LED驅(qū)動(dòng)需要不需要電容.docx

LED驅(qū)動(dòng)需不需要電容林偉濤信息工程學(xué)院Driving LEDs: To Cap or Not to CapIntroductionHigh-brightness LEDs are available today with forward currents more than 100 times greater than their predecessors. These new devices are not just high brightness, but are high power as well. Single die with dissipations of 5W and multi-die modules with power in excess of 25W are now available. The requirements of high efficiency and low dissipation dictate a switching power supply for this new generation of High-Brightness (HB), High-Power (HP) LEDs, as a voltage regulator and a current limiting resistor are no longer appropriate. High-brightness, high-power LEDs require a constant-current source to take full advantage of their ever-increasing luminous efficiency and vibrant, pure color. The topology of choice for this new breed of switching constant current sources is the basic buck converter. The most convincing argument for using a buck converter is the ease with which this simple DC-DC converter can be turned into a constant-current source. This article will explain the selection of, or possible exclusion of, an output capacitor when designing a buck regulator for constant-current drive of HB LEDs.Controlled CurrentThe buck regulator is uniquely suited to be a constant current driver because the output inductor is in series with the load. Regardless of whether a buck regulator is used as a voltage source or a current source, selection of the inductor forms the cornerstone of the system design. With an inductor in series with the output, the average inductor current is always equal to the average output current, and the buck converter naturally maintains control of the AC-current ripple. By definition, the LED drive is a constant load system; hence a large amount of output capacitance is not necessary to maintain VOduring load transients.No Output Cap Yields High Output ImpedanceIn theory, a perfect current source has infinite output impedance, allowing the voltage to slew infinitely fast in order to maintain a constant current. For switching regulator designers who have concentrated on voltage regulators, this concept may take a moment to sink in. Completely removing the output capacitor from a buck regulator forces the output impedance to depend on the inductor. Without any capacitance to oppose changes in VO, the output current (referred to as forward current, or IF) slew rate depends entirely upon the inductance, the input voltage, and the output voltage. (VO is equal to the combined forward voltage, VF, of each series-connected LED)LED manufacturers generally recommend a ripple current, IF, of 5% to 20% of the DC forward current. Over the typical switching regulator frequency range of 50 kHz to 2 MHz the ripple itself is not visible to the human eye. These limits come from increasing thermal losses at higher ripple current (a property of the LED semiconductor PN junction itself) and a practical limit to the inductance used. The percentages are similar to the recommended current ripple ratio in buck voltage regulators. Inductor selection for a fixed-frequency current regulator is therefore governed by the same equations as a voltage regulator:One difference is that the inductance used for current regulators without output capacitors tends to be higher because the drive currents for the emerging standards of 1W, 3W, and 5W HB LEDs are 350 mA, 700 mA, and 1A respectively. Modern buck voltage regulators tend to use inductors in the range of 0.1 H to 10 H with saturation currents from 5A to 50A. Current drivers at similar switching frequencies tend to require inductors ranging from 10 H to 1000 H and saturation currents ranging from 0.5A to 5A.The main goal of high output impedance is to create a system capable of responding to PWM dimming signals, the preferred method of controlling the light output of LEDs. The dimming signal might be applied to the enable pin of the regulator, in which case the output current can slew from zero to the target and back to zero without the delay of CO being charged and discharged. For even faster, higher resolution dimming, a shunt switch, usually a MOSFET, can be placed in parallel with the LED array, allowing the continuous flow of current at all times. Again, with no output capacitor to slow the slew rate, dimming frequencies into the 10s of kHz are possible. This is a critical requirement in applications such as backlighting of flat-panel displays, and the creation of white light using an RGB array.Using an Output Capacitor Reduces Size and CostSome amount of output capacitance can be useful as an AC current filter. Applications such as retrofitting of incandescent and halogen lights often require that the LED and driver be placed in a small space formerly occupied by a light bulb. Invariably the inductor is the largest, most expensive component after the LEDs themselves. For the sake of efficiency (especially important in cramped quarters), the designer generally chooses the lowest switching frequency that allows the solution (mostly the inductor) to fit. Allowing a large ripple current in the inductor and filtering the LED current results in a smaller, less expensive solution.For example, to drive a single white LED (VF3.5V) at 1A with a ripple current iFof 5% from an input of 12V at 500 kHz would require a 50 H inductor with a current rating of 1.1A. A typical ferrite core device that fits this application might be 10 mm square and 4.5 mm in height. In contrast, if the inductor ripple current is allowed to increase to 30% (typical for a low-current voltage regulator) then the inductance required is less than 10 H, and an inductor measuring 6.0 mm square and only 2.8 mm in height size can be used. The output capacitance required is calculated based on the dynamic resistance, rD, of the LED, the sense resistance, RSNS , and the impedance of the capacitor at the switching frequency, using the following expressions:ConclusionThe high brightness, high power LED represents the biggest change in lighting design since the introduction of fluorescent bulbs. Using LEDs requires a fundamental change in the complexity of electronics used for lighting systems. Currently a large portion of LED lighting design is retrofitting of incandescent, halogen, and fluorescent installations. Such systems rarely include sophisticated dimming control, and place a high value on small size. These are the applications where an output capacitor is a welcome addition to the driver circuit.In the future, the higher cost of LEDs for general lighting will be balanced by new levels of control over brightness, tone, and color. Lighting in homes and businesses will require fast PWM dimming, requiring current drivers to minimize or eliminate their output capacitance. These systems will draw upon experience from todays fast-dimming applications which have already shed the output capacitor to provide the best response time.LED驅(qū)動(dòng)需不需要電容現(xiàn)如今高量發(fā)光二極管的正向電流越來(lái)越大，遠(yuǎn)遠(yuǎn)高過(guò)原先二極管的百倍以上。這些新材料的LED燈已經(jīng)不單單只是高亮度了，而且功率越來(lái)越高。5W的單晶管損耗，甚至超過(guò)了25W的多晶片堆疊式的LED損耗現(xiàn)如今也是存在的。為滿足現(xiàn)如今的極高的LED驅(qū)動(dòng)效率和很低的損耗的要求，必須有新一代的高亮度的而且是高功率的開關(guān)電源驅(qū)動(dòng)器去實(shí)現(xiàn)。而以以往的僅僅靠電壓調(diào)節(jié)器和一個(gè)限流電阻的傳統(tǒng)驅(qū)動(dòng)器已經(jīng)沒(méi)法滿足現(xiàn)在的需求了。要求的高亮度而且高效率的發(fā)光二極管驅(qū)動(dòng)器需要一個(gè)穩(wěn)定的恒流源去保證能夠充分發(fā)揮光源的發(fā)光效率。同時(shí)能夠使光源的亮度和純度得到更充分的發(fā)揮。這種新型的自動(dòng)切換恒流源的拓?fù)涫枪こ淘O(shè)計(jì)中最基本的BUCK變換器。使用基本拓?fù)銪UCK變換器的最優(yōu)說(shuō)服力的理由是這個(gè)DC-DC變換器很簡(jiǎn)單便可以轉(zhuǎn)為恒流模式。這篇文章將解釋，選擇或者不使用一個(gè)輸出電容對(duì)設(shè)計(jì)一個(gè)高亮的發(fā)光二極管驅(qū)動(dòng)的BUCK恒流源將會(huì)產(chǎn)生的影響。被控電流BUCK變換器具有特有優(yōu)點(diǎn)對(duì)于恒流的LED的驅(qū)動(dòng)器，為什么？因?yàn)樵贐UCK變換器的基本結(jié)構(gòu)中，我們可以看到，他的電感是和負(fù)載相串聯(lián)的。先不管我們用BUCK變換器是作為恒流源使用還是作為電壓源使用，選擇具有電感串聯(lián)的形式設(shè)計(jì)驅(qū)動(dòng)系統(tǒng)是基礎(chǔ)。和一個(gè)輸出大電感串聯(lián)，這樣有一個(gè)優(yōu)點(diǎn)，就是電感的電流總是和負(fù)載的輸出電流相等，因此，BUCK變換器很容易控制輸出的交流紋波電流。LED驅(qū)動(dòng)器通過(guò)定義作為是一個(gè)恒定負(fù)載系統(tǒng)，當(dāng)負(fù)載發(fā)生變化的時(shí)候，大容量的輸出電容并不是維持穩(wěn)定輸出系統(tǒng)的必須條件。沒(méi)有輸出電容產(chǎn)生高輸出阻抗在理論上，一個(gè)完美的電流源必須擁有無(wú)限大的輸出電阻，可以保證極快的維持電流為一個(gè)常數(shù)。集中在研究電壓調(diào)節(jié)器的開關(guān)電源工程師在實(shí)現(xiàn)這些理論可能需要一些時(shí)間。從BUCK變換器中完全移除輸出電容，那么迫使完全靠電感來(lái)保證輸出阻抗。沒(méi)有用任何電容而要保證穩(wěn)定不變的輸出電壓VO，輸出電流（被成為正向電流或者IF）的變化率是完全取決于電感的、還有輸入電壓還有輸出電壓。（VO等同于連接起來(lái)的正向電壓，即串聯(lián)的LED壓降VF）LED制造商總是會(huì)推薦一個(gè)允許的紋波電流，IF，一般為5%20%的直流電流。在典型的開關(guān)電源穩(wěn)壓器，50KHZ到2MHZ的頻率范圍紋波，人類的眼睛是看不見的。這些在高的紋波電流時(shí)會(huì)有更高的熱損耗（LED半導(dǎo)體本質(zhì)是PN結(jié)），這些條件限制了電感的使用。這個(gè)百分率類似于BUCK電壓調(diào)節(jié)器中的紋波電流。電感器是按規(guī)定的頻率選擇的，因此，恒流器是和電壓調(diào)節(jié)器具有相同的方程的：L=VINVF=VIN-VFiLfsw一個(gè)不使用輸出電容只使用電感器的恒流源的不同之處在于其電感量更高一些，因?yàn)轵?qū)動(dòng)1W、3W、5W的 高亮的驅(qū)動(dòng)電流分別是350毫安、750毫安、和1安。現(xiàn)在的技術(shù)BUCK變換器傾向于使用電感量在0.1H到10H的電感，其飽和電流在5A到50A。當(dāng)前的電流源在現(xiàn)在的開關(guān)頻率范圍內(nèi)傾向于使用的電感量在10H到1000H，同樣其飽和電流的值是0.5A到5A。要達(dá)到高輸出阻抗的目標(biāo)，創(chuàng)建一個(gè)系統(tǒng)應(yīng)對(duì)PWM調(diào)光信號(hào)，實(shí)現(xiàn)控制LED是首先的方法。調(diào)光信號(hào)可能應(yīng)用是作用于調(diào)節(jié)器的使能端，可以實(shí)現(xiàn)輸出電流從零上升到目標(biāo)電流，再?gòu)哪繕?biāo)電流下降到零的目標(biāo)，而避免了因?yàn)橛休敵鲭娙菰跁r(shí)的充放電而導(dǎo)致的延時(shí)。為了有更快更高效的調(diào)節(jié)方式，通常在LED組上并聯(lián)一個(gè)MOSFET，可以保證電流總是連續(xù)的。同樣，沒(méi)有輸出電容去減緩轉(zhuǎn)換的速度，調(diào)