Engineers share: Street lighting system driven by digital LEDs

LED manufacturers recommend controlling the forward current to keep the light-emitting diode at the rated luminous flux and a specific color temperature. Given that 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 strictly limited by the junction temperature, especially for high-power LEDs; junction temperature is a well-known key parameter affecting quality and service life. Specifically, as the junction temperature increases, the forward voltage and output power will gradually decrease, and thermal drift will cause the critical current to increase.

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 overheating control functions, the lighting system has an increasing demand for LED drivers with specific control functions. If you want to add value to applications such as architectural lighting and street lighting, you also need to add remote control functions to the LED driver.

Because high power factor AC-DC converters can convert grid AC voltage into higher input DC voltage, general lighting LED drivers usually adopt standard buck topology, which is based on analog monolithic solutions with integrated power switch and has a maximum output current of 350mA. If the voltage is higher than 50/60V, the monolithic solution will not be able to cope with it due to chip technology limitations. Many lighting platforms require multi-output systems that use 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 sensing method based on a shunt resistor and an 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 control of peak current, which is not the best solution for high-quality lighting because LED light color drift is not allowed in many demanding lighting applications.

1 Innovative LED driver

STMicroelectronics has proposed 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), and complete failure prevention management (overcurrent protection, overvoltage protection, and short-circuit protection). The platform consists of two parts: the power supply part and the current controller. Among them, the current controller is a digital current controller. The maximum output power of the power supply circuit reaches 130W (48V, 2.7A), and the circuit consists of two stages: a front-end power factor corrector (PFC) based on the L6562AT and an LLC resonant converter based on the 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 efficiency (93.85% at full load) eliminates the need for heat sinks
c No electrolytic capacitors for long-term reliability
d Double insulation (SELV) in accordance with EN61000-3-2 Class-C (AC harmonics), EN55022-Class-B (EMI) and EN60950

The core of the current controller is a ground-referenced current sensing method. This algorithm is implemented by a general-purpose microcontroller and can adjust the output current of the inverting buck converter. This solution does not require a differential amplifier or error amplifier, nor does it require network filters and other external passive components.

LED manufacturers recommend controlling the forward current to keep the light-emitting diode at the rated luminous flux and a specific color temperature. Given that 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 strictly limited by the junction temperature, especially for high-power LEDs; junction temperature is a well-known key parameter affecting quality and service life. Specifically, as the junction temperature increases, the forward voltage and output power will gradually decrease, and thermal drift will cause the critical current to increase.

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 overheating control functions, the lighting system has an increasing demand for LED drivers with specific control functions. If you want to add value to applications such as architectural lighting and street lighting, you also need to add remote control functions to the LED driver.

Because high power factor AC-DC converters can convert grid AC voltage into higher input DC voltage, general lighting LED drivers usually adopt standard buck topology, which is based on analog monolithic solutions with integrated power switch and has a maximum output current of 350mA. If the voltage is higher than 50/60V, the monolithic solution will not be able to cope with it due to chip technology limitations. Many lighting platforms require multi-output systems that use 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 sensing method based on a shunt resistor and an 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 control of peak current, which is not the best solution for high-quality lighting because LED light color drift is not allowed in many demanding lighting applications.

1 Innovative LED driver

STMicroelectronics has proposed 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), and complete failure prevention management (overcurrent protection, overvoltage protection, and short-circuit protection). The platform consists of two parts: the power supply part and the current controller. Among them, the current controller is a digital current controller. The maximum output power of the power supply circuit reaches 130W (48V, 2.7A), and the circuit consists of two stages: a front-end power factor corrector (PFC) based on the L6562AT and an LLC resonant converter based on the 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 efficiency (93.85% at full load) eliminates the need for heat sinks
c No electrolytic capacitors for long-term reliability
d Double insulation (SELV) in accordance with EN61000-3-2 Class-C (AC harmonics), EN55022-Class-B (EMI) and EN60950

The core of the current controller is a ground-referenced current sensing method. This algorithm is implemented by a general-purpose microcontroller and can adjust the output current of the inverting buck converter. This solution does not require a differential amplifier or error amplifier, nor does it require network filters and other external passive components.

The mode of this 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 to the ground instead of the high-side switch like the standard buck topology. Therefore, in this solution, a microcontroller can be used to drive a logic level (5V) or super logic level (3.3V) power switch directly without any gate drive stage, which makes the overall solution simple and low-cost. Figure 1 shows the complete lighting solution.

Flexibility is the purpose of this solution, which can drive up to 16 output channels individually, from low power and low voltage to high power and high voltage. STMicroelectronics has a dedicated portfolio for street lighting, so this solution allows designers to cover a wide range of different LED drive systems with just one topology.

2 Current sharing detection: dedicated microcontroller peripheral

Current control is the differentiator of this platform. The solution uses microcontroller peripherals (high-resolution timers and fast analog-to-digital converters) to manage the current control process. The trigger/clock controller is one of the components of the timer architecture, and the analog-to-digital converter trigger circuit is a special function built into the trigger/clock controller. The four trigger source events (Reset, Enable, Up/Down, Count) of the analog-to-digital converter can be managed through the TRGO signal.

In this architecture, there is a triangular carrier aligned with the center of the PWM period, which triggers the analog-to-digital converter using the TRGO signal when the maximum calculated value is reached, which is exactly in the middle of the on-time (Ton/2) waveform period.

If continuous conduction mode operation can be guaranteed, this triggering operation and the subsequent analog-to-digital conversion process will calculate the current sharing value instead of estimating the current sharing through software processing during the current increase period, as shown in Figure 2b.

Figure 2: a) LED current during the on-time (Ton); b) ADC triggering operation during Ton/2

This triggering function is embedded in the timer architecture, so there is no CPU load as the conversion operations are managed by software before the conversion data can be used in the current loop to regulate the current via a standard PI controller.

In addition, the Ton/2 current value is not affected by the switching operation (Figure 3a), and the current detection accuracy is no longer a problem because there is no delay caused by the RC filter. The current adjustment waveform with PWM dimming function is shown in Figure 3b.

Figure 3: a) LED current (green waveform) and voltage across the parallel resistor (purple waveform); b) Current sharing control on the LED string

Once the conversion operation is completed, the current control immediately executes the End Of Conversion Interrupt Service Routine every 3 PWM cycles on a channel-by-channel basis to ensure the appropriate controller bandwidth. To minimize the current mismatch between channels caused by the control conversion time, when the controller is converting and adjusting one of the channels, it also controls the remaining channels with different sampling times.

In order to change the output luminous flux during the day and adjust the overall brightness of the lighting system, the platform also adds a dimming function in the LED rectifier circuit.

To fully analyze the digital current controller implemented in an inverting buck converter topology, the efficiency vs. current load is analyzed in Figure 4. At full load, the four channels achieve a total efficiency of 97%, which meets the primary energy saving requirement.

Figure 4: Energy efficiency curve

Finally, overvoltage protection, overcurrent protection and LED short-circuit protection (for application scenarios where maintenance personnel are present) further improve the performance and market competitiveness of this street lighting platform.

3 Conclusion

The advantages of this platform include: easy implementation of 1 to 16 output channels, 1W, 3W or high-power LED power modules controlled by software and flexible digital controllers, providing the best solution for energy-efficient street lighting systems with dimmable multi-light string architectures.