In recent years, the market for high-brightness light-emitting diodes (HB- LEDs ) has grown rapidly, and LED performance (efficacy, in lumens per watt or lm/W) has more than doubled, making it more suitable for many new applications, such as our handheld flashlights. Many revolutionary new products can be found in applications such as architectural lighting and street lighting. But LEDs are still facing challenges when they are more cost-effective than incandescent and compact fluorescent lamps. In fact, for many applications, the challenge of powering HB-LEDs based on a wide input voltage range is involved. This is especially true for general lighting applications such as track lighting, which use a 12 Vac or 12 Vdc power supply that can be loosely regulated. However, LEDs need to be driven by a current source rather than a voltage source because the forward voltage (rated 3.4 V) of the HB-LED may vary by more than ±20% depending on process tolerances and temperature.
In addition, in terms of the current lumens of a 1 W warm white power LED, typically 3 to 4 LEDs are required to replace the light output of a 20 W incandescent lamp. To achieve predictable and matched brightness and chromaticity, it is also necessary to drive the LED at a constant current. From a architectural point of view, the buck-boost topology meets this requirement, but it is not as common as a standard buck or boost topology. But with a thorough understanding, the buck-boost topology can also provide numerous advantages for cost-effective HB-LED lighting applications where the input voltage (Vin) overlaps the forward voltage (Vf).
Reference design overview
This reference design document describes the carefully constructed and tested GreenPoint 1 W to 5 W LED driver solution for MR16 LED replacement applications. This reference design circuit is suitable for driving HB-LEDs in a variety of lighting applications, but is sized and configured for MR16 LED replacement applications. This type of configuration is common in 12 Vac/12 Vdc rail lighting applications, automotive applications, low voltage AC landscape lighting applications, and work lighting applications such as cabinet lights and desk lamps that may be powered by standard off-the-shelf AC voltage wall adapters.
A key consideration in this reference design is the flat current stabilization across the input line variation and output voltage variation at 12 Vac input. This reference design circuit is based on ON Semiconductor's NCP3065 and operates at approximately 150 kHz in a non-isolated configuration. The NCP3065 is a monolithic switching regulator that supports a 12 Vdc or 12 Vac power input and is designed to provide constant current to the HB-LED. In addition to the NCP3065, this reference design incorporates an automatic detection circuit. The functional block diagram of this reference design is shown in Figure 1.
Figure 1: Block diagram of the GreenPoint reference design for 1 W to 5 W LED drivers from ON Semiconductor for MR16 LED replacement applications.
Basic power topology
The principle of a buck-boost converter is very simple. In the on state, the input voltage source is directly connected to the inductor (L), accumulating energy in the inductor. At this stage, capacitor C supplies energy to the output load. When off, the inductor is connected to the output load and capacitor through the output diode to transfer energy to the load.
Note that this is an inverting output with a negative output connected to the anode of the LED and a positive output connected to the cathode of the LED. In addition, when measuring with an oscilloscope probe, the ground of the probe is not grounded. The oscilloscope filter will need to be floated (remove the ground connection from the AC wall power supply), otherwise the ground loop/short circuit will cause the device to shut down.
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