Design of flyback switching power supply transformer

1. Power indicator

Input voltage: 90AC~264AC
Input frequency: f=50hz
Output voltage: 12v
Output current: 3A
Output power: po=36w
Design efficiency: 75%~80%
Input power: pin=48w 2. Set Transformer flux

Transformer at work when the temperature is high, in the process of the design of saturated core density bs=0.39t, in order to prevent the saturation of the core need to leave a certain margin, here in the design of the time to take the magnetic flux bm=0.25t 3. Calculate transformer turns ratio

According to the parameters of the MoS tube and diode of the BOM, considering a certain amount of headroom, by calculating the turn ratio n when the MoS tube is turned on and off
Switch ON:0.8VD>VINMAX/N+VO
Where VD is the diode reverse voltage maximum value
Switch off:0.8vmos>n* (VO+VF) +vinmax
Wherein the VMOS is the voltage withstand value of the MoS tube
Calculate N=8 (5.5~11.5) 4. Calculate the minimum input voltage

According to the formula:

where Tc=3ms 5. Calculate the maximum duty-free ratio

Maximum duty ratio, according to minimum input voltage
Dmax=n (VO+VF)/(N (VO+VF) +vinmin)
The maximum duty ratio is 0.48 6. Calculate peak current for primary inductor

ip=2po/(dmax*vinmin*η)
The peak current of the primary inductor is 1.818A 7. Calculating the RMS value of primary winding current

Irms=ip*√ (DMAX/3) =0.727a 8. Calculate the inductance of primary winding

lp=vinmin*dmax/(IP*F)
Calculate the primary inductance 450μh 9. Maximum on-time

tonmax=dmax/f
Calculation of the available time is 7.4us 10. Calculation of primary turns

np=vin*tonmax/(AE*BM)
Calculates the number of primary turns that can be 40 turns. Where the armature is given, AE is a known amount of 11. Secondary turns

ns=np/n
The number of turns that can be calculated is 5 laps. 12. Auxiliary Winding

Auxiliary winding According to the LD5530R data sheet, set the VCC voltage to 16V, there are:
ncc=vcc*ns/(VO+VF)
The calculation indicates that the auxiliary winding is 7 turns 13. One-time side winding diameter selection

Flyback transformer Winding Conductor current density 5a/〖mm〗^2~7a/〖mm〗^2, in the design of the current density of 6 A, then according to the sp=irms/j=0.727/6=0.121/〖mm〗^2
The diameter of the wire diameter is: dp=1.13*√0.121=0.3933mm 14. Wire diameter selection for secondary windings

Sp=irms/j=3/6=0.5/〖mm〗^2
ds=1.13*√0.5=0.79mm 15. The air gap size of the core

lg= (0.4π*〖np〗^2*ae)/LP
Calculation of the available air gap size is 1.291mm 16. Transformer Winding

Take a sandwich around half of the primary winding at 3 feet and 2 feet, which is 20 laps. Then on the 5, 6 feet around the auxiliary winding VCC, around 7 laps. Then on the 10, 8 feet around the secondary winding 5 laps. Finally on the 2 feet and 1 feet around the primary winding of the other half 20 laps, the last 2 feet of the line connected together. The windings of the transformer with the same name are determined according to the direction of the input and the winding of the skeleton pin.

References:
1. Baidu Encyclopedia
2. Research and design of single-ended flyback switching power supply _ Zhang Wei
3. Capacitor selection in power frequency rectifier capacitor filter circuit-Zhao Xiuko
4. Design and realization of multi-output flyback switching power supply _ Ox Force
5.OB Transformer design