Engineers share: Digital LED-driven street lighting system
LED manufacturers recommend keeping the LEDs at a nominal luminous flux and a specific color temperature by controlling the forward current. Since the brightness of the LED is proportional to the forward current value, this control method is the best LED power solution. In addition, the forward voltage and output power of the LED are severely limited by the junction temperature, especially for high-power LEDs; junction temperature is a well-known key parameter affecting quality and lifetime. To be precise, as the junction temperature increases, the forward voltage and output power will gradually decrease, and thermal drift will cause the critical current to rise.
In order to solve the thermal drift problem by reducing the forward voltage, improve the overall energy efficiency of the system, control the brightness through PWM and / or analog dimming technology, obtain anti-failure management and overheat control functions, the lighting system has a constant demand for LED drivers with specific control functions. improve. If you add value to applications such as architectural lighting and street lighting, you also need to add remote control to the LED driver .
Because high-power-factor AC-DC converters convert the grid AC voltage to a higher input DC voltage, common lighting LED drivers typically use a standard buck topology based on an analog monolithic solution that integrates a power switch. The maximum output current reaches 350mA. If the voltage is higher than 50/60V, the monolithic solution will not be able to do so because of chip technology limitations. Many lighting platforms require multiple output systems that use multiple drives, which adds complexity to the system architecture and layout design, resulting in increased design costs.
The main application limitations of standard solutions are related to current sensing methods based on shunt resistors and internal comparators. The comparator compares the current fed back from the sensitive resistor with the internal reference current value and then produces an output signal for controlling the gate drive circuit. This common analog control method enables peak flow control because LED light color drift is not permitted in many demanding lighting applications, so this method is not the best solution for high quality lighting.
1 innovative LED driver
STMicroelectronics proposes a cost-effective street lighting platform solution that meets lighting requirements. The solution has excellent performance, ultra-high energy efficiency (overall energy efficiency greater than 91% at full load), complete fail-safe management (overcurrent protection, overvoltage protection and short circuit protection). The platform consists of two major components: the power supply section and the current controller. Among them, the current controller is a digital current controller. The maximum output power of the power circuit reaches 130W (48V, 2.7A). The circuit consists of two stages: front-end power factor corrector (PFC) based on L6562AT and LLC resonant converter based on L6599AT.
The features of this design are as follows:
a Extended European input AC voltage range (177 ÷ 277 VAC – frequency 45 ÷ 55 Hz)
b Ultra-high energy efficiency (93.85% of full load) eliminates the need for heat sinks c Electrolytic capacitors, long-term reliability d in accordance with EN61000-3-2 Class-C (AC harmonics), EN55022-Class-B (EMI) and EN60950 double insulation (SELV) standard
The core of the current controller uses a ground-referenced current-sense method implemented by a general-purpose microcontroller that adjusts the output current of the inverting buck converter. This solution eliminates the need for differential amplifiers or error amplifiers, and eliminates the need for network filters and other external passive components.
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