Anatomy of Switching Power Supplies

Switching Power Supply Diagram

On Figures 3 and 4 you can see the block diagram of a switching power supply with PWM feedback used on PCs. In Figure 3 we show the block diagram of a power supply without PFC (Power Factor Correction) circuit – used by cheap power supplies – and in Figure 4 we show the block diagram of a power supply with active PFC circuit, which is used by high-end power supplies.

Figure 3: Block diagram for a switching power supply design with PWM (no PFC).

Figure 4: Block diagram for a switching power supply design with PWM and active PFC.

You can see what is the difference between a power supply with active PFC and one without this circuit by comparing Figures 3 and 4. As you can see, power supplies with active PFC don’t have a 110/220 V switch and also don’t have a voltage doubler circuit, but of course they have the active PFC that we will talk more about later.

This is a very basic diagram. We didn’t include extra circuits like short-circuit protection, stand-by circuit, power good signal generator, etc to make the diagram simpler to understand. If you want detailed schematics, see Figure 5. If you don’t understand electronics, don’t worry. This figure is just here for the readers that want to go deeper.

Figure 5: Schematics for a typical low-end ATX power supply.

You may be asking yourself where is the voltage regulation stage on the Figures above. The PWM circuit does the voltage regulation. The input voltage is rectified before passing the switching transistors, and what they send to the transformer is square wave. So what we have on the transformer output is a square waveform, not a sine waveform. Since the waveform is already square, it is very simply to transform it into a DC voltage. So after the rectification after the transformer, the voltage is already DC. That is why some times switching power supplies are also referred as DC-DC converters.

The loopback used to feed the PWM control circuit is in charge of making all the necessary regulation. If the output voltage is wrong, the PWM control circuit changes the duty cycle of the signal applied to the transistors in order to correct the output. This happens when the PC power consumption increases, situation where the output voltage tends to drop, or when the PC power consumption decreases, situation where the output voltage tends to increase.

All you need to know before moving to the next page (and that you can learn from paying attention to Figures 3 and 4):

• Everything before the transformer is called “primary” and everything after it is called “secondary”.
• Power supplies with active PFC circuit don’t have a 110 V/ 220 V switch. They also don’t have a voltage doubler.
• On power supplies without PFC, if the 110 V / 220 V is set to 110 V, the power supply will use a voltage doubler, in order to make the voltage always around 220 V before the rectification bridge.
• On PC power supplies two power MOSFET transistors make the switcher. Several different configurations can be used and we will talk more about this later.
• The waveform applied to the transformer is square. Thus the waveform found on the transformer output is square, not sine.
• The PWM control circuit – which is usually an integrated circuit – is isolated from the primary through a small transformer. Sometimes instead of a transformer an optocoupler (a small integrated circuit containing a LED and a phototransistor packed together) is used.
• As we mentioned, the PWM control circuit uses the power supply outputs to control how it will drive the switching transistors. If the output voltage is wrong, the PWM control circuit changes the waveform applied on the switching transistors in order to correct the output.
• On the next pages we are going to explore each one of these stages with pictures showing where you can find them inside a power supply.