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This paper presents a light-emitting-diode (LED) model based on the four-parameter Taylor series to describe the $V$ – $I$ characteristic of the LED at different junction temperatures. Therefore, in order to precisely model the LED nonlinear characteristics at different junction temperatures, an improved Taylor series-based LED model is proposed according to the four available parameters: 1) maximum operating point; 2) rated operating point; 3) knee point; and 4) temperature coefficient obtained from the LED $V$ – $I$ curves. Furthermore, in order to perform the dc and small-signal simulation and analyses of the LED drivers at different junction temperatures, the proposed model can be used to derive the LED dc and small-signal equivalent resistances at different junction temperatures. Finally, the measured results are compared with the modeled $V$ – $I$ curves, dc equivalent resistances, and small-signal equivalent resistances of the experimental LED at different junction temperatures.

There is currently a trend in power electronics to avoiding the use of electrolytic capacitors in LED drivers, provided that their expected life span is much lower than that of the LEDs, thus reducing the reliability of the entire LED fixture. This paper presents an optimization study about the determination of the minimal bus capacitance needed for reducing the power factor preregulator (PFP) stage dc-link capacitor down to values that are available in metalized-film technology. This optimization study will be applied to the buck converter, which is an interesting topology for high power factor (HPF) LED drivers due to its low component count, HPF, low output voltage, and lower voltage stress on semiconductors than the boost or buck–boost PFPs, which yields to lower efficiency decline under dimming operation. This way, a 70-W integrated buck–flyback converter is developed and proposed with an metalized-film polypropylene (MKP) dc-link capacitor, intended for outdoor applications where a high reliability is pursued, and capable of driving several LED lamps in different configurations. The experimental results show the accuracy of the theoretical predictions, confirming the feasibility of the buck converter as a high-performance PFP.

In this paper the integrated double buck-boost converter (IDBBC) operating with both stages in discontinuous conduction mode (DCM) is analyzed. The converter is intended to supply high-voltage LED lamps from the mains, typically in street lighting applications. A high input power factor is assured by operating the first stage in DCM, thus complying with current harmonic injection standards. In this work, the second stage is also operated in DCM as an option to the continuous conduction mode (CCM) that has been studied in a previous paper. The behavior of the converter with both stages in DCM is compared with that of the converter operation with the second stage in CCM. It is demonstrated that with both converters operating in DCM the filter capacitances are reduced even further, allowing for the use of long-life non-electrolytic capacitors. A laboratory prototype to supply a 70 W LED lamp with nominal voltage of 200 V from a 230 V/50 Hz mains maintaining the same design conditions as in the previo...

In this paper, a straightforward methodology to modeling high power factor (HPF) ac-dc converters is presented. The proposed methodology is based on the averaging technique directly applied to the converter under modeling. An averaged large-signal equivalent circuit is thus generated, which is useful to perform extremely fast simulations. From the large-signal model, both a steady-state and a small-signal analysis can be carried out to completely model the converter, achieving any desired steady-state characteristic and small-signal transfer function. The presented methodology is very intuitive and allows designers to achieve a natural understanding of the converter behavior. The methodology is illustrated with a HPF buck-boost converter operating in discontinuous conduction mode. Simulation and experimental results are provided to validate the proposed methodology.

A long-life high-power-factor integrated converter able to supply light-emitting diode (LED) lamps in high-pressure-sodium lamp retrofitting applications is presented. The proposed converter is the integration of a buck-boost converter with a buck converter, thus providing the necessary high input power factor and voltage transformation ratio to supply the low-voltage LED array from the mains. The analysis of the converter is carried out, and a design procedure is proposed. The design is also performed so that low capacitances are needed at the output of each semi-stage. This allows for the use of long-life film capacitors, thus matching the converter lifetime to that of the LED lamp. A laboratory prototype supplied from 230 V rms /50 Hz mains used to drive two LED arrays in series with a total output power of 100 W is presented.

This paper deals with the thermal characteristics of the I -V curve of GaN-based white light-emitting diodes (LEDs), focused on the variations of the dynamic resistance. The final goal of this study is to improve the static and dynamic operations of the LED driver within a wide range of temperatures. Four LEDs from different manufacturers were chosen for this study. The first part of this paper shows the thermal characterization of the forward voltage at a given injected current. After that, the experimental data are fitted in order to calculate the junction temperature accurately. Then, a small-signal analysis where the LEDs are supplied with dc current and an ac perturbation superimposed at the operation point under variable junction temperature is covered. This analysis allows the dynamic resistance to be experimentally determined for a wide junction temperature range. Furthermore, the experimental data have been fitted in order to establish the relationship between junction temperature and dynamic-resistance variation, so the dynamic resistance can be determined for a given operation point. Finally, an illustrative example is presented as a case study in order to analyze the implications of the dynamic resistance on the output-current ripple and on the closed-loop operation of a LED driver. The experimental results confirm that the junction temperature shift induces a variation in the dynamic resistance, which might have a significant effect on the output-current ripple and closed-loop performance in certain LED fixtures.

In the prior literature, the small-signal incremental impedance of fluorescent lamps has been characterized using a single-pole single-zero transfer function. This transfer function provides a good estimation of the lamp dynamics at rated power. However, when this model is used at low power levels, the accuracy is severely reduced, particularly at high perturbation frequencies (5-20 kHz). In this paper, a new double-zero double-pole transfer function is used to model the lamp dynamics with great accuracy even at low power levels. The best results have been obtained using a transfer function with two real zeros and a double complex pole. Using a new characterization procedure, the small-signal model of a 26-W compact fluorescent lamp has been obtained. All the terms of the transfer function (static gain, poles, and zeros) have been calculated at different power levels. Different polynomial expressions have been used to approximate these terms as power-dependent functions. The proposed characterization procedure also allows obtaining the equivalent resistance of the electrodes. This feature has been used to measure the effect of the extra heating imposed by IEC 60901 during dimmed operation.

This paper deals with the capability of pulse width modulation (PWM) dimming operation added to the high-power factor-integrated buck-flyback converter (IBFC), which was developed in previous works for LED lighting applications. First, the two main dimming techniques will be introduced, namely, analog dimming and PWM dimming. Then, the three main PWM dimming schemes found in the literature, that is, enable dimming, series dimming, and parallel dimming, will be briefly discussed. Afterwards, the IBFC topology will be tested performing both analog and enable dimming. In addition, a new proposal is introduced: the high-frequency series PWM dimming technique, which overcomes all the challenges faced when developing PWM dimming capabilities in low slew-rate constant-current fixed-frequency-controlled converters and offers all the advantages of PWM dimming over analog dimming while maintaining good efficiency. The proposed dimming-technique control loop, as well as the laboratory tests, will be presented, discussed, and compared to analog dimming and PWM dimming, showing satisfactory results in terms of dimming ratio, input current total harmonic distortion (THDI), and power factor.

In this paper, a digitally controlled electronic ballast is presented. The ballast corresponds to the so-called universal or multiwatt ballasts, which are intended to operate different lamps with different power ratings without performing any change in the hardware. Thus, the ballast must incorporate some kind of circuitry so that the rated power of the connected lamp can be identified. The ballast must also adjust some of its parameters in order to supply the lamp adequately, which usually means the following: 1) as close as possible to the nominal power and 2) with the best efficiency. In the proposed ballast, the parameters selected to adapt the power circuit to each lamp are the switching frequency and the resonant tank inductance. The resonant tank inductance is varied by means of a magnetic regulator whose inductance can be adjusted through a dc bias current. Due to the relatively high complexity of the proposed converter, a digital control circuit is developed to control both the resonant inverter used to drive the lamp and the dc-dc converter employed to supply the control winding of the magnetic regulator. Experimental results from a laboratory prototype for lamp powers ranging from 15 to 58 W are provided.

This paper presents an off-line integrated full ballast to supply a 35 W assembly of Power LEDs. The proposed solution integrates an input PFC stage (a flyback converter operating in DCM) and a DC-DC output converter (a buck converter) into a single switch power stage, operating with peak current control. As it will be shown, this control scheme maintains the current through the load constant, regardless of the instantaneous value of the DC link voltage. This issue allows the use of a small capacitor for the DC link, which enhances the overall system reliability. The complete ballast has full dimming capability, and all the analysis and design steps are presented, thus ensuring the fulfilling of the existing regulations. The novelty of the final solution comes from the simplicity and robustness of the control scheme in an integrated compact single-switch power stage. A final prototype of the ballast has been built and tested, and experimental results are shown in the last part of the paper. Finally, conclusions and future developments are shown

A low-cost 18W intelligent LED driver for energy savings is presented. The intelligent LED driver has a DALI interface that allows it to be included in an upper lighting smart grid. So, it must provide galvanic isolation. The proposed LED driver is based on a two stage scheme with 24V safety voltage. The first stage is based on a PFC Flyback converter operating in the boundary conduction mode, meeting the International Standard IEC61000-3-2 Class C. The second stage is based on 3×6W low-cost and efficient DC/DC buck converters for driving LEDs. The LED lamp has a built-in ambient light sensor that measures the lumen level of the ambient light and a low-cost microcontroller adjusts the emitted luminous flux in order to obtain the desired luminous level of the ambient light contributing to great energy savings. Experimental results validate the circuit proposed.

In this paper a methodology to modeling high-power-factor (HPF) ac-dc converters in LED driving applications is presented. The proposed methodology is based on the averaging methodology applied to the converter under modeling. An averaged large-signal equivalent circuit is generated in this manner, which will be useful to perform extremely fast computer simulations. From the large-signal model both steady-state and small-signal analysis can be carried out to completely modeling the converter, achieving any desired steady-state characteristic and small-signal transfer functions. The proposed methodology is very intuitive and allows designers to achieve a natural understanding of the LED driver performance. The modeling procedure is illustrated with a HPF buck-boost converter operating in discontinuous conduction mode (DCM) designed to supply a 70W LED lamp. Simulation and experimental results demonstrate the validity of the proposed methodology.

In this paper a new technique to reduce the LED current ripple and storage capacitance in high power factor (PF) off-line LED power supplies is investigated. The proposed methodology is based on the use of integrated power converters (IPC) to implement the power supply. Thus, the first stage operates in discontinuous conduction mode (DCM) to provide high PF. The second stage can be operated in either continuous conduction mode (CCM) or DCM. A family of IPCs is proposed to be used as LED lamp power supplies. A generalized analysis is carried out in order to obtain the important design equations and conditions. Finally, an example of an IPC is given, along with some experimental results to demonstrate the effectiveness of the proposed technique.

This paper deals with a topology for an off-line LED electronic driver. The use of electrolytic capacitors in the DC link is avoided, therefore extending lifespan and reliability. Given that the resulting output capacitance of the PFC stage is relatively small, a high LF ripple appears at the DC-link. This work proposes the use of a current-fed bidirectional buck converter as a second stage. With the proposed scheme, this stage processes only the AC power corresponding to this voltage ripple, thus increasing the efficiency with respect to a conventional two stage topology. In this approach, the output converter is connected in series with the LEDs assembly, yielding to low voltage stresses at the stage. With a given current control, this scheme ensures a constant current through the LEDs, and ability for AM and PWM dimming operation. A design procedure is shown, and the theoretical results have been validated in a built prototype for a 65W LEDs lamp.

Nowadays, different power generation strategies for grid-connected systems can be found, ranging from single high power plants (tens and hundreds of MW), to thousands of small wattage (below 1 kW) micro-generators. This paper studies a new power topology for the latter, based on a bidirectional Flyback converter. The comparison with the standard topology for this kind of converters is carried out, and the constraints for the control scheme are also studied.

One important aspect for the design of dimming ballasts for fluorescent lamps is the output power range that can be obtained. Some applications require that the output power can be reduced below 10%. However, in some ballast designs, there is no easy way to reduce the output power to the required minimum level without finding instabilities that could lead to light flicker, audible noise or arc extinction. In this paper, a new power-dependent small-signal model for fluorescent lamps is presented. This model is based in a double-pole double-zero approximation. The generalized multifrequency averaging technique is then used to analyze the lamp-ballast interaction in resonant inverters. This method was used to calculate the actual dimming range in two of the most widely used resonant configurations for a 26W compact fluorescent lamp.

In this paper a long-life high-power-factor integrated converter able to supply LED lamps in high-pressure-sodium (HPS) lamp retrofit applications is presented. The converter is made up by the integration of a buck-boost converter with a buck converter, which provides the necessary high input power factor and low voltage transformation ratio to supply the low-voltage LED array from the mains. The analysis of the converter is carried out and a design procedure is proposed. Also, the design is performed so that low capacitances are needed at the output of each semistage. This allows for the use of long-life film capacitors, thus matching the converter lifetime to that of the LED lamp. A laboratory prototype supplied from 230V/50Hz mains used to drive two LED arrays in series with a total output power of 100W is presented.

In this paper the integrated double buck-boost (IDBB) converter operating with both stages in discontinuous conduction mode (DCM) is analyzed. The converter is intended to supply high voltage LED lamps from the mains, typically in street lighting applications. A high input power factor is assured by operating the first stage in DCM, thus verifying harmonic injection standards. In this case, the second stage is also operated in DCM as an option to the continuous conduction mode (CCM) that has been studied in a previous paper. The behavior of the converter with both stages in DCM is compared with that of the converter operation with the second stage in CCM. It is demonstrated that with both converters operating in DCM the filter capacitances are reduced even further, allowing for the use of long-life non-electrolytic capacitors. A laboratory prototype to supply a 70W LED lamp with nominal voltage of 200V from a 230V/50Hz mains maintaining the same design conditions as in the previous work is presented. A comparison of both prototypes is also carried out.

This paper proposes an electronic driver for public lighting systems. The input stage is based on the integration of two flyback converters with different polarities, in order to avoid the use of the input diode bridge rectifying stage. The output stage is a boost converter with an output current control. The proposed circuit is introduced and analyzed, and a design example for a 50-W light-emitting diode lamp is presented. The most relevant simulation and experimental results are also shown, in order to validate the idea. A comparison in terms of efficiency for the input stage is carried out between the proposed topology and the conventional flyback converter, showing an efficiency enhancement in the proposed converter. However, the most important advantage of the proposed converter is outlined in the final part of this paper. This advantage comes from the avoidance of the input diode bridge, thus enabling the bidirectional power flow at the input stage. Therefore, the topology can be used to deliver energy into the power grid. In recent lighting applications, the use of micro renewable generators in lighting points has made the use of special electronics to handle the energy flows in the ballast necessary. The present topology is an alternative to standard bidirectional input stages.

This paper presents an off-line integrated full ballast to supply a 35W assembly of Power LEDs. The proposed solution is based in an input PFC stage and a DC to DC converter which provides a constant current through the assembly independently from the DC link voltage. The full ballast has full dimming capability, and all the analysis and design steps are presented, thus guaranteeing the fulfilling of the existing regulations. The novelty of the final solution comes from the simplicity and robustness of the control scheme in a compact power stage. A final prototype of the ballast has been built and tested, and experimental results are shown in the last part of the paper. Finally, conclusions and future developments are shown.

In this work, an interleaved inverter with peak current control is investigated to supply low wattage metal halide lamps. The proposed topology provides a low frequency square current waveform that theoretically has no high frequency ripple, thus allowing acoustic resonances free operation of these lamps. The series igniter stage equivalent is chosen so that its effect is neglected. The operation principle, the control scheme and the power stage are presented. After carrying out the analysis, a built prototype has been tested in the laboratory, showing a good behavior as a lamp ballast for these lamps. The obtained results are presented, along with conclusions and future developments.

In this paper the quadratic buck-boost (QBB) converter is proposed as a high-power-factor off-line power supply for power-LED lamps. The QBB converter features just one controlled switch and two inductors and is able to supply a solid-state lamp from the mains providing high power factor and good efficiency. In the paper, the QBB converter is analyzed and a design methodology is proposed. It is demonstrated that with a careful design of the converter, the filter capacitances can be made small enough so that film capacitors may be used. In this way, the converter mean time between failures (MTBF) can be made as high as that of the solid-state lamp. A design example for a 70W converter supplied from a 230V/50Hz mains for street lighting applications is shown. Finally, experimental results from a laboratory prototype are also presented.

In this paper a new solution to operate dc-dc resonant converters at constant frequency is investigated. The proposed converter is based on the use of a variable inductor to achieve output voltage control and regulation at constant switching frequency. The output voltage characteristics for two typical dc-dc resonant converters operating under this so-called magnetic control are analyzed. As an example, the design of a 48V input 5V/10A output, 500 kHz prototype was carried out. Also, the dynamic behavior of this prototype was investigated by performing a step response test. From this result, a compensator for operating in closed loop was designed and experimented. The obtained results showed that the proposed control method could be a good possibility to achieve constant frequency operation in dc-dc resonant converters.

This work proposes a new interleaved converter to supply Power LEDs. As an interleaved scheme, the converter provides a very low output current ripple, which makes it suitable for this application. The low inductor values used in the device allow the ability of attaining PWM dimming of the Power LEDs at high frequencies (20 kHz and higher). The main operation concept is explained, and a full design process is shown. An experimental setup, for a full diming range assembly of 10 Power LEDs has been built and tested, and experimental results are shown and discussed. Conclusions and limitations of the proposed converter are also commented.

This paper deals with the capability of dimming operation added to the Integrated Buck-Flyback Converter (IBFC) topology developed in previous works. Firstly, the three main dimming methods that have been identified by the authors such as enable dimming, shunt dimming and series dimming will be briefly commented. Afterwards, the IBFC topology will be tested performing enable dimming at 100 Hz and 200 Hz. Secondly, besides the main dimming methods, a new proposal is introduced: high frequency series PWM dimming, which overcomes the main issues faced when developing series PWM dimming in constant-current fixed-frequency controlled converters. The dimming control loop of this technique will be presented, as well as the laboratory tests performed, obtaining good results in terms of dimming ratio, harmonic content of the input current and power factor.

In pattern recognition systems the lighting method is critical to determine the success or failure of an algorithm. The goal of this work is to design a lighting workbench able to control the color and luminous flux allowing the vision systems programmers to verify the robustness of their algorithms. This paper describes the methodology to control an RGB chip to determine luminous flux and color of the whole lamp. An experimental prototype has been built and tested.

This paper presents a full ballast for driving an assembly of High Brightness LEDs, based on a new dc to dc converter. The converter provides a constant current waveform to the load parting from a DC bus voltage. The converter, operating with a peak current control, keeps the current through the LEDs constant independently of the bus voltage. This is carried out internally by self-adjusting the switching frequency of the converter. Thus, the requirements of the capacitance of the bus capacitor are greatly diminished, allowing the avoidance of electrolytic technology. Also, low frequency PWM dimming can be carried out easily. The present work shows the design procedure and the experimental results obtained after building a prototype of a full ballast for HB LEDs.

This paper proposes a novel complete electronic ballast for Low Wattage Metal Halide lamps. The lamp driver is based on the integration of a flyback PFC input stage and a buck-boost inverter stage. The resulting circuit supplies the lamps properly, operating from the mains facility. The main feature of the proposed ballast is that through an adequate integration of the stages, the ballast operates with only one controlled switch. This switch is shared by the active PFC input stage, by the output high-frequency inverter stage as well as by the igniter. The control of the ballast is carried out by adjusting the maximum current through the switch and by fixing the duty ratio. Thus, the resulting output characteristic is suitable for a proper operation of these discharge lamps, including ignition, warm-up stage and steady- state operation. The obtained high frequency square waveforms in the lamp avoid the apparition of acoustic resonances. A complete prototype has been built and tested, and experimental results are shown.

One of the most important advantages in using piezoelectric transformers (PT) is that they operate at high frequencies with a very good efficiency (1), tipically higher than 95%. This paper presents a DC/DC converter based on two piezoelectric transformers which duplicates the output voltage frequency ripple, maintaining the switching frequency. Also, the converter operates at a fixed frequency, and an alternative way to regulate the output power is shown. The converter is achieved by parallelizing two DC/DC converters, based on piezoelectric transformers. Of course, the size is increased, but on the other hand, the efficiency and the output ripple are improved. The obtained results evince an improvement in the overall efficiency, as well as a reduction in the output filter size, with a lower voltage ripple.

In this paper a digitally controlled electronic ballast is presented. The ballast corresponds to the so-called universal ballasts, which are intended to operate different lamps with different power ratings without performing any change in the hardware. Thus, the ballast must incorporate some kind of circuitry so that the rated power of the connected lamp can be identified. The ballast must also adjust some of its parameters in order to supply the lamp adequately, which means: (i) as closer as possible to the nominal power and (ii) with the best efficiency. In the proposed ballast, the parameters selected to adapt the power circuit to each lamp are the switching frequency and the resonant tank inductance. The resonant tank inductance is varied by means of a magnetic regulator whose inductance can be adjusted through a dc bias current. The PSoC technology from Cypress is used to implement the control stage. Besides a microprocessor, this technology integrates in a single chip both analog and digital modules that can be programmed, thus reducing the external component count to a minimum. Experimental results from a laboratory prototype for lamp powers ranging from 15W to 58W are provided.

New advances on magnetically-controlled electronic ballasts can grant the ability to dim the lamp and simultaneously guarantee output electrical isolation. By means of a variable transformer, whose model is presented, this new technique is developed and experimentally verified. In addition, a simple method for the large signal characterization of the transformer is described. A FH 14W/840 Osram lamp was chosen to validate this new technique and experimental results clearly show its adequate behaviour.

Due to the high rise in luminous efficiency that HB-LEDs have experienced in the last recent years, many new applications have been researched. In this paper, a streetlight LED application will be covered, using the Integrated Buck-Flyback Converter developed in previous works, which performs power factor correction (PFC) from a universal ac source, as well as a control loop using the LM3524 IC, which allows PWM dimming operation mode. Firstly, the LED load will be linearized and modeled in order to calculate the IBFC topology properly. Afterwards, the converter will be calculated, presenting the one built in the lab. Secondly, the converter will be modeled in order to build the closed loop system, modeling the current sensor as well in order to develop an adequate controller. Finally, experimental results obtained from the lab tests will be presented.

The first LED diode was created in 1969. Since then, they have been experiencing a continuous improvement in luminous efficiency and power dissipation, achieving current AIInGaP and InGaN devices for yellow to red, and blue to green, including white, colours respectively. The improvement experienced in both luminous efficiency and power dissipation makes them interesting for lighting application in automotive industry, where the advantages offered are numerous, from a lower time response in brake lights to an operating life much higher than that of the vehicle itself [1], [2]. In the other hand, the improvement experienced in luminous efficiency and power dissipation allows the construction of most of the signal devices using just a few emitters [1]. Due to the points stated above, a comparison among the most efficient red, amber/yellow and white Power LEDs currently available from the leading manufacturers is proposed, offering a comparison in terms of luminous efficiency in nominal test conditions, thermal resistance between the junction and the slug and also giving a comparison about the performance depending on junction temperature between the two most efficient LEDs for each colour.

Since the high rise in luminous flux power LEDs have experienced in the last years, they have become an overcoming alternative to incandescent bulbs for automotive signal lamps, and even an interesting choice for automotive headlamps, as most recent prototypes has shown, since the performance is near to that of HID lamps. This rise in luminous flux is based on both higher efficiency and forward current, being the latter the reason why switching regulators have become interesting for current control. Linear regulators usually work quite well in lamps where the forward current is kept at a low level, whereas the efficiency drops as forward current rises. Another drawback of linear regulators is that the load voltage must be lower than the supply voltage, otherwise the LED lamp does not work The advantages of switching regulators consist of both high efficiency whatever the working point is, and the capability of working in any condition of supply and load voltage. In this paper, three high efficiency-one switch-simple-switching converters such as buck, boost and buck-boost converter are discussed for the use in automotive LED lighting applications, both for 14 V network and 42 V Powernet system, the newest power bus system, consisting of a 14/42 V dual voltage bus.

A solution to operate DC-DC resonant converters at constant frequency is proposed. A variable inductor is used to achieve output voltage control and regulation without changing the switching frequency of the converter. The output voltage characteristic of the parallel resonant converter operating under this so-called magnetic control is briefly analysed. A 48 V input, 5 V output, 500 kHz, 25 W prototype is presented to prove the feasibility of the proposed control method.