After n DC pulse voltages, the maximum magnetic flux density Bm and the residual magnetic flux density Br in the transformer core can be basically stabilized above a certain value, that is, after the pulse sequence reaches a steady state, the magnetization process will follow the original A fixed local hysteresis loop on the curve repeats n points; at this time, the residual flux density is Br n (Br n = Br), and the change in the magnetic flux density is essentially the same regardless of whether the magnetic field strength increases or decreases. Obviously, where the local hysteresis loop is fixed depends on the value of the B value for a certain material. If B is large enough, the lowest point of the local hysteresis loop lies at the point Br of the remanent magnetic flux density point of the maximum local hysteresis loop. At this point, Br corresponds to the start of each input dc pulse and Bm corresponds to the end of each dc pulse.
It is understandable that the magnetic flux density does not continue to increase after reaching the maximum value Bm, because the magnetic flux density and the magnetic field strength can be either potential energy or potential energy, and the two can be converted to each other. The process of charging and discharging them with the capacitor is very similar. For example, when the power supply voltage charges the capacitor, the voltage across the capacitor rises. When the power supply is disconnected, the capacitor discharges to the load, and the voltage across the capacitor drops. When the capacitor charges and discharges the charge is completely equal. At this time, the voltage ripple across the capacitor will stabilize above a certain value.
The magnetic field strength increment is denoted by H, which corresponds to the magnetic flux density increment B on the fixed local hysteresis loop, that is, they can be expressed by the following relation:
The formula (2-10) refers to the relationship between the magnetic field strength increase H and the magnetic flux density increase B pulse static characteristics. Not established under DC conditions.
The correspondence between the magnetic field strength increment H and the magnetic flux density increment B can also be expressed by the following equation:
(2-11) In the formula, it is called the pulse static susceptibility, or the pulse transformer's pulse permeability. Due to the relatively small range of pulse permeability, we can also use the concept of average permeability for switching transformers. which is:
(2-12) In the formula, the average magnetic permeability of the switching transformer; The average magnetic flux density increase in the switching transformer core; The average magnetic field intensity increase in the switching transformer core.
The difference between the pulse permeability and the average permeability is that the amplitude and the width of the input pulse voltage of the pulse transformer are basically fixed, and it is a unipolar pulse. The area of ​​the hysteresis loop is relatively small, therefore, The core's pulse permeability can be seen almost as a constant; while the amplitude and width of the input pulse voltage of the switching transformer are not fixed, the area of ​​the hysteresis loop is relatively large, and the variation range of the core's permeability is relatively large. Also very large, especially double-shocked switching transformers, therefore, can only be described by the concept of average permeability.
Exciting current or magnetic field intensity when the transformer core is magnetized similar to the capacitor charging and discharging characteristics: When the magnetic field strength generated by the excitation current in the transformer primary coil magnetized transformer core, the magnetic flux density will increase, quite The capacitor is charged; when the excitation current in the primary coil of the transformer is zero, the primary and secondary coils of the transformer will generate a counter electromotive force, and the induced current will generate a reverse magnetic field to demagnetize the transformer core, so that the magnetic flux density The drop is similar to the case where the charge capacitor discharges the load.
The maximum magnetic flux density Bm and residual magnetic field in the transformer core when the magnetic flux density generated when the transformer core is magnetized is exactly equal to the increase (negative value) of the magnetic flux density generated when the transformer core is demagnetized The density Br will stabilize at a certain value.
At this point, we can say that the magnetization process of the transformer core has entered a basically stable state, that is, each input of a DC pulse voltage, the magnetic flux density in the transformer core will produce a magnetic flux density increase ΔB, ΔB = Bm- Br , when the DC pulse is over, the magnetic flux density returns from the maximum value Bm to the position of the residual magnetic flux density Br. In this way, the Br value corresponding to the magnetization curve is called remanence (or residual flux density), and the Bm value corresponding to the magnetization curve is called the maximum value of magnetic flux density.
However, the values ​​of the maximum magnetic flux density Bm and the residual magnetic flux density Br in the magnetization curve of the transformer core are not constant. They change with the amplitude of the input pulse voltage and the pulse width; only the magnitude of the input pulse voltage and When the pulse width remains substantially constant, the values ​​of the maximum magnetic flux density Bm and the residual magnetic flux density Br in the magnetization curve of the transformer core remain substantially unchanged.
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