# Four-Channel Buck-Type LED Driver with Automatic Current Sharing and Soft Switching
###### Abstract  
A buck-type LED driver together with automatic current sharing and high step-down voltage conversion ratio but without complex control is proposed. The proposed LED driver can not only achieve zero voltage switching (ZVS) turn-on by adding only one resonant coupled inductor, which resonates with parasitic capacitors of active switches, but also can obtain lower voltage gain and better conversion efficiency. In this paper, the operating principles and design considerations of the proposed converter are discussed in detail. In addition, the number of LED strings can be extended to more than four channels. Finally, the theoretical analysis and performance of the proposed LED driver are verified by simulations and experiments using a field-programmable logic gate array (FPGA) named EP3C5E144 CSN as a circuit control kernel.  
differential-mode transformer; LED driver; resonance; soft switching; zero voltage switching 2022  
K.-J.-F.-J.F.-J.F.-J.F  
As for the first current sharing method, the operational behavior is such that when the LED current is unbalanced, the voltage across the two ends of the differential-mode transformer is not zero. At this moment, the differential-mode transformer will be activated to force the currents on both sides to be as identical as possible. The literature [4] proposes that in order to ensure that the magnetic element meets the volt-second balance in the steady state, a diode will be added to the demagnetization path to complete the demagnetization and reduce the current error. This will reduce the overall efficiency.  
As for the second current sharing method, the ampere-second balance characteristic of the capacitor is used. In the steady state, the average current of the capacitor over one cycle is zero to achieve current balance. Compared with the first method, it has the advantages of (1) smaller size, (2) lower cost, (3) simpler structure, etc. However, every two LED strings must have a diode to control the direction of current flow, which also causes an increase in power loss.  
Generally, the switching power supplies can be divided into hard switching and soft switching according to their switching methods. For hard switching to be considered [10,11], when the switch is on and off, there will be additional switching losses from the switch.  
In order to reduce the switching loss, the related soft switching technology is proposed to avoid voltage and current overlap. Among them, soft switching includes zero voltage switching (ZVS) [12, 13, 14] and zero current switching (ZCS) [15, 16, 17, 18].  
Based on the aforementioned, resonant circuits have been proposed [19, 20, 21, 22], in which inductors and capacitors are added to form resonance. Since the voltage across both ends of the switch is a capacitive voltage, the capacitive energy must be transferred to the inductor in the form of resonance before the switch is turned on, so that the voltage across the switch is zero, and then the switch is turned on to achieve zero voltage conduction. A novel type of LED driver was proposed which is based on the LED driver [23], as shown in Figure 2, where four switches can achieve ZVS turn-on by using the parasitic capacitors of the MOSFETs and only one coupled inductor. Moreover, to make sure each LED string has the same current, the differential-mode transformers are used.  
In this paper, a four-channel buck-type LED driver with automatic current sharing and soft switching is presented, and its working principle is explained in detail. In Section 2, the operational principle is described. The specification considerations, together with how to control the LED proposed driver topology, are introduced in Section 3. Simulated and experimental results and discussions are presented in Section 4 as verification. Finally, some conclusions are given in the Section 5.  
Figure 1: Classification of LED current sharing methods.