Detailed Explanation of Working Principle of Single Excitation Transformer Switching Power Supply

During toff, the control switch K is turned off, and the current flowing through the primary coil of the transformer suddenly becomes 0. Because the current in the primary coil circuit of the transformer changes suddenly, and the magnetic flux in the transformer core cannot change suddenly, the current flowing through the secondary coil circuit of the transformer must also change suddenly to offset the impact of the current change in the primary coil of the transformer, or a very high back EMF voltage will appear in the primary coil circuit of the transformer, Break down the control switch or transformer.

If the magnetic flux in the transformer core Ñ„ If there is a sudden change, the primary and secondary coils of the transformer will produce an infinitely high back electromotive force, which will produce an infinite current, and the magnetic line of force generated in the current coil will resist the change of magnetic flux. Therefore, the change of magnetic flux in the transformer core will eventually be constrained by the current in the primary and secondary coils of the transformer.

Therefore, during toff when the control switch K is turned off, the magnetic flux in the transformer core is mainly determined by the current in the transformer secondary coil circuit, that is:

e2 =-N2*d Ñ„/ DT = - L2 * di2 / dt = I2R - K off period (1-64)

The negative sign in the formula indicates that the polarity of the back electromotive force E2 is opposite to the symbol in formula (1-62), that is, the polarity of the induced electromotive force generated by the secondary coil of the transformer when k is turned on and off is exactly opposite. Solve the order differential equation of formula (1-64):

In the formula, C is a constant. It is easy to calculate C by substituting the initial conditions into the above formula. When the control switch K suddenly changes from on to off, the current in the primary coil circuit of the transformer suddenly becomes 0, and the magnetic flux in the transformer core cannot change suddenly. Therefore, the current I2 in the secondary coil circuit of the transformer must be exactly equal to the current I2 (ton ) during the on of the control switch K, And the excitation current in the primary coil circuit of the transformer is converted to the sum of the secondary coil circuit current of the transformer.

(1-66) in the formula, the first item in brackets represents the current in the transformer secondary coil circuit, and the second item represents the current converted from the excitation current in the transformer primary coil circuit to the transformer secondary coil circuit.

Figure 1-16-a output voltage uo of single excitation transformer switching power supply is equal to:

(1-68) up - in the formula is the peak value of the counterattack output voltage, or the maximum value of the output voltage. It can be seen that at the moment when the control switch K is turned off, when the load of the transformer secondary coil circuit is open, the transformer secondary coil circuit will produce a very high back EMF. Theoretically, when the time t is equal to infinity, the output voltage of the secondary coil circuit of the transformer is 0, but this generally does not happen, because the off time of the control switch K can not wait that long.

It can be seen from equations (1-63) and (1-67) that the working principle of switching power transformer is different from that of ordinary transformer. When the switching power supply works in forward excitation, the working principle of the switching power supply transformer is basically the same as that of the ordinary transformer; When the switching power supply works in flyback, the working principle of the switching power supply transformer is equivalent to an energy storage inductor.

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