The following points should be considered when designing the structure of a switching power supply transformer: Minimal leakage flux is crucial to reduce winding leakage inductance. The structural design should facilitate winding and lead-out, simplifying installation and greatly aiding maintenance and production. Proper planning before design is essential to ensure sufficient space and heat dissipation. By comprehensively considering these factors in the design of the switching power supply transformer, its safety and lifespan can be significantly enhanced.
Material selection is crucial in the design of switching power supply transformers, and the choice of magnetic core is paramount. The choice of material varies depending on the application of the switching power supply transformer. The most widely used magnetic core is the manganese-zinc iron oxide core, while high-permeability cores are also used in power input filters. Soft ferrite cores are widely used today due to their advantages such as low cost, good adaptability, and good high-frequency performance.
The chip's operating frequency is set to 70kHz. This frequency can be set via an external RC circuit, and the operating frequency is equal to the switching frequency. This peripheral function is beneficial for designing switching power supplies and allows for external synchronization. Similar in function to the UC384X, the transformer core is EER28/28L. Generally, the operating frequency of AC-DC converters should not exceed 100kHz. This is mainly because excessively high switching power supply frequencies are detrimental to system stability and EMC compliance. Higher frequencies also increase di/dt and dv/dt. Besides the PI's 132kHz operating frequency, you can refer to other manufacturers' chips to summarize your own experience.
The selection of the magnetic core is based on the switching frequency and power, and is largely determined by experience. Of course, for calculations, you need to obtain more core parameters, including magnetic material, Curie temperature, frequency characteristics, etc., which requires gradual development. EE25, EER25, EER28, EFD25, EFD30, etc., are all suitable within the 20W to 40W range.
The transformer design calculations above have already determined the transformer's inductance. Now, we also need to obtain the corresponding number of turns to complete the transformer's operation.
1) Calculate the conduction time Ton. Periodic time T = Ton + Toff = 1/Fsw. Ton = T * Dmax. Fsw, where Dmax is a known value (70kHz). Substituting 0.45 into the above formula, we get Ton = 6.43us.
2) Calculate the number of primary turns of the transformer Np = Vin(min)*Ton/(ΔB × Ae) = 120Vdc * 6.43us/(0.2 * 82mm2) = 47 T (The number here must be rounded up; we cannot wind only half a turn or other non-integer turns in the transformer).
3) Calculate the number of turns and output voltage (Vo) of the 12V main output of the transformer:
12 Vdc rectifier diode voltage drop (Vd): 0.7
Vdc winding voltage drop (Vs): 0.5
Primary-side turns ratio (K) = Vi_min / Np = 120 Vdc / 47 T = 2.55 Output turns (Ns) = (Output voltage (Vo) + Rectifier diode voltage drop (Vd) + Winding voltage drop (Vs)) / Primary-side turns ratio (K) = (12 Vdc + 0.7Vdc + 0.5Vdc) / 2.55 = 6 T (rounded)
4) The calculation method for the number of turns of the transformer auxiliary winding (aux turning) output is the same as that for the 12V main winding output. Because the secondary feedback of ST VIPer53DIP needs to be lower than 14.5 Vdc, 12 Vdc is selected as the auxiliary voltage; Na = 6 T. At this step, we have basically obtained the main parameters of the transformer: Primary winding: 47T, Primary inductance: 0.77mH, Leakage inductance < 5% * 0.77mH = 39uH, 12V output: 6T, Auxiliary winding: 6T. Next, we just need to put forward the safety requirements such as the wire diameter, number of strands, pin positions, withstand voltage, etc. of the winding, and then send it to the transformer factory for prototyping. As for the calculation of the air gap and the return verification Dmax, these are all from textbooks. It is not recommended to blindly follow them. Be more flexible.
