Anatomy of Switching Power Supplies

The Secondary

Finally, the secondary stage. Here the outputs of the main transformer are rectified and filtered and then delivered to the PC. The rectification of the negative voltages (-5 V and –12 V) is done by conventional diodes, since they don’t demand a lot of power and current. But for the rectification of the positive voltages (+3.3 V, +5 V and +12 V) is done by power Schottky rectifiers, that are three-terminal components that look like power transistors but they have two power diodes inside. The way rectification is done depends on the power supply model and two configurations are possible, shown in Figure 27.

Recitifcation OptionsFigure 27: Rectification configurations.

Configuration “A” is more used by low-end power supplies. As you can see, this configuration needs three pins from the transformer. Configuration “B” is more used by high-end power supplies. Here only two transformer pins are used, however the ferrite coil must be physically bigger and thus more expensive, and that is one of the main reasons low-end power supplies don’t use this configuration.

Also on high-end power supplies, in order to increase the maximum current the power supply can deliver two power diodes can be connected in parallel, thus doubling the maximum current the circuit can handle.

All power supplies have a complete rectification and filtering circuit for the +12 V and +5 V outputs, so all power supplies have at least two circuits like the one shown in Figure 27.

But for the +3.3 V output, three options can be used:

  • Adding a +3.3 V voltage regulator to the +5 V output. This is the most common option on low-end power supplies.

  • Adding a complete rectification and filtering circuit like the one shown in Figure 27 for the +3.3 V output, but sharing the same transformer output used by the +5 V rectification circuit. This is the most common option for high-end power supplies.

  • Using a complete independent +3.3 V rectification and filtering circuit. This is very rare and would be found on very high-end and expensive power supplies. To date we’ve seen only one power supply using this option (Enermax Galaxy 1000 W, for the record).

Because the +3.3 V output usually uses the +5 V circuit totally (on low-end power supplies) or in part (on high-end power supplies), the +3.3 V output is limited by the +5 V output and vice-versa. That’s why PC power supplies have a “combined power” rating, stating the maximum power that these two outputs can pull together, in addition of each output maximum power (the combined power is lower than the sum of the +3.3 V and +5 V power ratings).

In Figure 28 you have an overall look at the secondary of a low-end power supply. Here you can see the integrated circuit in charge of generating the Power Good signal. Usually low-end power supplies use a LM339 or equivalent for this task.

You will find several electrolytic capacitor (far smaller than the ones found on the voltage doubler or active PFC circuit) and several coils. They are in charge of the filtering stage (see Figure 27).

SecondaryFigure 28: Power supply secondary stage.

For a better shot we cut all the wires and removed the two big filtering coils. In Figure 29, you can see the smaller diodes used on the rectification of the -12 V and –5 V lines, which have smaller current (and thus power) ratings (0.5 A each on this specific power supply). The other voltage outputs have current needs far above 1 A, requiring power diodes for performing the rectification.

Rectifying DiodesFigure 29: Rectifying diodes for the –12 V and –5V lines.

Author: Gabriel Torres

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.

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