High brightness LED driving challenge and security

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The challenge of high brightness LED driving in the general lighting market and ansenmey semiconductor solutions

in recent years, as green environmental protection groups continue to expand the boundaries of energy conservation and environmental protection, various regulatory and standard organizations continue to issue new energy efficiency standards. At the same time, terminal products continue to develop towards higher integration and smaller size. Reducing energy consumption and improving energy efficiency have become governments, industry organizations Semiconductor companies, electronic product manufacturers and consumers are a common focus

if we look at the power consumption in various application fields, it is estimated that about 19% of the global power is used for lighting. The proportion of electric energy consumption in this part is considerable. In view of this, the industry is constantly committed to finding more energy-saving and efficient lighting solutions for the general lighting market

comparison of different light sources in the general lighting market

from the perspective of specific applications, the general lighting market covers a wide range of fields, including building lighting, signs, landscape lighting, retail, signal lights, street lighting and residential lighting. In the general lighting market, the commonly used light sources include incandescent lamps, compact fluorescent lamps (CFL), linear fluorescent lamps, high intensity discharge lamps (HID) and emerging high brightness light emitting diodes (HB LED)

if we compare different light sources based on energy efficiency benchmarks, an important indicator for measuring lighting is the ratio of total output lumens to input power, measured in lumens per watt (lm/w), which is called energy efficiency. In different lighting solutions, the energy efficiency of incandescent lamps is relatively low. For standard 60 W incandescent lamps, the energy efficiency range is between 10 and 13 lm/w (total output is 600 to 800 LM). Comparatively speaking, the energy efficiency of CFL is much higher, with a typical energy efficiency of 55 to 60 LM/W. However, CFL belongs to omni-directional irradiation. When installed in lamps, the light will be direct, reverse or shielded, resulting in light loss, so that the net energy efficiency of 55 lm/w CFL lamps is only between 28 and 50 LM/W. Other light sources, such as hid, also have higher energy efficiency than incandescent lamps. A 100 W metal halogen HID lamp can produce an output of about 8000 lumens, that is, the energy efficiency is 80 LM/W; However, like CFL, the light output of HID is omnidirectional, and there will be a lot of loss in the light projection path

Figure 1: comparison of energy efficiency development trends of different light sources

comparatively speaking, LED is a new light source technology. The most common white LED is a phosphor plated blue LED that emits yellow light when excited. LED has higher and higher energy efficiency. The industry recently announced that the strongest white LED research and development capacity has reached 132 to 136 lm/w, and the color temperature has reached (4, 000k). In fact, in recent years, the industry is increasingly interested in using LEDs in the general lighting market. For general lighting, LEDs have many attractive characteristics. For example, it is essentially a low-voltage device. They are small in size, produce directional light, and can produce a variety of colors and white light. They produce infrared (IR) or ultraviolet (UV) radiation, and because they are solid-state devices, they are mechanically strong and mercury free, and can have a working life cycle of more than 50000 hours when properly designed and used. Compared with the standard incandescent lamp with a service life of 1000 hours, the service life of LED is much longer. Is there a facility to eliminate vibration for materials that require long-term continuous work, possibly the shell itself and the instrument itself? Inferior metals do not include 1. These characteristics of LEDs are particularly attractive for lighting applications with limited amounts of carbon, many extreme application environments, or applications that are geographically inaccessible or costly to shut down. As a result, high brightness LEDs have begun to replace incandescent lamps in a variety of applications; The applications that can be seen in the daytime include central parking warning lights and traffic lights

led general lighting requirements and led driving challenges

for the application of LED in general lighting, we need to analyze its requirements from the perspective of system. In general, led solid-state lighting system involves the following requirements:

* LED light source: the light source is compact and efficient, providing a wide range of colors and output power

* power conversion: AC wall socket, battery The power supply of solar cells is efficiently converted to a safe low-voltage DC power supply

* control and drive: 3. The price of hydraulic universal testing machine uses electronic circuits to stabilize and control the LED

* thermal management: in order to achieve a longer working life, node temperature control is very important, Need to analyze heat dissipation

* optical devices: focusing light to where it is needed requires the use of lenses or light guide materials

these requirements are very important when developing energy-efficient LED general lighting solutions. Among them, LED control and drive is the focus of this paper. For LED driver, the main challenge it faces is the nonlinearity of LED. This is mainly reflected in the fact that the forward voltage of the LED will change with the current and temperature, the forward voltage of different LED devices will vary, the LED "color dot" will drift with the current and temperature, and the LED must work within the range required by the specification to achieve reliable work. The main function of LED driver is to limit the current within the range of working conditions, regardless of the change of input conditions and forward voltage. Figure 2 shows the basic working circuit diagram of LED driver

Figure 2: basic working circuit diagram of LED driver

for LED driver circuit, in addition to constant current and stable current, it also faces some other key requirements. For example, if LED dimming is required, pulse width modulation (PWM) technology needs to be provided, and the typical PWM frequency for LED dimming is 1 to 3 kHz. In addition, the power processing capacity of the LED drive circuit must be sufficient, and the function must be strong, which can withstand a variety of fault conditions, and it must be easy to realize. It is worth mentioning that since the LED is always in the "on" state at the most appropriate current, its color will not drift

since the number of LEDs that need to be used in the system is often more than one, this involves the problem of LED configuration. In general, it is strongly recommended to drive a single string of LEDs, because this can provide the best current matching, regardless of the forward voltage change or the output voltage "drift". Of course, users can also configure LEDs in parallel, series, parallel cross connection, etc. If the parallel configuration is adopted, the circuit needs "matched" led forward voltage; If one LED fails and opens, other LEDs may be overdriven. Accordingly, multi-channel parallel or series, parallel cross connection technology can be used to try to reduce the risk of failure

led drive application examples

depending on the specific application, LEDs may use different power supplies to supply power, such as AC lines, solar panels, 12 V automotive batteries, direct current power supply or low-voltage AC system that defines the construction measures of XPS panels for exterior wall external insulation systems, or even batteries based on alkali and nickel or lithium-ion batteries

1) using AC off-line power supply to supply power to led

in the application of using AC off-line power supply to supply power to LED, it involves many different applications, such as electronic ballast, fluorescent lamp replacement, traffic signal lamp, LED bulb, street and parking lighting, building lighting, obstacle lamp and sign, etc. In these applications of driving high-power LED from AC main power supply, there are two common power conversion technologies, that is, using flyback converter when galvanic isolation is required, or using a relatively simple buck topology when isolation is not required

in terms of flyback converter, different flyback converters of ansenmey semiconductor can be used according to different output power. For example, on semiconductor ncp1013 is suitable for compact design applications with power up to 5 W (current 350 Ma, 700 Ma or 1 a), ncp1014/1028 can provide continuous output power up to 8 W, and NCP1351 is suitable for general-purpose applications with higher power than 15 W

taking ncp1014/1028 as an example, this is an off-line PWM switching regulator launched by ansenmey semiconductor. It has an integrated 700 V high-voltage MOSFET. It adopts 350 ma/22 VDC transformer design and 700 ma/17 VDC configuration. The input voltage range is 90 to 265 vac. It has the characteristics of output open circuit voltage clamping, frequency jitter to reduce electromagnetic interference (EMI) signals and built-in thermal shutdown protection. It is suitable for LED ballasts, building lighting Applications such as display backlight, sign and channel lighting and work lights. The application design diagram of ncp1014/1028 is shown in Figure 3 below. It is worth mentioning that this design has the function of open circuit output protection, which will clamp the output to 24 V in case of open circuit. In this design, the current and open circuit voltage can be adjusted by simply changing the resistance/zener diode combination. It is worth mentioning that if another optional transformer is used for 230 VAC AC lines, NCP1014 can provide power up to 19 W and NCP1028 can provide power up to 25 W

Figure 3: application diagram of ansenmey semiconductor offline second-generation LED driver ncp1014/1028

in lighting applications, if the output power is required to be higher than 25 W, the LED driver is facing the problem of power factor correction (PFC). For example, the requirements of the International Electrotechnical Commission (IEC) of the European Union for lighting (power greater than 25 W) have provisions for total harmonic distortion (THD). In the United States, the solid-state lighting standard of the "Energy Star" project of the Department of energy has mandatory requirements for PFC (regardless of the power level), that is, for residential applications, the power factor is required to be higher than 0.7, while for commercial applications, the power factor is required to be higher than 0.9. This standard is voluntary and not mandatory, but some applications may require good power factor. For example, public utilities will promote the large-scale application of LEDs, and LEDs applied in public facilities are expected to have a high power factor; Moreover, when public institutions own or provide LED street lamp services, whether the LED has a high power factor (usually greater than 0.95) depends on the wishes of public institutions. If they are willing, the corresponding LED drive solution must meet this requirement

Figure 4: comparison of different architectures in LED drive applications requiring PFC

in such applications that may require PFC controller, the traditional solution is the two-stage scheme of PFC controller + PWM controller. This scheme supports modularization and simple certification, but there will be a compromise in the overall energy efficiency. For example, assuming that the energy efficiency of the AC-DC section is 87% to 90%, and the energy efficiency of the DC-DC section is 85% to 90%, the total energy efficiency is only 74% to 81%. With the continuous improvement of LED technology, this architecture is expected to be transformed into a more optimized and energy-efficient solution. According to different requirements, there are many options, such as pfc+ non isolated step-down, pfc+ non isolated flyback or half bridge LLC, ncp1651/ncp1652 single-stage PFC scheme

in such applications that may need PFC controller, the traditional solution is the two-stage scheme of PFC controller + PWM controller. This scheme supports modularization and simple certification, but there will be a compromise in the overall energy efficiency. For example, assuming that the energy efficiency of the AC-DC section is 87% to 90%, and the energy efficiency of the DC-DC section is 85% to 90%, the total energy efficiency is only 74% to 81%. With the continuous improvement of LED technology, this architecture is expected to be transformed into a more optimized and energy-efficient solution. There are many options according to different requirements

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