National Semiconductor's high-power LED lighting system solutions

LEDs are becoming more and more popular due to their higher efficiency and longer lifespan . Therefore, power supplies need to be more efficient and have at least the same lifespan as LEDs. National Semiconductor has come up with a solution that guarantees 90% efficiency and an extremely long lifespan.

The European standard EN61000-3-2 has strict limits for lighting fixtures with power losses exceeding 25W in terms of THD (Total Harmonic Distortion) . In addition, these lighting fixtures need to meet power factor requirements. To do this, active PFC (Power Factor Correction) needs to be included to ensure that the input current matches the input voltage.

The following article discusses how LED drivers meet these requirements and explains how an AC/DC buck converter can efficiently drive 30 high-brightness LEDs in a string.

In addition, this article will also give a schematic diagram of another implementation method, which uses an isolated AC/DC power supply and an LM3464 with dynamic headroom control. This solution has extremely high efficiency by avoiding electromagnetic interference.

Introduction

LED is increasingly used in lighting renovation, industrial lighting, commercial lighting, street lighting and many other fields. Its efficiency and service life have been proven. In order 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. This solution does not require galvanic isolation and improves operating efficiency. The total system efficiency depends more on the AC/DC isolation transformer. Although the flyback PFC is economical, its efficiency is difficult to exceed 85%. In the first solution, an insulating glue layer or ceramic layer is used between the heat sink and the LED for isolation. Since the transformer does not need to be isolated, its efficiency becomes higher.

Figure 1: Block Diagram

The main purpose of this power supply is to convert the rectified AC input into a DC regulated current. The following is an optimized example showing how to drive 30 high brightness white LEDs at 35W at 350mA . The power supply provides protection for the LEDs, limits transient input voltages, and avoids current surges during hot plugging. The overall power consistency (power bus harmonics (EN), power bus interference, international safety standards, etc.) all complies with European standards (EN).

Below is an example of replacing a 35W T8 tube with a ballast, see Figure 2. The ballast is protected from failure in the event of an open circuit, short circuit, or overload of the LED string. It ensures that no component will overheat or burn out in the event of a fault, ensuring the stability of the design.

Figure 2: 35W T8 lamp replacement when using ballast

The main features of 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

? Output voltage is 100V±20% (depending on LED VF)

? Busbar harmonics meet EN61000-3-2 Class C standard requirements

? Electromagnetic interference (conductivity) complies with EN55022 standard requirements

? Electromagnetic interference (radiation) complies with the current EN55022 standard requirements

? Efficiency is 87%

? Comply with current safety standards

? Passive cooling method

? Temperature range -20 ° C ~ +65 ° C

? Use polymer capacitors to extend life

? Using 3 strings of 90 LEDs LCW_G5GP-GX-6S with current sharing OSRAM T8 tubes.

First stage PFC

Most basic AC/DC power supplies have harmonic distortion and poor power factor in the input line, making it difficult to meet the requirements of European standard EN-61000-3-2. This solution uses a PFC circuit to make the input current waveform the same sinusoidal as the input voltage waveform.

For ballast products, they comply with the European EN-61000-3-2 Class C standard. This standard applies to all lighting products, including active dimming devices with input power greater than 25W.

The PFC operates as a boost converter in critical conduction mode. It provides a relatively stable output voltage (380VDC) as the LED driver input voltage. The LED driver, as a constant current controlled buck converter, will be more suitable for rectified input voltage.

Due to the higher input ripple of the LED driver , smaller capacitors will be used in a 380 VDC environment. Electrolytic capacitors are not allowed to maintain long service life due to the higher failure rate.

This ballast uses EPCOS film capacitors instead of electrolytic capacitors, and limits the derating of all components according to internal indicators, which minimizes the failure rate and extends the service 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 step-down 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 lighting, and the dimming decoder allows a wider range of LED dimming. Figure 3 shows in detail the drain-source voltage and current of Q3 in a complete AC cycle of the LED driver.

The cycle can be divided into several different stages, the curve is as follows:

1. Closing stage

2.Conduction phase

3. Disconnection phase

4. During the disconnection phase, energy is transferred to the load

Figure 3: Drain-source voltage and current of Q3 during a complete AC cycle of the LED driver

This LED driver uses constant off-time control to regulate current through a string of LEDs. When the MOSFET turns on, the LED current through the inductor increases until it reaches a peak value defined by the reference voltage and the current sense resistor. After reaching this peak current, the MOSFET turns off and the diode conducts during the Toff period .

To drive more LEDs, some improvements were made, including the successful series connection of 60 LEDs. At an output power of 70W, the total efficiency can reach 92%, and multiple strings of LEDs (30 per string) can be driven by paralleling other LM3445s, but more cables will be needed to connect the LEDs.

Figure 4: Solution using multiple strings of LEDs

Another method is shown in Figure 4. The output voltage on the main power supply side is less than 60V, which meets the maximum voltage limit requirements of UL1310 Class 2. When the secondary voltage is required to be limited in the isolation system, the only option is multiple strings of LEDs.

On the secondary side, 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 being able to control multiple strings of LEDs, each with up to 15 LEDs connected in series.

The maximum average current value per channel can reach 500mA. Figure 4 shows how the LM3464 controls an isolated AC/DC primary offline power supply. The command issued by the LM3464 can dynamically adjust Vo to keep the voltage value through each linear regulator to the minimum. The regulation reference of Vo is the LED string channel with the highest reference voltage. Even if the drive current per channel reaches 350mA, the power efficiency of the LM3464 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 as the total power increases, EMI will become more and more difficult to control, and the only switching noise comes from the AC/DC part.

in conclusion

This article discusses different approaches to driving a larger number of LEDs. The first solution uses a PFC as a standard boost device to drive a string of 30 LEDs with an isolated heat sink, which is an attractive option for designers who need multiple strings and a larger number of LEDs. The example focuses on LED replacement, and detailed information on the LED tube is available on request. If designed correctly, in addition to high efficiency, LED tubes and ballasts have a long service life. For traditional lighting installations, maintenance costs have always been a large expense, and using LED tubes can minimize maintenance costs.

The second solution uses a primary power supply and a multi-channel linear regulator with dynamic headroom control. This solution is indeed attractive to engineers who want to equip each LED string with a dedicated power supply, but using a buck regulator in each LED string will cause problems. The LM3464 provides a smaller, cheaper, and simpler option while maintaining high power efficiency, high reliability and excellent electromagnetic compatibility ( EMC ).