The difference between linear power supply and switching power supply
According to the conversion principle, power supplies can be classified into linear power supplies and switching power supplies. When we classify linear power supplies and switching power supplies, we actually need to clarify whether it is AC/DC or DC/DC. Although this classification is aimed at distinguishing the principles of transformation. But are linear power supplies and switching power supplies that achieve AC/DC functions a complete process of converting AC to DC, and some of the circuits are composed of DC/DC.
Linear power supply and switching power supply for AC/DC
There are many textbooks, books, and articles that directly refer to linear power sources as "linear power sources for AC/DC". What is a linear power source? Linear power supply first reduces the voltage amplitude of AC power through a transformer, then rectifies it through a rectifier circuit to obtain pulsed DC power, and then filters it to obtain DC voltage with small ripple voltage.
The characteristics of AC/DC linear power supply and switching power supply are different as follows:
The linear power supply of AC/DC is first reduced by AC voltage using a power frequency transformer, and then rectified. After voltage reduction through a transformer, the voltage has become relatively low, and power chips such as a three-terminal voltage regulator can be used for voltage stabilization. The adjustment tube of the linear power supply operates in an amplified state, resulting in high heat generation and low efficiency (related to the voltage drop), requiring the addition of a bulky heat sink. The volume of power frequency transformers is also relatively large, and when producing multiple sets of voltage outputs, the transformer volume will be larger.
The adjustment tube of AC/DC switching power supply operates in saturation and cut-off states, resulting in low heat generation and high efficiency. The AC/DC switching power supply eliminates the need for bulky power frequency transformers. However, the DC output of the AC/DC switching power supply will have larger ripples, which may be improved by connecting a voltage regulator diode at the output end. In addition, due to the high peak pulse interference generated during the operation of the switch tube, magnetic beads need to be connected in series in the circuit to improve. Relatively speaking, the ripple of a linear power supply can be made very small. Switching power supplies can be achieved through different topological structures, such as voltage reduction, boost, and boost, while linear power supplies can only achieve voltage reduction.
Many early power adapters were relatively heavy, and their conversion principle was AC/DC linear power supply, which used a power frequency transformer internally. AC/DC linear power supply first uses a transformer to reduce the AC voltage. This type of transformer, which directly reduces the voltage in the mains, is called a power frequency transformer, as shown in Figure 1.9. Power frequency transformers, also known as low-frequency transformers, distinguish them from high-frequency transformers used in switching power supplies. Power frequency transformers were widely used in traditional power sources in the past. The standard frequency of mains power in the power industry, also known as mains power ("mains power" refers to the power supply mainly used by residents in cities), is 50Hz in China and 60Hz in other countries. A transformer that can change the voltage of alternating current at this frequency is called a power frequency transformer. Power frequency transformers are generally larger in size compared to high-frequency transformers. So the volume of AC/DC linear power supply implemented with power frequency transformers is relatively large.
AC/DC switching power supply requires first rectifying and filtering the AC power supply to form an approximate DC high voltage, and then controlling the switch to generate high-frequency pulses, which are transformed through a transformer. AC/DC switching power supply has higher efficiency and smaller size. One important reason for its small size is that high-frequency transformers are much smaller than power frequency transformers. Why is the higher the frequency, the smaller the transformer volume?
Transformer core materials have saturation limits, so there are limits to the peak magnetic field strength. The current, magnetic field strength, and magnetic flux of alternating current are all sinusoidal signals. We know that for sine signals of the same amplitude, the higher the frequency, the greater the peak of the signal's "rate of change" (the moment the sine signal crosses zero is the peak of the "rate of change", while the rate of change at the peak of the signal is 0). Meanwhile, the induced voltage is determined by the rate of change of magnetic flux. So, for the same voltage per turn, the higher the frequency, the smaller the peak magnetic flux required. But as mentioned above, the peak value of magnetic field intensity is limited. Therefore, if the magnetic flux requirement is reduced, the cross-sectional area of the iron core can be reduced. The above analysis assumes the same voltage per turn. And the voltage per turn is related to power. Therefore, assuming the same power. If the power is smaller, the current is also smaller, and the allowed wire is thinner, and the resistance is slightly higher, it is allowed to increase the number of turns. In this way, the voltage per turn is also reduced, which can also reduce the magnetic flux requirement. Then reduce the volume. Also, the above analysis assumes that the material is constant, that is, the saturation magnetic field strength is constant. Of course, if materials with higher saturation magnetic field strength are used, the volume can also be reduced. We know that compared to transformers of the same size decades ago, transformers nowadays have much smaller volumes because they now use new iron core materials.
According to Maxwell's equation, the induced electromotive force E in the transformer coil is
That is, the integral of the rate of change of magnetic flux density B over time over N wire turns with an area of Ac.
For transformers, the induced electromotive force E on the primary side of the transformer and the voltage U applied on the input side can be considered as a linear relationship. On the premise that the amplitude of U on the input side of the transformer remains unchanged, it can be considered that the amplitude of E also remains unchanged.
In addition, there is an upper limit for the magnetic flux density B of each type of magnetic core. The ferrite used for high-frequency applications is around a few tenths of a Tesla, while the iron core used for power frequency applications is around a level slightly greater than one, with a small difference.
Therefore, when the frequency increases, the rate of change in magnetic flux density dB/dt during each cycle increases significantly, provided that the peak change in magnetic flux density B is not significant. Therefore, smaller Ac or N can be used to achieve the same induced electromotive force E. A decrease in Ac means a decrease in the cross-sectional area of the magnetic core; A decrease in N means that the area of the empty window of the magnetic core can be reduced, both of which can help achieve a smaller volume of the magnetic core. The cross-sectional area of a high-frequency transformer is smaller, and the number of turns in the coil decreases, resulting in a smaller volume.
The adjustment tube of the switching power supply operates in saturation and cut-off states, resulting in low heat generation and high efficiency. AC/DC switching power supplies do not require the use of large power frequency transformers. However, the DC output of the switching power supply will have large ripples superimposed on it. In addition, due to the large peak pulse interference generated during the operation of the switching transistor, it is also necessary to filter the power supply in the circuit to improve the quality of the power supply. Relatively speaking, linear power sources do not have the above defects, and their ripple can be very small.
