Isolation high power factor LED driver applications

In recent years, many countries in the world are committed to the use of energy-efficient lighting programs to replace the low efficiency of incandescent lamps. With the continuous improvement of light-emitting diode (LED) in almost all aspects of luminous performance and cost, LED has become a very attractive solution in the field of general lighting. From the power supply point of view, LED lighting also includes high voltage AC-DC, medium voltage AC-DC or DC-DC, as well as different types of battery power supply. This article will focus on the study for low power (generally less than 30 W) scheme AC-DC driver example LED general lighting applications in PAR30, A lights and other applications.

Comparison of AC-DC power conversion topologies

For low power AC-DC LED power conversion, you can choose isolated or non isolated flyback buck different topology. The so-called "isolation" refers to the use of transformers and other electrical isolation between input and output. These two topologies have their own characteristics. In comparison, the non isolated topology design and circuit board configuration simple circuit board, small size, fewer components and higher energy efficiency, and the isolated topologies are easy to meet the safety requirements, but the design of complex magnetic circuit board, larger size. AC-DC LED drive scheme in this paper will focus on the isolation topology.

LED driver application requirements

LED for general lighting applications, the cost is relatively high, so the functional LED drive will undoubtedly be more popular. In addition, LED drive should also have high efficiency (low loss), with high reliability, electromagnetic interference (EMI) and the harmonic content of the power factor (PF) standard and flexible to adapt to environmental conditions, can be used for wide transformation has been applied to support the work of the traditional control method (compatible with traditional dimming) etc..

Among them, the power factor requirements, solid state lighting standards of the American Energy Star program of PFC with mandatory requirements (regardless of what level of power,) applicable to specific products, such as embedded lamps, cabinet lights and lamps etc.. The standard for residential applications require the power factor is higher than 0.7, according to the requirements of commercial application part of the power factor is higher than 0.9, while the overall type LED lighting requirement is the lamp power input power factor higher than 5 W greater than 0.7. Of course, not all countries are absolutely required to improve power factor in lighting applications, but some applications may have this requirement. For example, utilities may vigorously promote the commercial application of products with high power factor in public utilities.

Figure 1: a comparison of the traditional two - stage architecture with the improved single - stage architecture

NCL30000 isolated LED driver scheme for high power factor

To provide high power factor and meet other requirements, it is necessary to use high power factor circuit architecture. As a result, the traditional two segment architecture (PFC boost + pulse width modulation (PWM) flyback conversion) can not meet the requirements. Advantageously, such as on semiconductor NCL30000 power factor correction adjustable optical LED drives using a single segment architecture (see Figure 1), can provide up to 0.9 or even higher power factor. This device uses a 8 pin compact surface mount package, the use of critical conduction mode (CrM) flyback architecture, providing greater than 0.95 power factor in single-stage topology, power conversion section dedicated to DC-DC Province, helping to reduce the number of components, reducing the total cost of the system.

NCL30000 uses a constant turn-on time CrM is particularly suitable for isolated flyback LED applications, because the control principle is simple, even in low power, can also provide very high efficiency. NCL30000 operating temperature range of -40 to +125 DEG C, to ensure that most of the LED general lighting applications in different environmental requirements of the scope of work. NCL30000 has a typical 24 A (A) low starting current and low operating current of 2 typical mA, with high energy efficiency design. This device is also integrated leading edge blanking (LEB) circuit, preventing fault trigger, integrated fault treatment ability strong.

Figure 2: reference design simplified block diagram isolation type flyback LED driver based on GreenPoint NCL30000

With TRIAC dimming

In addition to providing high energy efficiency and high power factor required by the relevant standards, it is important to consider the specific terminal application requirements. For example, the most commonly used method to control the brightness of the light is the three terminal triac (TRIAC) dimming. But the TRIAC dimmer was originally aimed at the resistive load of incandescent lamps (incandescent lamps in the circuit as a function of resistance). Therefore, the LED driver compatible with TRIAC dimmer is also very important, which is conducive to the use of the existing wall TRIAC dimmer, saving expenses.

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

Advantageously, the trailing edge of the transistor NCL30000 compatible dimmer and dimmer in front of TRIAC, can provide more than 0.95 of the high power factor, the input current waveform looks like resistive load waveform, which is compatible with the TRIAC dimmer is very important, because with the interception of the oscilloscope waveform display, the optimization design of NCL30000 single-stage CrM flyback LED driver basic current waveform and the input voltage waveform to maintain phase (see Figure 3).

Demonstration board energy efficiency test results

A Morimi developed a semiconductor NCL30000 target power less than 18 W, supports 4 to 15 LED @ 350 mA and supports TRIAC dimming (based on pre allocation for 12 LED) reference design demo board. The input voltage range of energy efficiency test results NCL30000 demo board from 90 to 305 Vac display, wide input voltage range of the power factor reaches or exceeds 0.95 (see chart), and maintain excellent steady effect and high efficiency over wide LED positive voltage conditions (right).

Figure 4: demonstration board measuring efficiency results (left: THD of input current and power factor; the load flow and energy efficiency)

Application examples

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, there are unique challenges in the application of bulb replacement, such as heat dissipation, limited size and compatible dimming technology. A Morimi semiconductor has developed a number of light bulbs to replace the reference design, describes how to use the driver of the A Morimi semiconductor solutions to meet these challenges.

Figure 5: 11 W LED PAR30 lighting transformation based on sample NCL30000.

The PAR30 bulb replacement as an example, application notes "A Morimi semiconductor 11 W TRIAC dimmable PAR30 LED lamp driver" (AND8463/D) describes how to use NCL30000 to design LED PAR30 lamp single-stage power factor correction isolation flyback controller complete power conversion scheme for driving series parallel configuration of the 3 series of a total of 24 LED (8 LED each string, each string of the current 150 mA, 3 on a total of 450 mA). This reference design uses the market bought the LED heat sink and E27 Edison base, effective heat dissipation and limited size of the problem. The output power of the 11 W contains 115 version of Vac (power factor higher than 0.9, the input current of THD<20% and Vac (230) comply with the IEC61000-3-2 specification) in two versions, LED current accuracy of + 5%, LED voltage range 21 to 27 Vdc, energy efficiency is higher than 82%, compatible with TRIAC and electronic low voltage dimmer dimming, and comply with the FCC B conductive type EMI standard.

The PAR30 bulb is an example of a specific directional lighting. In reality, there are many omnidirectional lighting applications, such as A type lamps. LED type A lamp to standard A lamp size to replace the traditional A incandescent lamp, energy-saving lighting applications is an excellent commercial and residential alternative solutions, not only more and more environmental protection, easy maintenance and replacement. The traditional incandescent lamp has different power levels, such as W (40 light output of about 450 or 60 W (LM) light output of about 600 to 800 LM). Now the first-class LED warm white light effect in the range of lm/W 100. Considering the thermal effect and photoelectric conversion efficiency, to provide 450 LM light output, takes about 5 to 7 LED. A Morimi is currently being applied to the development of LED semiconductor type A lamp NCL30000 TRIAC dimming scheme.

Advantageously, the A Morimi semiconductor has been developed based on the critical conduction mode power factor correction controller NCP1607 developed by the A19 bulb LED driver reference (see Figure 6). This reference design input voltage range of 90 to 265 Vac, rated output power is about 6.4 W (16.5 @ 400 mA), the isolated single section type design, the input voltage range of power factor is higher than 0.9, the energy efficiency is higher than 80%, with IEC61000-3-2 C (input harmonic content standard THD<15%, FCC B) comply with the conductive type radiation standard, taking into account the design is consistent with UL8750 safety, and in accordance with the "Energy Star" for the specification for integral LED lamp 1.1 edition.

Figure 6: NCP1607 high power factor A19 bulb E26/E27 driver scheme.

Summary:

This paper focuses on the low power AC-DC LED general lighting applications, discusses the application of the design requirements of isolated LED driver, A Morimi introduced the corresponding single section type semiconductor isolation type high power factor LED driver program NCL30000, analysis of the NCL30000 to cooperate with the relevant requirements, such as support for TRIAC dimming, providing high efficiency, proper heat dissipation and meet energy efficiency standards, and shared the A Morimi semiconductor has developed or is developing the PAR30 and A type LED bulb replacement reference design to help design engineers to shorten the design cycle, accelerate product launch.