No abstract

Most commercial white LEDs are made from nitridebased blue LEDs coated with yttrium aluminium garnet phosphor, which produce spectra that shift in opposite directions under the influences of drive current and junction temperature changes. This property gives rise to different emitted spectra, hence chromaticity properties, when the LED is driven/dimmed by different current waveforms. By using a commercial white LED sample, LUXEON K2, the effects of drive current and junction temperature on the changes of chromaticity coordinates are studied experimentally. The impact of dc, pulse width modulation (PWM), and bilevel current waveform is discussed through a graphical analysis, followed by experimental verification. It is proven that dc offers the best color stability over dimming due to the counteracting influences of drive current and junction temperature variations, whereas an LED constantly suffers from noneliminable chromaticity changes when driven by the PWM. Theoretical explanations are given to justify these cases, and it is found that, for the case of dc drive, an ideal heat sink's thermal resistance can be selected based on a simple equation to minimize the overall chromaticity change over dimming. This paper provides an in-depth discussion on the relations between the chromaticity properties of phosphor-converted (pc) white LEDs and the driving/dimming methods used.

Multistring arrays of LED are increasingly used for high-luminance lighting applications. The parallel connection of multiple LED strings, however, gives rise to the issue of current balancing between the strings, as a common voltage applied to them does not guarantee an equal current sharing due to the manufacturing spread in electrical properties. In relation to the need of current balancing, the emergence of AC-LED has enabled a direct driving of LED with ac voltage/current, and capacitors, whose impedance are designed to dominate over the equivalent resistance of LED string, are employed to achieve quasi-lossless current balancing. Although high-efficiency resonant inverters represent an ideal choice for driving AC-LED, their properties tend to be changed significantly by the addition of current-balancing capacitors, which necessitates a reformulation of the conventional design approach. In this paper, the characteristics of an LCC resonant network, which is based on the conventional parallel resonant network and modified by the addition of current-balancing capacitors, are analyzed in depth, and a systematic design procedure for developing an LCC-based resonant inverter with current balancing is formulated. A 100-W prototype is constructed and its performance is tested to verify the proposed design procedure. It is also shown that, with the proposed LCC resonant network, the inverter is capable of reconfiguring itself and operating stably in the case of LED failures.

No abstract

No abstract

No abstract

No abstract

Power-factor pre-regulators are commonly used in electronic equipments for minimizing the injection of current harmonics into the mains power line. However, the bandwidth of conventional pre-regulators are usually configured to be very small in order to achieve near-unity power factor, and this inevitably gives rise to poor dynamic response. Ripple estimation/cancellation method is used to eliminate the double-line frequency component from the sampled output voltage before it propagates into the voltage control loop, hence the requirement of small bandwidth is not mandatory. The existing ripple estimation circuits, however, are only accurate under specific conditions, beyond which power factor will be degraded. In view of this, a new ripple estimation/cancellation network consisting of an amplitude tuner and a phase shifter based on switchedresistor circuits is proposed and verified experimentally on a 200-W boost power-factor pre-regulator. It is shown that the proposed ripple estimation network not only provides accurate ripple estimation over a wide range of operating conditions, it also gives a way to decouple the power factor of a pre-regulator from its controller bandwidth. With the aid of the proposed ripple estimation network, the desired features of fast dynamic response and unity power factor are both achieved. Index TermsPower-factor correction, fast response, ripple cancellation, unity power factor, low-frequency ripple, switchedresistor. 0885-8993 (c) dynamic response of the pre-regulator. Hence, it can be concluded that there is a trade-off between the power factor and the dynamic performance of a PFC pre-regulator. 0885-8993 (c)