Calculating Power Supply AC Input Current

Knowing a product’s AC input current rating is essential for determining appropriate circuit breakers, AC input cabling, and connectors. The AC input current for a DC power supply can be calculated using a few known product specifications, as follows:

3-phase AC Input Current Formula:

1-phase AC Input Current Formula:

where:

• AC Input Current is measured in Aac
• DC Output Power is measure in Watts
• AC Input Voltage is measured in Vac

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Product Specifications

Several parameters influence the power supply’s AC input current rating, each of which are described in this section.

Efficiency

Power supply efficiency is the ratio of output power to input power. In various power conversion stages, energy is loss to heat in components like diodes, switching devices like MOSFETS and IGBTs, magnetics, and wires.

In power supply specifications, efficiency is typically measured and specified at full load, which will provide the maximum efficiency 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.

Efficiency is typically expressed as a percentage, which must be used in decimal format for calculations.

DC Output 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. Even with infinite AC input current available, these products will not deliver more output power than their rating.

For 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. 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.

The maximum DC output power rating must be factored into the calculation for AC input current. If available, use the maximum DC output power rating provided by the power supply manufacturer for this calculation.

Power Factor

A power supply’s power factor is the ratio of true (or real) power, expressed in watts, to apparent (or reactive) power, expressed in vars. 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.

When designing a power supply, considerable effort is made to reduce reactive power draw to minimize the overall AC current draw. Magna-Power employs active power factor correction (active PFC) in its lower-power models, while its higher power models achieve a high power factor passively through large AC input inductance, providing a stiff AC input impedance; one of the benefits of Magna-Power’s signature current-fed topology.

Power factor is typically expressed as a decimal number less than 1.

AC Input Voltage Tolerance (Optional)

AC input current is typically specified at nominal AC input voltage. Often times, manufacturers–Magna-Power included–specify an AC input voltage tolerance, within which normal operation can be expected. If the power supply is operating off below nominal AC input voltage, higher AC input current draw can be expected. For example, if you operate at 10% below nominal AC input voltage, the AC input current draw will be 10% higher.

The AC input voltage tolerance is not factored into the formulas provided in this article, but can be factored in by either using the worse-case continuous AC input voltage that will be supplied to the power supply.

MagnaDC Power Supply AC Input Current Considerations

Given the very broad range of power ranges and models within the MagnaDC programmable DC power supplies, a few additional considerations are made for the purposes of specification consolidation.

Only one AC input current rating is provided for each AC input voltage within a certain power range, despite differences in efficiency ratings across models. For example, the SL Series 6 kW models have over 20 different models with efficiency ratings ranging from 90% to 95%, yet there is only one AC input current rating for each AC input voltage. In this case, Magna-Power utilizes the lowest efficiency rating from this power range (i.e. 90%) to ensure worst-case AC input current for that power range.

In addition, Magna-Power supports a wide range of AC input voltages to support applications worldwide, but consolidates key assemblies and magnetic designs with similar AC voltage ratings. On the AC input current ratings table, 208 Vac 3-phase and 240 Vac 3-phase ratings are grouped together as 208/240 Vac 3-phase. The AC input current rating is always specified for the lower of the two voltage, providing the worse-case AC input current rating.