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I am trying to understand how computer PSUs work. I found the following circuit in Wikipedia and I noticed that, unlike the 12V and 5V outputs, there is no winding in the transformer for the 3.3V output. Instead, a coil is connected to the 5V secondary before the diode.

I have some experience in swith mode power supplies but I did not see this circuit before. How does this 3.3V output circuit work?

Principal circuit diagram of a PC power supply

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2 Answers 2

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That's a simplified diagram how a power supply might work - not how all power supplies work.

Quote how it works from the Wikipedia article you linked :

In the most common design this voltage is generated by shifting and transforming the pulses of the 5 V rail on an additional choke, causing the voltage to rise delayed and rectified separately into a dedicated 3.3 V rail and getting the rising idle voltage cut by a device type TL431, which behaves similar to a zener diode.

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  • \$\begingroup\$ I am specially interested in this circuit because it is something new to me and I am curious about it. I also read the article in Wikipedia but it is still not clear to me how these 3.3V are achieve... Any application note or technical article will be greatly appreciated. \$\endgroup\$
    – ErnstOlch
    Apr 13, 2020 at 12:22
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I found a quite complete explanation of how the 3.3V voltage regulation works in this old post: Unconventional voltage regulation scheme in the secondary of an ATX power supply, how does it work?

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