Design of high frequency transformer is a difficult point in the design of power supply. The following is the example of the feedback type current transformer, and introduces a design method of high frequency transformer for power supply.

Design goal: the power input AC voltage between 180V~260V, frequency is 50Hz, the output voltage is DC 5V, 14A, power is 70W, the power supply operating frequency is 30KHz.

Design steps:

Calculation of the high frequency transformer primary peak current Ipp

As the current is not continuous power supply, when the power tube conduction, the current will reach the peak value, this value is equal to the peak current of the power tube. The relationship between the current and voltage of the inductor can be determined by the V=L*di/dt:

Input voltage: Vin (min) =Lp*Ipp/Tc

Take 1/Tc=f/Dmax, the above formula is:

Vin (min) =Lp*Ipp*f/Dmax

Among them: Vin: DC input voltage, V

Lp: high frequency transformer primary inductance value, mH

Ipp: transformer primary peak current, A

Dmax: maximum working cycle coefficient

F: power supply operating frequency, kHz

In a discontinuous current supply, the output power is equal to the energy stored in each period of the operating frequency:

Pout=1/2*Lp*Ipp2*f

The inductor voltage can be divided:

Pout/Vin (min) =Lp*Ipp2*f*Dmax/ (2*Lp*Ipp*f)

Thus available:

Ipp=Ic=2*Pout/ (Vin (min) *Dmax)

Where: Vin (min) =1.4*Vacin (min) -20V (DC ripple and diode drop) =232V, maximum working cycle coefficient Dmax=0.45. Then:

Ipp=Ic=2*Pout/ (Vin) (min) =2*70/ (*Dmax) =1.34A (232*0.45)

When the power tube is switched on, the collector is able to withstand the current.

Minimum working cycle coefficient Dmin

In the feedback current discontinuous power supply, the size of the work cycle coefficient is determined by the input voltage.

Dmin=Dmax/[(1-Dmax) *k+Dmax]