LED照明知識(shí)：PWM調(diào)光 LED照明論文（中英文對(duì)照）

1、led照明知識(shí)(第四部分)：pwm調(diào)光在本系列的第一部分中，我們了解了led光源及其驅(qū)動(dòng)需求的基本知識(shí)。在第二部分中，我們討論了當(dāng)一個(gè)常電流buck轉(zhuǎn)換器可以被用作led轉(zhuǎn)換模式驅(qū)動(dòng)的時(shí)候，為什么它能成為您的首選。在第三部分中，我們研究了大型led顯示及在其它轉(zhuǎn)換拓?fù)渲械膽?yīng)用空間?，F(xiàn)在，在本系列的總結(jié)篇中，作者將視角轉(zhuǎn)向如何最好地實(shí)現(xiàn)調(diào)光功能。 不管你用buck, boost, buck-boost還是線性調(diào)節(jié)器來驅(qū)動(dòng)led，它們的共同思路都是用驅(qū)動(dòng)電路來控制光的輸出。一些應(yīng)用只是簡(jiǎn)單地來實(shí)現(xiàn)“開”和“關(guān)”地功能，但是更多地應(yīng)用需求是要從0到100%調(diào)節(jié)光的亮度，而且經(jīng)常要有很高的精度。設(shè)計(jì)者

2、主要有兩個(gè)選擇：線性調(diào)節(jié)led電流（模擬調(diào)光），或者使用開關(guān)電路以相對(duì)于人眼識(shí)別力來說足夠高的頻率工作來改變光輸出的平均值（數(shù)字調(diào)光）。使用脈沖寬度調(diào)制（pwm）來設(shè)置周期和占空度（圖1）可能是最簡(jiǎn)單的實(shí)現(xiàn)數(shù)字調(diào)光的方法，并且buck調(diào)節(jié)器拓?fù)渫軌蛱峁┮粋€(gè)最好的性能。 圖1：使用pwm調(diào)光的led驅(qū)動(dòng)及其波形。推薦的pwm調(diào)光 模擬調(diào)光通?？梢院芎?jiǎn)單的來實(shí)現(xiàn)。我們可以通過一個(gè)控制電壓來成比例地改變led驅(qū)動(dòng)的輸出。模擬調(diào)光不會(huì)引入潛在的電磁兼容/電磁干擾（emc/emi）頻率。然而，在大多數(shù)設(shè)計(jì)中要使用pwm調(diào)光，這是由于led的一個(gè)基本性質(zhì)：發(fā)射光的特性要隨著平均驅(qū)動(dòng)電流而偏移。對(duì)于單色

3、led來說，其主波長(zhǎng)會(huì)改變。對(duì)白光led來說，其相關(guān)顏色溫度（cct）會(huì)改變。對(duì)于人眼來說，很難察覺到紅、綠或藍(lán)led中幾納米波長(zhǎng)的變化，特別是在光強(qiáng)也在變化的時(shí)候。但是白光的顏色溫度變化是很容易檢測(cè)的。 大多數(shù)led包含一個(gè)發(fā)射藍(lán)光譜光子的區(qū)域，它透過一個(gè)磷面提供一個(gè)寬幅可見光。低電流的時(shí)候，磷光占主導(dǎo)，光趨近于黃色。高電流的時(shí)候，led藍(lán)光占主導(dǎo)，光呈現(xiàn)藍(lán)色，從而達(dá)到了一個(gè)高cct。當(dāng)使用一個(gè)以上的白光led的時(shí)候，相鄰led的cct的不同會(huì)很明顯也是不希望發(fā)生的。同樣延伸到光源應(yīng)用里，混合多個(gè)單色led也會(huì)存在同樣的問題。當(dāng)我們使用一個(gè)以上的光源的時(shí)候，led中任何的差異都會(huì)被察覺到。

4、led生產(chǎn)商在他們的產(chǎn)品電氣特性表中特別制定了一個(gè)驅(qū)動(dòng)電流，這樣就能保證只以這些特定驅(qū)動(dòng)電流來產(chǎn)生的光波長(zhǎng)或cct。用pwm調(diào)光保證了led發(fā)出設(shè)計(jì)者需要的顏色，而光的強(qiáng)度另當(dāng)別論。這種精細(xì)控制在rgb應(yīng)用中特別重要，以混合不同顏色的光來產(chǎn)生白光。 從驅(qū)動(dòng)ic的前景來看，模擬調(diào)光面臨著一個(gè)嚴(yán)峻的挑戰(zhàn)，這就是輸出電流精度。幾乎每個(gè)led驅(qū)動(dòng)都要用到某種串聯(lián)電阻來辨別電流。電流辨別電壓（vsns）通過折衷低能耗損失和高信噪比來選定。驅(qū)動(dòng)中的容差、偏移和延遲導(dǎo)致了一個(gè)相對(duì)固定的誤差。要在一個(gè)閉環(huán)系統(tǒng)中降低輸出電流就必須降低vsns。這樣就會(huì)反過來降低輸出電流的精度，最終，輸出電流無法指定、控制或保證

5、。通常來說，相對(duì)于模擬調(diào)光，pwm調(diào)光可以提高精度，線性控制光輸出到更低級(jí)。 調(diào)光頻率vs對(duì)比度 led驅(qū)動(dòng)對(duì)pwm調(diào)光信號(hào)的不可忽視的回應(yīng)時(shí)間產(chǎn)生了一個(gè)設(shè)計(jì)問題。這里主要有三種主要延遲（圖2）。這些延遲越長(zhǎng)，可以達(dá)到的對(duì)比度就越低（光強(qiáng)的控制尺度）。 圖2：調(diào)光延遲。如圖所示，tn表示從時(shí)間邏輯信號(hào)vdim提升到足以使led驅(qū)動(dòng)開始提高輸出電流的時(shí)候的過渡延遲。另外，tsu輸出電流從零提升到目標(biāo)級(jí)所需要的時(shí)間，相反，tsn是輸出電流從目標(biāo)級(jí)下降到零所需要的時(shí)間。一般來說，調(diào)光頻率（fdim）越低，對(duì)比度越高，這是因?yàn)檫@些固定延遲消耗了一小部分的調(diào)光周期（tdim）。fdim的下限大概是120

6、hz，低于這個(gè)下限，肉眼就不會(huì)再把脈沖混合成一個(gè)感覺起來持續(xù)的光。另外，上限是由達(dá)到最小對(duì)比度來確定的。 對(duì)比度通常由最小脈寬值的倒數(shù)來表示： cr = 1 / ton-min : 1 這里ton-min = td + tsu。在機(jī)器視覺和工業(yè)檢驗(yàn)應(yīng)用中常常需要更高的pwm調(diào)光頻率，因?yàn)楦咚傧鄼C(jī)和傳感器需要遠(yuǎn)遠(yuǎn)快于人眼的反應(yīng)時(shí)間。在這種應(yīng)用中，led光源的快速開通和關(guān)閉的目的不是為了降低輸出光的平均強(qiáng)度，而是為了使輸出光與傳感器和相機(jī)時(shí)間同步。用開關(guān)調(diào)節(jié)器調(diào)光 基于開關(guān)調(diào)節(jié)器的led驅(qū)動(dòng)需要一些特別考慮，以便于每秒鐘關(guān)掉和開啟成百上千次。用于通常供電的調(diào)節(jié)器常常有一個(gè)開啟或關(guān)掉針腳來供邏輯電平

7、pwm信號(hào)連接，但是與此相關(guān)的延遲（td）常常很久。這是因?yàn)楣柙O(shè)計(jì)強(qiáng)調(diào)回應(yīng)時(shí)間中的低關(guān)斷電流。而驅(qū)動(dòng)led的專用開關(guān)調(diào)節(jié)則相反，當(dāng)開啟針腳為邏輯低以最小化td時(shí)，內(nèi)部控制電路始終保持開啟，然而當(dāng)led關(guān)斷的時(shí)候，控制電流卻很高。 用pwm來優(yōu)化光源控制需要最小化上升和下降延遲，這不僅是為了達(dá)到最好的對(duì)比度，而且也為了最小化led從零到目標(biāo)電平的時(shí)間（這里主導(dǎo)光波長(zhǎng)和cct不能保證）。標(biāo)準(zhǔn)開關(guān)調(diào)節(jié)器常常會(huì)有一個(gè)緩開和緩關(guān)的過程，但是led專用驅(qū)動(dòng)可以做所有的事情，其中包括降低信號(hào)轉(zhuǎn)換速率的控制。降低tsu 和 tsn要從硅設(shè)計(jì)和開關(guān)調(diào)節(jié)器拓?fù)鋬煞矫嫒胧帧?buck調(diào)節(jié)器能夠保持快速信號(hào)轉(zhuǎn)換而又優(yōu)

8、于所有其它開關(guān)拓?fù)渲饕袃蓚€(gè)原因。其一，buck調(diào)節(jié)器是唯一能夠在控制開關(guān)打開的時(shí)候?yàn)檩敵龉╇姷拈_關(guān)變換器。這使電壓模式或電流模式pwm（不要與pwm調(diào)光混淆）的buck調(diào)節(jié)器的控制環(huán)比boost調(diào)節(jié)器或者各種buck-boost拓?fù)涓??？刂崎_關(guān)開啟的過程中，電力傳輸同樣可以輕易地適應(yīng)滯環(huán)控制，甚至比最好的電壓模式或電流模式的控制環(huán)還要快。其二，buck調(diào)節(jié)器的電導(dǎo)在整個(gè)轉(zhuǎn)換周期中連在了輸出上。這樣保證了一個(gè)持續(xù)輸出電流，也就是說，輸出電容被刪減掉。沒有了輸出電容，buck調(diào)節(jié)器成了一個(gè)真正的高阻抗電流源，它可以很快達(dá)到輸出電壓。cuk和zeta轉(zhuǎn)換器可以提供持續(xù)的輸出電感，但是當(dāng)更慢的控制

9、環(huán)（和慢頻）被納入其中的時(shí)候，它們會(huì)落后。 比開啟針腳更快 即使是一個(gè)單純的無輸出電容的滯后buck調(diào)節(jié)器，也不能滿足某些pwm調(diào)光系統(tǒng)的需要。這些應(yīng)用需要高pwm調(diào)光頻率和高對(duì)比度，這就分別需要快速信號(hào)轉(zhuǎn)換率和短延遲時(shí)間。對(duì)于機(jī)器視覺和工業(yè)檢驗(yàn)來說，系統(tǒng)實(shí)例需要很高的性能，包括lcd板的背光和投影儀。在某些應(yīng)用中，pwm調(diào)光頻率必須超過音頻寬，達(dá)到25khz或者更高。當(dāng)總調(diào)光周期降低到微秒級(jí)時(shí)，led電流總上升和下降時(shí)間（包括傳輸延遲），必須降低到納秒級(jí)。 讓我們來看看一個(gè)沒有輸出電容的快速buck調(diào)節(jié)器。打開和關(guān)斷輸出電流的延遲來源于ic的傳輸延遲和輸出電感的物理性質(zhì)。對(duì)于真正的高速pwm

10、調(diào)光，這兩個(gè)問題都需要解決。最好的方法就是要用一個(gè)電源開關(guān)與led鏈并聯(lián)（圖3）。要關(guān)掉led，驅(qū)動(dòng)電流要經(jīng)過開關(guān)分流，這個(gè)開關(guān)就是一個(gè)典型的n-mosfet。ic持續(xù)工作，電感電流持續(xù)流動(dòng)。這個(gè)方法的主要缺點(diǎn)是當(dāng)led關(guān)閉的時(shí)候，電量被浪費(fèi)掉了，甚至在這個(gè)過程中，輸出電壓下降到電流偵測(cè)電壓。 圖3：分流電路及其波形。用一個(gè)分流fet調(diào)光會(huì)引起輸出電壓快速偏移，ic的控制環(huán)必須回應(yīng)保持常電流的請(qǐng)求。就像邏輯針腳調(diào)光一樣，控制環(huán)越快，回應(yīng)越好，帶有滯環(huán)控制的buck調(diào)節(jié)器就會(huì)提供最好的回應(yīng)。 用boost和buck-boost的快速pwm boost調(diào)節(jié)器和任何buck-boost拓?fù)涠疾贿m合p

11、wm調(diào)光。這是因?yàn)樵诔掷m(xù)傳導(dǎo)模式中（ccm），每個(gè)調(diào)節(jié)器都展示了一個(gè)右半平面零，這就使它很難達(dá)到時(shí)鐘調(diào)節(jié)器需要的高控制環(huán)帶寬。右半平面零的時(shí)域效應(yīng)也使它更難在boost或者buck-boost電路中使用滯后控制。另外，boost調(diào)節(jié)器不允許輸出電壓下降到輸入電壓以下。這個(gè)條件需要一個(gè)輸入端短電路并且使利用一個(gè)并聯(lián)fet實(shí)現(xiàn)調(diào)光變得不可能。在buck-boost拓?fù)渲?，并?lián)fet調(diào)光仍然不可能或者不切實(shí)際，這是因?yàn)樗枰粋€(gè)輸出電容（sepic，buck-boost和flyback），或者輸出短電路（cuk和zeta）中的未受控制得輸入電感電流。當(dāng)需要真正快速pwm調(diào)光的時(shí)候，最好的解決方案是一

12、個(gè)二級(jí)系統(tǒng)，它利用一個(gè)buck調(diào)節(jié)器作為第二led驅(qū)動(dòng)級(jí)。如果空間和成本不允許的時(shí)候，下一個(gè)最好的原則就是一個(gè)串聯(lián)開關(guān)（圖4）。 圖4：帶有串聯(lián)dim開關(guān)的boost調(diào)節(jié)器。led電流可以被立即切斷。另外，必須要特別考慮系統(tǒng)回應(yīng)。這樣一個(gè)開路事實(shí)上是一個(gè)快速外部退荷暫態(tài)，它斷開了反饋環(huán)，引起了調(diào)節(jié)器輸出電壓的無界上升。為了避免因?yàn)檫^壓失敗，我們需要輸出鉗制電路和/或誤差放大器。這種鉗制電路很難用外部電路實(shí)現(xiàn)，因此，串聯(lián)fet調(diào)光只能用專用boost/buck-boost led驅(qū)動(dòng)ic來實(shí)現(xiàn)。 總而言之，led光源的單純控制需要設(shè)計(jì)的初始階段就要非常小心。光源越復(fù)雜，就越要用pwm調(diào)光。這就需

13、要系統(tǒng)設(shè)計(jì)者謹(jǐn)慎思考led驅(qū)動(dòng)拓?fù)洹uck調(diào)節(jié)器為pwm調(diào)光提供了很多優(yōu)勢(shì)。如果調(diào)光頻率必須很高或者信號(hào)轉(zhuǎn)換率必須很快，或者二者都需要，那么buck調(diào)節(jié)器就是最好的選擇。下頁(yè)為英文原文參考：a matter of light, part 4 - pwm dimming by sameh sarhan and chris richardson, national semiconductor in part one of this series, we looked at the basics of led lighting sources and their driving requireme

14、nts. in part two, we discussed why a constant-current buck converter should be your first preference when it comes to switch-mode led drivers. in part 3, we investigated larger led displays and the applications space for other converter topologies. here in the concluding part of this series, the aut

15、hors take a look at how to best implement the dimming function. whether you drive leds with a buck, boost, buck-boost or linear regulator, the common thread is drive circuitry to control the light output. a few applications are as simple as on and off, but the greater number of applications call for

16、 dimming the output between zero and 100 percent, often with fine resolution. the designer has two main choices: adjust the led current linearly (analog dimming), or use switching circuitry that works at a frequency high enough for the eye to average the light output (digital dimming). using pulse-w

17、idth modulation (pwm) to set the period and duty cycle (fig. 1) is perhaps the easiest way to accomplish digital dimming, and a buck regulator topology will often provide the best performance. figure 1: led driver using pwm dimming, with waveforms.pwm dimming preferred analog dimming is often simple

18、r to implement. we vary the output of the led driver in proportion to a control voltage. analog dimming introduces no new frequencies as potential sources of emc/emi. however, pwm dimming is used in most designs, owing to a fundamental property of leds: the character of the light emitted shifts in p

19、roportion to the average drive current. for monochromatic leds, the dominant wavelength changes. for white leds, the correlated color temperature (cct) changes. its difficult for the human eye to detect a change of a few nanometers in a red, green, or blue led, especially when the light intensity is

20、 also changing. a change in color temperature of white light, however, is easily detected. most white leds consist of a die that emits photons in the blue spectrum, which strike a phosphor coating that in turn emits photons over a broad range of visible light. at low currents the phosphor dominates

21、and the light tends to be more yellow. at high currents the blue emission of the led dominates, giving the light a blue cast, leading to a higher cct. in applications with more than one white led, a difference in cct between two adjacent leds can be both obvious and unpleasant. that concept extends

22、to light sources that blend light from multiple monochromatic leds. when we have more than one light source, any difference between them jars the senses. led manufacturers specify a certain drive current in the electrical characteristics tables of their products, and they guarantee the dominant wave

23、length or cct only at those specified currents. dimming with pwm ensures that the leds emit the color that the lighting designer needs, regardless of the intensity. such precise control is particularly important in rgb applications where we blend light of different colors to produce white. from the

24、driver ic perspective, analog dimming presents a serious challenge to the output current accuracy. almost every led driver uses a resistor of some type in series with the output to sense current. the current-sense voltage, vsns, is selected as a compromise to maintain low power dissipation while kee

25、ping a high signal-to-noise ratio (snr). tolerances, offsets, and delays in the driver introduce an error that remains relatively fixed. to reduce output current in a closed-loop system, vsns, must be reduced. that in turn reduces the output current accuracy and ultimately the output current cannot

26、be specified, controlled, or guaranteed. in general, dimming with pwm allows more accurate, linear control over the light output down to much lower levels than analog dimming. dimming frequency vs. contrast ratio the led drivers finite response time to a pwm dimming signal creates design issues. the

27、re are three main types of delay (fig. 2). the longer these delays, the lower the achievable contrast ratio (a measure of control over lighting intensity). figure 2: dimming delays.as shown, tn represents the propagation delay from the time logic signal vdim goes high to the time that the led driver

28、 begins to increase the output current. in addition, tsu is the time needed for the output current to slew from zero to the target level, and tsn is the time needed for the output current to slew from the target level back down to zero. in general, the lower the dimming frequency, fdim, the higher c

29、ontrast ratio, as these fixed delays consume a smaller portion of the dimming period, tdim.the lower limit for fdim is approximately 120 hz, below which the eye no longer blends the pulses into a perceived continuous light. the upper limit is determined by the minimum contrast ratio that is required

30、. contrast ratio is typically expressed as the inverse of the minimum on-time, i.e., cr = 1 / ton-min : 1 where ton-min = td + tsu. applications in machine vision and industrial inspection often require much higher pwm dimming frequencies because the high-speed cameras and sensors used respond much

31、more quickly than the human eye. in such applications the goal of rapid turn-on and turn-off of the led light source is not to reduce the average light output, but to synchronize the light output with the sensor or camera capture times. dimming with a switching regulator switching regulator-based le

32、d drivers require special consideration in order to be shut off and turned on at hundreds or thousands of times per second. regulators designed for standard power supplies often have an enable pin or shutdown pin to which a logic-level pwm signal can be applied, but the associated delay, td, is ofte

33、n quite long. this is because the silicon design emphasizes low shutdown current over response time. dedicated switching regulations for driving leds will do the opposite, keeping their internal control circuits active while the enable pin is logic low to minimize td, while suffering a higher operat

34、ing current while the leds are off. optimizing light control with pwm requires minimum slew-up and slew-down delays not only for best contrast ratio, but to minimize the time that the led spends between zero and the target level (where the dominant wavelength and cct are not guaranteed). a standard

35、switching regulator will have a soft-start and often a soft-shutdown, but dedicated led drivers do everything within their control to reduce these slew rates. reducing tsu and tsn involves both the silicon design and the topology of switching regulator that is used. buck regulators are superior to a

36、ll other switching topologies with respect to fast slew rates for two distinct reasons. first, the buck regulator is the only switching converter that delivers power to the output while the control switch is on. this makes the control loops of buck regulators with voltage-mode or current-mode pwm (n

37、ot to be confused with the dimming via pwm) faster than the boost regulator or the various buck-boost topologies. power delivery during the control switchs on-time also adapts easily to hysteretic control, which is even faster than the best voltage-mode or current-mode control loops. second, the buc

38、k regulators inductor is connected to the output during the entire switching cycle. this ensures a continuous output current and means that the output capacitor can be eliminated. without an output capacitor the buck regulator becomes a true, high impedance current source, capable of slewing the out

39、put voltage very quickly. cuk and zeta converters can claim continuous output inductors, but fall behind when their slower control loops (and lower efficiency) are factored in. faster than the enable pin even a pure hysteretic buck regulator without an output capacitor will not be capable of meeting

40、 the requirements of some pwm dimming systems. these applications need high pwm dimming frequency and high contrast ratio, which in turn requires fast slew rates and short delay times. along with machine vision and industrial inspection, examples of systems that need high performance include backlig

41、hting of lcd panels and video projection. in some cases the pwm dimming frequency must be pushed to beyond the audio band, to 25 khz or more. with the total dimming period reduced to a matter of microseconds, total rise and fall times for the led current, including propagation delays, must be reduce

42、d to the nanosecond range. consider a fast buck regulator with no output capacitor. the delays in turning the output current on and off come from the ics propagation delay and the physical properties of the output inductor. for truly high speed pwm dimming, both must be bypassed. the best way to acc

43、omplish this is by using a power switch in parallel with the led chain (fig. 3). to turn the leds off, the drive current is shunted through the switch, which is typically an n-mosfet. the ic continues to operate and the inductor current continues to flow. the main disadvantage of this method is that

44、 power is wasted while the leds are off, even through the output voltage drops to equal the current sense voltage during this time. figure 3: shunt fet circuit, with waveforms.dimming with a shunt fet causes rapid shifts in the output voltage, to which the ics control loop must respond in an attempt

45、 to keep the output current constant. as with logic-pin dimming, the faster the control loop, the better the response, and buck regulators with hysteretic control provide the best response. fast pwm with boost and buck-boost neither the boost regulator nor any of the buck-boost topologies are well s

46、uited to pwm dimming. thats because in the continuous conduction mode (ccm), each one exhibits a right-half plane zero, which makes it difficult to achieve the high control loop bandwidth needed in clocked regulators. the time-domain effects of the right-half plane zero also make it much more diffic

47、ult to use hysteretic control for boost or buck-boost circuits. in addition, the boost regulator cannot tolerate an output voltage that falls below the input voltage. such a condition causes a short circuit at the input, and makes dimming with a parallel fet impossible. among the buck-boost topologi

48、es, parallel fet dimming is still impossible or at best impractical due to the requirement for an output capacitor (the sepic, buck-boost and flyback), or the uncontrolled input inductor current during output short circuits (cuk and zeta). when true fast pwm dimming is required, the best solution is

49、 a two-stage system that uses a buck regulator as the second, led driving stage. when space and cost do not permit this approach, the next best choice is a series switch (fig. 4). figure 4: boost regulator with series dim switch.led current can be shut off immediately. on the other hand, special con

50、sideration must be given to the system response. such an open circuit is in effect a fast, extreme unloading transient that also disconnects the feedback loop and will cause the regulators output voltage to rise without bound. clamping circuits for the output and/or the error amplifier are required to prevent failure due to over-voltage. these clamps are difficult to realize with ex