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PC Power Supply topologies..etc.

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Member level 5
Jul 3, 2002
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What is the most used topologies for computer power supplies nowadays? I am asking about the ones in the 400-500W range. I assume most are switching power supplies, but what topology of switching power supply is used mostly?

Also, do they regulate one channel (like 5V line) only in their PWM control, and the other secondaries just track that, or do they regulate each channel seperately..? 5V, 12V, 3.3V..etc seperatley regulated?

Any schematics/articles/websites to describe how those supplies are built would be very helpful.

I have a book about switching power supplies, but it lists almost 30+ topologies in general, without saying which is mainly used in PC industry...

Thanks in advance....

hi gemini1706,

you can try ....there are some good design ideas listed there. hey can u tell me the name of the book u r currently referring(the book u mentioned in ur post)?


Thanks lithium for the useful link.
The book I am refering to is:
Switching Power Supply Design, by Abraham I. Pressman
$63.20 from Barns and Nobles website...

I will try to browse through the site you gave me, but if anybody want to help, please feel free to give some pointers. :)


Mainly forward convertors and some variations of it. Normally only one output, the LV 3.3V or 5V is directly regulated as this is a critical voltage.

The latest PC supplies are also required to have PFC (Power Factor Correction) to ensure that sine wave currents are drawn from the mains supply, curb interference and increase eff.

All ATX PS I have seen are built as half-bridge inverters.The voltage that is maintained is usually +5V.This means that if one overloads +5V output then +12V will rise to +13V (for example).Not a proper design from my point of view,but a cheap one.
You might not believe it but some of these cheap ones use bipolar transistors and switching freq. around 20kHz.Low-voltage rectifier is Shottky diode - based whilst it should be MOSFET syncronous rectifier.
So don't look at your PC-PSupply if you easily get frightened!

batdin said:
All ATX PS I have seen are built as half-bridge inverters.

Yes this reduces the stress on the switching devices but requires 2 as well as a split capacitor bank. It can still be a forward converter as I will explain below.

Some advantage of the 1/2 bridge is that you can switch the input on the capacitors with just one link to enable it to work on 110VAC with the caps and rectifier acting as a voltage doubler, and on 220VAC as a normal rectifier. Another is that you have a DC blocking capacitor between the devices and the transformer prim winding that will prevent flux-imbalance in the core.

One can also find a diagonal 1/2 bridge forward converter which will also employ 2 devices. "Forward" is only the way power is transferred to the output, so combinations of various primary and secondary arrangements are possible in applications.

Some manufacturers may even use a interleaved forward topology which again will have 2 lower current devices.

When a PFC block is used ahead of the converter there's really no need to go to 1/2 bridge as the output of the PFC can be low enough to accommodate the lower voltage devices directly.

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