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Minimise idle losses while meeting IEC 62368-1

25 Nov 2015  | Edward Ong

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Manufacturers today are under pressure to boost 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 stand-alone 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 that is needed is an update to the safety file, and one part covers a broad range of X capacitor values.

When AC voltage is applied, it blocks current flow in the X capacitor safety discharge resistors, reducing the power loss to less than 5 mW at 230 V AC. When AC voltage is disconnected, it automatically discharges the X capacitor by connecting the series discharge resistors. This operation allows total flexibility in the choice of the X capacitor to optimise differential mode EMI filtering and reduce inductor costs, with no change in power consumption. Designing with the device is simply a matter of selecting the appropriate external resistor values for the X capacitor value being used to achieve the necessary time-constant.

Safety tests
However, X capacitor discharge ICs have to be robust so they will not fail even under extreme conditions. This is why the IEC committee was so keen to ensure that proper testing provisions were included for these capacitors when it produced the IEC 62368-1 safety standard.

Although vital and mandatory, this certification can be difficult to achieve with some design techniques, such as using a flyback controller integrated with X capacitor discharge functions, where the designer is forced into using a complicated discharge path. The tests in the standard require the discharge function to happen even in the event of a single fault. Therefore, in this case with the discharge passing through many different components, system reliability is negatively impacted since if just one of those components fails then the discharge cannot happen safely.

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