Secondary Analysis

This power supply uses a mix between new and obsolete designs, showing us that the manufacturer instead of creating a new design from scratch adapted an old design.

The main difference between this power supply and newer (and better) models is how power is distributed. This power supply was projected when most of the power drawn by the computer was concentrated on the +5 V line and not on the +12 V line like it is today. We can say this because it uses a rectifier with lower specs for the +12 V line and the rectifier with higher specs for the +5 V line.

The +12 V rectifier is connected like in old power supplies (design described as “A” on the “Secondary” section from our Anatomy of Switching Power Supplies tutorial). The +5 V rectifier, however, uses an updated design, the same one used by modern power supplies (design described as “B” on the same tutorial).

Then it comes how +3.3 V is produced. It has a separated rectifier like all current power supplies, but the output of this rectifier is +5 V, so it uses a voltage regulator to decrease this +5 V to +3.3 V. This is a mix between old and new designs. Old ATX power supplies generated their +3.3 V output by using a voltage regulator connected to the +5 V output. New power supplies have a completely separated rectifier. So this power supply uses a mix of these two approaches.

The +12 V output is produced by one MBR20100CT Schottky rectifier, which can deliver up to 20 A (measured at 133º C), which equals to 240 W. The maximum current this line can really deliver will depend on other components, especially the transformer, the coil, the capacitor and the wire gauge used. It is also important to notice that almost all power supplies nowadays use two rectifiers connected in parallel on the +12 V line instead of just one.

The +5 V output is produced by one MBR4045PT Schottky rectifier, which support up to 40 A (measured at 125º C). So the maximum theoretical power the +5 V output can deliver is of 200 W. Of course the maximum current (and thus power) this line can really deliver will depend on other components, especially the transformer, the coil, the capacitor and the wire gauge used, as mentioned before.

The +3.3 V output is produced by one MBR3045PT Schottky rectifier, which support up to 30 A (measured at 105º C). Like we explained, the output of this rectifier is connected to a +3.3 V voltage regulator, controlled by an IPP09N03LA power MOSFET transistor, which is capable of handling up to 50 A at 25º C or 46 A at 100º C. Since in configurations like this the component with the lower current limit is the one that limits the circuit, in theory the +3.3 V output from this power supply can deliver up to 30 A or 99 W. As we explained the real limit depends on other factors.

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Figure 13: +12 V, +5 V and +3.3 V rectifiers.

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Figure 14: Power MOSFET transistor used on the +3.3 V regulator circuit.

As you can see on Figure 14 this power supply has a thermal sensor attached to its secondary heatsink. This sensor is used to control the fan speed according to the power supply internal temperature and also to shut it down in the case of an overheating situation on power supplies that implement over temperature protection (OTP).

On this power supply the big electrolytic capacitors from the voltage doubler are from Teapo (a Taiwanese company) and rated at 85º C, while the electrolytic capacitors from the secondary are from Teapo and Su’scon (another Taiwanese company) and rated at 105º C.

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