On the first part of the series of power supply decoupling, I hope to talk about the understanding of power supply decoupling from a qualitative point of view. I hope everyone can support it. We also ask you to correct your inadequacies.
Talking about the Decoupling of Power Supply - The Cause of Power Decoupling
The ideal power supply:
The voltage of the "ideal power supply" is stable, there is no noise, the output power is not limited, and the response speed is infinitely fast. That is, regardless of how the current consumed by the load changes, and at what speed, the voltage of the power supply should be a constant value. Without any impact on the load, but also fully meet the power requirements of the load, will not affect the normal operation of the load.
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A brief description of the commonly used power devices in actual circuits:
There are two types of actual power devices often used in electronic circuits:
1) Linear Regulator:
The principle of this power supply structure is generally shown below
Using the principle of negative feedback, the output voltage is divided by two resistors and fed back to the input of the “op ampâ€, and the other input of the op amp is connected to a fixed reference voltage Vref, according to the virtual short and false negative and negative Based on the principle of feedback, the op amp automatically adjusts the output voltage to adjust the on-resistance of the regulator (FET in the figure) so that Vout·R2/(R1+R2)=Vref is satisfied between the output voltages Vout and 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 or load current of the power supply changes, the resistance of the resistor changes accordingly, so that the load can be divided. constant.
2), switching power supply:
Take the step-up switching power supply as an example. Its general structure is as follows
The FET in the figure acts as a switch. When the FET is turned on, the power supply charges the inductor and capacitor and provides energy to the load. When the FET turns off, the energy stored in the inductor and capacitor forms a loop through the freewheeling diode to provide power to the load. By controlling the duty cycle of the FET switch, the output voltage can be regulated.
Commonly used in the electronic circuit power is the above two, these two have their own advantages and disadvantages, there are differences in the use of occasions, and each can continue to be subdivided into a variety of sub-categories, which is beyond the scope of power supply decoupling in this section. From the simple introduction above, we can see that these two practical power supplies have limitations.
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The difference between actual power and ideal power:
(1) There is a gap between the characteristics of the actual power supply device and the ideal power supply:
Responsive speed: It is said that the linear regulated power supply is stabilized by changing the on-resistance of the adjusting tube through negative feedback. However, the bandwidth of the op amp is limited, and the negative feedback process takes some time. So the actual linear regulated power supply requires a certain response time. In other words, when the input voltage or the current of the load changes too fast or the amplitude is too large, the power supply device cannot respond or cannot completely eliminate the change, and the load supply voltage will change, affecting the normal operation of the load.
Noise characteristics: Since the switching power supply operates in the switch state, that is, there is a sharp change in the current inside the circuit, the noise is larger than the linear regulator. And various semiconductor devices inside the power supply itself are also noisy. These noises and the undesired characteristics of the power supply will have a certain impact on the load.
Specific parameters describing the characteristics of the power supply device include linear adjustment rate, load adjustment rate, and output voltage noise.
(2) PCB traces have an impact on power quality:
We all know that high-speed PCBs are generally multi-layer boards, which have a dedicated power layer in order to reduce the parasitic inductance of the power line. The role of the inductor is to hinder the rapid change of the current. If the parasitic inductance of the power line is large, when the load suddenly needs current, it cannot be obtained from the power supply in time. The large 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 pins of the chip (the current of the capacitor can be mutated and the voltage can not be mutated).
(3) External interference has an impact on power quality:
The power line may also be affected by the electromagnetic interference of other signals and spaces on the board during the trace process, thereby increasing the noise. Decoupling capacitors can effectively filter out these noises
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to sum up
The ultimate goal of power supply decoupling is to allow the load to operate properly and bring the power supply characteristics closer to that of an ideal power supply - the ability to respond quickly to load current demand, voltage stability, and clean noise. Minimize the coupling interference between the various parts of the circuit through the power supply.
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