As we explained, one of the ways power supply manufacturers can cut costs on cheaper units is by using cheaper components. With semiconductor components (diodes and transistors) they accomplish this by using components with lower current (and thus power) limits.
On the primary side of the power supply, generic units usually use four discrete diodes instead of a rectifying bridge – which is a component that has four diodes inside. These diodes can be see on Figure 7, present on the previous page.
This generic 500 W unit uses four 1N5408 diodes, which can handle up to 3 A each, rated at 105º C. “Branded” power supplies use rectifying bridges that can handle at least the double from that. At 115 V this unit would be able to pull up only 345 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver only up to 276 W without any diode burning.
On the switching section generic power supplies use regular power BJT transistors instead of power MOSFET transistors, using the half-bridge configuration, which is the configuration traditionally used by power supplies without active PFC. On a generic unit it is expected that the amount of current each transistor can handle to be lower compared to “branded” units, as the manufacturer chooses to use cheaper components.
Our 500 W generic unit uses two 2SD13007K transistors. Unfortunately we couldn’t find its datasheet so we can’t comment on its maximum rated specs. The third transistor on Figure 8 is for the +5VSB power supply, which is independent from the rest of the power supply.
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Figure 8: Switching transistors.
Let’s now take a look on the secondary from this power supply.
