Switching power supply transformer design

As a transformer engineer, mastering the essential skills is crucial before you can contribute effectively to the work chain. First, it's important to understand the role of the transformer within a circuit and how voltage and current waveforms behave during operation. This foundational knowledge is key to innovating new transformer designs and troubleshooting issues that may arise in practical applications. In recent years, power transformers have evolved to meet more specialized demands, especially in low-power switching power supply designs, which now better align with civilian needs. In this session, we'll walk through the design and production process of a low-power power transformer, offering engineers valuable insights and practical guidance. **Detailed Design of a Switching Power Supply** The first step in designing a switching power supply is to review the specifications. While many are familiar with this process, it’s always helpful to revisit and analyze them carefully. For this example, we’ll focus on a conventional 12V 2A isolated switching power supply with a wide input voltage range. **1. Determine the Power Requirements** Based on the specific application, the appropriate topology is selected. A flyback topology is often a good choice for such a design, as it can meet most basic requirements efficiently. Note: I will use empirical formulas for calculations here. If needed, I can provide more detailed analysis and discuss further. **2. Selecting the PWM IC and MOSFET** Once the flyback topology is chosen, the next step is to select the right PWM IC and MOSFET for the initial schematic design. You can choose between discrete or integrated solutions: - **Discrete**: Offers greater flexibility in power matching but requires more time for design and debugging. - **Integrated**: Combines the PWM IC and MOSFET into one package, simplifying the design process and reducing development time—ideal for beginners or fast-paced projects. **3. Creating the Schematic** After selecting the components, the schematic (Sch) is created. For this example, I’ve used the ST VIPer53DIP, which integrates the MOSFET, making it a convenient choice. Before starting the design, it's always a good idea to consult the datasheet and verify the basic parameters. Most datasheets include a sample schematic, which serves as a solid foundation for your design. **4. Moving to PCB Layout** Once the schematic is complete, the next step is to define the parameters for the PCB layout. Different companies may have different workflows, so it's important to follow established processes and develop good design habits. This stage typically involves preliminary evaluation, schematic confirmation, and final sign-off before proceeding with calculations. Here is the corresponding schematic:

There's no need to add a resistor for the startup current since this chip already includes a high-voltage startup current source:

For chips like the UC384X that require an external startup resistor, you can calculate the value using the formula: **Rstart = (Vin(min) - Vdd) / Istart** Where: - Rstart = Startup resistor - Vin(min) = Minimum input DC voltage - Vdd = Chip supply voltage - Istart = Chip startup current This is a deep and critical aspect of the design. One advantage of the VIPer53 is that even if the optocoupler is removed (or R205 is disconnected), the power supply still functions normally. It then operates in primary-side regulation (PSR) mode, where the COMP pin forms a stable PWM control system based on the first-order inertia link. Here is the reference circuit diagram:

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