Low Power LED General Lighting Application Isolated High Power Factor LED Driver Solution

In recent years, many countries around the world have been working to replace energy-efficient incandescent lamps with more energy-efficient lighting solutions. With the continuous improvement of light-emitting diodes (LEDs) in almost every aspect of illuminating performance and cost, LEDs have become a very attractive solution in the field of general lighting. From the perspective of power supply, LED lighting also includes different types of high-voltage AC-DC, medium-voltage AC-DC or DC-DC, and battery power. This article will focus on driver solutions for low-power (typically less than 30 W) AC-DC LED general-purpose lighting applications and their examples in applications such as PAR30 and A-type lamps.

AC-DC power conversion topology comparison

For low-power AC-DC LED power conversion, different topologies such as isolated flyback or non-isolated buck can be selected. The so-called "isolation" refers to the electrical isolation between the input and output using a transformer or the like. Both of these topologies have their own characteristics. In comparison, the non-isolated topology design and circuit board configuration are simple, the board size is small, the number of components is small, and the energy efficiency is higher. The isolation topology is easy to meet the safety requirements, but the magnetic design is complicated and requires a large circuit. Board size. This article will focus on the AC-DC LED Driver solution in an isolated topology.

LED driver application requirements

For LED general lighting applications, the current cost is relatively high, so high-performance LED drivers will undoubtedly be more popular. In addition, LED drivers should also have high energy efficiency (low loss), high reliability, compliance with electromagnetic interference (EMI) and harmonic content or power factor (PF) standards, flexibility, adapt to wide environmental conditions, and can be retrofitted to existing Application, support for traditional control mode work (compatible with traditional dimming).

Among them, in terms of power factor requirements, the US ENERGY STAR project solid-state lighting standard has mandatory requirements for PFC (regardless of the power level), applicable to specific products, such as recessed lights, cabinet lights and table lamps. Wait. The standard requires a power factor greater than 0.7 for residential applications and a power factor greater than 0.9 for commercial applications, while integral LED bulbs require a lamp power factor greater than 0.7 for input power greater than 5 W. 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.

Figure 1: Comparison of traditional two-stage architecture with improved single-segment architecture

NCL30000 isolated LED driver solution with high power factor

To provide a high power factor while meeting other application requirements, it is necessary to use a high power factor circuit architecture. As a result, the traditional two-stage architecture (PFC boost + pulse width modulation (PWM) flyback conversion) can not meet the requirements. Advantageously, such as the ON Semiconductor NCL30000 power factor correction dimmable LED driver with a single-segment architecture (see Figure 1), can provide a power factor of 0.9 or higher. The device is packaged in a compact 8-pin surface mount package that uses a critical conduction mode (CrM) flyback architecture to provide a high power factor of greater than 0.95 in a single-segment topology, eliminating the need for dedicated DC-DC conversion power sections. Reduce component count and reduce total system cost.

The constant on-time CrM operation of the NCL30000 is particularly well-suited for isolated flyback LED applications because of its simple control principle and extremely high energy efficiency even at low power levels. The NCL30000 operates over a temperature range of -40 to +125 ° C, ensuring that it can be used in the different environmental operating ranges specified in most LED general lighting applications. The NCL30000 has a low startup current of typically 24 microamps (μA) and a low operating current of typically 2 mA for an energy efficient design. The device also integrates leading edge blanking (LEB) circuitry to prevent fault triggering and also integrates robust fault handling.

Figure 2: Simplified block diagram of the GreenPoint® reference design for an isolated flyback LED driver based on the NCL30000

With TRIAC dimming

In addition to providing the high energy efficiency and high power factor required by the relevant regulatory standards, it is also important to consider the specific end application requirements. For example, the most commonly used method for adjusting the brightness of light is triac dimming (TRIAC) dimming. But the TRIAC dimmer was originally intended for incandescent lamps with resistive loads (incandescent lamps act like resistors in the circuit). Therefore, it is also important that the LED driver is compatible with the TRIAC dimmer, which is beneficial for the user to use the existing wall TRIAC dimmer to save money.

Figure 3: NCL30000 dimming performance demonstration (dimming between 350 mA and 1.7 mA by changing the dimmer position)

Advantageously, the NCL30000 is compatible with tail-edge transistor dimmers and leading-edge TRIAC dimmers, providing a high power factor above 0.95, making the input current waveform look like a resistive load waveform, which is compatible with TRIAC dimming. The device is very important because the waveforms captured by the oscilloscope show that the basic current waveform of the optimized design of the NCL30000 single-section CrM flyback LED driver remains in phase with the input voltage waveform (see Figure 3).

Demo board energy efficiency test results

Based on the NCL30000, ON Semiconductor has developed a reference design demonstration board that targets less than 18 W, supports 4 to 15 LEDs @ 350 mA, and supports TRIAC dimming (preconfigured for 12 LEDs). Energy efficiency test results for the NCL30000 demonstration board with an input voltage range of 90 to 305 Vac show that the power factor at a wide input voltage range is at or above 0.95 (see image to the left) and maintains excellent stability over wide LED forward voltage conditions. Flow effects and energy efficiency (right).

Figure 4: Demo board measures energy efficiency results (left: input current THD and power factor; right: load current and energy efficiency)

Application example

For LED lighting applications, bulb replacement is an important application area, such as parabolic aluminized reflector (PAR) replacement (PAR20/30/38), type A lamp replacement (such as A19). But bulb replacement applications have unique challenges such as heat dissipation limitations, limited size, and compatible dimming techniques. ON Semiconductor has developed a number of light bulb replacement reference designs that show how to use ON Semiconductor's drive solutions to address these challenges.

Figure 5: Example of an 11 W LED PAR30 bulb retrofit based on the NCL30000.

Taking the PAR30 bulb replacement application as an example, ON Semiconductor's application note "11 W TRIAC Dimmable PAR30 LED Bulb Driver" (AND8463/D) describes how to design with the NCL30000 single-segment power factor correction isolated flyback controller. The complete power conversion scheme for the LED PAR30 lamp is used to drive 3 strings of 24 LEDs in series-parallel configuration (8 LEDs per string, 150 mA per string, 450 mA in 3 strings). This reference design uses commercially available LED heat sinks and the E27 Edison base to handle heat dissipation and size constraints. The target output power of this design is 11 W, including 115 Vac version (power factor higher than 0.9, input current THD <20%) and 230 Vac (according to IEC61000-3-2 specification), LED current stability accuracy ±5%, LED voltage range 21 to 27 Vdc, energy efficiency higher than 82%, compatible with TRIAC and electronic low-voltage dimmer dimming, and comply with FCC Class B conductive EMI standards.

The PAR30 bulb is an example of a specific pointing illumination. There are also many omnidirectional lighting applications in the real world, such as A-type lamps. The LED Type A lamp replaces the traditional A-type incandescent lamp with a standard A-size lamp. It is an excellent energy-saving alternative for commercial and residential lighting applications. It is not only more environmentally friendly, but also easier to maintain and replace. Traditional incandescent lamps have different power levels, such as 40 W (light output is about 450 lm) or 60 W (light output is about 600 to 800 lm). The efficacy of today's first-class warm white LEDs is in the 100 lm/W range. Considering thermal effects and photoelectric conversion efficiency, it takes about 5 to 7 LEDs to provide 450 lm of light output. ON Semiconductor is currently developing the NCL30000 TRIAC dimming solution for LED A-type lamps.

Advantageously, ON Semiconductor has developed the A19 bulb LED driver reference design based on the critical conduction mode power factor correction controller NCP1607 (see Figure 6). This reference design has an input voltage range of 90 to 265 Vac and a nominal output power of approximately 6.4 W (16.5 @ 400 mA). It is an isolated single-segment design with a power factor greater than 0.9 over the input voltage range and energy efficiency above 80. %, in line with IEC61000-3-2 Class C harmonic content standard (input THD <15%), comply with FCC Class B conductivity type radiation standard, the design takes into account the UL8750 safety regulations, and meets the "Energy Star" for the whole Specification for version 1.1 of the LED bulb.

Figure 6: High power factor A19 bulb E26/E27 driver solution based on NCP1607.

to sum up:

This paper focuses on the low-power AC-DC LED general lighting application, focusing on the application design requirements of the isolated LED driver, and introduces the corresponding single-segment isolated high power factor LED driver solution NCL30000 from ON Semiconductor, and analyzes how the NCL30000 cooperates with Requirements, such as support for TRIAC dimming, providing energy efficiency, proper heat dissipation, and compliance with relevant energy efficiency standards, and sharing PAR30 and A-type LED bulb replacement reference designs developed or under development by ON Semiconductor to help design engineers shorten design cycles