Closed-loop electronic controllers drive LED systems

Radiant Research, a UK-based company, was recently awarded a US patent (US 6,963,175) entitled "Illumination Control System", which describes a sophisticated method for driving arrays of LEDs. The patent is based on a GB patent (GB 2,369,730) granted in November 2002, with a priority date of August 2001.

Figure 1 Specifically, the patent describes an LED illumination control system that includes a microprocessor that controls a voltage-controlled current circuit driven by pulse-amplitude modulation (PAM). The system also includes a feedback loop to monitor current and temperature, to ensure fixtures operate within acceptable LED tolerances (see figure 1).

Radiant Research has licensed its patents and technology to a variety of lighting companies. At PLASA 2005, Tryka LED Ltd one of the first licensees, unveiled two new driver systems based on this technology, designed primarily for RGB mixing and effects lighting (see figure 2). Radiant is also working with a variety of other lighting manufacturers to develop a range of new lighting controllers and intelligent LED fixtures with superior performance to those currently available in the market.

Figure 2 Shah Islam, electronics design engineer at Radiant Research, explains, “Our technology enables the precise control of LED fixtures to maximize the light output performance and to extend the lifetime of fixtures without materially increasing the cost of the system. When a product that uses our technology is in the field, the manufacturer knows it will be responsive to environmental conditions, such as temperature, that usually reduce the LED life.”

Comparison of PWM and PAM

Keith Anderson, CEO of Radiant Research, describes the difference between PWM and PAM for driving LEDs (see Footnote for basic definitions of PAM and PWM):

LED driver control systems embedded within LED fixtures may be classified into three generations of increasing intelligence and sophistication:

• Generation 1: Open-loop electronic control systems are the simplest types, providing no control over the LED array. For example, if one or more LED(s) fail within a fixture and no action is taken by the control system then rapid failure can occur. This type of system will not prevent thermal or current runaway.

• Generation 2: Feedback electronic control systems are slightly more intelligent, being able to detect simple environmental conditions to safely power-down a fixture. For example, if an LED fixture begins to overheat due to a fault, the system will shut down the lamp.

• Generation 3: Closed-loop electronic control systems are active real-time systems that continuously monitor the fixture and its environment. The fixture output is adjusted according to a series of parameters, independent of conditions. For example, this type of controller can simultaneously monitor the LED temperature(s), room temperature, LED array brightness and color output, and ambient lighting conditions. It can then dynamically adjust the LED array output to maintain desired brightness, color, CRI and CCT values.

Most fixture manufacturers including Color Kinetics have historically based their lighting technology on generation 1 controllers and several now include thermostat (overheating) type functions. The Radiant Research patents are based on generation 2 & 3 controllers.

LED lighting fixture With PWM driving techniques, only frequency and duty cycle can be used to change LED intensity and this has limited application.

By using PAM, changes may be made to the frequency, duty cycle and more importantly the amplitude. This enables the control system to be used in applications other than architectural, stage & theatre lighting, such as machine vision lighting synchronization, mobile phone / digital camera flash and automotive.

The driver technology enables pulsing of LEDs for a variety of industrial applications, including traffic signals, positional lighting and displays. The patent offers the designer the opportunity to use pulsing, or to switch to DC, as both options are covered within the same technique. For example, if the duty cycle is set at 100%, the amplitude can still be controlled to vary the LED intensity.

Advantages of PAM over PWM

The technology allows for a change in the drive signal shape – for example from a square wave to a cycle wave – to enable a reduction in EMC and signal noise ratio (SNR). With PWM drivers, the square wave is fixed.

PAM/generation 3 controllers increase the lifetime of any LED fixture by maximizing the light output for any given thermal environment independently of the thermal design of the fixture. This allows for consistently-high output intensity irrespective of errors in design or manufacturing tolerances.

TV studio application Driving systems utilizing generation 1 & 2 controllers have to under-drive the LEDs to allow for over-compensated designs and errors in thermal design, and to take into account the environment in which the fixture is used.

Since generation 1 & 2 drivers cannot compensate for environmental variations, most fixture manufacturers drive (for example) 3-watt LEDs at a 2-watt rating to allow for unknown operating temperatures. Of course, this reduces the available light output by one third.

Since generation 3 drivers compensate for the temperature and lifetime variation of individual LEDs, the resulting effect is an extended useful life of the fixture with optimized light output.

The output of a typical red/amber LED can decrease by as much as 80% as the temperature rises, for the same forward current value. Therefore, if the color of an RGB light is fixed by the user, the actual output color will drift over a normal temperature range even when using PWM driving techniques. However, generation 3 controllers can compensate by using closed-loop feedback.

Shah Islam says "Once we explain and demonstrate to clients that changes in ambient temperature can significantly change the colour of an RGB fixture even if the DMX value remains constant, they suddenly understand why our patented techniques can enable LED drivers to precisely control fixtures."

Licensing & products

The Radiant Research technology allows licensees to utilize a variety of communication techniques, such as complex data networks using TCP/IP, DMX512, RF, Bluetooth and other industry protocols. The IPR protects the techniques for utilizing a variety of sensors covering thermal management, closed loop feedback, optical sensing for both color and intensity, and current feedback measurement.

"We have now developed a range of advanced LED driver solutions for the lighting industry covering 40W single fixtures right up to the most advanced 320W 36-channel drivers," says Keith Anderson. "Additional products soon to be launched include a wall-mounted lighting controller which offers over 400 control functions and can synchronize up to 30 different 36-channel drivers or over 4000 LEDs."

Footnote

Figure 3 As the name implies, pulse-amplitude modulation (PAM) is used to modulate the intensity of one or more LEDs by varying the amplitude of the drive pulse(s). Starting with a continuous train of pulses of constant frequency, duration and amplitude, the PAM signal is created by altering the pulse amplitudes to reflect the information being encoded. For a multi-LED system, the amplitude of the signal dictates the intensity of the LEDs.

In contrast, the pulse width modulation (PWM) technique controls each LED by varying the width of the drive pulse. PWM uses a fixed frequency signal with a repeating drive pulse. The width of each drive pulse is changed to encode the data. The intensity of each LED is defined by the duty cycle, which is the ratio of the on time (pulse width) to the maximum possible on time.

A major advantage of PAM is color mixing: all the pulses are synchronized so that all three LEDs (in an RGB system) switch on and off at the same time (with different amplitudes). With PWM, the pulse width for each LED varies so that they all turn on and off at different times during each cycle (see Figure 3). This means that the color mixing is not balanced, but is changing constantly. "Although PAM provides better color mixing, the human eye probably would not notice much difference," says Anderson. "However, any camera system would see a huge variation in both color and intensity."

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