照明技术概览第二部份 -- 采用降压的优点
The rapid adoption of LEDs in various applications makes simple drive solutions such as linear regulators impractical in many cases. In general, simple drive schemes continuously deliver power from the input source to the driver's output while using resistive elements to program the desired LED forward current. For the same LED current, the losses in these resistive elements increase considerably as the line voltages increase. For example, a linear regulator based LED driver yields 70 percent efficiency when supplying 1 amp from a 5-volt input source to a typical white InGaN LED (VF= 3.5V). Under the same operating conditions, the driver's efficiency will drop to approximately 30 percent when the input voltage increases to 12 volts. Such poor efficiencies require impractical thermal management schemes.
Switching regulators
Switching regulators improve the conversion efficiency. They interrupt the power flow while controlling the conversion duty cycle to program the desired output voltage or output current. Interrupting the power flow results in pulsating current and voltage and therefore it necessitates the use of energy storage elements (inductors and/or capacitors) to filter these pulsating waveforms. Contrary to linear regulators, switching regulators can be configured in different arrangements to realize voltage or current step-down (buck), step-up (boost) or both (buck-boost) functions. They are also capable of achieving high conversion efficiencies across wide input/output range. Replacing the linear regulator with a buck-based LED driver in the previous example yields 95 to 98 percent efficiency across the 5-to-12 volt input range.
The configuration flexibility and the efficiency improvements of switching regulators come at the expense of higher noise generation caused by the periodic switching events, as well as higher premiums and reduced reliability due to their perceived complexity. Constant-current LEDs favor regulator topologies that can be simply configured as a constant-current source. The selected topology should also combine high performance with minimum component-count to increase the driver's reliability and to reduce cost. It should also facilitate the use of various dimming techniques to take advantage of the LEDs dynamic light- tuning characteristic. Fortunately, the most basic step-down (buck) switching topology enjoys all these characteristics, making it the regulator of choice to drive LEDs whenever possible.
Constant-current power stage
