While the input voltage for ac-dc supplies is standardized over a range, the output voltages are most common at the specific values of 5, 12, 24, or 48 Vdc. Figure 2: Block diagram of typical switching power supply Dc Output Voltage Specifications Users should verify the power supply development team has evaluated the supply for use with a dc input voltage if they plan to use a dc input configuration. On many ac input supplies, a dc voltage can be applied to the input of the supply with the rated dc voltage range being calculated as 1.4 times the rated ac voltage range. Inside ac-dc power supplies, the ac input voltage is typically immediately rectified to a dc voltage. Figure 1: Relationship between 480 Vac and 277 Vac

A maximum voltage of 305 Vac for this configuration is calculated by applying the previously discussed 10% tolerance above the 277 Vac rated voltage. This voltage is obtained by connecting between the phase and neutral of a 480 Vac Wye configuration. One higher (and less common) voltage specification found in industrial applications is 277 Vac. The minimum standardized range value of 85 Vac is derived from allowing a 15% voltage droop of 100 Vac, while the 264 Vac maximum range value is derived from allowing operation at 10% above 240 Vac. Japan is a country with one of the lowest nominal input voltage requirements (100 Vac) while the UK, Canada and the US have among the highest nominal input voltage requirements (240 Vac). There are specifications for different countries, but they are similar enough that the single specification range can meet the needs of most applications. The required input voltage operating range for most low or medium power ac-dc supplies is standardized throughout the world to 85~264 Vac. The user should verify the ac input voltage range of the power supply is compatible with their target markets and specified application.

This includes the ac input range and the dc output specifications. When selecting an ac-dc power supply, the primary consideration is determining the system power requirements. Input Voltage Specifications for Ac-Dc Power Supplies

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