The switching section of switching mode power supplies can be built using several different configurations. We summarized the most common ones on 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 on 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.
click to enlarge Figure 19: Single-transistor forward configuration.
click to enlarge Figure 20: Two-transistor forward configuration.
click to enlarge Figure 21: Half bridge configuration.
click to enlarge Figure 22: Full bridge configuration.
click to enlarge Figure 23: Push-pull configuration.
