*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

protection, small size, high reliability and fast response speed. In the last generation of the LED light bulbs, the equivalent lighting effect is achieved with power consumption about 1/10 of incandescent lamp and 1/2 of the fluorescent lamp [8].

In the Section 3, the main topologies of the DC-DC switching driving circuits are

In the Section 4, the switching-type driving circuits and their interfacing with

In an LED device the emitted light follows the increment of the current. It is almost proportional to the supplied current. However, the relationship between voltage and light output is highly nonlinear. The direct voltage VF drop and the current IF are linked by an exponential function typical of the silicon diode. In **Figure 1a** the voltage–current characteristic is depicted for a white LED. The curve has been obtained by a variable voltage source with series resistance to control the diode direct current. In **Figure 1b** the circuit schematic of the LED characterization is shown. From the **Figure 1a**, the diode features rated voltage of 3.5 V and a rated current of 700 mA, while the threshold voltage Vth is 2 V. From **Figure 1b** the

> *RS* <sup>¼</sup> *VS* � *VF IF*

The circuit schematic of **Figure 1b** is also the basic linear LED brightness control. Referring to **Figure 1a** two control approaches can be performed. In the first methodology, the LED V-I curve is used to set the voltage needs to generate the requested forward current. In the second driving approach, the LED device is controlled with a constant-current source to drive the LED eliminating the high current changing due to little variations in forward voltage control. Indeed, the high slope of the voltage–current curve leads that a small change of voltage that can carry on a significant change of current through the diode consequently, a considerable change of the emitted light appears. To avoid any flickering, LEDs need a constant current source [13]. Furthermore, constant current control circuits are robustness

The LEDs can be driven by different kind of passive or active circuits. Furthermore, the active driver circuits can be classified in linear or switching topologies.

for the load short-circuit but suffer the load fully open conditions.

*(a) I-V curve characteristic for a white LED, (b) schematic of characterization circuit.*

(1)

presented and analyzed.

**Figure 1.**

**61**

**2. LED driver circuit basics**

the network AC and DC source are investigated.

*DOI: http://dx.doi.org/10.5772/intechopen.97098*

*Passive and Active Topologies Investigation for LED Driver Circuits*

resistance RS to obtain the requested current is

The design of the LED driver circuit plays a key role to achieve a performant light system. The light brightness is function to the supplied forward current. Therefore, LED is a current-driven device. The driver circuit must provide the correct level of current for the required brightness as well as comply with other characteristics such as:


Specifically, in AC connected LED driver a high PF leads a displacement power factor next to one and an input current with quite low total harmonic distortion (THD) [9, 10]. Furthermore, the LED light system must comply with the national and international standards and regulations concerning harmonic currents, such as the standard IEEE-Std-519 and the IEC 61000–3-2.

Other characteristics of the driver circuits concern the circuit structure. The circuits solutions can be passive or active topologies. Active circuits can be classified as linear or switching type. Furthermore, the driver circuits can be non-isolated if the output current is limited and a low voltage source is involved or can be isolated when the safe operative conditions are prevalent and a higher output current is requested [11]. Additionally, the information and communication technologies (ICT) are making the driving of solid-state lamps smarter and smarter, allowing to vary the brightness level (dimming) and the colors through remote and controlled communication systems by means of user interfaces developed according to the needs of the user [12].

This chapter is organized as follows.

In the Section 2, the basics of the LED driver are addressed. Furthermore, the main passive, and active circuit for the solid-state lighting driving are described and classified.

In the Section 3, the main topologies of the DC-DC switching driving circuits are presented and analyzed.

In the Section 4, the switching-type driving circuits and their interfacing with the network AC and DC source are investigated.
