When it comes to circuit design, one of the most crucial elements is the power supply design. Every electronic device requires a properly designed power supply circuit to function correctly.
There are two main types of power supplies in the market: linear power supply and switching power supply. Let’s discuss the differences between these two.
A linear power supply operates by converting high AC voltage (e.g., 220V) into low voltage (e.g., 12V or other lower AC voltages) using a transformer. This low voltage AC is then rectified and converted into pulsating DC voltage using a series of diodes.
After obtaining the pulsating DC voltage, it needs to be filtered with a capacitor to remove the fluctuations (often referred to as ripples). Subsequently, the filtered low-voltage AC is converted into stable DC voltage.
However, the resulting DC voltage may still contain some fluctuations. To correct this, a Zener diode or a voltage regulation circuit is employed. Finally, a relatively stable DC output can be obtained.
The whole process is the principle of linear power supply. What about switching power supplies? Different from the process of converting high-voltage alternating current to low-voltage alternating current in the linear power supply, the switching power supply directly converts the 220V AC input terminal into direct current DC, and then under the action of the high-frequency oscillating circuit, the switching tube is used to control the on-off of the current to form a high-frequency pulse current. Finally, with the help of an inductor (high-frequency transformer), the required stable low-voltage direct current is output. For power supply circuit design, generally the first thing that needs to be mastered is the switching power supply circuit, which obtains the voltage from the household AC voltage of 220V, converts it into a DC stable voltage, and then supplies power to the main board of the electronic device. And most electronic products on the market, such as computers and mobile phones, use switching power supplies. Then there is a simple example of a switching power supply (the process of converting AC to DC is reduced). As shown in the figure below, this is a simple switching power supply. The voltage of the direct current will change by using the continuous closing and opening of the switch.
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For example, according to the law of 0.5s on and 0.5s off, the 12V voltage can be changed to 6V, realizing the step-down effect.
The principle of the power supply circuit can be explained as follows:
When the switch in the circuit is initially closed, the 12V DC power is supplied. At this moment, the diode is in the off state, and the power supply begins to store energy in the inductor and capacitor while simultaneously delivering power to the load. However, since the current in the inductor cannot change abruptly, an opposing voltage is induced in the inductor due to the rise in resistance current. This opposing voltage will offset a portion of the power supply voltage, preventing the voltage at the load from reaching exactly 12V.
When the switch is turned off, the inductor acts as a canceller by providing power to the load, preventing a sudden change in current. As a result, even after the switch is turned off, the load still maintains a certain level of current for a short period of time.
However, over time, the inductive canceling effect gradually weakens and becomes a wire that no longer supplies 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, and the switch is repeated, a section of DC direct current that acts 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 the capacitor cannot be ignored. The capacitor mainly plays the role of energy storage and filtering in this circuit. Without it, the waveform will become very sharp.
The last is the switch in the circuit. In an actual circuit, a transistor is generally used as a switch, and it can switch tens of thousands of times within 1 second. Don’t underestimate this simple switching power supply, its efficiency can reach about 90%. Among them, its main loss is the equivalent resistance of the inductance and the capacitor; the conduction voltage drop of the diode; and the conduction resistance of the transistor.
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