StarTech.com WattSmart 650 W Power Supply Review

Secondary Analysis

This power supply uses four Schottky rectifiers on its secondary, but they were scratched as well, so we couldn’t identify them.

Figure 12: Two of the four Schottky rectifiers used on the secondary.

Figure 13: The other two rectifiers used on the secondary.

We, however, were more curious to find out what configuration was used by the two transformers. There are several ways to use two transformers on a PC power supply. On both of them all main outputs (+12 V, +5 V and +3.3 V) have independent transformer outputs and independent rectifiers.

And here we have one of the advantages of the dual-transformer architecture: the +5 V and +3.3 V outputs are completely independent. On power supplies using just one transformer, the +3.3 V and +5 V outputs share the same output from the transformer, even when they have independent rectifiers, so the maximum current these outputs can pull at the same time will depend on the transformer. With two transformers they are not only completely independent; they are obtained from different transformers: +5 V output comes from the first transformer while +3.3 V output comes from the second transformer.

The two main differences between the available dual-transformer architectures are how the primary and the secondary are obtained. On the primary side, the power supply can have just one switching section driving both transformers (which is cheaper) or one switching section for each transformer (which is better but more expensive). On the secondary side, the power supply can simply join the outputs of the two +12 V rectifiers, using just one filtering section (which is cheaper) or the power supply can have two independent +12 V outputs, each one with its own filtering stage (which is better but more expensive).

Usually power supplies with separated secondaries will also have separated switchers, while power supplies that join the +12 V outputs will also join the switchers in order to cut costs.

The advantage of the more expensive implementation is that you have two really independent +12 V power supplies, each one feeding different components. This implementation provides a higher current (and thus power) limit and also a “cleaner” output, i.e., less noise. This happens because noise produced at one +12 V rail won’t be propagated to the other rail.

In Figure 14 we show the difference between these two designs. StarTech.com WattSmart 650 W uses the cheapest approach. Another power supply that uses the same design is Tagan TurboJet TG1100-U95 1,100 W. Enermax Galaxy 1000 W uses the more expensive design.

Figure 14: The two different ways to implement two transformers.

This power supply thermal sensor is located on the secondary heatsink, as you can see in Figure 15. This sensor is used to control the fan speed according to the power supply internal temperature.

Figure 15: Thermal sensor.

On this power supply all the electrolytic capacitors are manufactured by Teapo (a Taiwanese company). The active PFC capacitor is rated at 85° C, while the electrolytic capacitors from the secondary are rated at 105° C.