Achieving High Efficiency When Driving LEDs with Constant Current

January 12, 2016
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Light Emitting Diodes (LEDs) are highly efficient light sources that are used in a wide variety of applications, from digital signage to providing illumination. There is little consistency between applications, with the number, type, color and configuration of the LEDs meaning driver circuits need to be designed specifically for the application.

Power Chain Performance is Critical to Ensure Overall Efficiency

LEDs are generally chosen because of their high efficiency. If these benefits are to be realized, then the power chain supplying the LEDs must also be optimized to minimize losses. In addition, the need to optimize the performance of the power architecture and components used, has led designers to look for other ways of driving towards higher efficiency, including moving from the traditional 12V or 24V bus voltage to a 48V bus, thereby taking advantage of the significant reduction in I2R losses.

The Two Categories of LED Driving Applications

LED applications fall into two broad categories. The first, where each LED needs to be controlled individually (eg digital signage applications), require dedicated LED drivers. In these applications, an efficient way of stepping down the bus voltage to the supply voltage of the driver, which can be 12 V, 5 V or 3.3 V, is critical. The ability to efficiently step-down the 48 V bus voltages to that required by the drivers in a single conversion stage makes components such as the Vicor ZVS Buck regulators ideal for this application.

The second category of applications, for example LED lighting, simply requires arrays of LEDs to be driven as efficiently as possible. Many applications, from lighting systems that need to match the intensity of different banks of LEDs to outdoor systems that need to control the level of illumination based on whether it is a sunny day or the middle of the night, require control of brightness. Eliminating the drivers would inevitably increase efficiency, but to control the brightness, however, the LEDs need to be driven by a constant current, rather than the constant voltage that is supplied by most buck converters.

Buck Converters with Constant Current Mode

Typical buck converters will step down the bus voltage to power LED drivers, but most cannot provide a constant current output, meaning they are unable to drive LED arrays directly. The PI354X family of ZVS Buck Regulators, however, offers a constant current mode, allowing accurate control of the LEDs’ brightness. Their wide input range of 36 V to 60 V makes them ideal for modern systems using a 48 V bus, and their high efficiency of over 95% ensures that the benefits of using LEDs are maximized. The family of products also allows the designer to choose the optimum configuration of LEDs, allowing for series and parallel connections depending upon the requirement of the application.

Constant Current Operation of the PI354X Family

The lighting (LGH) mode of the PI354X uses a separate current amplifier (called the LGH) and an internal 100mV lighting reference. Designers connect a shunt in series with the LEDs (see figure 1). When the voltage in this shunt exceeds 100mV, the current amplifier takes over, regulating the current to maintain the voltage drop across the shunt.

Configuration of PI354X in Constant Current Mode for LED Driving

Figure 1 – Configuration of PI354X in Constant Current Mode

To configure these ZVS Buck Regulators to operate in constant current mode, the designer simply needs to:

  • Ensure R1 and R2 are set so that there is no voltage loop interaction with the current loop – i.e. the EAIN pin should be held at a voltage above the internal 1 V reference.
  • Configure the shunt resistance to drop 100mV at the required current
  • Configure Rlgh and the parallel R-C filter to ensure stable operation. Full details of how to calculate these values is given in the PI354X data sheet.

The PI354X is an ideal solution for LED lighting applications, allowing power system designers to use a 48 V power bus that reduces I2R power losses, providing efficient single stage conversion and delivering a controlled constant current to the LEDs. The family of products also offers a range of additional features, such as over temperature protection.

 

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