On this page we will take an in-depth look at the primary stage of BFG LS-550 (New). For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one GBU1006 rectifying bridge in its primary, which can deliver up to 10 A at 100º C. This component is clearly overspec’ed: at 115 V this unit would be able to pull up to 1,150 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 920 W without burning this component. Of course we are only talking about this component and the real limit will depend on all other components from the power supply. LS-450 uses an 8 A bridge.

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Figure 9: Rectifying bridge.

On the active PFC circuit one SPW47N60C3 power MOSFET transistors is used, capable of delivering up to 47 A at 25º C or 30 A at 100º C in continuous mode (note the difference temperature makes) or 141 A in pulse mode at 25º C, presenting a resistance of 70 mΩ when turned on, a characteristic called RDS(on) – the lower this number the higher efficiency is. Like LS-450, this unit uses only one transistor on the active PFC circuit. The transistor used on LS-450 has lower current specs (to 32 A at 25º C or 20 A at 100º C).

The active PFC capacitor is Japanese from Chemi-Con and labeled at 105º C. This is good for two reasons. Usually manufacturers use 85º C capacitors here, so it is good to see a manufacturer using a capacitor with a higher temperature rating. Secondly, Japanese capacitors don’t suffer from leakage problems.

In the switching section, two STP20NM50FD power MOSFET transistors are used on the traditional two-transistor forward configuration. Each one is capable of delivering up to 20 A at 25º C or 14 A at 100º C in continuous mode (note the difference temperature makes) or 80 A in pulse mode at 25º C, with a maximum RDS(on) of 250 mΩ. BFG LS-450 uses transistors with lower current limits here (12 A at 25º C or 7.5 A at 100º C).

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Figure 10: One of the switching transistors, active PFC diode and active PFC transistor.

The primary is controlled by a CM6806A PFC/PWM combo controller.

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Figure 11: PFC/PWM combo controller.

Now let’s take a look at the secondary of this power supply.