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2.2.1 PC (Personal Computer) power supply

PC power supply is, in many cases, attached just like an "accessory of chassis" when chassis is bought. In reality, it's no exaggeration to say that it is the most critical part of PC.
Because, to tell the work of PC mentioned above in one word, it is "to convert AC delivered to household and stably provide computer devices such as a motherboard" with DC (direct current). If it becomes unstable or broken, all functions get crashed to lose important data.  

Is PC power supply special? No, No. In fact, many home appliances have power supply inside to convert AC to DC. Commonly seen AC adapter, for example, is externally placed type of built-in power supply. In daily life, we are not conscious of power supply of home appliances as it is built in them, and home appliances have no need to replace power supply because of no expandability of them.

Also, you cannot find out power supply for TV set at shops even when you want to replace. On the contrary, you may possibly replace PC power supply as PC has expandability to add HDD or replace CPU. Power supply is originally limited life component so that you need to replace at some stage.

<< Category and Differences of PC power supplies >>

PC power supplies are categorized mainly into 4 types, such as AT power supply, ATX power supply, SFX power supply, and EPS power supply. Almost every PC power supply at shops is ATX type. AT and SFX type is not so large. There are other type power supply for NLX or WTX too, but they are built-in in the chassis so that you can hardly get them individually.

ATX type provides you a wide range to choose from as there are many motherboards. SFX power supply is for microATX motherboard. AT type's application is getting limited mainly for FA (Factory Automation), but, as mentioned later, it is still used due to stable ON/OFF control of power supply. In addition, there are ATX12V・SFX12V specification as successor to ATX/SFX specification and, they are compatible with previous specifications. (-5V is not required for ATX12V). Now, let us explain the differences among PC power supplies.

Photo 2.1 AT type bottom left ATX type bottom right SFX type in the center
Photo 2.1
AT type bottom left
ATX type bottom right
SFX type in the center
5 dimensions for SFX power supply are offered so that you need to verify if it fits to chassis even with the same SFX specification. Outlet is provied to AT power supply.

First, take a look at dimension. AT type and ATX type have the dimension called PS/2 size. They may look identical. On the other hand, SFX type is less than 50% as small as PS/2 in volume. (Photo 2.1)
Each PC power supply has various shapes of main connector to be connected to motherboard and output voltages. This is because type of motherboard requires different voltages as shown in Table 2.1.

Table 2.1 Main connector to supply power to motherboard
Category

AT
power supply

ATX
power supply
ATX12V
power supply
SFX
power supply
ATX12V
power supply
Connector shape 6 pin
2 pcs.
20 pin
1 pc.
24 pin
1 pc.

20 pin
1 pc.

24 pin
1 pc.
Size (WxDxH) 150x140x86 150x140x86 100x125x63.5
Output voltage +5V, +12V, -5V, -12V +3.3V, +5V, +12V, (-5V), -12V, +5VSB
* -5V is optional for ATX12V
+3.3V, +5V, +12V, (-5V), -12V, +5VSB
* -5V is optional
+5VSB is always on for ATX and SFX type when AC is turned on.
This output is used to control power supply startup,
and to provide power to Wake ON LAN, Wake ON Modem Ring.


Ref. 1: Pinout assignment of main 20Pin connector for ATX type
Pin
Output
Output
Pin
11
3.3V
3.3V
1
12
-12V
3.3V
2
13
COM
COM
3
14
PS_ON
5V
4
15
COM
COM
5
16
COM
5V
6
17
COM
COM
7
18
-5V
PWR_OK
8
19
5V
5VSB
9
20
5V
12V
10
* COM is zero volt level of secondary output.
* For PS_ON, all outputs turn ON when this pin is activated by TTL Low.
* PWR_OK delivers TTL High (approx. 5V) when outputs become stable.
* -5V is optional for SFX type.


Ref. 2: Pinout assignment of main connector 6Pin 2 pcs. for AT type
Pin1
Output
1
P.G.
2
+5V
3
+12V
4
-12V
5
COM
6
COM
Pin2
Output
1
COM
2
COM
3
-5V
4
+5V
5
+5V
6
+5V
* P.G. is the same signal as PWR_OK of ATX type.


Ref. 3: Pinout assignment of peripheral connector and FDD connector
Pinout assignment of peripheral connector
Pin1
Output
1
+12V
2
COM
3
COM
4
+5V
Pinout assignment of FDD connector
Pin2
Output
1
+5V
2
COM
3
COM
4
+12V
* P.G. is the same signal as PWR_OK of ATX type.

Ref. 4: Pinout assignment of main 24Pin connector for ATX12V type
Pin
Output
Output
Pin
13
3.3V
3.3V
1
14
-12V
3.3V
2
15
COM
COM
3
16
PS_ON
5V
4
17
COM
COM
5
18
COM
5V
6
19
COM
COM
7
20
-5V
PWR_OK
8
21
5V
5VSB
9
22
5V
12V
10
23
5V
12V
11
24
COM
3.3V

12

* COM is zero volt level of secondary output.
* For PS_ON, all outputs turn ON when this pin is activated by TTL Low.
* PWR_OK delivers TTL High (approx. 5V) when outputs become stable.


Ref. 5:Pinout assignment of 12V Power Connector and 6Pin connector for PCI-Express
Pinout assignment of Power Connector for +12V
Pin1
Output
1
COM
2
COM
3
+12V
4
+12V
Pinout assignment of 6Pin connector for PCI-Express
Pin2
Output
1
+12V
2
+12V
3
+12V
4
COM

5

COM
6
COM


Ref. 6: Pinout assignment of 8Pin Processor Power Connector
Pin
Output
Output
Pin
1
COM
+12V
5
2
COM
+12V
6
3
COM
+12V
7
4
COM
+12V
8

AT and ATX power supply are dimensionally identical, but there are much difference in its shape and output power to be recognized. Also, ATX power supply includes outputs of SFX power supply and its connector shape is the same.
For this reason, ATX power supply is available to mother-board of microATX. PC power supply is equipped with peripheral connector (large 4Pin connector) and FDD connector (small 4Pin connector) other than main connector. The shape and output voltages are the same despite the difference of number of connectors. (Photo 2.2 and 2.3)

Photo 2.2 Connectors of ATX type
Photo 2.2 Connectors of ATX type
From left, main connectors (6Pin 2 pcs.), Peripheral connector, and FDD connector
Photo 2.3 Connectors of ATX and SFX type
Photo 2.3 Connectors of ATX and SFX type
From left, main connector (20Pin 1pc.), Peripheral connector, and FDD connector

 

Moreover, regarding ON/OFF control, it is categorized into 2 types.
One is ATX power supply which needs manual operation, second, ATX and SFX power supply which can be controlled by motherboard.

Faston terminals are normally equipped to AT power supply (Photo 2.4).
It allows power supply to turn on or off by operating external switch. In the case that outlet is equipped for monitor connection, ON/OFF of monitor can be controlled in conjunction with PC operation.
In addition, ON/OFF control of PC is secured while it does not depend on PC condition as supply mains can be turned on or off manually.
On the contrary, for ATX and SFX power supply, ON/OFF control is carried out by electric control from motherboard, voltage of control terminal of main connector, pin 14, to be exact. (Photo 2.5)

When TTL Low (same voltage as black cable that is zero volt) is given, PC becomes activated.
When PC with ATX specification is started up, a switch on the front panel is usually pressed down. In this operation, supply mains of PC power supply is not directly turned on, but a signal is given to mother-board to turn on PC power supply indirectly.
Also, this control allows the function to shut down automatically PC power supply when OS of Windows2000/XP shuts down (Figure 2.3). AT power supply has the advantage of fail-safe operation over ON/OFF control method that depends on motherboard. Fail-safe means that the nit operates to safe side when abnormal situation occurs. This is why AT power supply is adopted for FA use.

Photo 2.4 Faston terminals of AT power supply Connect a switch to those to turn on or off power supply manually.
Photo 2.4 Faston terminals of AT power supply Connect a switch to those to turn on or off power supply manually.

Photo 2.5 Pin 14 of main connector of 20Pin is for ON/OFF control.
Photo 2.5 Pin 14 of main connector of 20Pin is for ON/OFF control.

Fig 2.3  For AT power supply, ON/OFF of PC power supply is controlled manually.
Fig 2.3  For AT power supply, ON/OFF of PC power supply is controlled manually.
For ATX and SFX power supply, ON/OFF of PC power supply is controlled by signal from motherboard.

<< How to start up ATX power supply >>

Starting up ATX power supply by itself without connecting to motherboard can be carried out by giving pseudo signal.
This is useful to see where the problem that the unit does not start up comes from, power supply itself? or motherboard?
To make pseudo signal, turn PS_ON of main connector to TTL Low, or connect PS_ON to COM, providing that minimum load is given.

This is because just one transformer in PC power supply handles multiple outputs. Some units may not deliver 3.3V or 12V when 5V does not have approx. 2A of load.
Without any load, proper output may not be delivered when ON operation of power supply is carried out. Make sure to give minimum load. Many of our products can operate normally even with no load (zero amps.).

<< Is power supply which differs from the
specification of motherboard acceptable? >>


Some may wonder "ATX power supply is acceptable to motherboard of AT?" The answer is "YES."
Table 2.2 below shows the combination of different power supplies and motherboards to see what will happen.

Table 2.2 Cross combination of power supplies and motherboards
MB/PSU
AT
power supply
ATX
power supply
SFX
power supply
EPS
power supply
AT
OK
Conversion connector required
Conversion connector required
Conversion connector required
ATX
Unacceptable
OK
(*NOTE: 1)
(*NOTE: 2)
micro ATX
Unacceptable
OK
OK
(*NOTE: 2)

*NOTE 1: Many SFX power supplies do not have -5V that is required by ATX specification. If motherboard requires -5V, the power supply is unacceptable, but some power supplies have -5V output. So it depends.
*NOTE 2: In the case that EPS power supply is connected to motherboard of ATX/microATX, conversion connector may be required.

Any motherboards are acceptable to ATX power supply.
However, in the case that ATX power supply is connected to AT motherboard, conversion connector from ATX output connector to AT output connector is required as they are different in shape.
In this case, ON/OFF control of power supply must be carried out manually as motherboard is unable to handle it.

<< How about compatibility between power supply and motherboard?>>

Often asked question is the compatibility with motherboard.
We do not know whether we can call it compatibility, but some power supplies do not work with some motherboards.

We are sorry to say we cannot check in-depth every combination of them to see which board is acceptable, and which is not. For ATX, strict specification has been issued mainly by Intel, but it has some degree of freedom causing some variance among manufacturers.
For example, suppose the following requirement that "PW_OK signal shall start up 100 to 2000ms after +3.3V and +5V reach 95% when power supply has started up. PW_OK signal shall start up within 10ms after it begins to start."
We need to follow this requirement that is "period from A to B", "Within C." Sole power supply meets this of course, but some power supplies may not work with some motherboards.
According to our experience as particular case, power supply did not work at specific rise time (dv/dt).

Also, the power supply works well with some motherboard, but does not with another motherboard even from the same
manufacturer.
Well then, can we solve the problem by eliminating the degree of freedom completely?
No.
The severer the specification is, the harder the materialization is resulting in higher cost and loss of customer's benefit.
Is the problem like this being solved?, or mounting?
We do not know, but we keep you updated with information in-depth.

 

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