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

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 are investigated.

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

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

**Advantage Disadvantage**

• Not accurate current control • High power dissipation on the resistor

• Low Efficiency with increasing LED

• High power dissipation with heat-sink need with increasing LED power

for high power LED

• More complex design • Higher cost • EMI design constraint

power request

request • Lower efficiency

solutions are reported in **Table 1**.

**Figure 4.**

**Driver topology**

Active Linear

Active switching

**Table 1.**

**64**

*LED driver applications.*

**Current control method**

Linear control loop

PWM current control

Passive Resistor • Easy design procedure

• Low cost

• Accurate current control • Capability of dimming by current control • do not require electromagnetic interference (EMI) filters

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

• High efficiency • Dimming capability by PWM control

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

**Figure 6.**

*Block diagram of AC-DC LED driver circuits classification.*
