Multi-Channel LEDs Optimize Lighting Applications

­LEDs are now available in a wider range of colors and spectral choices, opening new possibilities in luminaire design for both outdoor and indoor lighting applications. However, to successfully select and use these new products without problems and realize the full benefits of their improvements, several design guidelines must be considered, and a few board-level assembly issues must be addressed.

Background for Multi-Channel LED Lighting                                          

Selecting the right LED for any application involves understanding several terms including correlated color temperature (CCT), color rendering index (CRI), lumens (light output) and others. These terms will be explained as they appear. For power engineers, lumens/watt (LPW) is probably the most relevant specification, since efficiency and efficacy are major concerns for every application. Multi-channel LEDs add more complexity to the LED selection process.

For LEDs, the term channel refers to an individual LED or group of LEDs that can be controlled independently. Typically, in a color-mixing design, each channel would consist of one or more LEDs of a single color, so channel may also refer to the color point used in mixing. One of the color points used in a color-mixing application is sometimes referred to as an anchor point because it is a fixed position in the color space which can be used to “pull” the final output color in a certain direction. The achievable color space that the mixed output can create is determined by plotting the area encompassed by the shape created by the anchor points. Figure 1 shows a 6-channel example that covers most of the CIE 1931 color space using royal blue, blue, cyan, green, lime and red LEDs.

The arrangement of a single color point per control channel is not always the case. For example, in advanced color mixing applications, a designer might mix multiple LED colors on a single control channel. This is typically done to create a virtual anchor point at a different point in the color space that is not otherwise achievable. It can also be done for CRI or efficacy reasons.

[The color rendering index or CRI describes a light source's ability to accurately reproduce color. Using up to 15 predefined test color samples (TSCs), a rendering score (Ri) is calculated for each color sample, with a value of 100 indicating an exact match. Commonly referred to as just “CRI”, the CRI Ra value is the average of R1 through R8. A CRI Ra value over 90 is commonly considered excellent color rendering for lighting applications.]

Color mixing systems with LEDs commonly use either a 3- or 4-channel approach. With 3 channels, using red, green and royal blue is the most common way to cover the widest range of possible colors. In contrast, with a 4-channel approach, luminaire designers often opt to add a white LED (2700 K to 7500 K CCT, near the black body line).  [The correlated color temperature or CCT measured in degrees K but simply indicated K, describes warm, orange-appearing (1,000 K) to cold, blue-appearing (10,000 K) lighting with a single number.] [The black-body line (BBL) is the common name of the Planckian locus, which describes the color of an ideal black-body radiator as it is heated. The change in colors, from red to yellow to white to blue, as temperature increases is a common physical phenomenon. White light with color points along the BBL will appear neutral in tone, free from green or pink hues.] White lies within the achievable range of the 3-channel RGB solution but adding a phosphor-converted white LED provides a more balanced spectrum and often better color rendering. In addition, it allows the device to operate in an easy 1-channel mode to produce good quality white light.

There are advantages and disadvantages to using different LED colors as the fourth channel in a 4-channel system, supplementing the existing three channels of red, green and royal blue. With an optimized design to achieve the highest possible CRI Ra across a standard range of white light, CCT targets near the BBL can be achieved. Different optimizations, such as maximizing LPW or CRI R9, are possible and may be more suited for a specific application.

LED Selection for Multi-Channel LED lighting                                                    

Multi-channel lighting designs are easiest to implement when the LEDs chosen share as many design features as possible, since items such as PCB footprints can be reused in the design. Cree LED offers two comprehensive LED families to cover a wide range of applications. For multicolor directional indoor and outdoor lighting, XLamp® XE-G (Element G) high-power LEDs offer up to 17 color options plus the full range of white CCT and CRI options. These new color options enable new 4-channel color-tunable solutions with more light output and more accurate color rendering than what could previously be achieved. Offered in a 1.6 x 2.05 mm package with a consistent 3A max current rating across all colors, these LEDs deliver an innovative building block approach to lighting system design, providing an entirely new performance standard for this category. Figure 2 shows an example of 4-channel design using XLamp XE-G LEDs.

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Figure 2: Four XLamp XE-G LEDs mounted on a Metal Core Printed Circuit Board (MCPCB) in a pinwheel configuration at approximately 200-μm edge-to-edge spacing

For low-density indoor lighting applications, J Series® 2835 white and color LEDs are optimized to deliver the best value with high efficacy. The J Series 2835 Color LED family features the widest array of colors available in mid-power LEDs, enabling the highest degree of optimization possible.

Electrical Design for Multi-Channel LED Lighting                                   

The primary barrier to creating high-channel-count color mixing applications is the availability of LED drivers. Four-channel LED drivers are available, but almost all of them are designed to operate low-power RGBW LED strips, and the color mixing functions are usually limited to producing saturated RGB colors in fixed pre-programmed patterns geared towards entertainment lighting. Few off-the-shelf solutions offer the flexibility to use color points other than RGBW, or to mix them to produce variable-CCT white light. For this reason, a custom driver solution is often the best choice for high-performance color mixing applications.

In typical single-channel LED drivers, the constant-current output to the LED is often tightly coupled to the AC-to-DC portion of the circuit. When creating multichannel drivers, it is more common to have a largely separate AC-to-DC section that provides a bulk DC supply (at common output voltages, such as 12, 24, 36, or 48 V) to several copies of a DC-to-DC constant-current LED driver section.

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Figure 3. Block diagram of a recommended multi-channel LED lighting power supply

The most common technique for modulating a channel’s output is pulse width modulation (PWM). This technique involves switching the output on and off at a frequency that is high enough to be imperceptible to the human eye. For lighting applications, Cree LED recommends at least 1kHz PWM frequency with at least 10 bits of resolution to avoid visible artifacts. To generate the PWM signal generally a microcontroller is needed.

Some basic criteria used to select an LED driver include integration of components, switching frequency, output current and maximum voltage. In some applications, a dedicated LED driver IC is available to create a suitable LED driver. In these cases, the freewheel diode, MOSFET, inductor and microcontroller should be considered together to achieve the desired level of control and output current for the system.

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Figure 4: A PCB using four copies of a multi-channel driver circuit using the onsemi NCV30161 buck LED driver and a PIC16F1575 microcontroller

In other cases, a constant-current output IC is not available, and the more common constant-voltage type must be adapted to form an LED driver. The IC regulates the output by maintaining the feedback pin (FB) at 1.0 V, and this voltage is set up to be ratiometric to the output voltage via a voltage divider. To convert this design to constant current, a signal needs to be created that is ratiometric to the LED current, and that outputs 1.0 V at the target current setpoint. This can be accomplished with an op amp set up in a non-inverting amplifier circuit and using a current-sensing resistor to measure the LED current.

The constant-voltage IC is now a constant-current LED driver with PWM-adjustable output. It should be noted that the PWM signal is now inverted logic, the lower the duty cycle, the higher the LED current. Another thing of note about this design is that, when well-tuned, the LED output is a continuous current, not pulsed as with the buck LED driver IC. This typically leads to higher efficacy from the LEDs. With this type of design, it may be necessary to also control the enable input of the IC in case the analog circuit is not able to fully turn off the LED string. There is very little bandwidth requirement for this application so nearly any single-supply op amp with appropriate voltage ratings can be used.

With the circuit design complete, the next steps involve tuning the circuit to deliver the desired output color configurations, coding the microcontroller firmware and testing the circuit across a range of temperatures.

Other Design Challenges                                                                                              

There are two recommendations for choosing how to arrange LEDs in a color-mixing application, when placing LEDs as close as possible to each other:

1. Avoid positioning colors with high short-wavelength content (Violet, Royal Blue, Blue or PC Blue) near components with heavy phosphor coatings (e.g., PC Lime, PC Yellow, PC Amber, PC Red Orange or PC Red). The light emitted from the short-wavelength colors can excite the phosphor in the neighboring components and cause a loss of blue saturation or unintended optical artifacts.

2. When mixing arrays of more than 4 colors to generate white light, position components in pairs that are on opposite sides of the black-body line. This will balance the hues going through the color-mixing optic to avoid asymmetric color. For example, Cyan and Red could be paired, or Green and Royal Blue.

PCB Material & Construction

In general, Cree LED’s recommended dimensions for arrays match those of the single component dimensions on the data sheet. Specifically, this means:

It should be noted that tightly packing these LEDs has tradeoffs which must be carefully considered in any new design. Generally, smaller LED-to-LED spacing will increase temperature, decrease lumen output and cause a color shift away from the binned color point.

Multi-Channel LEDs for Optimized Lighting                                                       

No matter what the lighting application or luminaire design, multi-channel LEDs can add product differentiation and flexibility to the end product. Specifically, color-mixing lighting applications that require high levels of light output and full control over the spectral content can benefit from the newest XE-G LEDs. However, luminaire designers must consider a few key design recommendations to obtain full advantage of these and other LED’s capabilities.

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