Principle and design of switching power supply

Flyback switching power supply, fixed switching transformer core area after switching transformer volt-second capacity by flux increment ⊿ B (⊿ B = Bm-Br) the size of the primary coil and the switching transformer turns number N1 to decide. The strength of the magnetic induction is determined by the magnetic field strength, i.e. the flux incremental ⊿ B is the magnetic field strength to determine. Figure 2-50, the dotted line B for the initial magnetization curve of the switching transformer core, the so-called initial magnetization curve is switching transformer core with magnetic magnetization curve in the first, once the switch transformer core band magnetic initial magnetization curve no longer exists. Therefore, in the switching transformer, switching the magnetization of the transformer core is generally not to carry out the work of the initial magnetization curve, but with the increase and decrease of the magnetic field strength, the strength of the magnetic induction will be along the magnetization curve AB and ba, or magnetization curve cd and dc change back and forth.

When the magnetic field strength increases, the magnetic field strength of the switching transformer core magnetizing; When the magnetic field strength is reduced, the magnetic field strength of the switching transformer core demagnetization. The strength of the magnetic induction is determined by the magnetic field strength when the magnetic field is increased from 0 to H1 of the corresponding magnetic induction intensity along the magnetization curve by Br1 ab increased Bm1; When the magnetic field strength is decreased by H1 to 0:00, which corresponds to the magnetic induction intensity by Bm1 along the magnetization curve ba down to Br1. Without considering the direction of the magnetic flux, the amount of change of the magnetic flux is ⊿ B1, i.e. the flux increments ⊿ B1 = Bm1-BR1. If the magnetic field strength is further increased, from 0 to H2 of the magnetization curve will be along curve cd and DC corresponding to the magnetic flux generated incremental ⊿ B2 = BM2-Br2. <br> Figure 2-50 can be seen, corresponding to a different magnetic field strength, i.e., different excitation current, the magnetic flux change amount is not the same, and the amount of change in magnetic flux with the magnetic field strength is not a linear relationship.

Mutual change function graph of magnetic flux density and magnetic field strength. Curve B is the curve of the corresponding change of the intensity and the magnetic field strength of the magnetic induction; curve μ is the magnetic permeability and the magnetic field strength corresponding to the curve of the variation; curve iμ change curve corresponding to the excitation current and the magnetic field strength. B = μH (2-147) iμ = Uτ / L (2-148) (2-147) and (2-148), wherein, B is the magnetic flux density, H is the magnetic field strength,  μ is the magnetic permeability, iμ excitation current, U is the voltage applied to the both ends of the primary coil of the switching transformer, L is the inductance of the primary coil of the switching transformer, the  τ is the pulse width. Permeability place is not the magnetic induction intensity or magnetic field intensity minimum or maximum place, but in place of the magnetic induction or magnetic field strength to an intermediate value. When the permeability reaches a maximum, the permeability increases with the magnetic induction or magnetic field strength, and decreased rapidly; when the permeability will be decreased to near 0, we believe that the switching transformer core begins to saturate, as shown in Bs and Hs. In this case, the switch transformer primary coil inductance will drop to 0, the excitation current becomes infinite.

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