0 Preface
Flashlights, MR-16 bulb upgrades, emergency lights, and almost any low-power white light lighting application are already using LED technology .
Next, streetlights may be the next area for large-scale application of LED technology. LED streetlight applications also represent a greater challenge than flashlights and low power applications.
1 Design requirements
LED street lighting will not happen overnight, because there are still major technical problems that need to be overcome. In addition to individual cases (such as solar cells), the input of street lights uses AC power (often referred to as "offline"), mostly 120 V or 230 V AC. For fluorescent tubes and high pressure discharge lamps, they have a wide range of optional off-line ballasts. But because the number of light-emitting elements is small, this circuit is simple. Few fluorescent lamps have more than four lamps, and high-pressure discharge lamps use at least one component. However, the LEDs are very different, even though the "high-power" LEDs, most of the power is only 0. 5W ~ 5W. Although there are some exceptions, for street lamps, it is usually necessary to use 100 or more 1W LEDs to emit the thousands of lumens of light they need.
The LED is a current-driven device, and a 1W white LED driven at 350 mA typically has a forward voltage VF of 3.0 V to 4. 0 V. The LED is a PN junction diode with very small dynamic resistance. Applying a voltage greater than three times VF to a diode results in an uncontrolled amount of current. If the LED is directly connected to the offline AC voltage, it will emit very bright light and will quickly fail. The term "driver" is used to describe a power conditioning circuit that converts an off-line voltage into a controlled DC current. The flashlight is likely to have been lost before it is used up. The application requirements of street lamps are obviously different. Therefore, long-term reliability and product service life are the main considerations for street lamps. LEDs have been promoted as the longest-lasting commercial light source, but if the lights can last for tens of thousands of hours, the matching drives must be able to last the same amount of time. This means paying more attention to all aspects of the power drive, from the system architecture to the choice of each circuit component.
2 DC bus voltage
One way to drive 100 LEDs is to use a single series chain, as shown in Figure 1. This ensures that the current through each LED is the same.
In addition, since the light output is proportional to the current, this is the best way to ensure that each device emits the same light output. The problem, however, is that the DC voltage can easily reach 400 V. Such high voltages can be fatal and require large and expensive components.
Figure 1 All 100 LEDs in series
Another way to drive 100 LEDs is to use a lower DC voltage. It is well known that costly topologies (such as flyback converters) can form good AC-DC stages (commonly referred to as ?? offline converters) because they combine step-down functionality with galvanic isolation and power factor correction PFC. The DC bus voltage is typically 60 V or less, in this respect because it is 48 V in telecom applications and on the other hand because of safety regulations (eg IEC definition of safety ultra-low voltage). The 48 V distribution voltage is higher than the logic voltage of the digital circuit and lower than the rectified offline voltage, so it is often referred to as the “intermediate DC busâ€.
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