The use of LEDs is becoming increasingly popular due to higher efficiency and longer life. Therefore, the power supply needs to be more efficient, at least with the same lifetime as the LED. National Semiconductor has come up with a solution that ensures 90% efficiency and long life.
In terms of THD (Total Harmonic Distortion), the European standard EN61000-3-2 has strict limits on lighting devices with power losses exceeding 25W. In addition, these lighting devices need to meet power factor requirements. To do this, an active PFC (power factor correction) is required to ensure that the input current matches the input voltage.
The operation of the LED driver to meet these requirements will be discussed below, and how the AC/DC buck converter can efficiently drive 30 strings of high-brightness LEDs.
In addition, this paper will also give a schematic diagram of another implementation method, that is, an isolated AC/DC power supply and a LM3464 with dynamic headroom control. This solution is extremely efficient by avoiding electromagnetic interference.
Introduction
LEDs are becoming more and more widely used in lighting retrofits, industrial lighting , commercial lighting, street lighting, and many other areas. Their efficiency and longevity have been proven. To provide higher reliability, it needs a good power supply to meet its needs. . Our first solution consists of two stages: a front-end PFC and an LM3445 LED driver .
Figure 1 shows the block diagram, which eliminates the need for galvanic isolation and increases efficiency. The overall system efficiency is more dependent on the AC/DC isolation transformer. Although the flyback PFC is economical, the efficiency is hard to exceed 85%. In the first solution, an insulating layer or a ceramic layer is used for isolation between the heat sink and the LED, and the efficiency is made higher because the transformer does not need to be isolated.
Figure 1, structure diagram
The main purpose of this power supply is to convert the rectified AC input to DC regulated current. The following is an example of how to drive 30 high-brightness white LEDs with a power of 35W at 350mA. This power supply protects the LEDs, limits transient input voltages, and avoids current surges during hot swapping. The total power supply consistency (power bus harmonics (EN), power bus interference, international safety standards, etc.) are all in accordance with European standards (EN).
Below is an example of replacing a 35W T8 tube with a ballast, see Figure 2. In the event that the LED string is open, shorted or overloaded, the ballast can be protected from failure. It ensures that no components will overheat or burn out under fault to ensure design stability.
Figure 2, a 35W T8 lamp is used for replacement when using a ballast.
The main features of the ballast and T8 are as follows:
• Suitable for European input voltage range, but can be further extended to a wider range from 85VAC to 265VAC.
• PF is 0.98
• Output current is 350mA
• The output voltage is 100V ± 20% (depending on the LED VF)
• Bus harmonics comply with EN61000 - 3 - 2 Class C requirements
• Electromagnetic interference (conductivity) meets the requirements of EN55022
• Electromagnetic interference (radiation) meets the requirements of the current EN55022 standard
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