How should the power supply be decoupled?

The ultimate goal of power supply decoupling is to ensure that the load can operate normally and make the power supply characteristics closer to the ideal power supply - one that can quickly respond to the current demand of the load, have stable voltage, and be clean and noise-free. Minimize coupling interference between various parts of the circuit through the power supply.

1.Ideal power supply:

The voltage of an "ideal power supply" is stable, without any noise, the output power is unlimited, and the response speed is infinitely fast. That is, no matter how the current consumed by the load changes and at what speed, the voltage of the power supply should be a stable value. It will not be affected by the load, fully meets the power supply requirements of the load, and will not affect the normal operation of the load.

2. A brief introduction to commonly used power supply devices in actual circuits:

There are two types of actual power devices commonly used in electronic circuits:

1.Linear regulator:

The schematic structure diagram of this kind of power supply is generally as follows:

Using the principle of negative feedback, the output voltage is divided by two resistors and then fed back to the input terminal of the "op amp". The other input terminal of the op amp is connected to a fixed reference voltage Vref. According to the virtual short virtual break and negative According to the principle of feedback, the op amp will automatically adjust the output voltage, thereby adjusting the on-resistance of the adjustment tube (FET in the picture), so that the output voltage Vout and Vref satisfy Vout·R2/(R1+R2)=Vref. It can be seen that the linear regulated power supply can be equivalent to a variable resistor connected in series between the power supply and the load. When the voltage of the power supply or the load current changes, the resistance of the resistor changes accordingly, so that the voltage distributed to the load constant.

2. Switching power supply:

Taking the boost switching power supply as an example, its general structure is as shown below

The FET in the figure acts as a switch. When the FET is on, the power supply charges the inductor and capacitor and provides energy to the load. When the FET is off, the energy stored in the inductor and capacitor forms a loop through the freewheeling diode to provide power to the load. Controlling the duty cycle of the FET switch can adjust the output voltage.

The two power supplies commonly used in electronic circuits are the above two types. Each has its own advantages and disadvantages, and there are also differences in usage scenarios. Each can be further subdivided into multiple subcategories, which is beyond the scope of power supply decoupling in this article. From the brief introduction above, we can see that both of these actual power supplies have limitations.

3. The difference between actual power supply and ideal power supply:

(1) There is a gap between the characteristics of actual power supply devices and ideal power supplies:

Response speed: As mentioned earlier, the linear regulated power supply achieves voltage regulation by changing the on-resistance of the regulating tube through negative feedback. However, the bandwidth of op amps is limited, and the negative feedback process takes a certain amount of time. Therefore, the actual linear regulated power supply requires a certain response time. In other words, when the input voltage or load current changes too quickly or with too large an amplitude, and the power device has no time to respond or cannot completely eliminate the change, the load's supply voltage will change, affecting the normal operation of the load.

Noise characteristics: Since the switching power supply operates in a switching state, that is, there is a sharp change in current inside the circuit, the noise is greater than that of a linear regulator. And the various semiconductor devices inside the power supply also have noise themselves. These noises and unideal characteristics of the power supply will have a certain impact on the load.

Parameters that specifically describe the characteristics of power supply devices include linear regulation rate, load regulation rate, output voltage noise, etc.

(2) PCB routing has an impact on power quality:

We all know that high-speed PCBs are generally multi-layer boards with dedicated power layers in order to reduce the parasitic inductance of the power lines. The function of the inductor is to hinder the rapid change of current. If the parasitic inductance of the power line is large, when the load suddenly needs current, it will not be able to obtain it from the power supply in time. The wide power plane can minimize parasitic inductance and improve the quality of the power supply.

Even so, parasitic inductance still exists. In order to provide the instantaneous current required by the chip, decoupling capacitors are often placed at the chip pins (the current of the capacitor can change suddenly, but the voltage cannot change).

(3) External interference has an impact on power quality:

During the wiring process, the power line will also be affected by electromagnetic interference from other signals on the board and space, resulting in increased noise. Decoupling capacitors can effectively filter out these noises