Let’s now take an in-depth look on the primary stage from Litepower 450 W. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.
This power supply uses three US4BK80R rectifying bridges connected in parallel on its primary. This is the first time we’ve seen a power supply using three bridges in parallel. Each one of them can deliver up to 4 A at 125º C if a heatsink is used – which is not the case. Without a heatsink attached, the maximum current drops to practically half of this, 2.1 A at 30º C. Since three of them are used, we have a maximum current of 6.3 A at 30º C. At 115 V this unit would be able to pull up to 725 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 580 W without burning these components. Of course we are only talking about these components and the real limit will depend on all other components from the power supply. Why FSP chose to use three bridges without heatsinks instead of just one with a heatsink is a mystery that probably only their accounting department can explain.
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Figure 9: Rectifying bridges.
On the active PFC circuit two FDPF16N50 power MOSFET transistors are used, each one capable of delivering up to 16 A at 25º C or 9.6 A at 100º C (see the difference temperature makes) in continuous mode or 64 A in pulse mode at 25º C.
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Figure 10: Active PFC transistors and diode.
The active PFC capacitor is from Teapo and labeled at 85º C.
On the switching section two FQPF13N50C power MOSFET transistors are used on the traditional two-transistor forward configuration. Each one is capable of delivering up to 13 A at 25º C or 8 A at 100º C (see the difference temperature makes) in continuous mode or 52 A in pulse mode at 25º C.
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Figure 11: Switching transistors.
The primary is controlled by a CM6805 PWF/PFC combo controller.
