Switching Transistors

The switching section of switching mode power supplies can be built using several different configurations. We summarized the most common ones in the table below.

Configuration Number of Transistors Number of Diodes Number of Capacitors Transformer Pins
Single-Transistor Forward 1 1 1 4
Two-Transistor Forward 2 2 0 2
Half Bridge 2 0 2 2
Full Bridge 4 0 0 2
Push-Pull 2 0 0 3

Of course we are just analyzing the number of components needed, there are other aspects that engineers should take into account when deciding which configuration to use.

The two most common configurations for PC power supplies are the two-transistor forward and the push-pull, and both use two switching transistors. The physical aspect of these transistors – which are power MOSFET transistors – can be seen in the previous page. They are attached to the heatsink found on the power supply primary section.

Below we show you the schematics for each one of these five configurations.

Single-Transistor ForwardFigure 19: Single-transistor forward configuration.

Two-Transistor FowardFigure 20: Two-transistor forward configuration.

Half-BridgeFigure 21: Half bridge configuration.

Full-BridgeFigure 22: Full bridge configuration.

Push-PullFigure 23: Push-pull configuration.

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.