**2. LED driver circuit basics**

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]. 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

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

• high reliability, necessary in cases of operation in difficult conditions (for example in transport at very low temperatures or generally, for continuity of

case of portable lighting systems with batteries as a source,

spikes from the power grid can occur in LED streetlights),

such as flicker with the fault-tolerant capability,

short circuit of converter power switches,

the standard IEEE-Std-519 and the IEC 61000–3-2.

This chapter is organized as follows.

interferences (EMI) contents.

needs of the user [12].

classified.

**60**

• high efficiency to reduce losses and improve performance and autonomy in the

• small volumes of the power converters for arranging the drive circuits to meet the demands of having LED bulbs, light fixtures, and modern lighting systems

• flexibility and precision in control to adjust the brightness to avoid phenomena

• surge protection necessary due to the vulnerability of light emitter diodes to over voltages and low resistance to reverse voltages (for example, high voltage

• additional protection functions such as input under-voltage, temperature or

• high power factor (PF), in the case of AC power supply with a satisfactory power quality waveform and consequently with low Electromagnetic

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

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

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

characteristics such as:

service needs),

as compact as possible,

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 resistance RS to obtain the requested current is

$$R\_S = \frac{V\_S - V\_F}{I\_F} \tag{1}$$

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

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.

**Figure 1.** *(a) I-V curve characteristic for a white LED, (b) schematic of characterization circuit.*

### **2.1 Passive drive circuits**

The quantities of current and voltage to be supplied to the LED to achieve the required brightness can be provided with different circuits. Passive LED drivers feature the exclusive use of passive components (e.g., resistors, capacitors, magnetic components) and silicon diodes. The simplest and most reliable circuits are passive ones. This simple and cost-effective circuits do not exhibit performance like linear or switching driver circuits and operate without precise control of the output current. They generally provide a DC current with AC current ripple but are still used in those cases where reliability and continuity of service are prevailing parameters comparing to dynamic performance and efficiency. Examples of applications are outdoor street-lights that operating in difficult environmental conditions where complex circuits can be more vulnerable. Use of an impedance between the ac line and the LED light bulb load to fix and limit the current is mandatory. The main drawbacks of these passive topologies are the low PF and THD featured, sometimes not enough to comply with the standards [14]. Passive LED drivers can be arranged in two main categories, lossy and lossless (ideal) passive circuits [15].

The lossy passive driver is usually composed of a transformer that lowers the mains voltage to one compatible with the number of LEDs to be driven (The load is generally composed of LED arrays), a bridge rectifier circuit which rectifies the alternating voltage, an electrolytic capacitor which reduces the AC ripple and finally a resistor in series with the LEDs. The current limitation is achieved by means of a simple resistor. In some LED driver applications, a linear circuit replaces the resistor [16]. The traditional passive circuit described is represented in **Figure 2a**. The stepdown transformer reduces the voltage drop on the resistor RS leading to an increase in the overall system efficiency. Furthermore, the transformer guarantees galvanic isolation. A large electrolytic capacitor CS is used to reduce the ripple appropriately in order to avoid flickering. The large value of CS necessary lead to pulsating input currents which high harmonics contents. Generally, the PF of such circuits is low and hardly is comply with the Class D limit [17].

voltage into a direct current source I0 to drive the LED load. The input capacitor Cin is useful for further improvement of the input power factor. The use of a power factor correction capacitor Cin is a standard method used in the magnetic ballast in

*(a) Passive driver circuit with valley-fill circuit. (b) Idealized waveforms of circuit operation.*

*Passive and Active Topologies Investigation for LED Driver Circuits*

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

A passive driver circuit using the Valley-fill topology is depicted in **Figure 3a**. The Valley-fill circuit has been widely used in ballast systems for powering gaseous discharge lamps. It allowed having a power factor of 95% without the need for additional control [19]. In the case of LED diode drivers, Valley-fill enhances the improvement of the ripple of the output voltage, maintaining an adequate quality of the current waveform at the input thanks also to the presence of the Lin. Also in the Valley-fill circuit the capacitors used are not electrolytic. The idealized waveforms of the main input and output voltages together with the power P0 are shown in **Figure 3b**. The analysis of the presented waveforms is reported in detail in [20].

In low power applications, linear regulators are used extensively. As power increases due to losses, these regulators are replaced by current regulated switching converters. In many applications, the LED diodes in single or in string configuration are used as indicators (smart home devices, LED displays, rear lights, directional lights in the automotive sector, animated LED circuits, etc.) with linear regulators

The principle of operation of a linear regulator is shown in **Figure** 4**a**. the necessary constant current is established by means of feedback through a sensing resistor and a comparator circuit which compares a reference voltage with the actual voltage on the sensing resistor. The necessary LED current is established by

> *ILED* <sup>¼</sup> *VREF RS*

This type of regulator is usually realized in an integrated way. The integrated solution is more attractive because reduces board space and component count, simplifying circuit and system designs [21]. In **Figure** 4**b** the application circuit of an integrated regulator (IC) with battery source (NUD4001 - On-Semiconductor)

(2)

the case of fluorescent lamps [18].

**Figure 3.**

**2.2 Active drive circuits**

the relation (2).

**63**

using dedicated integrated circuit (IC) devices.

for driving up to 500 mA of a LED strings is shown [22].

In this type of passive driver, the main cause of the efficiency reduction is the conduction losses of the RS resistor.

The "lossless" drivers use to limit the current of the LEDs ideally a lossless impedance (such as inductors and capacitors). An inductor positioned on the AC side can be used to limit the current as shown in **Figure 2b**. The inductor Lin produces an impedance that withstands the voltage difference between the input voltage VAC and the output voltage V0 required across the LEDs. The Lin impedance does not require the step-down transformer of the previous circuit solution. Furthermore, Lin acts as an input filter. As consequence on the DC side, after the rectifier bridge, it is possible to use a capacitor CS with a smaller capacity and therefore not electrolytic. The use of non-electrolytic capacitors allows a long life of the entire system. The LS on the DC side is used to convert the rectifier output

**Figure 2.** *Passive LED driver circuit. (a) Lossy passive circuit, (b) lossless passive circuit.*

*Passive and Active Topologies Investigation for LED Driver Circuits DOI: http://dx.doi.org/10.5772/intechopen.97098*

**Figure 3.** *(a) Passive driver circuit with valley-fill circuit. (b) Idealized waveforms of circuit operation.*

voltage into a direct current source I0 to drive the LED load. The input capacitor Cin is useful for further improvement of the input power factor. The use of a power factor correction capacitor Cin is a standard method used in the magnetic ballast in the case of fluorescent lamps [18].

A passive driver circuit using the Valley-fill topology is depicted in **Figure 3a**. The Valley-fill circuit has been widely used in ballast systems for powering gaseous discharge lamps. It allowed having a power factor of 95% without the need for additional control [19]. In the case of LED diode drivers, Valley-fill enhances the improvement of the ripple of the output voltage, maintaining an adequate quality of the current waveform at the input thanks also to the presence of the Lin. Also in the Valley-fill circuit the capacitors used are not electrolytic. The idealized waveforms of the main input and output voltages together with the power P0 are shown in **Figure 3b**. The analysis of the presented waveforms is reported in detail in [20].

#### **2.2 Active drive circuits**

**2.1 Passive drive circuits**

The quantities of current and voltage to be supplied to the LED to achieve the required brightness can be provided with different circuits. Passive LED drivers feature the exclusive use of passive components (e.g., resistors, capacitors, magnetic components) and silicon diodes. The simplest and most reliable circuits are passive ones. This simple and cost-effective circuits do not exhibit performance like linear or switching driver circuits and operate without precise control of the output current. They generally provide a DC current with AC current ripple but are still used in those cases where reliability and continuity of service are prevailing parameters comparing to dynamic performance and efficiency. Examples of applications are outdoor street-lights that operating in difficult environmental conditions where complex circuits can be more vulnerable. Use of an impedance between the ac line and the LED light bulb load to fix and limit the current is mandatory. The main drawbacks of these passive topologies are the low PF and THD featured, sometimes not enough to comply with the standards [14]. Passive LED drivers can be arranged

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

in two main categories, lossy and lossless (ideal) passive circuits [15].

and hardly is comply with the Class D limit [17].

*Passive LED driver circuit. (a) Lossy passive circuit, (b) lossless passive circuit.*

conduction losses of the RS resistor.

**Figure 2.**

**62**

The lossy passive driver is usually composed of a transformer that lowers the mains voltage to one compatible with the number of LEDs to be driven (The load is generally composed of LED arrays), a bridge rectifier circuit which rectifies the alternating voltage, an electrolytic capacitor which reduces the AC ripple and finally a resistor in series with the LEDs. The current limitation is achieved by means of a simple resistor. In some LED driver applications, a linear circuit replaces the resistor [16]. The traditional passive circuit described is represented in **Figure 2a**. The stepdown transformer reduces the voltage drop on the resistor RS leading to an increase in the overall system efficiency. Furthermore, the transformer guarantees galvanic isolation. A large electrolytic capacitor CS is used to reduce the ripple appropriately in order to avoid flickering. The large value of CS necessary lead to pulsating input currents which high harmonics contents. Generally, the PF of such circuits is low

In this type of passive driver, the main cause of the efficiency reduction is the

The "lossless" drivers use to limit the current of the LEDs ideally a lossless impedance (such as inductors and capacitors). An inductor positioned on the AC side can be used to limit the current as shown in **Figure 2b**. The inductor Lin produces an impedance that withstands the voltage difference between the input voltage VAC and the output voltage V0 required across the LEDs. The Lin impedance does not require the step-down transformer of the previous circuit solution. Furthermore, Lin acts as an input filter. As consequence on the DC side, after the rectifier bridge, it is possible to use a capacitor CS with a smaller capacity and therefore not electrolytic. The use of non-electrolytic capacitors allows a long life of the entire system. The LS on the DC side is used to convert the rectifier output

In low power applications, linear regulators are used extensively. As power increases due to losses, these regulators are replaced by current regulated switching converters. In many applications, the LED diodes in single or in string configuration are used as indicators (smart home devices, LED displays, rear lights, directional lights in the automotive sector, animated LED circuits, etc.) with linear regulators using dedicated integrated circuit (IC) devices.

The principle of operation of a linear regulator is shown in **Figure** 4**a**. the necessary constant current is established by means of feedback through a sensing resistor and a comparator circuit which compares a reference voltage with the actual voltage on the sensing resistor. The necessary LED current is established by the relation (2).

$$I\_{LED} = \frac{V\_{REF}}{R\_S} \tag{2}$$

This type of regulator is usually realized in an integrated way. The integrated solution is more attractive because reduces board space and component count, simplifying circuit and system designs [21]. In **Figure** 4**b** the application circuit of an integrated regulator (IC) with battery source (NUD4001 - On-Semiconductor) for driving up to 500 mA of a LED strings is shown [22].

#### **Figure 4.**

*(a) Operation principle of a linear regulator LED driver. (b) Schematic of actual IC (NUD4001) for a string LED driver applications.*

In the AC source the switching LED drivers are used specially in indoor application. For the topologies attached to the electricity grid, the power factor plays a crucial role. Two solutions are pursued. In the first solution, the PFC can be composed within a single stage together with the actual driving circuit. In this case, it is referred to as a single-stage driver (SS). The SS driver block schematic plus the filter capacitor CS are reported in **Figure 5a**. In the SS driver circuit, the filter capacitor is usually connected after the DC-DC converter, which is on the high-frequency side to obtain a high PF [26, 27]. In the second approach, the driver topologies have a two stage (TS). The first stage is a front-end PFC converter and the second stage is a DC-DC converter which controls the

*(a) SS switching driver block schematic. (b) TS switching driver block schematic.*

*Passive and Active Topologies Investigation for LED Driver Circuits*

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

requested current in a string of LED [28]. In **Figure 5b** the block schematic of a TS driver is depicted. In TS driver the filter capacitor is placed between the two semi-staged downstream from the PFC DC-DC converter to obtain high PF. SS drivers arrangement can significantly increase efficiency by dramatically reducing component cost and volume at the expense of more complex control. In the case of light bulbs of reduced power and small dimensions, single-stage topologies are certainly to be preferred. The TS solution provides accurate and flexible control also with dimming feature of the DC-DC converter separated by the PFC control design at the expense of additional circuitry and cost. Furthermore, with the second stage DC-DC converter, is possible the use of a not electrolytic capacitor. The second stage compensates the low-frequency ripple on the output voltage to achieve an AC-DC LED driver with a lifetime comparable to that of the LED devices. In the range of medium power, SS or TS topologies choice depending on the trade-off of the design constraints. Generally, TS approach is

In case of industrial environment and higher power request for high-brightness Light-Emitting Diodes (HB-LEDs), three-phase AC source can be supplied. In these applications three phase rectifier can be used to power the DC-DC converter. In three-phase AC source also a multi-cell converter solution may be used. In this topology approach, three single-phase converters in a star or delta connection to the three-phase power grid are arranged and linked in parallel connection at the output [29]. Finally, in **Figure 6a** block diagram classification of ac-dc LED drivers for both

The LED driver circuits topologies selection depending on three basic needs. The kind of energy sources (DC or AC), the power requirement and the galvanic isolation features. In the following, as the first study case, the converters for DC sources

suitable in higher power applications.

are investigated.

**65**

**Figure 5.**

single-phase and three-phase AC source are summarized.

**3. DC-DC converter topologies for LED driver circuits**


#### **Table 1.**

*Design constraint comparison for passive and active driver circuits.*

The ever increasing demand for LED systems with high brightness and improved energy efficiency, especially for portable power applications, has led to the introduction of more and more advanced switching LED current control drivers with various features and better current matching/regulation. The use of power devices in switching operation allows to overcome the limits of linear regulators regarding efficiency [23]. Furthermore, the increasing switching frequency of the last generation power devices allows using inductor and high-frequency (HF) transformer with reduced core size featuring compact volume [24]. Several converter topologies are available depending on the power range and other characteristics such as galvanic isolation need, size and cost-effectiveness, easy dimming capability, modular approach availability and efficiency target. In switching converters, the current and voltage control is achieved by pulse width modulation (PWM) strategy. In a battery source, the switching LED driver is effective in the management of multiple LED strings and array in several kinds of application such as in automotive or in portable electronics devices [25]. The main design features of switching driver circuit with pros and cons compared with linear and passive solutions are reported in **Table 1**.

*Passive and Active Topologies Investigation for LED Driver Circuits DOI: http://dx.doi.org/10.5772/intechopen.97098*

**Figure 5.** *(a) SS switching driver block schematic. (b) TS switching driver block schematic.*

In the AC source the switching LED drivers are used specially in indoor application. For the topologies attached to the electricity grid, the power factor plays a crucial role. Two solutions are pursued. In the first solution, the PFC can be composed within a single stage together with the actual driving circuit. In this case, it is referred to as a single-stage driver (SS). The SS driver block schematic plus the filter capacitor CS are reported in **Figure 5a**. In the SS driver circuit, the filter capacitor is usually connected after the DC-DC converter, which is on the high-frequency side to obtain a high PF [26, 27]. In the second approach, the driver topologies have a two stage (TS). The first stage is a front-end PFC converter and the second stage is a DC-DC converter which controls the requested current in a string of LED [28]. In **Figure 5b** the block schematic of a TS driver is depicted. In TS driver the filter capacitor is placed between the two semi-staged downstream from the PFC DC-DC converter to obtain high PF. SS drivers arrangement can significantly increase efficiency by dramatically reducing component cost and volume at the expense of more complex control. In the case of light bulbs of reduced power and small dimensions, single-stage topologies are certainly to be preferred. The TS solution provides accurate and flexible control also with dimming feature of the DC-DC converter separated by the PFC control design at the expense of additional circuitry and cost. Furthermore, with the second stage DC-DC converter, is possible the use of a not electrolytic capacitor. The second stage compensates the low-frequency ripple on the output voltage to achieve an AC-DC LED driver with a lifetime comparable to that of the LED devices. In the range of medium power, SS or TS topologies choice depending on the trade-off of the design constraints. Generally, TS approach is suitable in higher power applications.

In case of industrial environment and higher power request for high-brightness Light-Emitting Diodes (HB-LEDs), three-phase AC source can be supplied. In these applications three phase rectifier can be used to power the DC-DC converter. In three-phase AC source also a multi-cell converter solution may be used. In this topology approach, three single-phase converters in a star or delta connection to the three-phase power grid are arranged and linked in parallel connection at the output [29]. Finally, in **Figure 6a** block diagram classification of ac-dc LED drivers for both single-phase and three-phase AC source are summarized.
