Calculation of Turns Ratio between Primary and Secondary Coils of Transformer

Transformer Primary and secondary Coil Turns ratio calculation

The output voltage of a forward switching power supply is generally the average value of the pulsating DC, and the average value of the pulsating DC is related to the duty cycle of the control switch. Therefore, before calculating the turns ratio of the primary and secondary coils of the forward switching power supply transformer, the duty cycle D of the control switch must be determined first. After the duty cycle D is determined, the turns ratio of the primary and secondary coils of the forward switching power supply transformer can be calculated according to formula (1-77):

Uo = Ua = nUi × Ton/T = Upa × D —— the entire cycle (1-77)

From (1-77) we can get:

n=Uo/Ui*T —— Transformer turns ratio (1-97)

In the above formula, n is the turns ratio of the secondary coil to the primary coil of the forward switching power supply transformer, that is: n = N2/N1; Uo is the output DC voltage, Ui is the primary input voltage of the transformer, and D is the duty cycle of the control switch.

Under normal output load conditions, the duty cycle D of the forward switching power supply control switch is preferably set to about 0.5. In this way, when the load is relatively light, the duty cycle D will be less than 0.5. Although the energy storage filter inductor will be interrupted, the charging time of the energy storage filter capacitor will be shortened, and the discharge time will increase, the output current is relatively small, and the charging and discharging current of the energy storage filter capacitor is also very small, so the voltage ripple generated at both ends of the capacitor will not increase, but decrease; when the output load is relatively heavy, the duty cycle D of the control switch will be greater than 0.5. At this time, the current flowing through the energy storage filter inductor is a continuous current, the output current increases, the charging time of the energy storage filter capacitor increases, and the discharge time is shortened. Therefore, the voltage ripple generated at both ends of the capacitor will not increase much.

Therefore, if the energy storage filter inductor and energy storage filter capacitor in the forward switching power supply circuit are charged and the switch duty cycle is controlled, the output voltage ripple will be very small. The turn ratio n of the secondary back electromotive force energy absorption feedback coil N3 winding and the primary coil N1 winding of the forward switching power supply transformer is generally 1:1, that is: N3/N1 = 1. If n is greater than 1, the limiting protection effect of the feedback coil N3 winding and the rectifier diode D3 will be enhanced, but the current flowing through the feedback coil N3 winding and the rectifier diode D3 will also increase, thereby increasing the loss; if n is less than 1, the limiting protection effect of the feedback coil N3 winding and the rectifier diode D3 will be weakened, and the peak pulse can easily break down the power switch tube.

The calculation method of the number of turns of the secondary back-electromotive force energy absorption feedback coil N3 of the forward switching power supply transformer is very similar to the calculation method of the limiting voltage-stabilizing diode. However, the number of coil turns is exactly opposite to the breakdown voltage of the voltage-stabilizing diode. The higher the breakdown voltage, the weaker the limiting protection effect.

By the way, the current density of the transformer coil enameled wire is generally 2~3 amperes per square millimeter. When the operating frequency of the switching power supply is very high, it is better to make the current density smaller, or use multi-strand wire instead of single-strand wire to avoid the skin effect of the current in the conductor, increase the loss and make the wire heat up. In addition, most of the enameled wires currently used for winding transformers are not pure copper wires, so the resistivity is relatively large. Considering these factors together, the current density cannot be too high.