Abstract—Due to
their high reliability and luminous efficacy, high-brightness light-emitting
diodes are being widely used in lighting applications, and therefore, their
power supplies are required to have also high reliability and efficiency. A
very common approach for achieving this in ac–dc applications is using a
two-stage topology. The power factor corrector boost converter operating in the
boundary conduction mode is a very common converter used as first stage. It is
normally designed without electrolytic capacitors, improving reliability but
also increasing the low-frequency ripple of the output voltage. The
asymmetrical half-bridge (AHB) is a perfect option for the second stage as it
has very high efficiency, it operates at constant switching frequency, and its
output filter is small (i.e., it can be also easily implemented without
electrolytic capacitors). Moreover, the AHB is an excellent candidate for
selfdriven synchronous rectification (SD-SR) as its transformer does not have dead
times. However, the standard configuration of the SD-SR must bemodified in this
case in order to deal with the transformer voltage variations due to the input
voltage ripple and, more important, due to the LED dimming state. This
modification is presented in this paper. Another important issue regarding the
AHB is that its closed-loop controller cannot be very fast and it cannot easily
cancel the previously mentioned low-frequency ripple. In this paper, a
feed-forward technique, specifically designed to overcome this problem, is also
presented. The experimental results obtained with a 60-W topology show that
efficiency of the AHB may be very high (94.5%), while the inherent control
problems related to the AHB can be overcome by the proposed feed-forward technique.
(Index Terms—AC–DC converter, asymmetrical half
bridge, dc– dc converter, LED drivers, low-output voltage, self-driven
synchronous rectification (SD-SR).)
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