LEDs in high-power industrial lighting fixtures
This article discusses the practical requirements for common industrial LED lighting systems. It introduces the various designs of dimmable LED luminaires as well as some basic principles of LEDs themselves. Additionally, some of the challenges posed to today's power electronics design engineers are included.
LEDs in high-power industrial lighting fixtures
High-power industrial light-emitting diode (LED) luminaires use series-parallel strings of high-brightness (HB) LEDs to produce higher-quality lighting fixtures compared to traditional "ballast-based" systems. HB-LEDs are configured as series or series-parallel strings. They require constant current, constant voltage (CC-CV) power supplies operating in the 50W to 200W range. Industrial LED lighting systems operating on universal offline AC voltages must be able to maintain high power factor over a wide range of input and output operating conditions. For example, to meet the U.S. Department of Energy's (DOE) solid-state lighting Energy Star program, residential and commercial lamps larger than 5W are explicitly required to have power factors of 0.7 and 0.9 respectively. In addition, the European standard EN61000-3-2,
The block diagram shown in Figure 1 highlights the functional requirements of a typical industrial LED lighting system. Although this is not always the case, in this example the PFC functionality is shown outside the LED driver block. There are several widely accepted control methods for implementing PFC, but for power regulation in the 25W
An LED driver consists of a DC-DC converter; load control; LED-specific functions such as dimming; and any necessary protection functions. The DC-DC converter provides safe isolation from the AC power source while converting the high voltage PFC output into DC current suitable for the LED load requirements. When multiple series-parallel LED strings are used in high-power LED luminaires, LED load control is necessary for current sharing.
In some cases, the DC-DC and load control blocks (shown in Figure 1) can be combined, where the load control provides CC-CV feedback information to the DC-DC converter. During normal operation, the LED driver should work in CC regulation mode. However, there may be times when a driver should operate in CV mode.
For example, if the LED string voltage exceeds a certain rating, the driver needs to respond quickly and switch from CC mode to CV mode to protect the DC-DC converter power stage components. When the driver works in CV mode, the LED current should be limited or folded back to provide overcurrent protection for the LED load. In addition to power conversion and load control, the second function of the driver is to implement dimming functionality.
There are many different configurations available for operating dimmable LED luminaires. Industrial LED lighting systems typically use analog or PWM dimming interfaces in conjunction with occupancy sensors, daylight sensors, and other controls to optimize efficient light utilization. Analog dimming uses 1V-10V (or 0V-10V) DC control voltage to linearly adjust the LED brightness by controlling the LED DC current. The 1V-10V control voltage can adjust the LED dimming range from 100% (10V) to 10% (1V). Since the light output can only be dimmed up to 10%, a separate switch is required for on/off control.
High power factor, safe isolation, dimming compatibility, constant current accuracy and LED load control are some of the challenges faced by power electronics design engineers today. How do you begin to make the trade-offs between meeting these requirements while remaining efficient? What are the considerations between different power topologies suitable for LED drivers in this power range? What other CC-CV LED current sharing methods are used in the industry? What is Fairchild doing to meet these needs?
For answers, be sure to check out the 2014 Fairchild Power Seminar Series where we'll delve into the details and reveal a step-by-step design approach to a scalable 100W LED driver for high-power industrial lighting fixtures. Magnetic component design, gate drive, PFC and analog circuit design will also be presented in a manner that will hopefully be relevant to power electronics engineers whose interests may not be specific to LED driver applications.
High-brightness (HB) LEDs are the backbone of high-power industrial lighting fixtures, and this document discusses their capabilities and how they produce so much brightness. More in-depth details on this topic will be presented in the 2014 Fairchild Power Seminar Series, including a step-by-step design methodology for a scalable 100W LED driver for the same application.
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