D. Gacio

A Universal-Input Single-Stag2e High-Power-Factor Power Supply for HB-LEDs Based on Integrated Buck–Flyback Converter

Due to the high rise in luminous efficiency that high-brightness light-emitting diodes (HB-LEDs) have experienced in recent years, many new applications have been researched. This paper covers a streetlight LED application using the offline integrated buck-flyback converter (IBFC) developed in previous works. In this application, the converter is used to provide power-factor correction from a universal ac source. A control loop using the LM3524 IC has been implemented for testing the feasibility of Enabling pulsewidth modulation dimming mode. First, the LED load has been linearized and modeled in order to calculate the IBFC topology properly. Second, the converter has been designed and tested at the laboratory. The converter has also been modeled in order to design a fixed-frequency constant-current control. The dynamics of all the stages involved in the closed loop have been taken into account in order to develop an adequate controller. Finally, the experimental results obtained from the laboratory tests are discussed.

A Universal-Input Single-Stage High-Power-Factor Power Supply for HB-LEDs Based on Integrated Buck-Flyback Converter

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.

PWM Series Dimming for Slow-Dynamics HPF LED Drivers: the High-Frequency Approach

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.

Electronic driver without electrolytic capacitor for dimming High Brightness LEDs

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.

Effects of the Junction Temperature on the Dynamic Resistance of White LEDs

This paper deals with the thermal characteristics of the I-V curve of GaN-based white LEDs, focused on the variations of the dynamic resistance. The final goal of this study is to improve the static and dynamic operation of the LED driver within a wide range of temperature. Four LEDs from different manufacturers were chosen for this study. The first part of the 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 an 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.

Reducing storage capacitance in off-line LED power supplies by using integrated converters

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.

A long-life high-power-factor HPS-lamp LED retrofit converter based on the integrated buck-boost buck topology

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.

High frequency PWM dimming technique for high power factor converters in LED lighting

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.

Analysis and design of the integrated double buck-boost converter operating in full DCM for LED lighting applications

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.

Optimizing Universal Ballasts Using Magnetic Regulators and Digital Control

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.