When designing a power supply, considerable effort is made to reduce reactive power draw to minimize the overall AC current draw. Although the current associated with reactive power does not do work at the load, it still must be considered when sizing the AC current draw. Power FactorĪ power supply’s power factor is the ratio of true (or real) power, expressed in watts, to apparent (or reactive) power, expressed in vars. If available, use the maximum DC output power rating provided by the power supply manufacturer for this calculation. The maximum DC output power rating must be factored into the calculation for AC input current. Auto-ranging power supplies may offer an output operating profile that allows the product to achieve full power over a wider voltage and current range. Even with infinite AC input current available, these products will not deliver more output power than their rating.įor a typical programmable DC power supply, the maximum DC output rating is product of maximum DC output voltage rating times the maximum DC output current rating. Programmable DC power supplies are designed, rated, and tested to deliver a specific maximum DC output voltage rating, maximum DC output current rating, and maximum DC output power rating. For modern power supply designs, high efficiency can still be expected at lower-than-maximum operating conditions, but less than the rating at full load.Įfficiency is typically expressed as a percentage, which must be used in decimal format for calculations. In power supply specifications, efficiency is typically measured and specified at full load, which will provide the maximum efficiency rating. In various power conversion stages, energy is loss to heat in components like diodes, switching devices like MOSFETS and IGBTs, magnetics, and wires. Power supply efficiency is the ratio of output power to input power. Several parameters influence the power supply’s AC input current rating, each of which are described in this section.
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