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Lack of ‘off’ switches strains grid

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An Australian survey* has found that most new portable stereos have no “hard’ off switch, and that average passive standby power consumption for new portable stereos in 2003 was 2.0 W, with 95 percent of units measured in 2003 registering more than 1 W.

With the power grid struggling to keep pace in terms of installed power generation capability increasingly leading to “brown-outs” and “black-outs”, governments are looking at the issue of standby power in small appliances. International Energy Agency (IEA) regulations now require that power supplies with rated output under 75 W should draw no more than 1 W on standby.

Semiconductor vendor Fairchild (distributed by Arrow and Soanar ) says it is offering solutions to help designers meet this rigorous standard.

The company says there are several key sections of the power supply that can be optimised to minimise standby power consumption. They can be categorised into two types – conduction losses and switching losses.

A major area of conduction loss is the Rds(on) of the MOSFET. According to Fairchild, this can be minimised by selecting MOSFETs with lower Rds(on). Unfortunately, these devices tend to have a larger gate capacitance, which in turn increases the switching losses. However, depending on the output power rating, it is possible to select an MOSFET that strikes an appropriate balance between switching and conduction losses.

The company says that minimising switching losses in standby mode is achieved by employing burst mode operation. In standby mode, especially with the requirement of providing output power is minimised, it is possible to operate the power supply in “bursts”, whereby the output power is supplied in small periodic bursts of Pulse Width Modulated (PWM) operation, rather than continuous mode. This minimises both conduction and switching losses. While this feature has been employed in high-end designs, it is only recently that this feature is needed in power supply designs for output power under 100 W.

Fairchild Power Switch (FPS) products featuring burst mode operation for stand-by mode (under 0.1 W at 265 VAC) that meets the “1-Watt Initiative”. These integrated off-line power switches combine a fully avalanche-rated SenseFET (LDMOS) for current sensing (650 V or 700 V minimum breakdown ratings) with a voltage mode of PWM IC, and they are equipped with a built-in start-up switch, eliminating the need for an external start-up resistor.

Further information - *Conducted by the National Appliance and Equipment Energy Efficiency Committee in 2000, and repeated in 2002 and January 2003. See www.energyrating.gov.au/library/pubs/sb200306-stereos.pdf

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