Analysis of a Resonant AC‐AC LED Driver

The study analyses the performance of different LED circuit configurations feed from a low power resonant driver under pulse quasi-triangular currents. The considered LED driver topology is based on a bridgeless single-stage AC-AC converter with bidirectional switches and a parallel LC resonant tank. The converter performances are simultaneously analyzed in correlation with the most important features, such as the electric efficiency, luminous efficacy, power factor correction capabilities, and flickering implications.


Introduction
Nowadays, once with the improvements in process and technology, light-emitting diodes (LEDs) have become a very popular solution for lighting devices due to its superior efficacy performance. It is well known that many electronic converters are using an input rectifier and a high-electrolytic filtering capacitance [1,2]. In high frequency commutations, this electrolytic capacitors have reliability issues, and this fact is limiting the lifetime of the overall LED system [3][4][5][6][7]. In addition, the use of high capacitance electrolytic capacitors remains a problem in achieving high power density and high power factor. Refs. [8][9][10][11] propose different topologies of AC-DC converters capable to increase the lifetime of LED driver, by using film capacitors instead of electrolytic capacitors. The basic idea of Ref. [8] is to increase the conduction time of the input current consuming more at the peak and less at the valley of the input power. A high power factor LED driver topology consisting of a derivate topology from a two-cascade flyback converter is presented in Ref. [9]. Thus, the proposed single-stage, single-switch topology is able to provide high power factor without any unreliable electrolytic capacitors. Moreover, no feedback control circuit is required for minimizing the low-frequency ripple of the LED current. Ref. [10] proposes an AC/DC driver, which is able to provide constant current for LEDs and nearunity input power factor, as well. The idea of this chapter consists in modulating the input current, using pulsating current to drive LEDs and some energy storage elements to balance the power difference. Bearing in mind all these, it is unanimously accepted that in order to increase the lifespan of artificial LED lighting, the elimination of electrolytic capacitors is a must. Likewise, the need to increase the power factor of this electronic load together with constant output current is necessary measures for sustainable development of these technologies [12,13].
For AC-DC LED drivers, light flicker introduced by the low-frequency pulsating current represents a real problem for the performance of the system and can also have negative influence on human vision [14]. Ref. [15] presents a series-resonant converter (SRC), which can be used as a power control stage able to reduce the low-frequency ripple of the LED current. Additionally, a good performance of the system is gained due to the low switching losses of the SRC and by using film capacitors instead of electrolytic capacitance at the output of power factor correction (PFC) stage. Ref. [16] offers another method capable to obtain a low current ripple by using an average current modulator in series with the LED load. In Ref. [17], a flicker-free electrolytic capacitor-less single-phase AC-DC LED driver is being introduced. By using a bidirectional buck-boost converter, the topology is capable to limit the AC component of the pulsating current and let only the DC component to drive the LEDs. The idea of obtaining an output low-current ripple for avoiding flicker problem is also found in Ref. [18]. This work offers a two-stage flyback/ Buck converter topology for which a low output current ripple is obtained. Taking into account all the facts mentioned above, many research interests are related to the minimization of the lowfrequency current ripples first because of optical behavior and lifespan of the LED and second because of the lower efficacy of LED in high current ripples [19]. Given that, a low ripple for the LED current can be considered a good practice in designing of high quality LED lighting systems.
In some situations, such as direct AC LED lighting devices [20][21][22], the high current ripples prove not to be a problem at frequencies of 100 Hz or higher. At these frequencies, the light flicker is considered invisible for most people as is presented in [23][24][25]. A negative impact for human vision is the stroboscopic effect from flicker, which can be permanently avoided at frequencies higher than 300 Hz.
The present work introduces a new AC-AC resonant converter topology, wherein the main novelty consists on directly feeding from mains a resonant LC tank by two bidirectional switches. In comparison with Ref. [26], the advantage is the elimination of the input diode rectifier, which mainly is translated into achieving higher efficiency. Also, the driver topology is characterized by inherent constant current and high power factor; thus, no close-loop control is considered. The results are presented in correlation with: the electric efficiency, luminous efficacy, power factor correction capabilities, and flicker parameters implications.
The study is organized with a nomenclature section followed by an introduction. Section 3 analytically analyzes the proposed topology and its working principles, while Section 4 deals in the practical measurements of the proposed topology. Section 5 is dedicated for the conclusion, and some hints on future work to be done for further improvements are provided.

Considered LED driver topology
In this section, the considered LED driver topology is introduced. The circuit is subject to a patent application [27] and is based on an AC-AC quasi-resonant LC parallel converter driving an output LED stage. The considered topology is presented in Figure 1.
It can be seen in Figure 1 that the AC-AC converter is composed of two bidirectional switches, each having two MOSFETs connected with a common source. In this way, the transistors' control signals can be easily obtained from an IC (in this case, an IR21531 was used) or discrete self-oscillating driver. Using only two signals to control all four transistors is advantageous due to the simplicity and cost-related implications. In Figure 2, the presumptive waveforms of the main signals are displayed. From the upper part of the image, it can be seen that the transistors' command signals are represented by 50% duty-cycle signals.
In Figures 3 and 4 for the input positive and negative half-cycles, the main circuit states depicting the current paths and the activated switches are highlighted. It can be noted that six different stages can be found on both the positive and negative input half-cycles, as presented in Figure 2.
Referring to Figure 1, the resulted simplified circuit is exemplified in Figure 5. The transistors T1 and T2 are represented by the bidirectional switch S1, while T3 and T4 by the bidirectional switch S2.
The analysis is made by considering the positive cycle of the input alternative voltage. Starting from the simplified converter model in Figure 5, from the presumptive waveforms in Figure 2, and the current paths in Figure 3, not considering the switching time frames, three main time intervals can be identified for a half-cycle: (a) For the time interval defined in Figure 2 between t0 and t1, corresponding to Figure 3 state I, the switch S1 is conductive and S2 is in the OFF state, the converter equation is: Now, the circuit is in resonant mode, with no current in the LED (i LED ðtÞ ¼ 0):  For this time interval, the inductor current is equal to the capacitor current: (b) For the time interval between t 1 and t 2 , corresponding to Figure 3, state II, same as for previous interval, the switch S1 is in the ON state and the switch S2 in the OFF state: Because the direct biased LED is conductive, the capacitor current equals 0; thus, the coil current is equal to the LED current, as in: (c) For the time interval between t 2 and T/2, corresponding to Figure 3, state III, the switch S1 is in OFF state and the switch S2 is in ON state, the converter equation becomes: In this time interval, u LED ðtÞ > U LED , thus: Because the direct biased LED is still in conductive mode, the inductor current is equal to the LED current: At the time t = T/2, the direct biased LED current reaches the 0 value, and from this moment, the above presented behavior is being repeated for the remaining half cycle. As can be seen in Figure 4, the same behavior can be found during the negative cycle of the mains voltage.
It is known that the forward voltage of the LED changes with temperature, so in view of this: One can admit that u LED ð˚CÞ ≪ U AC 2 þ U LED , thus: Consequently, the proposed schematic has a current source behavior, with constant output current, regardless of the output LED load type/characteristics. From the input point of view, the circuit presents, to some extent, a natural corrective power factor function.
In Figures 6 and 7, the simulation results obtained with PSim 10 software point out the high power factor attained by the proposed circuit. The mains input current waveform for half a cycle is slightly liner/constant and, admittedly, the input current waveform shows, once more, the current source/constant current behavior of the converter.

Practical implementation of the AC-AC LED driver
The practical measurements have been done on the converter topology from Figure 1 (named the AC-AC type) using the following LED modules: CREE-XLamp CXA1304, CITIZEN-CLU028-1202C4-40AL7K3, and OPTOFLASH-OF-LM002-5B380. Because the Cree LED has an antiparallel diode, the circuit from Figure 8, named the AC-AC-1 type, is proposed where a fast diode, with low voltage drop, has been introduced in series with each LED strings.
To reinforce the presumptive waveforms from Figure 2, the experimental results for the direct AC-AC driver considering the Citizen LED module are shown in Figures 9-12, wherein the signals are being presented both at low and high frequency ranges.
On the upper part of Figure 9, the input voltage u AC ðtÞ and current i AC ðtÞ are represented. On the lower part, the low frequency representation of the output resonant tank voltage, u s ðtÞ and LED current, i LED ðtÞ is highlighted. Figure 10 presents the main output signals in relation with the transistor control signal u GS, T1, T2 . Therefore, the output voltage of the resonant tank, u s ðtÞ is represented in conjunction with the LED voltage, u LED ðtÞ and LED current, i LED ðtÞ.  Figure 11 highlights the voltage at the drains for the bidirectional switch composed of T1 and T2 transistors in conjunction with the gate control signals and the output LED current. One can notice that the gate signal used for the T1 and T2 transistors is applied after the voltage at the drains was lowered close to zero, thus ZVS (zero voltage switching) is attained. This represents one of the most important aspects in using resonant converters, where the switching losses are   Analysis of a Resonant AC-AC LED Driver http://dx.doi.org/10.5772/67472 narrowed to a minimum value. Moreover, the ZVS helps limiting the EMI levels caused by high-frequency switching converters. Figure 12 shows, on the upper part, the output current of the resonant tank composed of the LED current, i LED ðtÞ, and the resonant capacitor current, i C ðtÞ. On the lower part of the picture, the LED String 1 current and the LED String 2 current are being represented.
The general performances of the converter with all the three LED modules are centralized in: Table 1 for the Cree LED, Table 2 for Citizen LED, and Table 3 for the OptoFlash LED. The power measurements were completed by the use of the precision power analyzer KinetiQ PPA2530. The flicker measurements have been performed with the light sensor OPT101 from Texas Instruments. From these waveforms, the percent flicker and the flicker index were deducted.
For all the LED types, the converter components and the switching frequency were kept the same, and since the forward voltage of the modules was different, dissimilar input power values have been obtained. Analyzing the electric efficiency, it can be observed that higher efficiency is achieved at higher input power, regardless of the LED type used. Also, the power factor is negatively influenced by the lower input power level.
The implementation of control loops was not an objective of the present study. Thus, the study states that for low power, acceptable performances in terms of light quality, high efficacy, and long lifespan with no capacitive filtering, the single-stage AC-AC with two antiparallel LED The proposed solution was closely analyzed, considering most of the known issues and good practices. The need for high power factor together with constant output current is the feature of an LED drive that is not the subject of discussion. Also, high efficacy, light quality, and lifespans in a cost-efficient technology are targets for high performance LED devices. The study attempts to address all aspects presented above, but as a well-known general rule, some of the above criteria are more important than others, which are defined by the target application.

Conclusion
The present research introduces a method of using the benefits of soft switching, by the implementation of a resonant converter in controlling the current for LED lighting devices. The topology is a single-stage AC-AC converter that is capable of obtaining high power factor in an inherent way, with no feedback control loop. What is more, the circuit has a strong current source behavior; thus, no imperative constant output current control is required. All the characteristics are inherently attained, with no control loops; thus, there is room left for further improvement. Mainly related with the AC-AC stage, the feasibility of the suggested solution is increased if electronic components manufacturing companies are willing to introduce bidirectional controlled switches in a single-chip technology for all power/voltage range applications.
Future work can consider the output circuits with low or high capacitance filtering, closed loop constant current controls, discrete self-oscillating control circuit, and higher switching frequencies for lower inductance needed for the resonant coil.

Nomenclature
Author details