LED manufacturers recommend to control the forward current to maintain the rated luminous flux and specific color temperature of LED. Since the brightness of LED is directly proportional to the forward current value, this control method is the best LED power solution.
In addition, the forward voltage and output power of LED are strictly limited by junction temperature, especially for high-power LED; Junction temperature is a well-known key parameter affecting quality and service life.
To be exact, with the increase of junction temperature, the forward voltage and output power will gradually decrease, and the thermal drift will lead to the increase of critical current.
In order to solve the thermal drift problem by reducing the forward voltage and improve the overall energy efficiency of the system, the brightness is controlled by PWM and / or analog dimming technology to obtain the function of failure prevention management and overheat control. The demand of lighting system for LED Driver with specific control function is increasing. If it adds value to the application of building lighting and street lighting, it also needs to add remote control function in the LED driver.
Because the high power factor AC-DC converter can convert the grid AC voltage into higher input DC voltage, the general lighting LED driver usually adopts standard buck topology. This driver is based on an analog monolithic solution integrated with a power switch, and the maximum output current reaches 350mA.
If the voltage is higher than 50 / 60V, the single chip solution will not be competent due to the limitation of chip technology.
Many lighting platforms need multi output systems with multiple drivers, which will increase the complexity of system architecture and layout design, resulting in increased design costs.
The main application limitation of the standard solution is related to the current detection method based on parallel resistor and internal comparator. The comparator compares the current fed back from the sensitive resistor with the internal reference current value, and then generates an output signal for controlling the gate drive circuit.
This commonly used analog control method realizes the peak current control, because the LED light color drift is not allowed in many demanding lighting applications, so this method is not the best solution for high-quality lighting.
Innovative LED driver
STMicroelectronics proposes a cost-effective street lighting platform solution to meet lighting requirements. The scheme has excellent performance, ultra-high energy efficiency (total energy efficiency at full load is more than 91%), and complete failure prevention management (over-current protection, over-voltage protection and short circuit protection).
The platform consists of two parts: power supply and current controller. Among them, the current controller is a digital current controller.
The maximum output power of the power supply circuit is 130W (48V, 2.7A). The circuit consists of two stages: front-end PFC based on l6562at and LLC Resonant Converter Based on l6599at.
The features of this design are as follows:
Extended European input AC voltage range (177 ÷ 277VAC – frequency 45 ÷ 55Hz）
Ultra high energy efficiency (93.85% at full load) eliminates the need for radiators
No electrolytic capacitor, long-term reliable
According to en61000-3-2class-c (AC harmonic), en55022-class-b (EMI) and en60950 double insulation (SELV) standards
The core of the current controller is to use a current detection method based on the reference of the ground wire. This algorithm is implemented by a general microcontroller, which can adjust the output current of the reverse buck converter. This solution doesn’t need differential amplifier or error amplifier, let alone network filter and other external passive components.
The mode of the reverse buck topology is continuous conduction mode (CCM). The reason for choosing CCM mode is that the power switch of the reverse buck topology is connected with the ground wire, rather than the upper arm switch as the standard buck topology. Therefore, in this solution, the microcontroller can be directly used to drive a logic level (5V) or super logic level (3.3V) power switch without any gate drive stage, which makes the overall solution simple and low-cost. Figure 1 shows the complete lighting solution.
Figure 1: LED street lighting solutions
Flexibility is the purpose of this solution. From low power and low voltage to high power and high voltage, the solution can drive up to 16 output channels independently. STMicroelectronics has a dedicated product portfolio for street lighting, so the solution allows designers to use only one topology to cover a variety of LED drive systems.
Current sharing detection: dedicated microcontroller peripherals
Current control is the difference of this platform. The solution uses microcontroller peripherals (high resolution timer and fast ADC) to manage the current control process.
Trigger / clock controller is one of the components of timer architecture. Trigger circuit of ADC is a special function built in trigger / clock controller. Through trgo signal, four trigger source events (reset, enable, up / down, count) of ADC can be managed.
In this architecture, there is a triangle carrier which is aligned with the center of PWM cycle. When the maximum value is reached, the triangle carrier uses trgo signal to trigger the ADC. The maximum value is just in the middle of the on time (ton / 2) waveform cycle.
If the continuous conduction mode operation can be guaranteed, the trigger operation and the subsequent analog-to-digital conversion process will calculate the current sharing value, instead of estimating the current sharing value by software processing during the current increase period, as shown in Figure 2B.
Figure 2: a) LED current during on (ton); b) ADC trigger operation during ton / 2
This trigger function is embedded in the timer architecture, because before the conversion data can be used in the current loop to adjust the current through the standard PI controller, the conversion operation is managed by software, so it will not increase the CPU load.
In addition, the current value of ton / 2 is not affected by the switching operation (Fig. 3a). Since there is no delay caused by the resistance capacitance filter, the current detection accuracy is no longer a problem. The current adjustment waveform with PWM dimming function is shown in 3B.
Figure 3: a) LED current (green waveform) and voltage on shunt rheostat (purple red waveform); b) Current sharing control of LED light string
Once the switching operation is finished, the current control immediately executes the end of conversion interrupt service routine every three PWM cycles channel by channel to ensure the appropriate controller bandwidth. In order to minimize the current mismatch between channels caused by controlled switching time, when the controller switches and adjusts one of the channels, it also uses different sampling time to control the other channels.
In order to change the output luminous flux and adjust the overall brightness of the lighting system in the daytime, the platform also adds the dimming function in the LED rectifier circuit.
In order to comprehensively analyze the digital current controller based on reverse buck converter topology, the energy efficiency and current load are compared and analyzed in Figure 4. At full load, the four channels can achieve 97% of the total energy efficiency, which can meet the main energy saving requirements.
Figure 4: energy efficiency
Finally, over-voltage protection, over-current protection and led short-circuit protection (with the application scenario of maintenance personnel) further improve the performance and market competitiveness of this street lighting platform. The advantages of the platform include: 1 to 16 output channels can be easily realized, and 1W, 3W or high-power LED power modules controlled by software and flexible digital controller can provide the best solution for high-energy street lighting system with dimmable multi lamp string architecture.
Editor in charge: lq6