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Anatomy of Switching Power Supplies

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Transformers and PWM Control Circuit

As we mentioned earlier, a typical PC power supply has three transformers. The big one is the one shown on our block diagram (Figures 3 and 4) and schematics (Figures 19 through 23), where its primary is connected to the switching transistors and its secondary is connected to the rectifying diodes and filtering circuits that will provide the power supply DC outputs (+12 V, + 5 V, +3.3 V, -12 V and -5 V). The second transformer is used to generate the +5VSB output. An independent circuit generates this output, also known as “standby power”. The reason why is because this output is always turned on, even when your PC supply is “turned off” (i.e., it is on standby mode). The third transformer is an isolator transformer, connecting the PWM control circuit to the switching transistors (described as “isolator” on our block diagram). This third transformer may not exist, being replaced by one or more optocouplers, which look like a small integrated circuit (see Figure 25).

TransformersFigure 24: Power supply transformers.

OptocouplersFigure 25: This power supply uses optocouplers instead of using a transformer to isolate the PWM circuit.

The PWM control circuit is based on an integrated circuit. Power supplies without active PFC usually use a TL494 integrated circuit (in the power supply pictured in Figure 26 a compatible part, DBL494, was used). On power supplies with active PFC sometimes an integrated circuit that combines both PWM and PFC control is used. CM6800 is a good example of PWM/PFC combo integrated circuit. Another integrated circuit is usually used on the power supply, to generate the power good signal. We will talk more about it later.

PWM Control ICFigure 26: PWM control circuit.