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Light-emitting diodes accept become the lighting antecedent of best for about all beam applications. Incandescents, fluorescents, CFLs, and added types are crumbling abroad in favor of the absolute LED. LEDs are now brighter and use beneath ability than any added lighting source. And their prices accept decreased to an adequate level. The aftereffect has been added acceptance of LEDs in home, auto, and added applications.
Perhaps the capital claiming for designers is acquirements how to ability the LEDs. For some simple applications, it’s aloof applying a advanced bent voltage through a current-limiting resistor. However, active LED strings and authoritative accuracy crave abundant added adult circuitry. Then there are the affairs of active LCD backlighting. In addition, automotive applications appeal appropriate measures because of the acute power-supply altitude that occur. Here’s a attending at these applications and accompanying solutions.
Powering an LED
A aloft point to accent is that the LED accepted allegation be controlled to set the accuracy level. Therefore, the best adorable LED disciplinarian is a accepted source. A simple adjustment is apparent in Figure 1. The resistor sets the accepted level. Such beeline accepted sources are satisfactory for low-power applications, but they blow too abundant ability and calefaction back assorted LEDs are acclimated and aerial accuracy is necessary. As a result, designers accept begin that the best drivers are switch-mode types.
1. A linear-current-source LED disciplinarian works best in low-power applications.
Basic LED Disciplinarian Topologies
Modern LED disciplinarian circuits resemble a switch-mode regulator or dc-dc converter. Best accepted disciplinarian varieties use some of the aforementioned topologies as acclimated in these basal regulator dc-dc advocate circuits. Figure 2 shows the best accepted topologies, all of which use MOSFET switches.
2. Accepted dc-dc advocate regulator switch-mode topologies are the blade (a) and addition (b).
The blade disciplinarian (Fig. 2a), which is the best common, has an achievement that’s consistently lower than the input. The addition disciplinarian (Fig. 2b) produces an achievement that’s college than the input. The buck-boost can aftermath an achievement voltage that’s either college or lower than the ascribe voltage.
There are several variations of the buck-boost, such as the amphibian buck-boost, the SEPIC, and the Cuk. The SEPIC and Cuk both crave two inductors that add bulk and booty up added space. The SEPIC (shown in Fig. 4 below) is discussed later.
A accepted allegation in LED lighting is dimming—controlling the accuracy over a ambit from abounding off to the abounding brightness. Assorted concealment techniques accept been developed appliance both beeline and agenda methods. In both, the accuracy is bent by the boilerplate advanced accepted in the LEDs. Over the accepted ambit of best LEDs, the accord amid boilerplate accepted and accuracy is linear. Only at the aerial and low levels of accepted does the accord become nonlinear.
3. A linear-current regulator disciplinarian IC sets the accuracy by applying a dc ascribe ascendancy voltage.
Linear concealment is able with a linear-current regulator IC (Fig. 3). A dc voltage controls the circuit, bearing a beeline accepted and accuracy variation. Concealment ratios from 10:1 to 250:1 can be realized. While beeline concealment is the simplest and easiest to implement, it may not fit the application. Some use cases crave a added concealment adjustment that could be as aerial as 1000 or added to 1. Furthermore, beeline concealment may aftermath an causeless blush about-face in blush LEDs over the concealment range. It’s accessible to abate that with pulse-width-modulation (PWM) dimming.
The boilerplate accepted in the LEDs can be assorted with PWM by alteration the assignment aeon of the activated concealment pulses. The abundance of the pulses allegation be fast abundant to anticipate the animal eye from acquainted beam in the light. A beating abundance of 200 Hz or college is usually sufficient. The beating is activated to a switching regulator blazon of concealment IC; the switching abundance of the regulator is decidedly college than the concealment frequency. The adapted accepted achievement is activated anon to the LED. Added arrange use an alien MOSFET to acceleration up the concealment acknowledgment and abbreviate nonlinear accuracy response. Figure 4 shows one arrangement.
4. LED concealment is able with a switching accepted regulator appliance a capricious duty-cycle PWM ascribe from a microcontroller or added source. The alien apparatus accredit faster switching at college frequencies.
LEDs are broadly acclimated to backlight aqueous clear displays (LCDs), such as those acclimated for archetypal laptops. One accepted adjustment is an adjustment of 60 white LEDs fabricated up of six strings of 10 series-connected LEDs. Each cord needs a disciplinarian that can be activated for dimming. Ablaze guides and diffusers are acclimated to advance the ablaze analogously beyond the screen.
Backlights allegation be cool ablaze to accord acceptable accuracy to an LCD. As a result, they about absorb a ample bulk of power. Back selecting a driver, ability is an important factor.
The two basal means to ascendancy concealment are beeline accepted aberration and PWM. The beeline adaptation works well, but it’s bound because of the adversity in carrying actual baby currents. The nonlinearity of the LEDs banned the minimum accuracy level.
A broadly acclimated disciplinarian is the dc-dc advocate with PWM. The PWM access varies the assignment aeon of activated pulses to ascendancy the boilerplate LED accepted and the brightness. Admitting this adjustment can accomplish actual baby assignment cycles and acceptable low accuracy resolution, it suffers from disability and aerial switching babble at the college assignment cycles. The band-aid is a amalgam access that uses both beeline accepted aberration and PWM methods to accomplish the optimum concealment ascendancy over the abounding ambit of brightness. There are LED disciplinarian ICs that absorb this amalgam concealment method.
Most of the lighting in new cartage is via the LED. Accepted automotive applications are daytime active lights, fog and headlamps, and about-face signals. Because of the advanced achievement aberration of the capital 12-V array antecedent beneath altered conditions, appropriate disciplinarian circuits are bare to advance brightness.
The capital ability bus voltage is usually a bit over 13 V during accustomed array charging. Beneath balmy crank conditions, this voltage can bead to about 7 or 8 V. And in amount dump conditions, the voltage may acceleration to 16 V. Transients can aiguille at about 42 V. In general, LED drivers allegation to be acceptable regulators.
One of the best regulator/dc-dc advocate topologies to accord with this book is the SEPIC. Its agreement produces a connected achievement voltage alike admitting the ascribe voltage may alter aloft or beneath the achievement voltage. Figure 5 shows the basal configuration. Its key appropriate is two inductors. A adopted adjustment is to use accompanying inductors that anatomy a agent with a 1:1 ratio. Such an access makes the ambit added able and minimizes amplitude on the PCB.
5. A SEPIC regulator disciplinarian ambit is a acceptable fit for automotive lighting applications. Its buck-boost adequacy readily deals with the advanced voltage fluctuations of the array accumulation bus.
A archetypal switching abundance is 350 kHz. Back the MOSFET Q1 turns on, the ascribe voltage is activated to the ascribe inductor autumn activity as a alluring field. The 1:1 coupling puts the ascribe voltage beyond the additional inductor; therefore, the alternation capacitor C2 accuse to the ascribe voltage. The achievement voltage is supplied by the antecedent allegation on the achievement capacitor C3.
When the MOSFET is switched off, the polarities of the inductor voltages reverse. During this time, the blow inductor L2 accuse the achievement capacitor. The ascribe voltage, alternation inductor L1, and alternation capacitor C2 voltages amalgamate and allegation the achievement capacitor C3 to abutment the achievement voltage. Feedback and PWM ascendancy of Q1 aren’t shown.
One SEPIC LED disciplinarian IC on the bazaar is Texas Instruments’ TPS92691-Q1. It can accumulation a connected 12 V at 3 A beneath broadly capricious ascribe conditions. Also accessible is the PMP15027 advertence architecture appliance this IC.
Learn Added to Architecture Better
TI offers a complete alternation of training videos that present the fundamentals of LEDs and disciplinarian topologies, additional analog and PWM concealment methodologies. Added videos awning LCD operation and active as able-bodied as backlighting. A video on automotive applications discusses the SEPIC disciplinarian cartography and explains the architecture challenges in ambidextrous with situations like algid crank, balmy crank, or amount dump. Learn aloof what you allegation at your own pace.
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