High power LED lighting constant current drive power supply design

n today's global energy shortage, energy conservation has become an irresistible trend. Also advocates energy conservation and emissions reduction, and at the same time, the country has the end of the 2008 Beijing Olympic Games and the upcoming 2018 Shanghai world expo are in green energy saving as the theme, this gives the development of China's LED lighting industry has brought great historic opportunity. High power LED high photosynthetic efficiency, low power consumption, long life, high stability, light color pure, good safety, controllable strong wait for an advantage, is gradually instead of the light source, began to widely used Yu Quancai displays, traffic lights, car lights, background light sources, landscape lighting, lighting, special work to become a new generation of green light source in the field of lighting. China's LED lighting market will grow rapidly from 4.85 billion yuan in 2007 to 9.81 billion yuan in 2010, according to local authorities. Experts believe that China's LED lighting industry will have a new development peak around 2010.
Raise the question
Generally speaking, the power of high-power LED is at least 1W, and the common ones are 1W, 3W, 5W, 8W and 10W. It is called "green light source", and it is developing towards high current (300mA ~ 1.4a), high efficiency (60 ~ 120lm/W) and brightness adjustable. However, high-power LED luminous intensity is determined by the current through the LED, too strong electricity will cause the decline of the LED too weak electricity will affect the LED luminous intensity, so the LED driver needs to provide constant current power supply, to ensure the safety of high power LED to use, also need to satisfy the requirement of the expected brightness, and ensure the consistency of each LED luminance and chrominance. Therefore, the power source used to drive the light bulb (tungsten wire), fluorescent lamp, energy saving lamp, sodium lamp, etc. Is not suitable for the direct drive of high-power LED. It is also necessary to solve the problem of reducing pressure, isolation, PFC (power factor correction) and constant current problem by using the power LED with the power of municipal power.
At present, there are thousands of models on the market of special chip of the high-power LED constant current drive, there is a wide peng (ADDtek), point (SITI) crystal, crystal chow (SCT), CRC silicon (PT), in the United States abroad the superfamily (Supertex), Texas instruments (TI), the United States, the half, the UK's jet (Zetex), and other well-known manufacturers. Most dedicated chip using hysteresis type converter, low voltage input range, step-up, step-down, PWM control, power switch can be internal or external, can achieve 1.5 A output current, internal overvoltage, undervoltage, open/short circuit protection circuit and temperature, etc.
As shown in figure 1, the key feature of the hysteresis converter is self-oscillation, which means that the frequency will vary with the input voltage, LED current, and the number of driving leds. However, this converter often runs in continuous mode, which means that the inductance will never be saturated, nor will it completely run out of current, and the MOSFET will continue to have current to maintain the LED brightness after the shutdown. But the disadvantage is that duty ratio and frequency under the condition of changing resistance RCS of impedance is different, compared to the current flowing through the RCS, and LED the actual current is not completely consistent, there is a deviation detection data.