By BizLED Bureau:
Jan 15, 2016: With use of smart controls and effective strategies, power saving can be easily done with minimal power consumption. Here, we will cover various techniques and technologies involved.
With the advancements in LED lighting, dimming capability is increasingly becoming a requirement for lamps and luminaries. Currently, there are two ways to control an LED light. The first way is to reduce the ?direct current? to the LED string. The second is PWM (pulse-width modulated) dimming which involves to turn the string on and off at a quicker rate (usually on the order of 1 kHz), modulating the light on and off quickly.
An important point here is that light from some LEDs changes with fluctuations in their direct current(dc). These LEDs then use the latter dimming method. However, LEDs with color content being relatively constant with direct current(dc) can use any method.
The question lies in how we implement the input dimming methods for LEDs. Often used method is a digital addressable lighting (Dali) type interface called 0-to-10-V linear dimming. It is a wireless control method such as Zigbee or equivalent which are preferable to the dimming method for traditional incandescent lights. It incorporates a triac (also known as phase-cut) dimming, which plugs out sections of the ac waveform to lower the electricity delivered to a bulb. However, there are some issues when used with the ac/dc conversion circuits consisting of fluorescent and solid-state lighting ballasts.
The main problem is that triacs, by their very nature, The triac require a minimum amount of current to stay on once they are triggered due to their default nature. For ac/dc converters without PFC, current flows from the input source when the voltage reaches its peak value pr a value close to it. Therefore, such circuits need a holding current. If the holding current used is dissipative (that is, it is dissipated through a resistive element rather than moved to the input or output), it may require as much as 5 W of extra power to keep the triac on. This will reduce the efficiency of your system as power is being wasted here.
It is a common misconception that adding triac dimmers to LED fittings in a house will reduce the building’s power consumption. Dimmers have the ability to reduce energy when they are used, dimming lights where needed so that less overall power is used. It is more advisable to use dimmers at a place which needs adjustable light.
However, if the area doesn’t need adjustable light, install appropriate lumen output to meet application needs and leave stop there. For higher power levels, PFC lets the lamp power also act as the holding current. It could almost be said that if you need triac dimming, you would do well to provide PFC as well. The exception to this practice is when PFC in active state is used. Active PFC makes the load appear to be a resistor (i.e. the ac input current is in phase with the ac input voltage), which draws a uniform holding current.
Voltage or current mode
LEDs are current-fed devices by nature though most of us are designed to thinking in terms of voltage. Even after choosing voltage mode (constant voltage output) ballast, it will require additional circuitry to control the current delivered to the LEDs. This added circuit is most likely provided by extra circuitry within a lamp. The current mode (constant current output) ballast makes the most sense if you are working with care LEDs or extremely low-cost LEDs.
The nearby flow chart will help recognize the best topology for ac-input LED ballast circuit needs that are in wide use. Which leads us to a question whether an off-the-shelf ballast is preferable to a custom design. There is not a clear answer but it is always a trade off.
A power supply/ballast is designed at rated output current to provide rated power and no peak performance. But suppose the ballast is designed to provide a 750 mA constant-current output at maximum, and you configure it for 450 mA constant current output. There is wasted size and needless extra expense. Future off-the-shelf ballasts may provide configurable output voltages with output voltages.
A ballast designed to provide 450 mA would use smaller transistors optimized for 450 mA output instead of 750 mA. Configurability may be the best trade-off. Producing a large volume of configurable ballasts will cost less than using multiple custom ballasts.
Many companies do not have the expertise to design ballast but do have the capability of manufacturing one or collaborate with the manufacturer to build one). Vastly available help of applications engineers of semiconductor companies that make offline LED controller ICs and devices. International Rectifier has a dedicated LED applications group working exclusively on evaluation boards for end-point circuits.
Some may be able to provide everything needed for a circuit to pass UL certification – or even provide example ballast circuits that have already been UL-approved. The local field application engineer has the skill to customize an evaluation board design for specific applications.