Adapter Design Calculation 23 Steps
1. Input: 100-264V
2. Output: 12V1.5A
3. Efficiency: 84% (80.2% energy efficiency, for ease of production of 84% was chosen)
4. Vcc: 14V (select VCC turn-on threshold)
5. Working frequency: 60K (PFM)
6. Dmax: 0.45 (duty cycle greater than 0.5 will bring loop instability defect, so we are controlled within 0.5)
7. ΔB: (Bs-Br) * n = ΔB = (390-55) * 0.6 = 0.2T
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 = 0.205cm^4
- EF25 AP = 0.2376cm^4, AE = 51.8 mm²
10. Np Calculation:
- Np = VINmin * ton / ΔB / AE = 108 * 7.5 / 0.2 / 51.8 = 78.18T (rounding to 79T)
11. NS Calculation:
- NS = (Vo + Vd) * (1 - Dmax) * NP / (VINmin * Dmax)
- = (18 + 0.6) * (1 - 0.45) * 78 / (108 * 0.45) = 11.12T (rounding to 11T)
12. N Calculation:
- N = Np / Ns = 79 / 11 = 7.18T
13. Iav Calculation:
- Iav = Po / η / Vinmin = 18 / 0.84 / 108 = 0.198A
14. Ipk Calculation:
- Ipk = Ipk1 + Ipk2 = Iav * 2 / Dmax = 0.198 * 2 / 0.45 = 0.88A
15. ΔI Calculation:
- CCM Ip2 = 3Ip1, DCM Ip1 = 0
- 0.88 / 4 = IP1 = 0.22
- 0.22 * 3 = IP2 = 0.66
- ΔI = Ip2 - Ip1 = 0.66 - 0.22 = 0.44A
16. Current RMS (CCM):
- Irms = 0.88 * 0.512 = 0.45A
17. Lp Calculation:
- Lp = Vinmin * ton / ΔI = 108 * 7.5 / 0.44 = 1.8mH (we actually use less than the calculated one, 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 calculation: CCM 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 is 5-7
22. Ds Calculation:
- Secondary diameter: Ds = (3.75 / 3.14 / 7) * 2 = 0.82mm
- The situation can not be reduced by 70% = 0.57
- J current density is 6-8
- Skin depth: The wire diameter does not exceed 2 times the skin depth. If it exceeds, multi-stranded is needed. δ = 66.1 / √f cm = 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. Primary winding must be in the innermost layer to reduce distributed capacitance and EMI.
2. The beginning of the primary winding should be connected to the MOSFET drain for shielding.
3. Primary winding is designed to be less than 2 layers to minimize distributed capacitance and leakage inductance.
4. Secondary winding with the largest output power should be close to the primary to reduce leakage inductance.
5. Feedback winding is generally at the outermost layer to improve stability.
6. Shield design: Add a shielding layer between primary and secondary to reduce common mode interference.
7. Copper shielding tape can be used around the transformer to inhibit leakage magnetic field.
8. Safety test: After winding, three layers of insulating tape are wrapped, core inserted, varnish immersed, and tested for safety.
Component Selection:
1. Fuse: If = Iav / 0.6 * 2 = 0.66A, 250V fuse can be used.
2. Varistor: V1ma = 1.2 * 374 / 0.85 / 0.9 = 487.9V
3. Input large capacitor: 33uF capacitor is selected.
4. X capacitor: Class 2 selects X2 capacitor.
5. Y capacitor: CY = 0.25 / 2 / 3.14 / 60 / 264 * 10^-6 = 2.5nF max, choose 222 / 400V.
6. Filter inductor: 20mH is commonly chosen.
7. Bridge stack selection: BR1 = 5 * Iav = 0.99A, choose 1A1KV.
8. RCD absorption: Typical configuration includes 150K resistor, 102 capacitor, and a slow tube.
9. CS resistor: Vcs = Rcs * Ipk * 1.2, try to take a bit lower to avoid saturation.
10. VCC capacitor: Choose a capacitor close to Vcc foot, all low single point grounding.
This detailed design process ensures efficient and safe operation of the adapter. Each step is crucial for optimizing performance, reducing losses, and ensuring compliance with safety standards. Practical testing and adjustments are essential to fine-tune the system for real-world applications.
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