Adapter Design Calculation 23 Steps
1. Input: 100-264V
2. Output: 12V1.5A
3. Efficiency: 84% (80.2% energy efficiency, for ease of production, 84% was chosen)
4. Vcc: 14V (select VCC turn-on threshold)
5. Working frequency: 60K (PFM)
PFM is used when no-load mode is in the down mode, which reduces switching and conduction losses. When the IC load is lower than a certain level, it enters sleep mode, reducing static current consumption. However, PFM has a less favorable dynamic response compared to PWM.
6. Dmax: 0.45 (duty cycle greater than 0.5 may cause loop instability; we control it within 0.5)
7. ΔB: (Bs-Br) * n = (390-55) * 0.6 = 0.2T
Bs: 390mT/100°C
Br: 55mT
Each parameter has a different safety value of 0.3T max.
CCM continuous mode, current is not zero, ΔB becomes smaller. n takes 60% ΔB value based on personal habits.
8. Vinmin, Vinmax Calculated:
Vinmin = Vacmin * 1.2 = 90 * 1.2 = 108V
Vinmax = Vac * 1.414 = 374V
9. Core Selection:
AP = [(Po/η + Po) * 10000] / (2 * ΔB * f * J * Ku)
= [(18/0.84 + 18) * 10000] / (2 * 0.2 * 60 * 1000 * 400 * 0.2)
= 394285.7 / 1920000 = 0.205 cmâ´
f = 60 * 1000 (Hz)
J = current density = 400
Ku = winding coefficient = 0.2
EF25 AP = 0.2376 cmâ´, AE = 51.8 mm²
Design experience: A small Ae value leads to high efficiency and low temperature. The core area should be small to reduce thermal differences and improve radiation.
10. Np Calculation:
Primary turns: Np = VINmin * ton / ΔB / AE
Np = 108 * 7.5 / 0.2 / 51.8 = 78.18T → round to 79T
11. NS Calculation:
Secondary turns: NS = (Vo + Vd) * (1 - Dmax) * NP / (VINmin * Dmax)
= (18 + 0.6) * (1 - 0.45) * 78 / (108 * 0.45) = 11.12T → round to 11T
12. N Calculation:
Turns ratio: N = Np / Ns = 79 / 11 = 7.18T
13. Iav Calculation:
Average current: Iav = Po / η / Vinmin = 18 / 0.84 / 108 = 0.198A
14. Ipk Calculation:
Peak current: Ipk = Iav * 2 / Dmax = 0.198 * 2 / 0.45 = 0.88A
15. ΔI Calculation:
Current change rate: ΔI = Ip2 - Ip1 = 0.66 - 0.22 = 0.44A
16. Current RMS (CCM):
Irms = 0.88 * 0.512 = 0.45A
17. Lp Calculation:
Primary inductance: Lp = Vinmin * ton / ΔI = 108 * 7.5 / 0.44 = 1.8mH
We use an empirical value of 0.7 * 1.8 = 1.26mH
18. Verify Saturation:
ΔB = Lp * Ipk / Np / Ae = 1.26 * 0.88 / 79 / 51.8 = 0.27T < 0.3T
19. Ipks Calculation:
Secondary peak current: Ipks = Ipk * N = 0.88 * 7.18 = 6.3A
20. Irmss Calculation:
Secondary RMS: Irms = 6.3 * 0.566 = 3.57A
21. Dp Calculation:
Primary wire diameter: Dp = (Irms / π / J) * 2 = (0.45 / 3.14 / 6) * 2 = 0.3mm
J = current density = 5-7
22. Ds Calculation:
Secondary diameter: Ds = (3.75 / 3.14 / 7) * 2 = 0.82mm
Skin depth: Wire diameter should not exceed 2 times skin depth. If it does, multi-stranded wire is needed. δ = 66.1 / √f = 0.269mm
Multi-strand calculation = 0.7 / number of shares = 0.57 / 1.414 = 0.4mm * 2
23. Nvcc Calculation:
Feedback winding: Va = (Vo + Vd) / Ns = 12.6 / 11 = 1.145V/T
Nvcc = Vcc / Va = 14 / 1.145 = 12.22T → take 12T
Transformer Winding Rules:
1. The primary winding must be in the innermost layer to reduce distributed capacitance and EMI.
2. The start of the primary winding should be connected to the MOSFET drain to shield the rest of the windings.
3. The primary winding should be less than 2 layers to minimize distributed capacitance and leakage inductance. Add an insulation layer between layers.
4. The secondary winding with the largest output power should be close to the primary to reduce leakage inductance. Use multi-strand if necessary.
5. The feedback winding is generally at the outermost layer for strong coupling with the secondary and improved stability.
6. Shielding between primary and secondary can reduce common-mode interference. Wrap a layer of enameled wire between them.
7. Copper shielding tape around the transformer can help suppress leakage magnetic fields. It should be connected to Vd.
8. Safety test: After winding, wrap three layers of insulating tape, insert the core, immerse in varnish, and test. For 110V, withstand 2000V AC for 60s. For 220V, 3000V AC.
Component Selection:
1. Fuse: If = Iav / 0.6 * 2 = 0.66A → 250V fuse.
2. Varistor: V1ma = 1.2 * 374 / 0.85 / 0.9 = 487.9V.
3. Input large capacitor: 2Po = 36uF → choose 33uF.
4. X capacitor: Class 2 uses X2, larger capacity improves conduction effect.
5. Y capacitor: Choose Y1 or Y2, leakage current < 0.25mA.
6. Filter inductor: 20mH is a common choice.
7. Bridge stack: Vd = 2√2 * Vinmax = 747V.
8. RCD absorption: Use 150k resistor, 102 capacitor, and a slow tube.
9. CS resistor: Vcs = Rcs * Ipk * 1.2. Avoid saturation.
10. VCC capacitor: Place it close to Vcc pin for stable startup.
Debugging Experience:
Adjust the balance between efficiency and Vds. Use sandwich winding to reduce Vds. Ensure the transformer design is verified through multiple methods.
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