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Some power supplies can operate under either 110 V or 220 V input, with different rated power though. What enables this behavior? Are there relays to react depending on the inputs and lead to different circuits for different inputs?

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  • \$\begingroup\$ Useful search term: universal input power supply \$\endgroup\$
    – winny
    Jul 30, 2022 at 19:38

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Most AC to DC power supplies first rectify and (partially) smooth the input AC into a high voltage DC with ripple. If the input AC is 110 volts (which is an rms value) the high voltage DC might be something like 145 VDC average. It might be more or less depending upon the smoothing capacitors and the power drawn. If the input AC is 220 volts, the high voltage DC might be something like 290 VDC average.

This high voltage DC will be passed into a switching DC-DC converter which will give a regulated output, of 5 VDC or 12 VDC, or whatever it is rated. The output of the switching DC-DC converter is monitored, and there is a feedback loop which adjusts the DC-DC converter to ensure the output is what the converter was designed for. Thus, the input could be 145 VDC with ripple, and the output a smooth 5 VDC. Or the input could be 290 VDC with ripple, and the output a smooth 5 VDC.

How this is accomplished is with pulse-width modulation (PWM). There are many types of switching topologies, but "chops" up the DC into "on" and "off" periods, that energize inductors and capacitors, leading to the output voltage. The ratio between the "on" period to the sum of the "on" and "off" periods is called the duty cycle. The feedback into the DC-DC converter adjusts the duty cycle, and in so doing, the output voltage is adjusted to be constant. Each topology of switching regulator has a relationship between the duty cycle, the current drawn, and the ratio between the input voltage and the output voltage which is unique to that topology. However, in any DC-DC converter topology, the output voltage is regulated by regulating the duty cycle of the switching element(s).

So, if the DC-DC converter is fed with 145 VDC, it will switch with a different duty cycle than if/when it is fed with 290 VDC. As long as the power supply is designed to handle both input voltages, (and probably including the entire range in-between), the end result is a fixed output voltage independent of the input voltage.

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  • \$\begingroup\$ Additionally, many modern power supplies have active power factor correction, which is essentially a boost converter. The bus voltage in these supplies is typically boosted to ~380V , regardless of input voltage. \$\endgroup\$ Aug 4, 2022 at 14:32

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