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PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

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Old 11-05-2007
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Default PSU 101: How A Power Supply Works

We hope this helps some of you out there. If anyone has any input, please feel free to comment. We are not necessarily experts, but simple hobbyist with a thirst for knowledge and a desire to teach others. Beaucoup special thanks go out to Oklahoma Wolf who pretty much wrote this whole section and he took all of the pictures. -- jon

• How the PC Power Supply Works

a. Transient Filter:

A computer power supply depends on stable power coming in to do its job. That's why the better built units employ a transient filter between the power connector and the rest of the circuitry. These use a combination of metal oxide varistors (used to control potentially damage-causing power surges), capacitors, and coils to take as much interference out of the incoming AC power as possible. They also have the benefit of acting in reverse, keeping the PSU from adding its own interference to the line.


Above is the transient filter portion of a Topower built BFG 800W. Photo by Oklahoma Wolf and coutesy of Hardware Canucks.


BFG 800W's 400V 470uF primary capacitor and PFC coil shown above. Photo by Oklahoma Wolf.

b. Input Rectifier:

Located immediately after the transient filter (if present), the input rectifier is responsible for converting AC to DC power. Self-contained bridge rectifiers are most commonly used, sometimes in pairs in higher watt designs, but some cheaper units get by with four diodes in a bridge arrangement as well. Most power supplies lacking active PFC will have a switch on the back to go from 120V input to 240V input. This switch is part of the input rectifier, doubling the input voltage when closed and allowing the circuit to operate as a straight rectifier when open. If an active PFC circuit is employed, the voltage doubler is omitted and the PFC circuitry takes over its duties instead.


There are two bridge rectifiers back to back on this primary heatsink (left). There are also three switchers on the left for PFC.
Photo by Oklahoma Wolf. Used by permission from Hardware Canucks.



This is the other side of the primary heatsink showing the two main switchers that make up the double forward
convertor arrangement in this Topower built BFG 800W power supply. The diode to the far left is part of the PFC circuit.
Photo by Oklahoma Wolf. Used by permission from Hardware Canucks.


c. Primary Switchers:

Once the incoming power has been converted to DC, it now needs to be converted into high frequency AC power. This, being the heart of a SMPS design, is done to allow lots of power to be rectified using far less space than an old linear power supply would need. There are several ways to do this, but the most common arrangements use a pair of power transistors for the main voltages and a single smaller transistor to handle standby voltages.

d. Primary Transformers:

Now that the incoming power has been regulated to DC and then converted to high frequency AC, step down transformers are needed to drop the voltage to the point where they can be regulated back to DC for the output. Usually, this means one large transformer for 3.3V, 5V, 12V, -12V, and -5V; and one smaller transformer connected to the standby switcher for the standby voltage. But, sometimes an extra transformer will make its way in there as well, most often for extra 12V capacity these days.


This Huntkey Titan 650W has had it's heatsinks removed so we can easily see the transformers.

e. PWM Control Circuit:

This is the circuit used to control the switching frequency in a power supply. Often, these can be based on an all in one integrated circuit that can also handle various protection modes or APFC duties. These circuits are connected to the primary switching transistors using a small transformer or by optocouplers to isolate the controller from the transistors.


This Champion CM6800G is a commonly used PWM/PFC controller.
Photo by Oklahoma Wolf. Used by permission from Hardware Canucks.


f. Secondary Stage:

Once the high frequency AC power has been reduced to the proper voltage, it must be converted back into DC. This is done using a combination of rectifiers (usually two diodes back to back in one package) and filters. Often, the 3.3V output will be taken either from the 5V regulated output or will be taken from the 5V output at the transformer. However, in some very high end designs the 3.3V will get its own transformer output as well.


There are a total of nine rectifiers on this secondary heatsinks. Four on one side, five on this side. These regulate
+12V, +5V and +5VSB for a Topower built BFG 800W power supply. Photo by Oklahoma Wolf. Used by permission from Hardware Canucks.

The secondary outputs can be either group regulated or independently regulated. The difference between the two usually lies in the filtration stage. In a group design, two outputs share a filtering coil while an independent design usually has one coil for each main output. However, most of the time the design isn't truly independent as the 3.3V and 5V usually still share one transformer winding.
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Last edited by jonnyGURU; 11-18-2007 at 12:52 PM.
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Old 11-05-2007
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Could you add images of different PSU internals highlighting the different parts. I think it would be good to have a visual here.
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Old 11-06-2007
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Quote:
Originally Posted by mpilchfamily View Post
Could you add images of different PSU internals highlighting the different parts. I think it would be good to have a visual here.
I think I have some photos Wolf took that we can add.

Actually, he's getting a new iron soon and a very interesting specimen we could probably use for photos here.
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Old 11-06-2007
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A small point about the regulation.

The filtering identifies group or Indy regulation but it can also be identified by knowing what kind of PWM and diodes are being used. If the PWM can control the duty cycle of all lines it is independently regulated. In other words, nature of the PWM control defines regulation.

Of course, you guys know more about this than I ever will, so take it with a grain of salt.
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Old 11-06-2007
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Quote:
Originally Posted by Super Nade View Post
If the PWM can control the duty cycle of all lines it is independently regulated. In other words, nature of the PWM control defines regulation.
Not too sure about that one - the PWM controller only seems to affect the switching transistors. I'm a little fuzzy about that part of it yet, but the datasheet of the CM6800 that was posted over at Badcaps doesn't really seem to have any connection to the secondary, yet there are many supplies using this chip with indy regulation. Indy is more about how many outputs there are at the secondary side of the transformer and how well the filtering can keep the voltages seperate.

Then again, truly independant regulation all the way back to the transformer is still not all that common... some of these cheaper indy designs still use common outputs from the transformer, which is possibly why a lot of them still have combined 3.3V/5V ratings.
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I'll have to get back to you on this one Wolf. I distinctly remember reading about certain popular push-pull topologies where control was achieved by varying the duty cycle (single transformer type). I thought the cheap way of throwing a Vreg on the 5.5V rail was not true indy regulation.
Hmm...
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Old 12-11-2009
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I think these are voltage regulators, not rectifiers, in the bottom photo for (f).
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Old 05-27-2011
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Which part is responsible for the crude wattage output? Lets say you have two power supplies from same manufacturer with identical design, one is rated 550W other is 750W where would be the difference inside those PSU?
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Old 05-27-2011
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Well... assuming the rectifying bridge, APFC and primary switching MOSFET's are beefed up enough to handle a wide range of wattages, it would be either the MOSFET's or Schottky diode rectifiers (depending on the PSU's topology) on the secondary side that would dictate the PSU's over all output capability.

The PSU above uses Schottky diodes. See the picture under "f. Secondary Stage". An LLC resonant topology would use MOSFET's here instead.
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Old 01-14-2012
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This guide needs to be updated to include DC-DC PSU's, I think? :P
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