In the first part of this series, we learned the basics of LED light sources and their drive requirements. In the second part, we discussed why it can be your first choice when a constant current Buck converter can be used as an LED conversion mode driver. In the third part, let's examine the large LED display and its application space in other conversion topologies.
LED light source manufacturers and designers often mention that the most obvious advantage of solid-state lighting applications is “the fruit that hangs very low on the treeâ€. For example, garden path lighting or MR16 cup lights often only need some or even just one LED (Figure 1). For low voltage photos, the most common voltages are 12 VDC, 24 VDC and 12 VAC. These applications often use a Bulk regulator. Although Bulk is the first choice, as mentioned above, in LED lighting applications, as the number of LEDs increases, Boost regulators are also being used more and more. Designers are no longer satisfied with flashlights or individual cup light applications, but are turning their attention to large-scale general lighting and lighting systems that reach thousands of lumens. Such as street lights, apartment and commercial lighting, stadium lighting and interior and exterior decorative lighting.
Figure 1: Bulk and Boost LED drivers with Vo calculation: buck: VO = nx VF, VO < VIN; boost: VO = nx VF, Vo > VIN.
Still need constant current
As with linear and Buck-derived LED drivers, the main technical challenge in Boost LED driver design is to provide a controlled forward current IF for each LED in the array. Ideally, each LED has a single set of chains installed to ensure the same current through each device. The Boost regulator is the easiest option when boosting the input DC voltage to a high DC output voltage because it allows more LEDs to be connected in series at a given voltage. Designers of general lighting systems typically need to design the line voltage to 110 VAC or 220 VAC. Single-stage non-isolated converters (buck, boost, or various buck-boost topologies) can be directly driven using the corrected output of the AC voltage if power factor correction (PFC), isolation, and line harmonic filtering are not required. Long strings of LEDs in series.
However, in many cases, we need to use an intermediate DC bus voltage that is generated by an AC/DC regulator that uses a universal AC input and PFC, isolation, and filtering. Including legal requirements, a low intermediate voltage bus reduces dielectric breakdown and arcing problems, making maintenance personnel safer. The EU has proposed the most stringent legal regulations in the world: any source above 25 watts must have a PFC. In the absence of a few years, the same rules have been made in North America and Asia. Safety standards such as UL and CE electrical regulations limit the AC/DC supply output voltage supplied to the boost LED drive. Usually the voltage is specified as 12 and 24V, sometimes 48V. These intermediate voltage buses rarely exceed 60V, which means UL Class 2 is the highest value of the DC voltage.
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