LED drive challenges and on semiconductor solutions in the general lighting market

In recent years, with the continuous expansion of energy conservation and environmental protection group green line, various specification standards organizations continue to release new energy efficiency standards, at the same time, the terminal products continue to a higher degree of integration and the smaller size of the direction of development, reduce energy consumption, improve energy efficiency has become a common concern of many countries in the world of government and industry organization, semiconductor, electronic product manufacturers and consumers.

Comparison of different light sources in general lighting market

From the point of view of specific applications, the general lighting market covers a wide range of areas, including building lighting, signs, landscape lighting, retail, signal lights, street lighting and residential lighting, etc.. In the general lighting market, the current commonly used light source, including incandescent lamps, compact fluorescent lamps (CFL), linear fluorescent lamps, high intensity gas discharge lamp (HID), as well as new high brightness HB (light-emitting diode), etc..

If we based on energy efficiency benchmark to compare the different light source, then for an important indicator of lighting is the total output lumens and input power ratio in lumens per watt (lm/W) to measure, called energy efficiency (efficacy). In different lighting solutions, incandescent energy efficiency is relatively low. For standard 60W incandescent lamps, the energy efficiency ranges from 10 to 13 lm/W (total output is from 600 to 800 LM). Relatively speaking, CFL energy efficiency is much higher, typical energy efficiency of 55 to 60 lm/W. However, CFL is a full direction of illumination, installed in the lamp, the light will be direct, reverse or shadowing, resulting in light loss, making the 55lm/W of the CFL lamp energy efficiency is only between 28 to 50 lm/W. The other light sources, such as HID also has a higher energy efficiency than incandescent lamp, output 1 100W metal halide lamp HID can produce about 8000 lumens, namely the energy efficiency in 80lm/W; however, like CFL, HID light output is in the light path direction, there will be a lot of loss.

Comparatively speaking, LED is a new light source technology. The most common white LED is the phosphor plated (yellow light LED) when excited. LED has more and more high energy efficiency, the industry recently announced the strongest white LED R & D capabilities to reach 132 to 136 lm/W, color temperature (4500-6000K). In fact, in recent years, the industry will continue to increase interest in the LED for the general lighting market. For general lighting, LED has many attractive features.

LED general lighting requirements and LED driver challenges

For the application of LED in general lighting, it is necessary to analyze the requirements of the system from the point of view of the system. Overall, the LED solid-state lighting system involves the following requirements:

LED light source: compact and efficient light source to provide a wide range of colors and output power

Power conversion: the AC wall socket, battery, solar cell power supply to the safe conversion of low-voltage DC power supply

Control and drive: the use of electronic circuits for LED voltage regulation and control

Thermal management: in order to achieve longer working life, the junction temperature control is very important, need to analyze the heat dissipation

Optical devices: focus light on where it is required to use a lens or light guide material

In the development of energy-efficient LED universal lighting solutions, these aspects of the requirements are very important. Among them, LED control and drive is the focus of this paper. For LED drivers, it faces a major challenge lies in the nonlinear LED. This is mainly reflected will change with the temperature and current in the forward voltage LED, positive voltage LED devices have different LED, "color" with the current and temperature drift, and LED must work in the range of the specification requirements in order to achieve reliable work. The main function of the LED driver is to limit the current in the operating conditions, regardless of the input conditions and how the forward voltage changes.

For the LED drive circuit, in addition to constant current, it is also faced with some other key requirements. For example, if you need LED dimming, you need to provide pulse width modulation (PWM) technology, and the typical PWM frequency for LED dimming is 1 to 3 kHz. In addition, LED drive power handling circuit must be sufficient and strong, can withstand a variety of fault conditions, and be easy to implement. It is worth mentioning that, because the LED in the most appropriate current is always in the "on" state, so its color will not drift.

Since the number of LED in the system needs to be more than one, which involves the configuration of the LED problem. In general, it is strongly recommended to drive a single string LED, because it can provide the best current matching, and with the forward voltage change or output voltage "drift" irrelevant. Of course, users can also be connected in parallel or in series, parallel connection and other ways to configure LED. In the case of a parallel configuration, the circuit needs to "match" the LED forward voltage; if a LED fails to open, the other LED may be driven. Accordingly, multiple parallel or series connection and parallel connection technology can be used to reduce the risk of failure.

LED driver application sample

According to the specific application, LED may use different power supply, such as AC lines, solar panels, 12V car battery, DC power supply or low voltage AC system, and even the alkali nickel batteries or lithium ion batteries based on.

1) using AC off-line power supply for LED

In the application of AC off-line power supply for LED applications, involving a wide range of different applications, such as electronic ballasts, fluorescent lamps, traffic lights, LED lights, street and parking lighting, building