AC Input

Larger power supplies may use three phase power. These can be more economical and a bit more efficient than single phase supplies, although the ripple frequencies will be higher.

Isolation: specified as the DC or AC voltage that can be applied between input and output without the supply failing. Typical numbers are 500 to 1500 V. The power supply’s isolation between input and output or chassis comes from the isolation provided by the supply’s transformer.

Some power supplies contain large filtering capacitors which essentially present a short to the rectifier when the supply is first powered on. Some power supplies have circuits to minimize the inrush current or spread it out over time (a “soft start”).

The hold-over specification defines how long the AC input can go away and the power supply will still stay in regulation. The charge stored on the filter capacitors is used to supply the power while the AC input is off.

As the cost of energy increases, power supply efficiency becomes more important. Efficiency is the output power divided by the input power and of course will always be less than 100% (it’s usually converted to a percent). The best supplies can be 90% efficient or better. Linear power supplies are typically much less efficient than switching mode power supplies.

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