Design of single-stage CrM TRIAC dimming LED driver using NCL30000

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

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

Of course, not all countries absolutely mandate improved power factor in lighting applications, but some applications may have this requirement. For example, utilities may strongly promote the commercial use of products with high power factor in utilities. In addition, when utilities own/maintain street lights, they can decide whether they want a high power factor (usually greater than 0.95+), depending on their wishes.

1) Establish maximum load design goals with reference to surrogate standards

Take the ENERGY STAR solid-state lighting standard as an example. This standard contains the overall requirements for determining the efficacy of the luminaire; in fact, this standard is a system-level standard that involves selected LEDs, field operating temperatures, optical components, and drives. Power conversion energy efficiency, etc. Lamp developers can therefore trade off LED choices, use of optical components, thermal management solutions, drive topologies, and designs to meet overall requirements. The following table lists the key system requirements for recessed lights in the Energy Star version 1.1 residential and commercial application solid state lighting specification version 1.1.

Table 1: Key Requirements for Recessed Lights in the ENERGY STAR Version 1.1 Residential and Commercial Solid State Lighting Specifications

The most common downlights are larger aperture type recessed lights. For residential and commercial applications, in addition to differences in power factor, designers have the flexibility to use neutral and warm white LEDs. As can be seen from the minimum requirements in Table 1, to obtain a minimum output of 575 lumens, the maximum input power threshold is approximately 16.4 W.

Since there is no directly applicable LED driver energy efficiency standard, consider the "Energy Star" version 2.0 external power supply (EPS) standard as a substitute standard. According to the EPS 2.0 standard, the minimum energy efficiency requirement for a standard power supply rated between 1 and 49 W is 0.0626 × ln(Pno) + 0.622. Therefore, the minimum energy efficiency of a 12 W rated power supply meeting this standard is 77.7%, and that of a 15 W power supply is 79.1%. Since the LED luminaire standard is based on the energy efficiency of the input socket, it is necessary to convert the driver energy efficiency target into an effective LED load. In order to increase some design margins, we set the minimum target energy efficiency to 80%. As a result, the LED load is 16.4 W × 80%, which is 13.1 W.

In this way, we have determined the maximum load design goal. LED efficacy is subject to LED manufacturers as well as drive current and operating temperature. ON Semiconductor's GreenPoint® reference design selects a constant current of 350 mA to support most high-brightness power LEDs on the market. Another factor to consider is that the luminaire developer can choose a wide range of LEDs. The higher the efficacy of the selected LED, the less LEDs are required. Therefore, the GreenPoint® reference design has a higher energy effect at 50% to 100% of rated load. As LEDs improve, the same basic power supply design can be easily modified to drive fewer LEDs, providing far greater than the minimum required luminaire efficacy.

2) Other design requirements

Once the basic design requirements are identified, other system factors related to the end application requirements need to be considered. For example, although there is no requirement in the standard, it is important to be compatible with existing line dimming solutions. Therefore, the design should be optimized for triac TRIAC wall dimmers. There are many challenges to TRIAC dimming, but one factor that designers may easily overlook is that the drive should be able to start and work with a low chopped AC input waveform. Moreover, the size of the drive power supply should match the junction light fixture junction box. Should also pay attention to a person's factor requirements. Although the LED actually illuminates in an instant, the driver is designed to have a specific start-up time. Regardless of the LED luminaire, this aspect should not be worse than CFL, or even better. Therefore, we can use CFL as a reference. In the "Energy Star" CFL bulb requirements, the maximum start-up time under rated conditions is 1 second, so we will set the LED driver's design time in terms of startup time to 0.5 seconds. Because this design is for residential or commercial applications, the specifications we set are more challenging. Table 2 summarizes the key design goals of this GreenPoint® reference design.

Table 2: Key Design Objectives

3) Design approach: Provide high power factor with single-stage solution

To achieve high power factor, power efficiency targets, and compact size, it is necessary to use a single-segment topology with high power factor. Due to the lower power target, the traditional two-stage topology (PFC boost + flyback conversion) is not sufficient. Therefore, we used a CrM flyback topology based on the ON Semiconductor NCL30000 Critical Conduction Mode (CrM) flyback controller.

The single-stage topology saves dedicated PFC boost sections, helping to reduce component count and overall system cost. However, with a single-stage topology, the system will also be affected, such as no primary high-voltage energy storage, and the output voltage retention time is short. In addition, the output ripple is high, more low-voltage output capacitors must be used to meet the maintenance requirements, and the dynamic load response is slow. Advantageously, this does not pose a problem for many LED lighting applications because LED lighting applications have no system maintenance time requirements and the ripples are fed into the average light output and are not noticeable to the human eye.