Isolated LED driver solution based on NCL30000 with PF greater than 0.95

　　LED driver application requirements

　　For LED general lighting applications, the current cost is still relatively high, so high performance-to-price ratio LED drivers will undoubtedly be more popular. In addition, LED drivers should also have high energy efficiency (low loss), high reliability, meet electromagnetic interference (EMI) and harmonic content or power factor (PF) standards, be flexible, adapt to wide environmental conditions, be retrofittable for existing applications, support traditional control methods (compatible with traditional dimming), etc.

　　Among them, as far as the power factor requirements are concerned, the solid-state lighting standards of the US "Energy Star" project have mandatory requirements for PFC (regardless of the power level), which are applicable to specific products such as recessed lights, cabinet lights and table lamps. The standard requires a power factor higher than 0.7 for residential applications and higher than 0.9 for commercial applications, while the requirement for integral LED bulbs is that the power factor of bulbs with input power higher than 5W is greater than 0.7. Of course, not all countries have an absolute mandatory requirement to improve the power factor in lighting applications, but some applications may have such requirements. For example, utilities may vigorously promote the commercial application of products with high power factors in public facilities.

　　NCL30000 isolated LED driver solution with high power factor

　　To provide high power factor and meet other application requirements at the same time, it is necessary to use a high power factor circuit architecture. In this way, the traditional two-stage architecture (PFC boost + pulse width modulation (PWM) flyback conversion) cannot meet the requirements. Fortunately, the ON Semiconductor NCL30000 power factor correction dimmable LED driver adopts a single-stage architecture (see Figure 1) to provide a power factor of 0.9 or even higher. This device adopts a compact 8-pin surface mount package and uses a critical conduction mode (CrM) flyback architecture to provide a high power factor greater than 0.95 in a single-stage topology, saving a dedicated DC-DC conversion power stage, helping to reduce the number of components and reduce the total system cost.

　　Figure 1 Comparison between the traditional two-stage architecture and the improved single-stage architecture

　　The constant on-time CrM operation adopted by the NCL30000 is particularly suitable for isolated flyback LED applications because the control principle is simple and can provide extremely high energy efficiency even at low power levels. The NCL30000 has an operating 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 and a low operating current of typically 2mA, combined with a high energy efficiency design. This device also integrates a leading edge blanking (LEB) circuit to prevent fault triggering, and also integrates strong fault handling capabilities.

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

　　With TRIAC dimming

　　In addition to providing high energy efficiency and high power factor required by relevant standards, it is also important to consider specific terminal application requirements. For example, the most commonly used method for controlling light brightness is TRIAC dimming. However, TRIAC dimmers are originally designed for incandescent lamps with resistive loads (incandescent lamps act like resistors in the circuit). Therefore, it is also important that LED drivers are compatible with TRIAC dimmers, which helps users use existing wall-mounted TRIAC dimmers and save costs.

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

　　The good thing is that the NCL30000 is compatible with both trailing-edge transistor dimmers and leading-edge TRIAC dimmers, and can provide a high power factor greater than 0.95, making the input current waveform look like that of a resistive load, which is very important for compatibility with TRIAC dimmers, because the waveform captured with an oscilloscope shows that the basic current waveform of the optimally designed NCL30000 single-stage CrM flyback LED driver remains in phase with the input voltage waveform (see Figure 3).

　　Demonstration Board Energy Efficiency Test Results

　　ON Semiconductor has developed a reference design demonstration board based on NCL30000 with a target power of less than 18W, supporting 4 to 15 LEDs at 350mA and supporting TRIAC dimming (pre-configured for 12 LEDs). The energy efficiency test results of the NCL30000 demonstration board with an input voltage range of 90 to 305Vac show that the power factor under a wide input voltage range reaches or exceeds 0.95, and maintains excellent current regulation and high energy efficiency under wide LED forward voltage conditions.

　　Figure 4 Demonstration board measurement results 　　Application example

　　For LED lighting applications, bulb replacement is an important application area, such as parabolic aluminized reflector (PAR) replacement (PAR20/30/38), A-type lamp replacement (such as A19), etc. However, bulb replacement applications have unique challenges, such as heat dissipation limitations, size restrictions, and compatible dimming technologies. ON Semiconductor has developed some bulb replacement reference designs to introduce how to use ON Semiconductor's driver solutions to meet these challenges.

　　Figure 5: Example of 11W LED PAR30 bulb modification based on NCL30000

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

　　PAR30 bulbs are an example of specific directional lighting. In reality, there are also many omnidirectional lighting applications, such as A-type lamps. LED A-type lamps replace traditional A-type incandescent lamps with standard A-type lamp size. They are excellent energy-saving alternative solutions for commercial and residential lighting applications. They are not only more environmentally friendly, but also easier to maintain and replace. Traditional incandescent lamps exist in different power levels, such as 40W (light output of about 450lm) or 60W (light output of about 600 to 800lm). Today, the luminous efficacy of first-class warm white LEDs is in the range of 100lm/W. Considering the thermal effect and photoelectric conversion efficiency, about 5 to 7 LEDs are required to provide 450lm light output. ON Semiconductor is currently developing the NCL30000 TRIAC dimmable solution for LED A-type lamps.

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

　　Fortunately, ON Semiconductor has developed an 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 265Vac, a rated output power of approximately 6.4W ( 16.5@400mA ), and an isolated single-stage design. Within the input voltage range, the power factor is higher than 0.9, the energy efficiency is higher than 80%, and it complies with the IEC61000-3-2C class harmonic content standard (input THD <15%), and complies with the FCCB class conductive radiation standard. The design takes into account compliance with UL8750 safety regulations and meets the Energy Star 1.1 specification requirements for integrated LED bulbs.

　　Summarize

　　This article focuses on the application design requirements of isolated LED drivers for low-power AC-DC LED general lighting applications, introduces ON Semiconductor's corresponding single-stage isolated high power factor LED driver solution NCL30000, and analyzes how NCL30000 meets relevant requirements, such as supporting TRIAC dimming, providing high energy efficiency, proper heat dissipation and compliance with relevant energy efficiency standards. It also shares ON Semiconductor's PAR30 and A-type LED bulb replacement reference designs that have been developed or are being developed to help design engineers shorten the design cycle and speed up product launch.