By Fabrizio Di Franco, Akshat Jain, STMicroelectronics
This Design Note describes the operation of a new demonstration board, the STEVAL-LLL004V1 from STMicroelectronics, which implements a complete circuit for converting a mains AC input to a DC output for driving LEDs with a maximum load of 75W. The board, as shown in Figure 1, shows how such a system can be entirely digitally controlled, an approach which gives much greater design flexibility and lower cost than the conventional analogue alternative.
The circuit is capable of dimming LEDs down to 0.5% of their maximum brightness. The power quality parameters are within the acceptable limits set by the IEC 61000-3-2 standard for universal AC mains.
Operation of the power converter circuit
The LED driver has two conversion stages. The frontend is a Power Factor Correction (PFC) converter which provides a regulated DC output. The downstream stage consists of a buck converter and modified buck converter connected in parallel, as shown in Figure 2. The 32-bit microcontroller implements digital control of the PFC stage and the two DC-DC converters.
The main advantage of digital control is the flexibility to tune settings on the fly, for any given operating condition, without any modification of the hardware. By contrast, an analogue control scheme can only be tuned for a narrow range of operating conditions.
The digital architecture also supports advanced features such as:
• choice of analogue or digital dimming
• choice of 0 to 10V dimming control or remote wireless control
• high dimming resolution
• temperature monitoring
In addition, protection and communication functions tend to be cheaper in a digital system since they can be implemented with a single IC. Digital control also provides more stability than an analogue circuit in noisy conditions.
The STEVAL-LLL004V1 LED driver’s three power stages, operating in transition mode for high efficiency, are controlled by a STM32F071CB microcontroller.
The MCU as well as the gate-drive section, including the PM8841D low-side gate driver, PM8834 dual low-side gate driver and L6395D high- and low-side driver, are supplied by a VIPer013LS offline converter, which manages the auxiliary switch-mode power supply in a flyback topology.
The driver’s use of an STD11N60M2-EP, a 600V MDmeshTM M2 EP power MOSFET, helps it to achieve overall efficiency of >91% at full load.
The MCU performs zero-crossing detection of the inductor current and of other relevant signals for all the power converters, and controls the MOSFET gate-drive signals. The board includes a comprehensive set of safety functions: short circuit, open circuit, input under-voltage and input over-voltage protection.
Choice of dimming functions
The STEVAL-LLL004V1 may be configured to perform either digital, PWM, or analogue dimming. In digital dimming, the current to the LEDs turns on and off at a fixed frequency, normally higher than 100Hz to avoid visible flicker. The LEDs always turn on at the nominal current level. The average current to the LEDs is the product of the total nominal current and the duty cycle of the dimming function. The brightness level can be adjusted by changing the duty cycle. In analogue dimming, the LEDs’ current remains continuously on, but its value changes.
Each technique is appropriate for different application requirements, and the benefits of each are shown in Table 1.
|Digital dimming||Analogue dimming|
|No colour shift as LED current remains the same||Colour shift as LED current changes|
|Possible current inrush problems||No inrush current|
|Linear change in brightness||Inferior linearity|
|Lower optical-to-electrical efficiency||Higher optical-to-electrical efficiency|
|Frequency limitations and concerns||No frequency concerns|
Table 1: Advantages and disadvantages of analogue and digital dimming
Digital control algorithm
The LED driver’s control algorithm runs on the STM32F071CB MCU, a member of the STM32 family. A Proportional-Integral (PI) control loop has been implemented for the PFC stage, improving the stability of the control loop, line transition and dimming operation, and reducing current and voltage overshoot at start-up. The buck converters work in hysteretic mode.
The dimming technique and control functions can be selected using the board’s toggle switches. Digital dimming uses one of the MCU’s timer peripherals running at 500Hz.
Analogue dimming is implemented using an internal comparator in the MCU and a DAC peripheral. The current threshold, the inductor peak current, at the non-inverting end of the internal comparator is adjusted by means of the DAC.
While the DC-DC converters operate in transition mode and control the inductor’s peak current level, the LEDs can only be dimmed to a limited extent. To perform dimming at low brightness levels, both converters are made to operate in discontinuous mode, as shown in Figure 3.
The overall efficiency, power factor, and Total Harmonic Distortion (THD) of the STEVAL-LLL004V1 have been tested at various loads. At full load and operating from a 230V AC supply, it achieves efficiency higher than 91%. Figures 4, 5 and 6 illustrate the driver’s performance in terms of efficiency, power factor and THD.
Operation is flicker-free down to the lowest 0.5% dimming level using either digital or analogue dimming. Test results show that the driver achieves high efficiency, a power factor near unity and low THD under a wide range of input-voltage and load conditions.