Power supply circuit design: detailed explanation of linear power supply and switching power supply

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Power supply circuit design: detailed explanation of linear power supply and switching power supply [Copy link]

 

Power supply circuit design: detailed explanation of linear power supply and switching power supply

When it comes to circuit design, one particularly important item that can be singled out is power supply design. After all, all electronic devices must have the support of a power circuit to work properly.

This time, I will briefly introduce the power supply circuit design so that ordinary people can have a general understanding of power supply design.

As for power supplies, there are two main types on the market: linear power supplies and switching power supplies.

So what is the difference between these two power supplies?

The working principle of a linear power supply is to first convert 220V or other AC voltage into low voltage electricity (12V or other smaller AC alternating current) through a transformer, and then correct and rectify it through a series of diodes, and convert the AC alternating current into a pulsating voltage.

After obtaining the pulsating voltage, it is necessary to filter the pulsating voltage, which is usually done through a capacitor, and then convert the filtered low-voltage alternating current into DC direct current.

However, the DC power at this time is not pure enough and will have certain fluctuations (generally called ripple), so a voltage regulator diode or voltage rectification circuit is needed for correction.

Finally, a relatively stable DC output can be obtained.

This whole process is the principle of linear power supply.

What about switching power supplies?

Different from the linear power supply that first converts high-voltage AC into low-voltage AC, the switching power supply directly converts the 220V AC input into DC, and then uses a switching tube to control the on and off of the current under the action of a high-frequency oscillation circuit to form a high-frequency pulse current.

Finally, with the help of inductor (high frequency transformer), the required stable low-voltage DC power is output.

For power circuit design, the first thing you need to master is the switching power supply circuit, which obtains voltage from the household AC voltage 220V, converts it into a DC stable voltage, and then powers the electronic device motherboard. And most electronic products on the market, such as computers and mobile phones, use switching power supplies. Then here is a simple switching power supply example (shortening the process of AC to DC conversion).

As shown in the figure below, this is a simple switching power supply. The voltage of the DC power will change by continuously closing and opening the switch.

For example, according to the rule of 0.5s closing and 0.5s opening, the 12V voltage can be changed to 6V, achieving a voltage reduction effect.

So what is the principle behind it?

In fact, when the switch is just closed, 12V DC power is supplied. At this time, the diode is cut off, and the power supply starts to store energy in the inductor and capacitor, and supply power to the load at the same time. However, because the inductor current cannot change suddenly, the inductor will induce a voltage opposite to the power supply in order to increase the resistance current. This inductance will offset part of the power supply voltage, making the voltage at the load end unable to reach 12V. However, as time goes by, the inductance offset effect will weaken, and the voltage at the load end will slowly reach 12V.

Then when the switch is turned off, the inductor has a counteracting effect. In order to prevent the current from changing suddenly, it will act as a power source to supply power to the load. Therefore, after the switch is turned off, the load still has current for a short time.

However, over time, the inductance canceling effect gradually weakens and becomes just a wire that no longer provides power.

Therefore, in this continuous process, if the time difference between the opening and closing of the switch can be controlled, and the current does not become zero, by repeatedly switching on and off, a DC current that operates at 6V but is not particularly stable can be obtained, and the effective value is almost close to 6V.

Therefore, in order to obtain a stable voltage, the role of capacitors cannot be ignored.

The capacitor in this circuit mainly plays the role of energy storage and filtering. Without it, the waveform will become very sharp.

The last is the switch in the circuit. In actual circuits, a transistor is generally used to act as a switch, which can be switched tens of thousands of times within 1 second.

Don't underestimate this simple switching power supply, its efficiency can reach about 90%. Its main losses are the equivalent resistance of inductance and capacitance, the conduction voltage drop of the diode, and the conduction resistance of the transistor.

This is a simple switching power supply design. Of course, in actual applications, the design of the switching power supply will be more complicated in order to achieve higher accuracy, higher performance, and more stability.

Jacktang

6V DC current works but is not particularly stable, and its effective value is almost close to 6V.

Why?