Optical feedback extends white LEDs' operating life
PWM-controller loop stabilizes light output.
Bjoy Santos, Intersil Corp, Milpitas, CA; Edited by Brad Thompson and Fran Granville -- EDN, January 18, 2007
Regardless of its color, an LED's light output varies as a function of forward current and ambient temperature. As Figure 1 shows, an LED's light output can vary by as much as 150% over its operating-current range. In response, a designer's first attempt to solve the problem focuses on driving the LEDs with a constant current. The most common white-LED-driver circuits use an inductor-based dc/dc boost-converter topology similar to the circuit in Figure 2. A current-feedback controller ensures that the voltage across current-sensing resistor R1 remains constant. As a result, the controller varies the voltage across the entire string to maintain the LEDs' current constant without regard to the LEDs' actual light output.
Driving series-connected white LEDs with a current source relies on the assumption that, at constant current, an LED's light output remains constant. Unfortunately, all LEDs exhibit a nonlinear decrease in brightness as a function of operating time. Although less obvious in colored LEDs that find use as indicators, the decrease in brightness of a white-LED-illuminator-array source becomes noticeable over an extended period. Brightness also varies as a function of temperature, which can affect an illuminator's performance over an extended-temperature range (upper curve, Figure 3).
To compensate for LED-output variations due to aging and temperature fluctuations, the control loop needs more information in addition to voltage or current data. Adding an ambient-light sensor and optical feedback to the control loop can ensure that a white LED's light output remains uniform and consistent over time and temperature variations. An optical sensor can measure the LED's light-output intensity and provide a feedback signal for the control loop, which can adjust the current to produce a relatively constant light output. As the LEDs' light outputs decrease, increased current compensates for aging and temperature-induced variations (lower curve, Figure 3).
The circuit in Figure 4 includes an optical-feedback loop based on Intersil's ISL29000 light-to-current optical sensor, IC2, which senses changes in the LEDs' light output and decreases the feedback voltage applied to IC1, the current controller, an Intersil EL7630. The pulse-width-modulated controller then increases the LED-drive current's duty cycle, boosting the LED current until the feedback voltage reaches its nominal value. As ambient temperature decreases, the LEDs' light output tends to increase, and IC2 delivers a higher feedback voltage to the controller, which responds by lowering the duty cycle to decrease the LEDs' current and thereby compensates for the decrease in temperature.
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well said Mark,
Any chance of scribbling a little circuit for guidance?
My expertice laid in coil winding techniques.
I suppose I could make a saturable reactor to do what you have suggested,a chip and a couple of components sounds highly cost effective.
GF
geoffrey whellams - 2009-14-3 14:50:00 PDT -
Well, having observed how Christmass tree light were wired.I cranked up the DC volts on a single jumbo bright LED and checked its opereating voltage. It seemed to like about 3 volts, so I figured if I put about 35 in series with a diode I could plonk the string accross the mains. The string seemed to work rather well except for the half wave ripple. which made them flicker at the 60 hrtz. I suppose I could play arround with a choke and capacitor to smooth things out a bit.
How complicated does this have to get? am I shortening
the operating life with a long string?
geoffrey whellams - 2008-15-12 10:31:00 PST -
Yes. The voltage across the series of LED is a decreasing function of temperature at constant current. However, it is not the voltage we are interested in. LEDs'' luminance is an increasing function of temperature at constant current. So we need to compensate for this relation. The only way to obtain luminance information is by using a light sensor and feedback the information to the driver. This optical feedback loop ensures that the light intensity of the LEDs remain constant over time and over temperature.
Bjoy Santos - 2007-7-2 23:00:00 PST -
This seems overly complicated. The voltage across the LEDs is a decreasing function of their temperature at constant current and provides a direct feedback voltage for the temperature of the LEDs if the 5 ohm resisters are put on the other end of the strings. A simple resistive divider and power MOS transistor then will compensate with negative feedback and filtering of the supply voltage. A single inverter stage (cmos) can drive the gate of the Power transistor if an inversion is needed.
Mark W. Levi - 2007-18-1 17:18:00 PST
