For 37-42 inch TVs, which require a larger LED backlight array and therefore a higher LED supply voltage, using a dedicated backlight PSU can be more efficient than boosting a lower DC voltage. A quasi-resonant controller such as the NCP1379, supplying a set of linear LED drivers such as the CAT4024 is an effective solution, allowing the output voltage supply variation to follow the LED’s requests due to aging and temperature. A Flyback PSU without PFC is capable of supporting a wide input supply voltage range from 90 to 264 Vac.
Finally, in higher-power applications such as the 95-120W power supplies typically required for existing applications up to 46 inches, PFC is required. The ON Semiconductor NCP1611 PFC controller operates in Current Controlled Frequency Foldback (CCFF) mode. At high current, it operates in standard critical-conduction mode (CrM), which allows the size of the transformer secondary coil to be reduced, and provides a follower boost feature that enhances the output capability of the audio/processing and backlight power converters. At light load a timer is enabled which controls the dead time to the next switching cycle based on the current demand. In contrast to CrM, as the load is reduced the switching frequency is also reduced which decreases the switching losses. The switching frequency can also be clamped above about 20kHz eliminating audible noise typically heard in CrM designs. This combination provides about 10 percent improvement of efficiency at light load and an improved design.
shows a simplified power supply design combining these enhancements. A dedicated backlight supply using NCP1379 and CAT4204 replaces the LED boost converter, there is no separate standby supply, and an NCP1236/46 supplies both the audio and processing subsystem. The NCP1611 PFC controller can be added if required. The final design is more efficient due to the elimination of multi-stage power conversion and smaller due to the elimination of components. This approach will enable faster and more robust designs with similar PSU solutions while supporting a wide range of TV screen sizes and power.
Figure 4: Generic power supply design saving conversion losses and component costs
Offering eye-catching thin designs and enhanced user experiences, today’s flat-panel televisions are highly valued and highly watched by their owners, but are also increasing concerns to reduce power consumption and increase energy efficiency. New Energy Star standards place more emphasis on operating power by limiting average power consumption according to screen size. Simpler, optimized power supply architectures featuring high-efficiency topologies and controllers can help designers meet these demands at a competitive price.
About the authors:
. Tim Kaske
is product marketing manager at ON Semiconductor
. Jean-Paul Louvel
is senior applications engineer at ON Semiconductor
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