LM3423 realizes solar LED street light driver

1. Introduction

The problem with traditional solar street light drivers is that the battery life is short, and the boost converter does not have any protection device to prevent short circuits. An input fuse must be added to provide protection to avoid short circuits. Using the LM3423 chip from NI (National Semiconductor), the solar LED street light driver designed for 12/24V battery input can effectively solve the problems of traditional methods and improve social and economic benefits.

2. Problems with Traditional Boost Drivers

1. The power system structure of solar LED street lights

Figure 1 Solar street light power system structure

The structure of a solar LED street light is shown in Figure 1. When the solar panel is exposed to sunlight, its internal PN junction will form new electron-hole pairs, which can generate DC current in a loop. This current flows into the controller and charges the battery in some way. The battery will be charged during the day and provide energy to the LED at night.

Solar street lights use a boost working mode, which can reduce the number of LED drivers in the system and save system costs. The boost structure makes the number of LEDs in series no longer limited by the withstand voltage of the IC, but is determined by the MOS, so more LEDs can be connected in series. Since the output voltage of most solar cells is not high, solar street lights are more suitable for using a boost structure.

The constant current design using the current mode can make the output current unaffected by changes in the input voltage, allowing the street light to maintain the same brightness when the battery is fully loaded or running low.

2. Problems with traditional boost drive

Figure 2 Traditional boost drive circuit

(1) Short battery life: In Figure 2, even if the main switch of the boost converter is turned off, a current leakage path will still be formed between Vin and Vout. If any load is connected to the output end, and the leakage current Ileak is close to Vin/Rload, the existence of the leakage current will keep the battery in working state, which will shorten the battery life.

(2) No short-circuit protection device: In Figure 2, when the output is short-circuited, the current passes through the inductor, rectifier and ground, and is only limited by the printed circuit board trace resistance, rectifier parasitic resistance and inductor DCR resistance. There is no other protection measure. Even if the main switch is turned off, this problem still exists. Therefore, an input fuse must be added to provide protection to avoid short circuits.

3. Design of LM3423 boost drive circuit for solar street lights

1. LM3423 chip function introduction

The LM3423 chip developed by National Semiconductor has an input voltage range of 4.5V to 75V, which is suitable for an extremely wide input voltage range; 50KHZ fast PWM dimming; internal reference voltage 50mV-1.25V; maximum operating frequency of 1MHZ; internal drive capability of 750mA, and ultra-low current shutdown of less than 2uA. It can be used to configure the controller of the low-side external MOSFET in boost, floating buck, floating buck/boost and SEPIC topologies [2]; uses a peak current mode controller and a predictive off-time design to regulate the LED current. The combination of a peak current mode controller and a predictive off-time design simplifies the loop compensation design while providing inherent input voltage feedforward compensation; has an integrated driver for controlling the external dimming FET and the system's "zero current" shutdown feature; and has an LED status output flag, a fault flag, a programmable fault timer and a logic pin for controlling the polarity of the dimming driver.

2. Avoid leakage current

The boost LED driver circuit designed using LM3423 is shown in Figure 3. In Figure 3, when the EN pin commands the boost converter to fail, the main switch, nDIMFET pin and FLT pin will be turned off, and the current flowing from Vin will approach zero, effectively preventing any leakage current from flowing into the LEDs connected in series, because the boost converter has failed at this time. Since the leakage current no longer exists, the battery can be fully charged during shutdown, which can effectively extend the battery life.

Figure 3 LM3423 boost LED driver circuit

3. Provide LED short circuit protection

In Figure 3, if the series LED is connected to the ground, forming a short circuit, the input P-FET driven by the FLT pin (Q1 in Figure 3) will be turned off immediately, making the input path open, so that LED short-circuit protection can be easily achieved without an external fuse.

IV. Conclusion

After testing, the boost efficiency has been greatly improved. With the widespread application of solar energy, the continuous improvement of solar LED street lights, and the continuous improvement of cost performance, solar LED will be widely used in urban and rural road lighting systems, providing environmentally friendly and economical resources for society.