Manufacturers today are under pressure to improve energy efficiency and finding ways to reduce system losses when the system is idle (no load) has become an important consideration. In equipment ranging from televisions and computers to microwave ovens, fridges, air conditioning systems and printers, every milliwatt saved when the device is waiting idle between operations is vital.
A significant amount of idle power is consumed by the power supply, especially in the EMI filter section. A good example is the energy consumed by the safety discharge resistor placed across the X capacitor. Another contributor to losses is any high-voltage resistor divider network, such as a feedback resistor network. These networks still operate during standby mode and their impact on power consumption can be significant.
To calculate this impact, consider a 1 MΩ discharge resistor. If 230 VAC RMS is applied across it, the loss is equal to the square of the RMS voltage across the resistor divided by the resistor value - in this case 53 mW. This is a common scenario for a 90 W notebook computer, for example. In a 200 W power supply where larger X-capacitors are required, the loss can go up to around 125 mW.
One of the standard circuit techniques to eliminate such idle losses is to open a path during standby mode using an electronically-controlled switch which takes the network offline. ICs are available to perform this function, such as the CapZero-2 two-terminal X capacitor discharge IC from Power Integrations. These devices are rated at 1 kV and can readily withstand surge voltages. CapZero-2 X capacitor discharge ICs are easy to design in and can even be retrofitted into old designs, such as a 200 W power supply in an appliance. The standalone devices can be simply dropped into the circuit, in series with the discharge resistor on the board and losses are immediately reduced. CapZero-2 ICs are already safety certified so all