The design used on this power supply is simply ridiculous. Instead of using a modern design using MOSFET transistors, its primary uses the same old design used by AT power supplies. Yes, this wasn’t a typo: we are talking about old AT power supplies. We didn’t even cover this design on our Anatomy of Switching Power Supplies tutorial, as we though nobody was still using it!
The main problem with this design is efficiency. FET transistors have high impedance, and the higher the impedance, the less power the component will draw from the circuit for its own operation – meaning less consumption and energy waste. Since this power supply uses regular transistors on its switching section, it cannot have a high efficiency – power supplies using regular transistors have a typical efficiency between 50% and 60%.
A power supply with 60% efficiency means that 40% from what it pulls from the power grid are wasted inside the power supply. For example, if your computer is pulling 300 W from the power supply, the power supply is pulling 500 W from the AC outlet – the rest is consumed by the power supply and wasted as heat. Yes, this is very bad, meaning a higher electricity bill.
Power supplies without PFC from competing companies do not use this design anymore; all of them use MOSFET transistors using one of the configurations describe on our Anatomy of Switching Power Supplies tutorial. It is simply unthinkable of using this obsolete approach nowadays.
Well, let’s take a better look on the primary. It uses one KBL406 rectifying bridge, which can deliver up to 4 A (rated at 50º C). No heatsink was used to cool down this component. This is the most low-end rectifier we’ve seen on a power supply to date.
On the switching section two 2SC2625 NPN power transistors are used using the very same configuration used by very old AT power supplies, as we mentioned before. Each transistor has a maximum rated current of 10A @ 25º C (or 20 A peak current).
click to enlarge
Figure 12: Two power NPN transistors are used on the switching section.
On Figure 13 you can find the schematics of a very old AT power supply. Young Year YP-AB primary stage uses exactly the same schematics. The secondary is a little bit different though and we will talk about it next.
click to enlarge
Figure 13: Schematics of a very old AT power supply. This power supply uses the same design on its primary.
This power supply uses three power Schottky rectifiers on its secondary section, one for each positive output: +3.3 V, +5 V and +12 V. The only advantage of this power supply compared to 100% “generic” units is that on this unit the +3.3 V output has a separated rectifier – sharing, however, the same transformer output as the +5 V output. In old ATX power supplies, a voltage regulator connected to the +5 V output provided the +3.3 V output.
click to enlarge
Figure 14: Power rectifiers used on the secondary.
The +12 V output uses one STPR1020CT Schottky rectifier, which supports up to 10 A (@ 110º C). So the +12 V output has a maximum theoretical power of 120 W, an outrageous discrepancy from what is printed on the power supply label (we will talk more about this on the next page). The maximum current this line can really deliver will depend on other components, especially the transformer, the coil, the capacitor, the wire gauge and even the width of the printed circuit board traces used.
The +5 V output uses one SBL3040PT Schottky rectifier, which supports up to 30 A (@ 95º C). So the +5 V output has a maximum theoretical power of 150 W, another discrepancy from what is written on the power supply label that we will talk about on the next page.
The +3.3 V output also uses one SBL3040PT Schottky rectifier (30 A @ 95º C), so the +3.3 V output has a maximum theoretical power of 99 W, another discrepancy we will talk about on next page.
Even though the +5 V line and the +3.3 V line have separated rectifiers, they share the same transformer output. So the maximum current both lines can deliver will depend a lot on the transformer.
