TECH NOTES: Vicor's flexible, accurate & efficient power solutions for LED backlighting applications

White light can come from a single white LED or be generated by three separate R-G-B LEDs with a color spectrum that closely matches the color filters of the LCD pixels. This results in a great improvement in light efficiency and a wide color range for a more vivid display. Currently LCD with CCFL backlight can generate only 70-80% of NTSC colors while newer displays based on LED backlight will generate all the colors in the NTSC specification plus additional colors. By using the ultra-fast switching time of LEDs, the backlight intensity can be modulated to enhance picture contrast and reduce the smearing effect due to fast-moving images.

To replace CCFL in small size (19” typical) LCD monitors, tri-color LED devices can be deployed alongside the housing to replace the CCFL tubes. Typically, only the light source (from CCFL to LED strings) is changed; the housing, light guide and optical films remain unchanged. For larger size LCD (20” or above), where higher light throughput is required, an LED matrix can be deployed directly behind the LCD panel. The matrix size, typically in hundreds, varies in accordance with panel size. To ensure homogeneous illumination, a special diffractive diffuser is used over standard optical film. As LED technology improves, the number of LEDs in the string or matrix can be reduced as luminous output (lumens/W) increases, further reducing overall material and system cost.

Of course, there are challenges to deal with, such as maintaining spectral consistency due to temperature change and the rate of LED aging. However, the future is promising. Key players in the portable electronic industry plan to roll out more models with LED backlight. Large screen TV manufacturers have committed resources to make LED backlight more popular in the consumer markets.

There has been a recent breakthrough in backlight panel technology – the utilization of the newly available high-brightness white LEDs in a backlighting panel for LCDs. These new LEDs require a 4 Vdc power supply at 200 W in a very small package size. This design is based on the patented technology of a company that develops and markets innovative High Dynamic Range (HDR) image technology. This technology makes it possible to build much brighter displays, as much as ten times brighter than a conventional LCD, by using intensive video processing algorithms to adjust the brightness of the LEDs. Every single LED in the backlight is individually addressable so the light levels can be changed dynamically frame by frame and specifically microregion by microregion. This technology for enhanced dynamic range displays enables them to display pictures much more vividly with much darker darks and much brighter brights than previously possible. An existing application involves large-format displays (37 inch and 46 inch high-definition displays) for high-end imaging applications.

New architecture

These new LEDs require a 4 Vdc @ 200 W power supply in very small package size. Earlier designs were powered from a 5.5 V power supply at 500 A, an unwieldy amount of current to distribute around inside the display due to I2R distribution losses with associated large and expensive cabling and connectors.

A V•I Chip™ BCM™ bus converter is now designed into this high-power LED application to address the current distribution issue. The BCM utilizes a patented Sine Amplitude Converter™ (SAC) topology with state-of-the-art power density, efficiency and low noise. With a footprint of just 1.1 sq. inch and weighing 15g, the BCM converter provides an isolated, stepped-down voltage to power niPOLs (non-isolated point of load converters), in this case LED driver ICs. Due to its fast response and low output noise, the need for limited life bulk capacitors at the load is reduced or eliminated, resulting in saving of board space, materials and total system cost.

Now the system distributes 48 V everywhere at a much lower current. At the point of load, it is locally stepped down to high-current at 4 V. On each of the large format boards there are four boards in one system. In total, there are 16 BCM modules running in each system. The result is 4 V at 200 A on each board. In contrast, the previous design using the large centralized power supply would need to provide 800 A for each board incurring significantly more distribution losses. Of course, such a supply is dangerous as well due to the high currents involved. That is why 20 A of 48 V are distributed to each board, which is much more manageable and quite fusible.

Well-suited solution

Figure 1 Several factors make the BCM converter the right solution. First, due to the small size and high efficiency (>94%), no special heatsinking is required. Second, it runs at 48 V – an easily routed, safe (SELV) voltage. There is a family of BCM devices providing different standard output voltages which are optimal for different applications. Typically, in the LED backlight application, the downstream LED driver ICs require an input voltage trimmed to 4.1 to 4.2 V. As the BCM module is an unregulated converter and not a regulator, designers are able to increase the input voltage to get to the designated output voltage.

Other LED Backlight Options
Providing a Constant Current for Powering LEDs

Other backlights may require a constant current driving an array of high-power LEDs connected in series. Generally, constant current is required to ensure predictable luminosity and chromaticity levels. PRM™ regulators and VTM™ voltage transformers, though originally designed to provide a regulated voltage from an unregulated input, are able to provide an extremely efficient and accurate constant current with a simple feedback circuit.

In a traditional DC DC converter – the functions of regulation and voltage transformation are combined limiting the ability to optimize the system. The PRM regulator and VTM transformer are individual components and can be separated or ‘factorized’ to enable higher power densities, higher efficiencies and more efficient, flexible system designs.

Using the PRM regulator and VTM transformer for constant current provides several advantages over conventional approaches. The implementation of a VTM module provides point-of-load current multiplication as the output current is proportional to its input current by the following equation:

I_OUT = I_IN / K

where K is the K-factor (transformer ratio) of the VTM module.

Most LEDs can be driven with a single PRM + VTM pair. The PRM regulator is pre-configured with an internal voltage loop to regulate its output voltage to a set value. The external constant current circuit is designed to work in conjunction with the internal voltage control loop, changing the PRM voltage reference in order to regulate the VTM output current.

The SC voltage is buffered and fed to the error amplifier through a resistive divider represented by the gain block of 0.961. R₆₈ forms the top half of the voltage-sensing resistive divider. The bottom half of the divider is formed by adding a resistor from the OS pin to SG (R_OS). Equation 2 defines the regulator output as a function of V_SC and R_OS. From Equation 2, it is seen that for a given R_OS resistor, adjusting the SC voltage will determine the output voltage. This is the method by which the external current control circuit will control the output.

Where: V_SC is the voltage at the SC pin. R_OS is the resistance from OS to SG of the PRM regulator. R₆₈ is the internal resistor.

Current Control Circuit

The recommended current control circuit is shown in Figure 4. As the VTM transformer is a current multiplier, the output current can be regulated by its input current. The advantage of this approach is that the current can be sensed prior to the current multiplication stage (at the higher voltage), reducing the I²R power dissipation in the external shunt. In addition, the control circuitry remains on the primary side (linked to the PRM regulator’s output), eliminating the need for isolating the feedback signal.

Figure 5 The error amplifier will adjust the PRM output voltage until V_SENSE is equal to the reference voltage V_REF. This forces the VTM transformer’s input current, and hence, its output current to be constant as determined by the V_REF. The resulting current may be 99.7% accurate directly from the PRM regulator and up to 98.7% when measured at the VTM transformer output, as shown in figure 5.

A single PRM regulator with a simple external current sense circuit can be used as constant-current source (at 26 - 55 V). The VTM unit transforms this ‘factorized’ bus voltage to the voltage that matches different color LEDs from 0.8 to 55 V (for instance, 6 V for a blue LED, 14 V for amber and 24 V for green – though this varies by LED manufacturer).

The flexibility of Factorized Power Architecture™ (FPA) allows the same regulator to drive different transformers (of different K factors) for different color LEDs. Also, if the LED color voltage requirement is constant, the same VTM transformer may be used while the PRM regulator may be changed for a different input source voltage. Furthermore, by ‘factorizing’ the conversion, only the VTM transformer is required at the LED load, minimizing the voltage droop and power dissipation. The PRM regulator may be placed elsewhere, perhaps on a separate PCB and linked via a cable or a connector. Conclusion

References & Further Reading

The FPA Overview: An introduction to FPA is available at: www.vicorpower.com/fpawp.

The Application Note: 018, Providing a Constant Current for Powering LEDs using the PRM and VTM is available at: www.vicorpower.com/ccdemo.

Trademarks

PRM regulator, VTM voltage transformer, BCM bus converter, Factorized Power Architecture (FPA), Sine Amplitude Converter (SAC) and V•I Chip are trademarks of Vicor Corporation.