Figure 3: Typical current LCD-TV power supply strategy
In total, four power stages are involved in driving the LED backlight array; their cumulative energy losses can reduce the total efficiency of the backlight supply to less than 65 percent. Similarly three stages are needed to supply the audio and processing subsystems, implying overall efficiency of around 70 percent assuming a typical efficiency level for each stage. The diagram also shows the dedicated standby supply typical of traditional TV power supplies, which makes a further contribution to energy losses and bill-of-materials costs.
Next-generation power supply design
A number of new power supply schemes are emerging to help designers improve the energy efficiency of LED-backlight TVs across the range of standard screen sizes from 26 inches to 46 inches and larger. Some key innovations, taking advantage of new device technologies for high-efficiency conversion and control, are enabling power supply designers to eliminate circuitry such as the dedicated standby power supply and simplify or design-out the boost converter for the LED backlight, thereby saving energy losses and bill-of-materials costs.
Typical TV power ratings in the 26-inch size class are low enough that Power-Factor Correction (PFC) is not required. A single flyback converter, using a controller such as the ON Semiconductor NCP1236, can be used to supply the backlight as well as the audio and signal-processing subsystems. This can be achieved using only two outputs of 5V and 24V. The NCP1236 can supply up to 4A at 5V to power the TV’s signal processing circuitry, while also providing 24V to drive a 10WRMS/8Ω
half-bridge audio amplifier as well as the backlighting drivers. This device has its own high-voltage startup circuity, and can be used in cost-sensitive applications where active X2 capacitor discharge is not required.
For a small backlight array, a relatively low boost voltage up to around 40V is adequate, and the design is able to use an integrated boost converter and driver.
The traditional dedicated standby power supply is also being eliminated in next-generation designs. Instead, a secondary-side OFF-mode controller such as the NCP4353/4 can be used to detect no-load conditions, control entry into low-power modes, and provide power with a reduced voltage to a standby LDO regulator. This approach reduces standby power to below 75mW and no-load power to less than 30mW when used with the NCP1246 flyback controller, which has built-in self-supply capability that allows control over start-up current when waking from standby mode. The two devices communicate to each other through the same optocoupler that is used for the feedback network so no extra circuitry is required. In addition, the NCP1246 has a dedicated OFF mode that may be used with the NCP4353/4, if desired, to signal the NCP1246 to enter sleep mode. Together, the devices can reduce no-load input power consumption to better than 10mW. A further feature of the NCP1246 is active X2 capacitor discharge capability, which triggers discharging of the capacitor after detecting that the equipment has been unplugged from the AC supply. This is an effective safety feature in many consumer electronics applications.