Increasing the performance of LED Drivers

The market for LED drivers in commercial and industrial lighting applications will grow over $ 4.3 billion over the next few years, mainly driven by the high adoption rate of modular recessed luminaires for the commercial lighting market. Large companies, including Texas Instruments, Infineon, Analog Devices and Maxim, will continue to dominate much of the microelectronic design of drivers.

Introduction

Whenever we are dealing with LEDs it is essential to have a driver, to be able to drive it at its best. This applies in general, regardless of the application case. The concept is valid in the automotive case or for more general lighting projects. It is also particularly convenient to use a multi-topology LED driver, also because this allows support for different input and output voltage values, which in turn depend on the application case.

LED drivers are used to provide a constant current for various applications, such as indoor commercial lighting and street / industrial lighting. For power conversion, numerous topologies are used by primary market companies. When considering LED driver topologies to convert the AC input voltage to a constant regulated current source for LED loads, it is useful to view the applications in three general power levels: low power applications that require less than 20 watts in input; medium power and high power over 50 watts input. With the increase in power rating, drivers with a dedicated stage for power factor correction and one for flyback are the most common. Other common topologies include a single-stage flyback and Buck solutions with constant current output. The type of topology widely used depends on the features implemented as well as the company’s experience in the dedicated sector. The critical parameter is the LED current. High-brightness LEDs work with several hundred mA, and this current must be kept constant. The advances in lighting technology have significantly reduced the price of LEDs thus making the burden of the most significant power driver costs in the overall cost of the system.

Maximum efficiency, low cost, input and output voltage range, THD (Total Harmonic Distortion) and regulatory requirements are all factors that must be considered when choosing the appropriate driver. Total harmonic distortion measures the noise introduced into the electrical signals by electronic devices. In general, the crossing of a signal in a system can undergo variations in terms of frequencies and amplitude, thus presenting distortion. Many LEDs, in particular, high-brightness LEDs (HB LEDs), are increasingly used in a variety of array configurations for display backlighting and digital signage. A single LED requires driving current between 20 and 60 mA and has a voltage drop from 1.8 to 4 V (nominally 1.8 V for a red LED, 2.2 V for yellow, 3.5 V for green, 3.6 V for blue and about 4.0 V for white). The designer is faced with a different mix of challenges in these three power ranges, starting from the cost, space to mount the driver, efficiency, the complexity of the design and the power factor.

ICs solutions

The choice of LED drivers must be able to satisfy the electrical, optical and thermal characteristics of the system, necessary to ensure correct operation. An example is the TPS92691, an integrated dedicated to current sensing, of rail-to-rail type based on an N-type MOSFET. It is used in applications in the automotive field for example in the driving of low beam or position lights. Other applications see him engaged in street lighting or floodlights (figure 1).

ILD2111 is a configurable buck regulator from Infineon, designed as a constant current generator. Some critical parameters, such as the protection functions, can be configured via a dedicated single-pin UART interface. The ILD2111 buck regulator can be controlled (dimming) via an external PWM signal. The controller typically uses a buck topology in a continuous conduction mode (CCM). The device automatically selects an optimal operating window concerning the switching frequency and the output current ripple. This guarantees maximum efficiency under various application conditions and can be customized with different parameters. The controller offers overload protection features, as does intelligent overtemperature protection (figure 2).

GaN Technology

The emergence of new materials at the bandgap level such as gallium nitride (GaN) with high switching frequency, will significantly improve the efficiency of the LED driver.

The characteristics of the GaN allow new solutions, simplifications, and improvements and are increasingly used in different areas, not only in the military or telecommunications sectors but above all in the lighting. However, despite the use of GaN, it brings certain improvements, it also imposes new design challenges and compromises to be accepted to avoid problems of primary importance.

Semiconductors of all types are characterized by the spacing between atoms in the crystal lattice. A difficulty with using silicon as a substrate is that atoms are not spaced at the same distance as those of a GaN layer. Growing GaN directly on the silicon would lead to a mismatch that could cause deformation, and this would only be detected through sporadic dislocations that could cause leakage currents and general deterioration of the LED’s performance. The turning point required for GaN cultivation on silicon is to use a buffer layer that can offer a better match to the silicon lattice, and then gradually transpose the buffer level into GaN. This buffer technology is the basis of the new GaN-on-Si technology.

Converters for LED lighting to operate at maximum efficiency must have a form factor as small as possible and must be able to withstand high temperatures since they are located near the light source. The physical-chemical characteristics of the GaN allow the semiconductor to withstand the higher temperatures better and at the same time allow the switching of the power supply to be reduced. Furthermore, the possibility of using significantly higher switching frequencies allows a smaller form factor to be used and therefore greater efficiency. However, it is these very high switching frequencies that can cause problems that, if not taken into consideration, could be so significant as to eliminate all the advantages that the material offers. Among these problems, we have various losses inside the circuit.

The 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.

Conclusion

As the market grows, advances in driver technology will determine winners and losers. The driver circuits will become more sophisticated, allowing significantly more power density with ever smaller packages.

The considerable growth of the LED lighting market flows differently in the various segments.  Performance and durability, two of the critical factors for the adoption of LEDs, will not evolve without efficient driver power electronics, a need that must vary greatly even for different market segments.

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