Analyzing how to choose the best topology for white light LED drivers

The era of incandescent light bulbs is coming to an end. Throughout the 20th century, Edison's incandescent light bulb stood the test of time and became the standard general lighting tool. But new lighting technologies - especially light-emitting diodes ( LEDs ) - will eventually replace incandescent and fluorescent lamps.

As the world tries to save money on its energy budget due to rising energy costs, incandescent lighting technology is clearly on the wrong side of the fence. 97% of the energy consumed by an incandescent bulb is wasted. Fluorescent bulbs are slightly better, but still waste 85% of the energy. Moreover, both lamps have an average lifespan of only about 5,000 hours. In addition, fluorescent lamps use toxic mercury and produce a harsh color light. Neither technology can compare to white light LEDs - which not only last 10 times longer, use no toxic substances, and can produce light in almost any color. What's more, their light conversion efficiency is just as good as that of fluorescent lamps.

Table 1: Comparison of various lighting technologies

Therefore, in general lighting applications, the transition to LED technology will greatly reduce energy consumption. A recent study by the U.S. Department of Energy predicts that by 2025, widespread white LEDs will save the world 10% of electricity and $100 billion in savings. Sandia National Laboratories in the United States said that such energy savings would mean that power plants around the world would emit 350 million tons less carbon dioxide each year. Government leaders are starting to take notice. For example, Australia recently announced a decree to end the use of inefficient incandescent lamps as part of its plan to reduce greenhouse gas emissions and lower household energy costs.

Although white LEDs are an ideal solution for today's large-scale lighting, designers still face considerable challenges in getting the electronics that drive them into every light bulb. First, space constraints require LED drivers to be small and efficient. There is also the issue of heat dissipation, which has a significant impact on the reliability of lighting equipment and limits design density. Finally, designers must carefully consider the EMI impact of their products.

Since drivers are not readily available, designers can use buck and buck/boost switch- mode power supply (SMPS) converters based on non-isolated commercial off-the-shelf (COTS) inductors for low- power (≤3 W) lighting . Both circuits do not require a transformer and offer many other advantages. This article compares the two topologies and discusses the trade-offs of each.

2 topologies

Figure 1 shows a LinkSwitch-TN configured as a basic buck converter (1a) and a basic buck/boost converter. The LinkSwitch-TN simplifies converter design and reduces component count by integrating a power MOSFET, oscillator, simple on/off control scheme, a high voltage switched current source, frequency jittering, cycle-by-cycle current limiting and thermal shutdown circuitry on a monolithic IC. It is self-powered through the DRAIN pin, so no bias supply and associated circuitry is required. As a cost-effective alternative to linear and capacitive non-isolated power supplies in the 360mA range or less, the LinkSwitch-TN offers best-in-class linear regulation and complex regulation, with higher efficiency than passive solutions and better power factor than capacitive solutions.

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

The buck converter shown in Figure 1a has many advantages. First, it maximizes the output power for a selected LinkSwitch-TN device and inductor value. It also reduces the voltage stress on the power switch and the freewheeling diode. In addition, the average current flowing through the output inductor in this buck converter is slightly lower than the average current flowing through the output inductor in a buck/boost converter.

The buck/boost converter configuration has one major advantage over the buck converter: its output diode is in series with the load. In a buck converter, if the MOSFET is shorted, the input is directly connected to the output. If the MOSFET is shorted in a buck/boost converter, the reverse biased output diode blocks the path between the input and output.

In both converters, the AC input is rectified and filtered by D1, D2, C1, C2, RF 1 and RF2. The two diodes enhance the resistance to line surges and transmit EMI. Designers must use a fuse on RF1, but only one fuse on RF2. The on/off control in Linkswitch-TN is used to adjust the output current. Once the current into the feedback pin exceeds 49 μA, the MOSFET switch will be disabled to prepare for the next switching cycle.

Minimize heat

Thermal management is a major challenge for LED driver designers. Although LEDs are more efficient than incandescent lamps, at 3W their circuits can reach a temperature level that threatens the integrity of the device. In addition, integrating the driver into a standard GU10 lamp holder can create significant challenges for heat dissipation. At this point, the only way to dissipate heat is to conduct it to the base of the lamp. In the solution discussed above, LinkSwitch-TN can add a thermal shutdown circuit to turn off the power MOSFET when the core temperature exceeds 142°C, thereby protecting the LED from damage. Once the core temperature drops by 75°C, the MOSFET can automatically restart.

The buck/boost topology is slightly less efficient than the buck topology because power is not delivered to the output every time the MOSFET switch is on. Therefore, it generates more heat, but the difference is not huge.

Table 2: Source pin temperature as a function of input voltage

To ensure that the circuit topology meets thermal regulation requirements, Power Integrations designers installed a power supply assembly into the socket and measured the temperature of the source pin on the LNK306DN (a member of the Linkswitch-TN product family). The LNK306DN is designed to regulate the load current to 330mA to drive three series-connected LEDs. Its input is a universal input range of 85" to 265VAC.

Ideally, the power supply pin temperature should not exceed 100°C. However, as shown in the above graph, at room temperature of 25°C, the source pin temperature rises sharply as Vin increases, and exceeds 100°C when Vin reaches 265 VAC. Therefore, designers need to perform additional heat dissipation, such as adding a heat sink on the top of the U1 SO-8C package, to meet thermal management requirements.

Controlling EMI

LED driver circuits Must comply with the EN55022B/CISPR22B standard for conducted EMI. These requirements present another major challenge to designers, given the high switching frequency of the switcher IC and the limited size of the GU10 lamp holder. The EMI noise current loop in the buck/boost circuit topology extends from the MOSFET to the output diode, output capacitor and back to the input capacitor, while the EMI noise current loop in the buck topology starts from the MOSFET through the freewheeling diode and returns to the input capacitor. Therefore, noise reduction in buck/boost designs is relatively more difficult.

Figure 2: LED filter and circuit board

To meet industrial EMI specifications, Power Integration engineers separated the driver into two boards: the first is a converter board at the top, and the other is an input rectifier/EMI filter board at the bottom. Then, they placed a Faraday shield between the two boards. The shield is connected to the converter board and consists of a single-sided copper-plated PCB , the other end of which is located on the input rectifier/EMI filter board at the bottom. When this design is used to drive three LEDs, the conducted EMI is about 7 dBμV lower than the industrial EMI standard requirements at an input voltage of 230VAC.

Figure 3: EMI results (converter board)

From a cost perspective, the two topologies have similar advantages. A typical design requires only about 25 components and can use low-cost, off-the-shelf inductors rather than custom transformers.

There is an important distinction in the design of the current sensing feedback loop. The current loop will limit the LED current in normal operation. Designers can solve the current sensing problem by directly using the FB pin to sense the voltage drop across the sense resistor. However, since there is a voltage of 1.65V at the FB pin, this may result in an undesirable result, that is, heat dissipation inside the GU10 housing. Therefore, designers using the buck circuit topology also need to add some low power signal components for the feedback loop. These components generally include 2 ceramic capacitors, 2 NPN surface mount transistors , and 4 precision thick film resistors. However, the total cost of adding these components is very low.

Summarize

Compared with traditional lighting technologies, LEDs undoubtedly have many advantages, including lower energy consumption, longer service life and lower maintenance rate. However, engineers who develop driver equipment for large-scale LED applications also face many challenges. Designers need to carefully consider the pros and cons of the two topologies mentioned above to abandon bulky transformers and meet the requirements of the technology in terms of thermal management, EMI and form factor.