Power supply circuit design optimized for digital light projectors

　　Standby power consumption requirements range from 1W to 15W worldwide, depending on the energy saving plan and TV type. For example, in order to obtain the EPA's Energy Star certification, a digital TV must consume less than 3W in standby mode. One obvious way to reduce standby power consumption is to minimize the power consumption required by the system while in standby mode. Unfortunately, there is often nothing the power supply designer can do about this, and they are burdened with having to deliver approximately 300 mW from a limited input power budget. While this may seem easy to implement, PFCs and 250W mains supplies often draw enough power when running at no load to cause losses well above acceptable limits. Therefore, it is very necessary to disable all unused power supplies (including PFC) during standby. Typically this is accomplished by gate controlling the bias supply to the power supply controller. Fortunately, IC vendors have noticed the need for efficient light-load controllers and now offer controllers designed specifically for these applications. Figure shows an example of PFC and green mode flyback converter standby power supply. This circuit uses the energy-saving UCC28600 to minimize power consumption in standby mode. The UCC28600 is capable of entering burst mode operation at light loads and provides a signal to disable the bias supply to the PFC controller.

　　Figure The UCC28600 controller (which implements a flyback converter as a standby power supply) enters burst mode at light load while providing a signal to disable the bias supply to the PFC controller. The circuit shown in the figure is sufficient to switch the standby power supply Power consumption drops below 3W, but not enough to obtain input power below 1W. The PFC controller requires a resistor divider to sense the AC line voltage and the PFC output voltage. These resistors can easily dissipate over 200mW of power. In addition, the leakage current of the PFC output capacitor can also cause another 200mW of unnecessary losses. Adding these losses together can bring standby losses well above acceptable limits. In these cases, it may be necessary to use relays to disconnect the AC power to the PFC and all downstream converters. This relay can be used with a dedicated standby power supply. Additionally, the relay can be a solid-state relay as long as it does not require significant bias power when the system is in standby mode.