bulb designers. The EMI noise current loop in the buck-boost circuit topology, running from the MOSFET to the output diode, the output capacitor and back to the input capacitor is longer than that in the buck configuration which runs from the MOSFET through the free-wheeling diode and back to the input capacitor. That, in turn, makes noise reduction slightly more difficult in the buck-boost design.

Figure 2. LED filter and circuit board diagrams

To meet industry EMI specs, Power Integration engineers opted to split the drive electronics into two boards: a converter board on top and an input rectification/EMI filter board on the bottom. They then placed a Faraday shield between the two boards. Electrically connected to the converter board, the shield was comprised of a single-sided, copper-clad PCB built to the same dimensions of the bottom input rectification/EMI filter board. Using this design to drive three LEDs, tests indicated that conducted EMI at a worst-case input voltage of 230Vac was approximately 7dB?V below industry EMI requirements.

Figure 3. EMI results (converter board)

From a cost standpoint both circuit topologies offer similar advantages. A typical design requires approximately 25 components and, importantly, allows the use of low-cost, off-the-shelf inductors instead of custom transformers.

One important distinction lies in the design of the current sense feedback loop. Current feedback limits the LED current during normal operation. Designers can address the current sense requirement using the FB pin directly to sense the voltage drop across the sense resistors. Since the FB pin has a voltage of 1.65V, however, this can result in unacceptable dissipation inside the GU10 enclosure. As a result, designers using the buck circuit topology may have to purchase a few additional low-power signal components for the feedback loop. This additional investment typically includes two ceramic capacitors, two NPN surface mount transistors and four precision thick-film resistors. It is important to note, however, that together these components represent an extremely small incremental cost.

LED lighting clearly offers a number of promising advantages over traditional lighting technologies including lower energy consumption, longer life and lower maintenance. However, engineers building the drive electronics for high-volume LED applications face a number of imposing obstacles. By carefully considering the advantages and disadvantages of each of the circuit topologies described above, designers can eliminate the bulk and expense of a transformer and still meet this emerging technology's thermal, EMI and footprint requirements.

About the author
Silvestro Fimiani
is product marketing manager of appliance and industrial applications at Power Integrations.

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