Overall power supply efficiency depends on sweating the small stuff

I talked with David Norton, VP of marketing at TDK-Lambda power supplies last February at APEC in Palm Springs about the drive in all power supplies towards increasing efficiency.

There are two reasons for the efficiency push: One is coming from government regulations that serve to raise the overall efficiency level of all power supplies in the market to meet minimum standards. Many customer companies have requirements that the products that they purchase meet Energy Star standards at a minimum. Energy Star compliance is necessary (but not sufficient) for being considered now in the market place.

The second reason is that for some hardware, like datacom servers, the cost of the hardware is exceeded in four years by the cost of the energy it consumes. Containing power costs is a significant cost saver for many businesses.

Norton’s observation about improving efficiency was that the power supply companies that were the best at it made significant overall efficiency improvement by sweating the small stuff. One example he gave was in selecting the lowest practical R_on for every load switch in an ac-dc power supply.  (These aren’t the power switcher MOSFETs, but the tiny ones that disconnect a whole power section and turn it on or off.)

I was reminded of this conversation when I saw Power Integrations’ new CAPZero X capacitor discharge switch, announced yesterday. X capacitors are used at the input of ac-dc power supplies to filter the differential mode EMI noise. They’re used across the ac input and can store energy for quite a while after the ac power is removed. To prevent this energy storage and potential resulting shock for anyone who inadvertently touches the terminals after the ac power is off, resistors are added to meet safety agency requirements that the X capacitor be discharged with a time constant of 1 second. But these resistors present a constant load and resulting power loss when the ac is connected. 

CAPzero acts as a smart switch when placed in series with these discharge resistors. The switch is open when the ac is connected eliminating the constant power loss of the discharge resistors. When the ac is disconnected, the switch automatically closes and the X cap discharged through the resistors. It’s a simple addition to any power design that relies on an X cap filter. UPDATE: CAPZero devices are available in an SO-8 package at $0.40 each for 10,000-piece quantities.

Like Norton says, sweating the small stuff adds up to significant system level gains in efficiency.