Select the core bobbin: see 'Derivation of the High-Frequency Transformer AP-Method Formula'
AP is the basis for core selection, Ae is the core cross-sectional area, Aw is the core window area,
fs is the power supply switching frequency, determined by the main control circuit; take the minimum operating frequency at full load
Quick approximation of flyback transformer AP value at 80kHz: AP >= Pin/360, unit is CM4
For the 30W case:
Calculate the primary winding inductance Lp, turns Np, and flux density change ΔB,
Primary winding inductance Lp calculation (based on the inductor energy storage formula + inductor voltage formula)
Note: If the actual transformer Lp is smaller than the calculated value, it means the actual duty cycle at AC90V is below 0.4,
e.g. at D=0.3 the theoretical Lp would be only 56% of that at 0.4. A small duty cycle increases the RMS
currents of both primary and secondary, reducing power supply efficiency.
Primary winding turns Np calculation
Note: Once the core cross-section Ae, operating frequency fs and duty cycle Dmax are fixed, the primary turns Np depend only on ΔB.
If the actual primary turns are fewer than the theoretical value, ΔB increases and the transformer core loss grows.
For a given Lp and Dmax, the actual operating ΔB= Vdc_min*Dmax / (Ae*fs) can be computed.
For the 30W case at an 80kHz operating frequency:
Calculate the primary/secondary turns ratio Np_s and secondary turns Ns
Primary/secondary turns ratio calculation, based on transformer volt-second balance; the duty cycle is maximum at the lowest DC input voltage,
Quick calculation: Np_s=85/Vout, valid when D=0.4
Calculate the auxiliary/secondary turns ratio Na_s and auxiliary turns Naux
According to the operating voltage VDD required by the control IC, Vaux is generally chosen as the middle of the VDD operating range,
for example, the VDD supply voltage range of the JW1520AC controller is 8~28V
Calculate primary/secondary peak currents Ip_max, Is_max and RMS currents Ip_rms, Is_rms
Although the operating currents of the transformer windings are not physical parameters needed for making the transformer,
computing the theoretical winding currents is essential for checking the transformer current waveforms and calculating copper loss heating.
Calculate the transformer losses
See 'Electromagnetic Loss Calculation for Power Inductors and Transformers'
K, a, b are constants; refer to the manufacturer datasheet. Reference values for the PC95 core:
f in kHz, Bac in mT, Ve in cm3, Pcore in mW
For the 30W (20V 1.5A) case:
= =0.42W