In LED lighting, matching the driver topology to the power level can be challenging. There are a number of topologies to choose from, but which one will give the right balance of size, performance, and cost? A quick look at the specific requirements for each power range — low, middle, and high — yields recommendations for the best power-switching technology for each power range.
Low-power lighting
Low-power lighting includes candle, R-lamps and incandescent replacements running at under 20W. These are small bulbs mostly used in residential and retail environments. Size and cost are the two biggest concerns, so using a primary-side regulated (PSR) topology is a good choice. A PSR controller delivers precise control of the output voltage and current by using only information from the primary-side of the LED lamp controller. This removes the output current sense losses and eliminates the secondary-feedback circuitry. The result is a design that meets tighter constant output current requirements with fewer external components. Efficiencies can start at 80 percent for a 20W LED load, so the thermal loss is about 4W. Figure 1 gives a sample circuit, using the FL103 Fairchild FPS™ power switch device that is a controller with PSR function.

FL103 PSR topology in a low-power application

Figure 1. The PSR topology in a low-power application

When power factor correction (PFC) is needed, Figure 2 is an example circuit using the FL7730 dimmable single-stage PFC PSR offline LED driver.

FL7730 PSR topology with PFC in a low-power application

Figure 2. The PSR topology with PFC in a low-power application

Middle-power lighting
In the middle-power range, LED bulbs are designed for use in down-lights and L-lights operating at up to 50W. To meet regulatory requirements, power factor correction (PFC) is a required element. Adding the circuitry for PFC can make for a tight fit. A good way to save on footprint and cost is to use a single-stage PFC topology, since it doesn’t use a large electrolytic capacitor after the full rectification diode. Figure 3 shows a flyback converter using the FL6961 PFC controllerthat operates in Critical Conduction Mode (CCM) and has functions such as soft-start and a cycle-by-cycle current limit.

Simplified schematic of a single-stage PFC topology, using the FLS6961

Figure 3. Simplified schematic of a single-stage PFC topology, using the FLS6961
[Image is from Fairchild App Note AN-9737, Figure 1]

The small capacitor (C1) in the figure acts as a noise filter to attenuate high-frequency components. Efficiencies up to 84 percent are possible with a 50W load, leaving 8W as thermal loss.
High-power lighting
High-power lightingincludes lamps that operate above 100W, such as street lights and other outdoor applications. These designs need the best efficiency at a reasonable cost. Including PFC functionality at the input stage lets the design meet international regulations for harmonics. For the downstream converter, a resonant switching technique, which processes power in a sinusoidal manner, can lower switching losses and reduce noise. A relatively new technique, called LLC resonant switching, improves efficiency at high voltages. The LLC technique (see Figure 4) builds on the more familiar L-C resonant network by using a shunt inductor across the transformer primary winding which is naturally provided by the transformer magnetizing inductance. This improves regulation at light loads.

A half-bridge LLC resonant converter

Figure 4. A half-bridge LLC resonant converter
[Image from AN-9730, Figure 2]

Figure 5 shows a high-voltage driver concept with a PFC input stage followed by an LLC topology. The PFC/LLC combination can produce an efficiency of up to 92 percent. In a 400W LED driver, that means a thermal loss of 32W.

Figure 5. A power topology for high voltages, combining a PRC input with LLC resonant control
[Image taken from Fairchild Semi website: http://www.fairchildsemi.com/applications/diagrams/lighting_high_power.html]

Fairchild’s half-bridge LLC resonant controllers include the FLS-XS series, which combines power MOSFETS with fast recovery-type body diodes, a high-side gate-drive circuit, an accurate current-controlled oscillator, frequency limit circuit, soft-start, and built-in protection functions.
Conclusion
The power range of the LED bulb influences the requirements for a driver’s cost, size, and efficiency. For low-power designs, consider a PSR flyback, but in middle-power applications, a single-stage PFC topology is probably the best option. For high-power installations, an LLC topology, used in combination with a PFC input stage, is likely to provide the right balance of efficiency and cost. Fairchild supports all these topologies and has innovative solutions for every power range.

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