隨著最近更高功率及效率的新LED出現(xiàn)，LED被迅速應用到新的領域，例如手電筒及車載設備中。高功率LED甚至被應用到了長期由白熾燈和熒光燈占據(jù)的環(huán)境照明領域中。為高功率LED供電，最好方法是電流源。因為絕大部分能源，包括電池、發(fā)電機和工業(yè)干線都是電壓源，而很少有電流源。于是就需要在LED和供電電源中間插入一些電路。這個電路像串聯(lián)電阻一樣簡單，但是考慮到能源效率和其他因素，最好的選擇是高效率帶電壓反饋的電流源。由于LED電流大于0.35A，因此電感式繼電器通常是最好選擇。
為了達到高效率和小型化的目標，基于單功率IC繼電器衍生出了一系列的電路。電路設計者通過減少使用體積較大的部件，如外部晶體管，開關，大容量電容以及限流電阻等，并在維持正常運作的情形下盡可能將高亮度的燈光傳遞地越遠越好，以此來達到高效及小型化的目標。
圖1、圖2、圖3中的電路均適合應用在由三到四塊鎳氫電池或鎳鎘電池組成的電源中。圖4和圖5中的電路適合于供電系統(tǒng)線電壓為12V、24V或是42V的車輛中。圖4和圖5的電路也可以應用在包括控制和應急子系統(tǒng)的24V電源和通信所用的－48V電源的工業(yè)系統(tǒng)中。
這些電路的設計者們在設計時都有同樣的觀念：完整的單一模式集成電路繼電器和微功率運放。運放將最終的1.25V反饋到集成電路上。雖然節(jié)點都是以標準電壓拓撲為目標，但是運放可以使其與微弱的電流敏感電壓及有些許不同的電流拓撲結構相匹配。所有的這些電路都不需要外部的功率開關。這種設計去除了通常在開關電源附近的大濾波電容，因為沒有必要對LED電流的高頻諧波進行平滑處理。所有電路最普遍的選擇是在運放的輸入端引入可調偏置，依靠IC通過一個電阻和一個由內部調節(jié)電位計，來增加亮度調節(jié)能力。
高頻開關調節(jié)器給LED的基本調節(jié)電路供電。如圖1所示電路，輸入電壓范圍為3.6V 到6.5V，可以提供高達1A的電流驅動LED，并用一個電流敏感電阻來控制電流調節(jié)閉環(huán)回路。圖2中所示電路與圖1中比較類似，但是電流敏感電阻被電感的寄生電阻代替。與圖1中電路功能相同，圖2電路也可以將3.6V 到6.5V的輸入電壓轉換成驅動LED高達1A的電流。
對圖3的單LED電路，MAX1685的啟動電源決定了輸入范圍，最低到2.7V。相對于圖1和圖2中的1A電路而言，最大電流能力為0.5A。輸入電壓上限仍為6.5V。一旦圖3電路開始運行，即使輸入電壓降到1.7V，仍可以驅動LED。以上三種電路可以應用在由堿性電池、三或四塊鎳氫/鎳鎘電池、鋰電池驅動的前燈、手電筒和其它便攜式燈光設備中。
圖4和圖5中電路適用于輸入電壓為8V到50V。假定一個12V系統(tǒng)中的所有部件都完全確定，由于集成電路輸入端電壓VIN最高可以達到76V，因此這兩個電路有負載抑制。如果將輸入電壓的最小值提高到11.5V，那么最大輸出電流為1A，最多可以驅動串連的三個LED。圖4與圖5中的電路很相似，除了圖5中用電感作為電流敏感元件。這樣的不利之處是由于銅的溫度系數(shù)較大，造成輸出電流對環(huán)境的依賴性很大。電感線圈是由銅纏繞而成的，外界溫度變化1°C，它的直流阻抗就會變化千分之3.9。因此，當外界溫度變化10°C的時候，輸出電流就會減少大約4%。
英文原文：
High-power LED drivers require no external switches
Suiting a variety of applications, these circuits transform a switching regulator into a current source for driving power LEDs.
Alfredo H Saab and Steve Logan, Maxim Integrated Products, Sunnyvale, CA; Edited by Charles H Small and Fran Granville -- EDN, 7/19/2007
As the latest generation of new LEDs achieves higher levels of power and efficiency, use of these devices extends to new areas, such as flashlights and vehicular applications. High-power LEDs are finding use even in ambient lighting, long the sole province of incandescent bulbs and fluorescent tubes. A current source is the best way to power LEDs. Becausemost energy sources, including batteries, generators, and industrial mains, look more like voltage sources than current sources, LEDs require that you insert some electronic circuitry between them and the source of power. This circuitry can be as simple as a series resistor, but a better choice, considering energy efficiency and other factors, is a high-efficiency, voltage-fed current source. For LEDs with currents greater than 0.35A, an i
nductive switching regulator is usually the best choice.
This Design Idea presents a series of circuits based on single-power-IC switching regulators, with efficiency and miniaturization as the main objectives. The circuits’ designers approach these objectives by minimizing the use of large components, such as external power transistors, switches, high-value capacitors, and current-sense resistors, and by maintaining regular operation by delivering constant, high-intensity light over as extended a range as possible.
The circuits in figure 1, figure 2, and figure 3 are suitable for applications in which the power source comprises three or four alkaline, NiMH (nickel-metal-hydride), or NiCd (nickel-cadmium) cells. Those in figure 4 and figure 5 are for vehicular applications in which the nominal line voltage for the power-distribution system is 12, 24, or 42V. The circuits of figure 4 and figure 5 are also useful in industrial systems that include a 24V distribution line for control andemergency subsystems and in telecom applications for which the system power is distributed as a –48V line.
The designers of these circuits based them on the same concept: a fully integrated, single-die-IC switching regulator and a micropower operational amplifier. The op amp drives the 1.25V feedback terminal on the IC. Although that node targets the topology of a standard voltage regulator, the op amp matches it to the much smaller current-sense voltage and the slightly different topology of a current regulator. None of the circuits requires the use of external power switches. The design eliminates the use of the large-valued filter capacitors you usually find in a switching regulator, because there is no need to smooth out high-frequency ripple in the LED current. Common to all circuits is the option of adding a dimming capability by introducing adjustable bias at an op-amp input through a resistor and a potentiometer powered from the internal regulator—the VD or CVL terminal, depending on the IC.
A high-frequency switching regulator powers the basic regulator circuit for LEDs (Figure 1). It operates with input voltages of 3.6 to 6.5V, drives a single LED with currents as high as 1A, and uses a current-sense resistor to control the current-regulation loop. The circuit of Figure 2 is similar, but, in place of a current-sense resistor, it employs the parasitic resistance of the inductor as a current-sensing element. Like the circuit in Figure 1, it operates with 3.6 to 6.5V inputs and drives one LED with currents as high as 1A.
For the single-LED circuit of Figure 3, the starting voltage of the MAX1685 defines the input range, which goes as low as 2.7V. Its maximum current capability is 0.5A versus 1A for the circuits in figure 1 and figure 2. The upper operating limit remains 6.5V. Once this circuit is operating, it maintains power to the LED even for input voltages as low as 1.7V. Applications for the circuits of figure 1, figure 2, and figure 3 include headlights, flashlights, and any other portable lights powered by three or four alkaline primary cells, three or four NiMH/NiCd secondary cells, or a single lithium secondary cell.
The circuits of figure 4 and figure 5 operate over 8 to 50V. Assuming a 12V system in which all the components are properly specified, these circuits can survive load dumps, thanks to the 76V absolute maximum rating for the IC’s input-power terminal, VIN. The maximum available current is 1A, and the circuits can drive as many as three LEDs in series, provided that you increase the lower limit of the operating range to 11.5V. These two circuits are similar, except for the use of the inductor resistance as a current sensor in Figure 5. The disadvantage of using the inductor resistance in this way is the resulting dependence of output current on temperature, due to the large temperature coefficient of copper resistivity. The inductorwinding is made of copper, and its dc resistance has a first-order temperature coefficient of 3.9 parts/1000/°C. As a result, the regulated current decreases about 4% for each 10°C increase in operating temperature. |
