Open access peer-reviewed chapter

Analysis of a Resonant AC‐AC LED Driver

By Teodosescu Petre Dorel, Szekely Norbert Csaba, Sabau Madalina Sabina and Bojan Mircea

Submitted: September 15th 2016Reviewed: January 13th 2017Published: July 12th 2017

DOI: 10.5772/67472

Downloaded: 675

Abstract

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.

Keywords

  • light‐emitting diodes
  • converters
  • resonant conversion
  • soft switching

1. 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 [37]. In addition, the use of high capacitance electrolytic capacitors remains a problem in achieving high power density and high power factor. Refs. [811] 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 near‐unity 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 low‐frequency 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 [2022], 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 [2325]. 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.

2. 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.

Figure 1.

Considered LED driver.

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.

Figure 2.

Presumptive waveforms of the AC‐AC LED driver.

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.

Figure 3.

The six main time intervals for the input positive half‐cycle.

Figure 4.

The six main time intervals for the input negative half‐cycle.

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.

Figure 5.

Simplified, model‐based schematics of the consider LED driver.

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:

us(t)=LRESdiL(t)d(t)+uLED(t)E1

Now, the circuit is in resonant mode, with no current in the LED (iLED(t)=0):

uc(t)=1CRESic(t)dtE2

For this time interval, the inductor current is equal to the capacitor current:

iL(t)=iC(t)E3

(b) For the time interval between t1 and t2, 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:

us(t)=LRESdiL(t)d(t)+uLED(t)E4

Now, uLED(t)>ULED(the forward voltage of the direct biased LED), and for the considered time frame where us(t)=UAC2, the above equation becomes:

diL(t)d(t)=UAC2ULEDLRESconst.E5

Because the direct biased LED is conductive, the capacitor current equals 0; thus, the coil current is equal to the LED current, as in:

iL(t)=iLED(t)E6

(c) For the time interval between t2 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:

us(t)=LRESdiL(t)d(t)+uLED(t)E7

In this time interval, uLED(t)>ULED, thus:

diL(t)d(t)=UAC2ULEDLRESconst.E8

Because the direct biased LED is still in conductive mode, the inductor current is equal to the LED current:

iL(t)=iLED(t)E9

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:

diL(t)d(t)=UAC2+ULED±uLED(˚C)LRESE10

One can admit that uLED(˚C)UAC2+ULED, thus:

iLED(t)=UAC2+ULED±uLED(˚C)LRES const.E11

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.

Figure 6.

Simulated low frequency representation of the input voltage/current and output current iLED.

Figure 7.

Simulated high frequency representation of the output current waveforms: output resonant tank current; LED string 1 current; LED string 2 current.

3. 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.

Figure 8.

Proposed AC‐AC‐1 type converter.

General characteristics and converter components used are: IR21531 self‐oscillating IC, IRF640 transistors, STPS2L40 high‐frequency diode, LRES – 2 mH resonant coil, CRES – 2.2 nF resonant capacitor, C1‐C2 100 nF voltage divider, L1 – 4 mH input filter, and 82 kHz switching frequency.

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 912, wherein the signals are being presented both at low and high frequency ranges.

Figure 9.

Low frequency signals representation: the input voltage uAC(t); the current iAC(t); the output resonant tank voltage, us(t); the LED current, iLED(t).

Figure 10.

High frequency signals representation: transistor control signal uGS, T1, T2;. The output voltage of the resonant tank, us(t); LED voltage, uLED(t); LED current, iLED(t).

Figure 11.

High frequency signals representation: the gate control signals (upper part); the voltage at the drains for the T1‐T2 bidirectional switch; the output LED current (lower part).

Figure 12.

High frequency signals representation: the LED current, iLED(t) and the resonant capacitor current, iC(t) (upper part); LED String 1 current and the LED String 2 current (lower part).

On the upper part of Figure 9, the input voltage uAC(t)and current iAC(t)are represented. On the lower part, the low frequency representation of the output resonant tank voltage, us(t)and LED current, iLED(t)is highlighted. Figure 10 presents the main output signals in relation with the transistor control signal uGS, T1, T2. Therefore, the output voltage of the resonant tank, us(t)is represented in conjunction with the LED voltage, uLED(t)and LED current, iLED(t).

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 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, iLED(t), and the resonant capacitor current, iC(t). 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.

Input power [W]Electric efficiency [%]System efficacy [lm/W]Power factorTHD [%]Flicker indexPercent flicker [%]
AC‐AC
AC‐AC‐15.0379.6102.390.91628.00.35100

Table 1.

Practical measurements—LED‐Cree.

Input power [W]Electric efficiency [%]System efficacy [lm/W]Power factorTHD [%]Flicker indexPercent flicker [%]
AC‐AC7.991134.360.9529.80.3100
AC‐AC‐17.9790.3128.680.95229.10.30100

Table 2.

Practical measurements—LED‐Citizen.

Input power [W]Electric efficiency [%]System efficacy [lm/W]Power factorTHD [%]Flicker indexPercent flicker [%]
AC‐AC7.1589.797.450.95228.80.34100
AC‐AC‐17.2488.296.910.926290.34100

Table 3.

Practical measurements—LED‐Optoflash.

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 strings could be an interesting solution. For upgraded results, mainly related with light quality, output rectifying solutions with high capacitance filtering are solutions to be investigated. The inherent current source behavior of the converter and high power factor are naturally being accomplished. For further improvements, pure sinusoidal input current shape building and dimming functions can be implemented by means of variable frequency closed‐loop control.

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.

4. 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.

Acknowledgments

This work was supported by a grant from the Romanian National Authority for Scientific Research, MEN‐UEFISCDI, project PN‐II‐PT‐PCCA‐2013‐4‐0914, No. 60/2014 ‐ in the frame of the project with the title”Innovative electric power grid protection device against low power factor electronic loads” (IEDPFC).

Nomenclature

CRESResonant tank capacitor
DDuty cycle; diode
iLEDInput current for the LED arrangement circuit
iACAlternative input current
iCResonant tank capacitor current
iLResonant tank inductor current
LRESResonant tank inductor
SnBidirectional switch n
tTime
uACAlternative input voltage
uDD,T1,T2Drains voltage for the T1‐T2 bidirectional switch
uGSGate‐source control voltage
uLEDLED voltage
ULEDLED forward voltage
uSAC‐AC converter output voltage

© 2017 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Teodosescu Petre Dorel, Szekely Norbert Csaba, Sabau Madalina Sabina and Bojan Mircea (July 12th 2017). Analysis of a Resonant AC‐AC LED Driver, Optoelectronics - Advanced Device Structures, Sergei L. Pyshkin and John Ballato, IntechOpen, DOI: 10.5772/67472. Available from:

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