Anatomy of Switching Power Supplies

The Secondary (Cont’d)

In Figure 30 we have an example of the components that are attached to the heatsink found on the secondary stage of a low-end power supply.

SecondaryFigure 30: Components found on the secondary heatsink of a low-end power supply.

From left to right, you can find:

  • A voltage regulator integrated circuit – though it has three terminals and looks like a transistor, it is an integrated circuit. In the case of our power supply it was a 7805 (5 V regulator), in charge of regulating the +5VSB output. As we mentioned earlier, this output uses a circuit that is independent from the standard +5 V line (see Figure 5 for a better understanding), as it will continue delivering +5 V to the +5VSB output even when your PC is “turned off” (standby mode). That is why this output is also called “standby power.” The 7805 IC can deliver up to 1 A.
  • A power MOSFET transistor for regulating the +3.3 V output. In the case of our power supply the one used was a PHP45N03LT, which can handle up to 45 A. As we mentioned in the previous page, only low-end power supplies will use a voltage regulator for the +3.3 V output – which is connected to the +5 V line.
  • A power Schottky rectifier, which is simply two diodes stuck together in the same package. In the case of our power supply the one used was a STPR1620CT, which can handle up to 8 A for each diode (16 A total). This rectifier is used for the +12 V line.
  • Another power Schottky rectifier. In the case of our power supply the one used was an E83-004, which can handle up to 60 A. This specific power rectifier is used for the +5 V and + 3.3 V lines. Since +5V and +3.3 V lines use the same rectifier, their added current cannot be greater than the rectifier’s maximum current. This concept is called combined power. In other words, the +3.3 V line is generated from the +5 V; the transformer doesn’t have a 3.3 V output, differently from what happens to all other voltages provided by the power supply. This configuration is only used on low-end power supplies. High-end power supplies use separated rectifiers for the +3.3 V and +5 V outputs.

Now let’s take a look at the main components used on the secondary stage of a high-end power supply.

SecondaryFigure 31: Components found on the secondary heatsink of a high-end power supply.

SecondaryFigure 32: Components found on the secondary heatsink of a high-end power supply.

Here you can find:

  • Two power Schottky rectifiers for the +12 V output connected in parallel, instead of just one like on low-end power supplies. This configuration doubles the maximum amount of current (and thus power) the +12 V output can deliver. This power supply uses two STPS6045CW Schottky rectifiers, which can deliver up to 60 A each.
  • One power Schottky rectifier for the +5 V output. On this particular power supply one STPS60L30CW was used, which supports up to 60 A.
  • One power Schottky rectifier for the +3.3 V output, being the main difference between high-end and low-end power supplies (as we have just shown you, on low-end power supplies the +3.3 V output is generated through the +5 V line). On the portrayed power supply the circuit used was a STPS30L30CT, supporting up to 30 A.
  • One voltage regulator from the power supply protection circuit. This kind of feature varies depending on the power supply model.

Note that the maximum currents we published are for the components only. The maximum current the power supply can actually deliver will depend on the other components that are attached to them, like the coils, the transformer, the gauge of the wires used and even the width of the printed circuit board traces.

Just as an exercise, you can calculate the maximum theoretical power for each output by multiplying the rectifier maximum current by the output voltage. For example, for the power supply pictured in Figure 30 its maximum theoretical power for its +12 V output is of 192 W (16 A x 12 V). But keep in mind what we’ve just said on the above paragraph.

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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