Design of Offline High Power Factor TRIAC Dimmable LED Driver

According to the International Energy Agency (IEA), 19% of the world's electricity consumption is used for lighting. Therefore, in recent years, countries around the world have been committed to replacing inefficient incandescent light sources with more energy-efficient solutions. As light-emitting diodes (LEDs) continue to make rapid progress in lumen output and light efficiency, while the average cost per lumen of light output is also decreasing, combined with the advantages of LEDs in high directivity, long life and low maintenance costs, LED lighting (also known as solid-state lighting, or SSL) has become an extremely attractive alternative solution.

Energy efficiency regulations for solid-state lighting

In order to promote energy conservation, government agencies or regulatory organizations around the world have formulated different LED lighting standards, mainly in terms of power factor (PF) requirements. For example, the European Union's International Electrotechnical Union (IEC) stipulates the total harmonic distortion performance of lighting applications with a power greater than 25 W, and other international standards in some regions also apply this regulation.

In addition, the U.S. Department of Energy has developed and published the "Energy Star" standard for solid-state lighting fixtures. This voluntary standard includes a series of requirements for common residential and commercial lighting fixtures (such as recessed lights, cabinet lights and table lamps), covering minimum lumen output, overall luminous efficacy, reliability targets, light color temperature and a series of other key system-level requirements. It is worth noting that this standard does not directly include power efficiency requirements, but includes power factor requirements, that is, regardless of the power level, residential applications require a PF greater than 0.7, commercial applications require a PF greater than 0.9, and integrated LED lighting requires a PF greater than 0.7.

Of course, not all countries have an absolute mandate to improve power factor in lighting applications, but some applications may have this requirement. For example, utilities may strongly promote products with high power factor for commercial applications in public facilities. In addition, when utilities own/maintain street lights, they can decide whether to require products with high power factor (usually greater than 0.95+) based on their own wishes.

13 W LED Downlight Design Example

1. Establish the maximum load design target by referring to the alternative standard

Take the Energy Star solid-state lighting fixture standard as an example. This standard includes overall requirements for determining the luminous efficacy of the fixture. In fact, this standard is a system-level standard involving the selected LED, field operating temperature, optical components, driver power conversion efficiency, etc. Luminaire developers can therefore make trade-offs in the selection of LEDs, the use of optical components, thermal management solutions, driver topology and design to meet the overall requirements. The following table lists the key system requirements for recessed lights in the Energy Star 1.1 version of the residential and commercial solid-state lighting specification 1.1.

Table 1: Key requirements for downlights in the ENERGY STAR 1.1 version of the residential and commercial solid-state lighting specification.

The most common downlights are the larger aperture type. For residential and commercial applications, designers have the flexibility to use neutral and warm white LEDs, except for the difference in power factor. From the minimum requirements in Table 1, it can be seen that to obtain the minimum output of 575 lumens, the maximum input power threshold is about 16.4 W.

Since there is no directly applicable LED driver energy efficiency standard, the Energy Star 2.0 external power supply (EPS) standard can be considered as an alternative standard. According to the EPS 2.0 standard, the minimum energy efficiency requirement for standard power supplies with a rated power between 1 and 49 W is 0.0626×ln(Pno)+0.622. Therefore, the minimum energy efficiency of a 12 W rated power supply that meets this standard is 77.7%, and the minimum energy efficiency of a 15 W power supply is 79.1%. Since the LED lamp standard is based on the input socket energy efficiency, it is necessary to convert the driver energy efficiency target into an effective LED load. In order to add some design margin, we set the minimum target energy efficiency to 80%. In this way, the LED load is 16.4 W×80%, or 13.1 W.

This way, we have determined the maximum load design target. LED efficacy is limited by the LED manufacturer as well as the drive current and operating temperature. ON Semiconductor chose a constant current of 350 mA for this GreenPoint? reference design, which supports most high-brightness power LEDs on the market. Another factor to consider is that luminaire developers can choose from a wide range of LEDs, and the higher the efficacy of the selected LEDs, the fewer LEDs are required. Therefore, the efficiency of this GreenPoint? reference design is higher at 50% to 100% of the rated load. As LED efficacy increases, the same basic power supply design can be easily modified to drive fewer LEDs, thereby providing luminaire efficacy far above the minimum requirement.

2. Other design requirements

Once the basic design requirements are determined, other system factors related to the end application needs need to be considered. For example, although not required by the standard, compatibility with existing line dimming schemes is important. Therefore, the design should be optimized for triac wall dimmers. There are many challenges with TRIAC dimming, but one factor that designers may easily overlook is that the driver should be able to start and operate under low chopped AC input waveform conditions. In addition, the driver power supply should be sized to match the wiring box of the downlight fixture. There is also a human factor requirement that should be noted. Although LEDs actually light up in an instant, the driver design must allow for a specific startup time. Regardless of the LED fixture, this performance should be as good as or better than CFL. Therefore, we can use CFL as a reference. The "Energy Star" CFL bulb requirement specifies a maximum startup time of 1 second under rated conditions, so we will set a design target of 0.5 seconds for the startup time of the LED driver. Since this design is for residential or commercial applications, the specification target we set is more challenging. Table 2 summarizes the key design goals of this GreenPoint® reference design.

Table 2: Key design objectives.

3. Design approach: Use a single-stage solution to provide high power factor

To achieve high power factor, power efficiency targets and compact size, it is necessary to use a single-stage topology with high power factor. Due to the low power target, the traditional two-stage topology (PFC boost + flyback conversion) cannot meet the requirements. Therefore, we used a critical conduction mode (CrM) flyback topology based on the ON Semiconductor NCL30000 CrM flyback controller.

The single-stage topology saves the dedicated PFC boost stage, helping to reduce the number of components and lower the total system cost. However, the system will also be affected by the single-stage topology, such as the lack of primary high-voltage energy storage and the shorter output voltage holding time. In addition, the output ripple is higher, more low-voltage output capacitors must be used to meet the maintenance requirements, and the dynamic load response is slower. Fortunately, this is not a problem for many LED lighting applications, because LED lighting applications have no system maintenance time requirements, and the ripple is merged into the average light output and is not noticeable to the human eye.

Designing for high power factor (PF>0.95) helps to easily meet the commercial lighting requirements of SSL lamps and makes the input current waveform look like the waveform of a resistive load. This is very important for compatibility with TRIAC dimming, because TRIAC dimmers are originally used for incandescent lamps, and incandescent lamps act like resistors in the circuit, that is, they act as resistive loads. The waveform captured by the oscilloscope shows that the basic current waveform of the optimized single-stage CrM flyback power supply remains in phase with the input voltage waveform.

Figure 1 shows a simplified functional block diagram of ON Semiconductor's single-stage high power factor flyback topology based on the NCL30000. As can be seen from Figure 1, the secondary side of the isolated flyback has a constant current constant voltage (CCCV) control module. This module has two main functions. One is to tightly regulate the constant current of 350 mA and provide feedback to the primary side to adjust the on-time and regulate the constant current flowing through the LED; the second is to enter the constant voltage control mode when an open circuit event occurs, and generate a regulated fixed voltage in the event of a fault. The open circuit voltage regulation is the 60 Vdc maximum voltage limit of the UL1310 Class 2 power supply. In addition, it can limit the power to avoid damage to the LED when the output is accidentally shorted.

Figure 1: Simplified block diagram of the GreenPoint® reference design for a single-stage CrM flyback LED driver based on the NCL30000.

4. Test results

The test results show that the performance of this reference design exceeds all the design targets listed in Table 2, as shown in Figure 2 (see reference [1] for details). Figure 2 shows the power factor and input current total harmonic distortion of the LED driver over the line voltage range of 90 to 135 Vac. It can be seen that the power factor of this reference design is very high (exceeding the minimum power factor requirement of 0.9 for commercial lighting) and the total harmonic distortion is low (<20%). Figure 3 shows the LED energy efficiency under different load conditions. The energy efficiency at the four operating points of 25%, 50%, 75% and 100% is averaged, and the total average energy efficiency is 80.7%; in the critical operating area of ​​50% to 100% load, the energy efficiency range is 81.1% to 82%. This not only exceeds the 80% energy efficiency target set by this reference design, but also exceeds the 79.1% energy efficiency requirement of the EPS 2.0 standard for 15 W power supplies. The loss source includes the energy consumption of the 15 ohm current limiting resistor required to support TRIAC dimming in the input EMI stage.

Figure 2: Power factor and total harmonic distortion for 90 to 135 Vac input line voltage conditions.

Figure 3: Energy efficiency under different load conditions at an input voltage of 115 Vac.

Summarize:

There are many challenges in designing an offline LED driver that meets all the requirements of the next generation of solid-state lighting products. This reference design document shows that ON Semiconductor's single-stage CrM TRIAC dimming LED driver GreenPoint? reference design based on NCL30000 meets all key performance indicators, such as the "Energy Star" version 1.1 commercial and residential solid-state lighting power factor requirements, and even the reference version 2.0 external power supply efficiency requirements under critical load conditions. This reference design also provides system developers with the flexibility to increase or decrease power to meet the requirements of different power applications. This approach allows designers to flexibly respond to the improvement of LED light efficacy, allowing them to design luminaires with fewer LEDs but still provide the expected light output.