Everything is difficult at the beginning, switching power supply design should be step by step.

**Introduction:** Many people find it challenging to switch power supplies, but it's not as difficult as it seems. Designing a switching power supply isn't inherently hard, but achieving precision and fine-tuning is. Once you start, things become much simpler when you accumulate experience and use a discrete structure for design. Everything feels tough at the beginning, and that’s why I’m here to guide you step by step through the process of designing a switching power supply. The first step in designing a switching power supply is to understand the specifications. Many have encountered this before, and you can also propose your own requirements for reference. I will help analyze them. I’ll be guiding you through a typical isolated switching power supply with a wide input range (85–265Vac) and an output of 12V/2A. **1. Determine the Power and Choose the Topology** Based on specific requirements, you should choose the appropriate topology. A flyback converter usually meets most needs. I'll use empirical formulas for calculation, and if you need a more detailed analysis, we can discuss it further. **2. Select the PWM IC and MOSFET for Preliminary Circuit Design** Once the flyback topology is chosen, selecting the corresponding PWM IC and MOSFET becomes essential. You can choose between discrete or integrated solutions. Discrete offers more flexibility in power matching, but the design and debugging cycle is longer. Integrated solutions save time by combining the PWM IC and MOSFET into one package, making them ideal for beginners or projects requiring fast development. **3. Create the Schematic** After choosing the chip, you begin drafting the schematic (SCH). For this example, I’ve selected the STVIPer53DIP, which includes an integrated MOSFET. It’s always a good idea to review the datasheet before starting to confirm key parameters. Whether using PI’s integrated solution or separate components like 384x or OBLD, the datasheet is your main reference. Most datasheets include a basic schematic that serves as a foundation for your design. **4. Define the Parameters** Once the schematic is complete, define the parameters needed for PCB layout. Different companies may have different processes, so following the right workflow is important. This stage may involve preliminary evaluation, schematic confirmation, and other steps before finalizing calculations. **5. Determine Switching Frequency and Select the Core for Transformer Design** The operating frequency of the chip is set at 70kHz. The frequency can be adjusted via external RC components, and it directly affects the switching frequency. This setup helps improve the performance of the power supply and allows for external synchronization, similar to the UC384X function. For the transformer core, I've selected EER28/28L. Generally, AC-DC converters operate below 100kHz because higher frequencies can lead to instability and poor EMC performance. While some chips like PI operate at 132kHz, experience plays a big role in core selection. Factors like magnetic material, Curie temperature, and frequency characteristics are important and should be considered carefully. Here’s a list of cores suitable for various power ranges: - **Less than 5W**: ER9.5, ER11.5, EE8.3, EE10, EE13, RM4, GU11, EP7, EP10, UI9.8, URS7 - **5–10W**: ER20, EE19, RM5, GU14, EFD15, EI22, EPC13, EF16, EP13, UI11.5 - **10–20W**: ER25, EE20, EE25, RM6, GU18, EPC17, EF20 - **20–50W**: ER28, ETD28, EI28, EE28, EE30, EF25, RM8, GU22 - **50–100W**: ER35, ETD34, EE35, EI35, EF30, RM10, GU30, PQ26 - **100–200W**: ER40, ER42, ETD39, EI40, RM12, GU36, PQ32 - **200–500W**: ER49, ETD49, EC53, EE42, EE55, EI50, RM14, GU42 - **Over 500W**: ER70, ETD59, EE65, EE85, GU59, PQ50, UU66 **6. Transformer Design Calculation** Given: - Input: 85–265Vac - Output: 12V / 2A - Switching Frequency: 70kHz - Core: EER28/28L - Core Area: Ae = 82mm² Set Parameters: - Efficiency η = 80% - Max Duty Cycle Dmax = 0.45 - ΔB = 0.2 T Calculations: - Po = 12V × 2A = 24W - Pin = Po / η = 30W - Vin(min) = 85 × 1.414 ≈ 120Vdc - Vin(max) = 265 × 1.414 ≈ 375Vdc - Iav = 30W / 120Vdc = 0.25A - Ipeak = 4 × Iav = 1A - Lp = (Vin(min) × Dmax) / (Ipeak × Fsw) = 770μH Next, calculate the number of turns: - Ton = Dmax / Fsw = 6.43μs - Np = (Vin(min) × Ton) / (ΔB × Ae) = 47T - Ns = ((Vo + Vd + Vs) / K) = 6T - Na = 6T Wire diameter: - Primary: φ0.25mm - Auxiliary: φ0.25mm - Secondary: φ0.4–0.5mm (3 strands) **7. Input and Output Capacitor Calculation** - Cin = 1.5–3 × Pin = 45–90μF - Cout = 200–300 × Io = 400–600μF Choose 47μF/400V for input and 470μF/16V for output. Use a CLC filter for better ripple suppression. **8. PCB Layout** Generate a netlist from the schematic, define the board outline, and load the component library. Place critical components like the transformer, capacitors, and ICs first. Keep loops small, ground traces wide, and minimize high dv/dt areas. **9. Finalize Transformer Details** Define the transformer with proper winding ratios and insulation. Ensure proper isolation between primary and secondary sides. Use sandwich winding to reduce leakage inductance. **10. Commissioning Process** After soldering, perform a static check, then power up gradually. Use an oscilloscope to monitor waveforms and ensure stability. Avoid connecting the oscilloscope ground to the PE line without isolation. **Final Summary:** Designing a switching power supply is simpler than it seems. Starting with a single-chip solution helps eliminate many uncertainties. As you gain experience, moving to a discrete design becomes easier. With practice, you'll master the process and build reliable power supplies efficiently.

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