LED lighting will replace mainstream incandescent lighting and other lighting technologies, occupying a dominant position in the market. But the transition from old technology to new technology will take many years. During this time, the challenge for LED lighting designers was to ensure that the new design was compatible and reliable with existing controllers and cabling architectures originally developed for incandescent lighting. This article describes a solution that can be applied to both low-power and high-power LED lighting systems. It is proven and mature.
LED bulb construction
An LED lighting bulb contains one to a dozen or more LED chips, which are usually connected in series. The luminance of each chip is determined by the amount of current passing through it. Due to the series connection, each LED chip in the bulb will automatically pass the same current, but the voltage on each chip is different. The forward voltage drop of the LED is typically 3.4V, but will vary from 2.8V to 4.2V. LEDs can be categorized to limit voltage fluctuations, but this adds cost and the forward voltage drop still varies with temperature and time of use. To provide consistent light output, LED lighting bulbs must be driven by a highly regulated, constant current source. As an alternative to incandescent lamps, the lamp must be integrated into the lamp housing.
Typical integrated LED lamps include drive circuitry, LED bundles, and enclosures that provide both mechanical protection and heat dissipation for the driver and LED chips.
The requirements for LED drivers are very strict. It must be energy efficient, must meet stringent EMI and power factor specifications, and be safe to withstand various fault conditions. One of the most difficult requirements is to have a dimming function. Due to the mismatch between the characteristics of the LED lamp and the dimming controller designed for incandescent lamps, it is prone to poor performance. The problem may be slow start, flicker, uneven illumination, or flicker when adjusting the brightness. In addition, there are problems such as inconsistent performance of individual units and audible noise emitted by LED lamps. These negative conditions are usually caused by false triggering or premature shutdown of the controller and improper control of the LED current.
Dimming controller
The lighting controller works in either line dimming or PWM dimming. The simplest line dimming method is the leading edge thyristor controller. This is currently the most commonly used lighting control method, but unfortunately, the use of thyristor controllers to dim LED lights can cause a lot of problems. More advanced line dimmers are electronic leading or trailing edge dimmers. PWM dimmers are used in professional lighting systems.
When using a leading edge TRIAC dimmer, dimming control is achieved by changing the phase angle of the thyristor on each half cycle. The input power of the bulb is a function of the phase angle of the dimming signal, and the phase angle varies from approximately 0° to 180°.
One of the important parameters of thyristors is the holding current (IH). This is the minimum load that the thyristor must maintain to remain conductive without the use of a gate drive. In order to maintain stable operation of the thyristor, the current cannot be zero, and the typical value of IH is between 8 mA and 40 mA. Therefore, phase angle dimmers for incandescent lamps typically have a specified minimum load, typically 40W at 230V rated AC voltage. This is to ensure that the current flowing through the internal thyristor is always above the specified holding current threshold. Since the power consumption of LED lighting is very low, maintaining current will become a problem.
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