In the last few years, LEDs have become more and more established on the market as ideal lighting solutions (in very different sectors: automotive, domotics, home appliances) thanks to the many guaranteed advantages (higher efficiency, energy saving, lifetime longer duration, lower costs) compared to other technologies.

Despite this, designers continue to face new challenges on a daily basis: in fact, there is an increasing demand for compact, efficient drivers with low electrical noise, with high light intensity regulation (dimmer) ratios, and advanced protection functions such as polarity inversion, short circuit to GND or Vcc, overcurrent or overvoltage protection, overtemperature protection. All parameters guarantee the correct relationship between current and luminous flux.

The LED drivers are essentially power supplies that supply power to the LEDs in a manner appropriate to their operation, but this functionality can be controlled in a more or less advanced way through devices equipped with logical functionalities.

LED Driver

The accuracy of the source and the fluctuations in terms of voltage and current are the fundamental design parameters for the correct driver. The constant current LED (DC) drivers maintain a constant electric current through an electronic circuit with a variable voltage and are often the most popular choice for LED applications. Constant voltage (CV) LED drivers are power supplies and used to operate multiple LEDs in parallel, for example, LED strips.

To further complicate the technological scenario, two other factors also contribute: the various lighting applications require many different types of drivers; each specific driver must also meet many technical requirements which are reflected in the LED drive design phase. These include temperature and humidity, voltage range, electromagnetic interference and compatibility (EMC), as well as the reliability requirements dictated by the qualification tests.

The power supply is correct if it allows producing a high-quality light, to obtain the maximum light conversion efficiency (in terms of lumens per watt) and to extend the life of the LEDs. The quality of the LED light is determined primarily by precise regulation of the current.

The LED lighting applications differ primarily in their power: generally, a distinction is made between low power applications (up to 20 watts), medium (20 to 50 watts) and high (over 50 watts). An example of low power applications is the domestic light bulb, while at the other end of the range, we find the lighting of large outdoor areas such as parking lots.

ICs Driver

LT3762 is a high-efficiency synchronous LED controller boost driver with output voltage up to 60V. This new Analog Devices driver integrates an internal programmable PWM LED signal generator and spread spectrum frequency modulation for a low EMI noise level. Internal PWM generation provides a 250: 1 ratio dimming, but it is also possible to implement 3000: 1 external dimming or analog dimming. Other features include the current-controlled mode with cycle-per-cycle limit, adjustable switching frequency between 100kHz and 1MHz, programmable lockout Undervoltage, open LED protection and short-circuit LEDs with fault condition indicators, overcurrent LED protection and thermal shutdown (figure 1).

Figure 1

Figure 1: Typical application circuit with the LT3762 [Source: Analog Devices]

A1569 is a LED driver with Hall sensor; it is the ideal solution for large household appliances, consumer electronics, and auxiliary lighting and automotive interior lighting, being the AECQ-100 qualified device. This solution is indeed UNIQUE in its kind: it is the first (highly integrated) solution that "combines" a Hall effect sensor with a system switch function, with a linear and programmable current regulator, in a single integrated circuit (SO8) able to supply up to 150 mA to "control" one or more LEDs. A dropout voltage of only 800mV also characterizes the device, thus ensuring a minimum voltage of Vin of only 7V (Fig.1). The solid state switch based on the omnipolar Hall effect technology; integrated into the device supports silent activation, without contact and is a significant upgrade compared to mechanical switches subject to failure many times. The switch is highly sensitive (Operating Point S = 40 G, TYP) and can support a wide range of mechanical configurations and different enclosures with various air gaps and degrees of mechanical misalignment (Figure 2).

Figure 2

Figure 2: Typical application circuit with the A1569 IC [Source: Allegro]

GaN Technology

Silicon has been the dominant material for power management since the late 1950s. The advantages that Silicon had, compared to other semiconductors, can be summarized as follows: lower cost, greater reliability, and greater ease of use. With the arrival of the new millennium, the development of new technologies, both in consumer and military market, has led to a demand for high power and high operating frequencies. Silicon material, with its constantly reducing margins for improvement, was not able to meet those requirements. The research has therefore focused on the development of new materials that could open a new path to transistors, restricting the research on materials which feature high breakdown voltage and high electron mobility. These studies led to the choice of Gallium Nitride (GaN).

The main benefits of GaN technology can be summarized as follows:

  • Lower resistance and capacity. GaN technology features an extremely high output power density, enabling the creation of devices with reduced loss and size and simplifying the PCB layout during the design phase;
  • High breakdown electric field. It not only almost eliminates the need for voltage converters, but potentially improves also the efficiency;
  • A very large bandgap, which allows operating even at high temperatures. HEMTs show better performance than other devices.

To operate at maximum efficiency, LED lighting converters shall have a form factor as small as possible and shall be able to withstand high temperatures since they are located very close to the light source. GaN physical-chemical characteristics allow the semiconductor to better withstand high temperatures, at the same time reducing the power supply size. Moreover, significantly higher switching frequencies and greater efficiency can be achieved. Very high switching frequencies can, however, generate some issues that, if not taken with the right consideration, could be so significant as to eliminate all the advantages that the material offers. Among those issues are the various losses we have inside the circuit: the most obvious of which is the switching losses. That type of loss is not the only one since other components inside the circuit (like diodes and inductors) also have considerable frequency-dependent losses. The main idea to avoid such losses is to use a Buck quasi-resonant converter (Figure 3), with the aim of not increasing the size and number of components. This type of switching allows having a high efficiency of the voltage converter. What we will obtain is a reduction in the voltage that passes through the transistor at power up to about twice the output voltage. By using a soft switching technique, we can further improve the circuit behavior, since when voltage drops to zero the overlaps between voltage and current are eliminated and switching losses are minimized. This type of technique significantly reduces the switching loss in Gallium Nitride FETs.

Figure 3

Figure 3: schematics of a quasi-resonant Buck converter

Conclusion

The electronics industry offers drivers based on a wide variety of circuit solutions to cope with all the different lighting applications and, at the same time, meet all the technical requirements. All major semiconductor manufacturers offer integrated circuits for intelligent LED lighting management. The market also offers various microcontrollers equipped with peripherals aimed explicitly at driving LEDs; devices of this type are produced, for example, by STMicroelectronics and Infineon.