How does a switching power supply reduce its power output? For example, suppose a CPU ends some task, reducing its power consumption. What happens internal to the computer's power supply to cause it to put out less power?

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    \$\begingroup\$ I'd argue any question about how an electric device works is squarely on topic. This question isn't asking in any way how to use a switching power supply: it's asking a very specific question about how it works. Just because the answer is obvious doesn't mean it should be closed. \$\endgroup\$
    – Phil Frost
    Commented Jan 7, 2014 at 15:09
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    \$\begingroup\$ I have reworded the question to be (I hope) clearer. If I've lost meaning somewhere, I apologize. \$\endgroup\$ Commented Jan 7, 2014 at 17:23

1 Answer 1


The power supply's job is to maintain a constant voltage. When the CPU "switches off logic", less current is required from the power supply to maintain that voltage. The current demand from the CPU is constantly changing under normal operation, so switching power supplies have feedback mechanisms to adjust the current they produce such that they maintain the correct output voltage. Thus, if the CPU enters a special low power mode, the power supply need not do anything extraordinary. It just does what it always does: maintains the output voltage.

  • \$\begingroup\$ Most (all) ATX power supplies are constant-voltage, not constant-current supplies. The reduction in load by the PC causes the rail to rise (slightly), which produces an error voltage and causes the voltage feedback loop to adjust the duty cycle to reduce the energy delivered to the secondary in order to maintain voltage regulation. \$\endgroup\$ Commented Jan 7, 2014 at 17:51

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