# A High Power Factor LED Driver with Intrinsic Current Balancing Capability
## 1 Introduction  
Light-emitting diodes (LED) have a much longer lifespan than traditional light sources, due to continuous innovation and advancement in manufacturing techniques and materials. Generally, the service life of LEDs can reach about 100,000 h, whereas fluorescent lamps and incandescent bulbs typically last only a few thousand hours. In addition, LED has many benefits, including compact size, high energy efficiency, good color rendering, fast response speed, and so on. These advantages mean that LEDs are widely used in many light applications [1; 2; 3]. Nowadays, the power level of a high-brightness LED is approximately hundreds of milliwatts to several watts. For large luminous lighting systems, a large number of LEDs are required to meet high power requirements. These LEDs are connected in combinations of series and/or parallel connections [4; 5]. Even for the same type of LED, the voltage-current characteristic curve of each LED will inevitably have slight differences. Moreover, connecting multiple LEDs in series will amplify the difference between each LED string, resulting in unbalanced LED current in each string. What is more serious is that LED has a negative temperature coefficient characteristic, that is, its forward voltage decreases with the increase of temperature. This negative temperature coefficient characteristic will aggravate the current imbalance between the strings and even cause the LED to burn out due to thermal runaway. Therefore, the study of current balancing techniques for parallel-connected LEDs is an important topic for LED lighting apparatuses. This helps to extend the life of the LEDs and ensures that they all operate at their maximum efficiency. For lower power lighting equipment, current-mirror technology is usually used to achieve current balancing [6; 7; 8]. Current-mirror technology uses transistors in series with each LED string. These transistors arecontrolled to have the same current, i.e., the same current in the LED strings. Since these transistors are not operated as switches and their conducting voltages are high, resulting in large conduction losses. The active current-balancing techniques use logic circuits to regulate the current of each string of LEDs [9; 10; 11; 12]. This method requires a more complex circuit and is more expensive because all LED strings require separate current control loops and drive circuits. Passive current-balancing techniques use passive components such as diodes, inductors, capacitors or transformers [13; 14; 15; 16; 17; 18]. Compared to active techniques, passive techniques are simpler because no additional active switches and complex control circuitry are required. Among them, some techniques use multiple transformers with primary windings connected in series so that the secondary windings can have the same current [13; 14]. Other methods place each LED string in series with a capacitor and/or inductor with high impedance. Using high-impedance inductors or capacitors can effectively alleviate the influence of current imbalance caused by differences in LED equivalent resistance [15; 16]. Nevertheless, these approaches require the use of bulky transformers, inductors, or capacitors. References [17; 18] used the ampere-second balance principle of capacitance to automatically balance the current between LED strings. The disadvantage is that, when expanding the number of LED strings, it is necessary to add resonant inductors and capacitors, resulting in a large increase in the number of components. A series resonant converter has the advantage of low switching losses due to zero-voltage switching on (ZVS) when it is designed to operate with an inductive load. In addition, its resonant capacitor can block the DC component of the input voltage so that the resonant current flowing through it is a pure AC component. In other words, the forward average value of the resonant current is the same as the reverse average value.