Calculation of the number of turns of the primary coil of the push-pull switching power supply transformer

The switching transformer used in the push-pull switching power supply has two primary coils, both of which are excitation coils, but the directions of the magnetic lines of force generated by the current flowing through the two coils are exactly opposite. Therefore, the push-pull switching power supply transformer belongs to a dual-excitation switching power supply transformer; in addition, the secondary coil of the push-pull switching power supply transformer will be induced by the magnetic field generated by the two primary coils at the same time, so the secondary coil of the transformer has both forward and reverse voltage outputs; the push-pull switching power supply has multiple working modes, such as: AC output, rectifier output, DC regulated output, and other working modes, and various working modes have different requirements for transformer parameters.

Calculation of the number of turns of the primary coil of the push-pull switching power supply transformer

Since the core of the push-pull transformer is alternately excited by the current flowing through the two windings of the transformer primary coil N1 and N2, the magnetic induction intensity B of the transformer core can change from the negative maximum value -Bm to the positive maximum value +Bm. Therefore, the range of variation of the magnetic induction intensity of the push-pull transformer core is several times larger than that of the single-excitation transformer core, and flux saturation is not easy to occur.

The core of a push-pull transformer generally does not need an air gap. Therefore, the magnetic permeability of the transformer core is much higher than that of a single-excitation transformer core. In this way, the number of turns of each coil winding of the push-pull transformer can be greatly reduced, so that the volume of the transformer core and the total volume of the transformer can be relatively reduced.

The calculation method of the push-pull switching power supply transformer is basically the same as that of the forward or flyback switching power supply transformer, except that the range of the magnetic induction intensity of the transformer core is different. For a transformer core with bidirectional magnetic polarization, the value range of its magnetic induction intensity B can vary from the negative maximum value -Bm to the positive maximum value +Bm.

For the calculation method of the switching power supply transformer, please refer to the content in the previous "1-6-3. Calculation of parameters of the forward transformer switching power supply circuit" in the "2.1 Calculation of the number of turns of the transformer primary coil".

According to formula (1-95):

Formula (1-150) and formula (1-151) are the formulas for calculating the number of turns of the primary coil N1 winding of the dual-excitation switching power supply transformer. In the formula, N1 is the minimum number of turns of the primary coil N1 or N2 winding of the transformer, S is the magnetic conductive area of ​​the transformer core (unit: square centimeters), Bm is the maximum magnetic induction intensity of the transformer core (unit: Gauss); Ui is the voltage applied to both ends of the primary coil N1 winding of the transformer, in volts; τ = Ton, is the on-time of the control switch, referred to as the pulse width, or the width of the power switch tube conduction time (unit: second); F is the operating frequency, in Hertz. Generally, when the dual-excitation switching power supply transformer works in the forward and reverse output, its volt-second capacity must be equal. Therefore, the number of turns of the primary coil N1 winding of the transformer can be directly calculated using the operating frequency; the values ​​of F and τ should reserve a margin of about 20%. The exponents in the formula are used in unified units. If different units are selected, the values ​​of the exponents will be different. Here, the CGS unit system is used, that is: the length is centimeters (cm), the magnetic induction intensity is Gauss (Gs), and the magnetic flux unit is Maxwell (Mx).

1-8-1-4-2. Calculation of the primary and secondary turns ratio of the push-pull switching power supply transformer

A) Calculation of the primary and secondary turns ratio of the AC output push-pull switching power supply transformer

If the push-pull switching power supply is used for DC/AC or AC/AC inverter power supply, that is, converting DC into AC output, or rectifying AC into DC and then converting it into AC output, this kind of inverter power supply generally does not need to adjust the output voltage, so the circuit is relatively simple and the working efficiency is very high.

The output voltage of the push-pull switching power supply used for inverter is generally a square wave with a duty cycle of 0.5. Since the waveform factor of the square wave (the ratio of the effective value to the half-wave average value) is equal to 1, the effective value Uo of the square wave is equal to the half-wave average value Upa, and the amplitude Up of the square wave is also equal to the half-wave average value Upa. Therefore, as long as the half-wave average value of the output voltage is known, the effective value can be known, and then according to the half-wave average value, the turn ratio of the primary and secondary coils of the push-pull switching power supply transformer can be obtained.

According to the previous analysis, the output voltage uo of the push-pull transformer switching power supply is mainly determined by the forward voltage output by the secondary coil N3 winding of the switching power supply transformer. Therefore, according to one of the equations (1-128), (1-129), and (1-131), the half-wave average value of the output voltage of the push-pull transformer switching power supply can be obtained. Thus, the turns ratio of the primary and secondary coils of the inverter push-pull switching power supply transformer is obtained:

n=N3/N1 =Uo/Ui =Upa/Ui —— Transformation ratio, when D is 0.5 (1-152)

Formula (1-152) is the formula for calculating the turns ratio of the primary and secondary coils of the inverter push-pull switching power supply transformer. In the formula, N1 is the number of turns of one of the two windings of the primary coil of the switching transformer, N3 is the number of turns of the secondary coil of the transformer, Uo is the effective value of the output voltage, Ui is the DC input voltage, and Upa is the half-wave average value of the output voltage.

Formula (1-152) does not take into account the working efficiency of the transformer. When the working efficiency of the transformer is taken into account, it is best to multiply the right side of Formula (1-152) by a coefficient slightly greater than 1.

B) Calculation of the primary and secondary turns ratio of the DC output voltage non-regulated push-pull switching power supply transformer

The DC output voltage non-adjustable push-pull switching power supply is a DC/AC inverter power supply with an additional rectifier filter circuit after the AC output circuit. The duty cycle of the control switches K1 and K2 of this DC output voltage non-adjustable push-pull switching power supply is the same as that of the DC/AC inverter power supply, which is generally 0.5. Therefore, the turn ratio of the primary and secondary coils of the DC output voltage non-adjustable push-pull switching power supply transformer can be directly calculated using formula (1-152). That is:

n=N3/N1 =Uo/Ui =Upa/Ui —— secondary/primary transformation ratio, when D is 0.5 (1-152)

However, when low voltage and high current are output, the voltage drop of the rectifier diode must be considered.

C) Calculation of the primary and secondary turns ratio of the DC output voltage adjustable push-pull switching power supply transformer

The function of the push-pull switching power supply with adjustable DC output voltage requires that the output voltage is adjustable. Therefore, the duty cycle of the two control switches K1 and K2 of the push-pull transformer switching power supply must be less than 0.5; because the push-pull transformer switching power supply has voltage output in both forward and reverse states, so under the same output voltage (average value), the duty cycle of the two control switches K1 and K2 is equivalent to being half as small. When the adjustable range of the output voltage is required to be the maximum, the duty cycle is best taken as 0.25. According to equations (1-140) and (1-145), it can be obtained:

Formulas (1-153) and (1-154) are the formulas for calculating the turns ratio of the primary and secondary coils of the push-pull switching power supply transformer with adjustable DC output voltage. In the formula, N1 is the number of turns of the primary coil N1 or N2 winding of the transformer, N3 is the number of turns of the secondary coil of the transformer, Uo is the DC output voltage, and Ui is the DC input voltage.