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Old 10-20-2012, 05:32 PM   #1
Wester547
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Default Questions on SMPS Architecture

I have a question regarding something that's been on my mind lately.

The transistors/MOSFETs that are used as the DC-DC converters of the 12V rail to the other rails (5V and 3.3V)... since you specified what they can handle, do they contribute any power to the 5V and 3.3V rail at all despite that the 12V rail's rectifiers rectify those rails through DC-DC conversion? If so, how much?

Last edited by Wester547; 10-29-2012 at 02:25 PM.
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Old 10-20-2012, 06:03 PM   #2
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^
The MOSFETS are entirely responsible for the 5v and 3.3v output. They just steal some of the already rectified 12V output and reduce it. In this case, both of the FETS are rated at 60A at 25*C, so the 5v and 3.3v rails can handle up to 120A at that temperature.
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Old 10-20-2012, 06:15 PM   #3
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So, what you're saying is...

If you had, say, for example, a 300W PSU with the following configuration on the secondary (with only one toroidal choke):

Two MOSPEC S30D40Cs on one end
One MBR20H100CT (12V)
One STP40FN03L

That even though the STP40 part is used as the MOSFET to DC-DC convert the 5V output (the MOSPECs) that generates the 3.3V rail (in other words, because of the lack of a second secondary choke, the 3.3V rail is generated by that MOSFET), that even though the STP40 part does not rectify the 3.3V rail (but instead regulates it) that it still provides most of its power? Slightly different example but it's using 5V to 3.3V DC-DC conversion. Gabriel says in the Anatomy of PSUs article that in DC-DC conversion, one rectifier powers the 5V and 3.3V rail and that a MOSFET transistor is used for conversion.

Last edited by Wester547; 10-20-2012 at 07:29 PM.
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Old 10-20-2012, 09:56 PM   #4
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Quote:
Originally Posted by Wester547 View Post
So, what you're saying is...

If you had, say, for example, a 300W PSU with the following configuration on the secondary (with only one toroidal choke):

Two MOSPEC S30D40Cs on one end
One MBR20H100CT (12V)
One STP40FN03L

That even though the STP40 part is used as the MOSFET to DC-DC convert the 5V output (the MOSPECs) that generates the 3.3V rail (in other words, because of the lack of a second secondary choke, the 3.3V rail is generated by that MOSFET), that even though the STP40 part does not rectify the [/COLOR]3.3V rail (but instead regulates it) that it still provides most of its power?
Yes. With DC-DC conversion, the 12V (or 5V) rail supplies power to the FET, and the FET reduces the voltage and supplies that to the output.

12V Rectifier -------------------> 12V output
5V Rectifier --------------------> 5V output
................'-->3.3V MOSFET ---> 3.3V output

OR

12V Rectifier ----------------->12V Output
.................'--5V MOSFET ----> 5V output
.................'--3.3V MOSFET---> 3.3V Output

The DC-DC rails don't require rectification, since their source voltage is already rectified.

Last edited by c.hegge; 10-20-2012 at 09:58 PM.
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Old 10-20-2012, 10:07 PM   #5
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But would the 3.3V rail and its MOSFET be stealing any power from the 5V rectifiers in that case?

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Old 10-21-2012, 01:15 AM   #6
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Yes. It's entire output is stolen from the 5v rail
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Old 10-21-2012, 01:16 AM   #7
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In that case, the MOSFET that generates the 3.3V rail, despite being capable of 40A, does not contribute any power amperage wise after all to the 3.3V rail?
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Old 10-21-2012, 12:41 PM   #8
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Yes, it does. Saying it doesn't is like saying that your PSU doesn't contribute any power to your PC, because it's all done by the mains. Really, Your PSU's entire output is taken from the mains. Some of the mains output goes to your PC, some goes to other devices. Similarly, some of the 5V output would go to the 3.3V rail, some would go to the other 5V devices.
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Old 10-21-2012, 12:44 PM   #9
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If that's the case, then.... in my example.... how much power does the 3.3V rail get from the 40A MOSFET that generates it? ^^;
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Old 10-21-2012, 10:10 PM   #10
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All of it. Think of it as a PSU inside the PSU.
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