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 • Efficiency is 87% • Meet current safety standards • Passive cooling • Temperature range -20 ° C ~ +65 ° C • Use polymer capacitors to extend life • Use a 3-string 90 LED LCW_G5GP-GX-6S OSRAM T8 tube with shared current. First level PFC Most basic AC/DC power supplies generate harmonic distortion and poor power factor in the input line, making it difficult to meet the requirements of the European standard EN-61000-3-2. The solution uses a PFC circuit that makes the input current waveform and the input voltage waveform the same as a sine wave waveform. For ballast products, it complies with European EN - 61000 - 3 - 2 Class C standard. This standard applies to all lighting products, including dimming devices with active input power greater than 25W. The PFC acts as a boost converter and operates in critical conduction mode. It provides a relatively stable output voltage (380VDC) as the LED driver input voltage. The LED driver acts as a constant current controlled buck converter and will be more suitable for rectifying the input voltage. Due to the higher input ripple of the LED driver, a smaller capacitor will be used in a 380 VDC environment. Due to the high failure rate, electrolytic capacitors are not allowed to maintain a long service life. This ballast uses EPCOS film capacitors instead of electrolytic capacitors to limit the amount of down-conduction for all components based on internal specifications, which minimizes failure rates and extends the life of the entire system. The second stage is the LED driver, which uses the LM3445 constant current controller. The LM3445 is an AC/DC buck constant current controller with adaptive constant off-time, compatible with triac dimming and pulse width modulation (PWM) signals. The LM3445 provides a constant current value for high power LED illumination, and the dimming decoder allows for a wider range of LED dimming. Figure 3 details the drain-to-source voltage and current of the LED driver for a full AC cycle. The cycle can be divided into several different phases, the curve is as follows: Closed stage 2. Conduction phase 3. Disconnection phase 4. Disconnect phase, energy transfer to load Figure 3. The drain-source voltage and current of Q3 in a complete AC cycle of the LED driver. The LED driver uses constant off-time control to regulate current through a string of LEDs. When the MOSFET is turned on, the LED current through the inductor increases until it reaches the peak defined by the reference voltage and current sense resistor. After reaching this peak current, the MOSFET turns off and the diode turns on during Toff. In order to drive more LEDs, some improvements were made, including the successful serial connection of 60 LEDs. At an output power of 70W, the total efficiency can reach 92%. By connecting other LM3445s in parallel, multiple strings of LEDs (30 per string) can be driven, but more cables will be needed to connect the LEDs. Figure 4. A solution using multiple strings of LEDs. Another method is shown in Figure 4. The output voltage on the main power side is lower than 60V, which meets the requirements of the UL1310 standard Class 2 for the highest voltage. When limiting the secondary voltage required in an isolation system, the only choice is multiple strings of LEDs. In the secondary, the LM3464 is a multi-channel LED driver controller. As a linear regulator, each LM3464 can control up to four external power N-MOSFETs, thus enabling control of multiple strings of LEDs with up to 15 LEDs in series. The maximum average current per channel is up to 500mA. Figure 4 shows how the LM3464 controls the isolated AC/DC primary off-line power supply. The LM3464 can be used to dynamically adjust Vo to keep the voltage through each linear regulator at a minimum. . Vo's regulation reference is the LED string channel with the highest reference voltage. Even with a drive current of 350mA per channel, the LM3464's power efficiency can exceed 95%. An important difference between Figure 1 and Figure 4 is that the LM3464 does not require a new switching frequency, which is extremely important for controlling EMI because EMI becomes more and more difficult to control as the total power increases, and the only switching noise comes from AC/ DC part. in conclusion This article discusses different ways to drive a larger number of LEDs. The first solution uses PFC as a standard booster to drive a string of LEDs (30) with isolated heat sinks, which is an attractive option for design engineers who need multiple strings of LEDs. The example focuses on LED replacement, and the details of the LED tube can be provided on request. If the design is correct, in addition to high energy efficiency, LED tubes and ballasts have a long service life. For traditional lighting installations, maintenance costs have always been a significant expense, and the use of LED tubes can minimize maintenance costs. The second solution uses a multi-channel linear regulator with primary power and headroom control. The engineer wants to equip each string of LEDs with one A dedicated power supply, but using a buck regulator in each LED string can cause problems. For them, this solution is really attractive. The LM3464 offers a smaller, cheaper, and simpler option while maintaining high power efficiency, high reliability and excellent electromagnetic compatibility (EMC). DC Capacitor For Electric Furnace DC Capacitor for Electric Furnace DC Capacitor For Electric Furnace,DCMJ DC Filter Capacitors,DCMJ Pulse Capacitors,DCMJ DC Support Capacitors YANGZHOU POSITIONING TECH CO., LTD , https://www.yzpstcc.com