The power transformer design is perhaps one of the most critical step in designing a converter, especially one that uses a Secondary-Side Regulated (SSR) flyback configuration. Fairchild’s Flyback/Forward PWM design tool helps make quick work or designing and prototyping the power transformer. The tool walks you through the steps of configuring an SSR flyback converter, including magnetic core selection, bobbin, wire size, winding method, and more. The tool also includes a Simplis simulation model that lets you verify circuit operation once the design is done, and you can issue a Bill of Materials (BOM) based on the circuit parameters.
You can access the Flyback/Forward PWM tool from the main page of the Fairchild Semiconductor website (www.fairchilsemi.com). Using the dropdown menus along the top of the page, navigate from Tools & Support to Design Tools, and then to Flyback/Forward PWM. This is what we’re calling the SSR design tool.
The next screen is the log-in page for “My Fairchild Account.” Once you’re logged in, you’ll see the first page of the design tool, shown below.
The left-hand side of the screen shows the nine steps in the design process.
- Design Requirements
- Controller Selection
- Core & Turns
- Power Semiconductors
- Output Filters
- Snubbers & Input
The right-hand side of the screen shows the first page for Step 1: Design Requirements. We’ll go through each step page by page.
Step 1. Design Requirements
Enter the type of input (AC or DC), and specify the input voltage range. If the input is AC, enter the line frequency.
1b. Output DC
Enter the number of outputs on the load side. If there are multiple outputs, enter the output voltage and current for each one.
1c. Output Regulation
Enter the requirements for output ripples, over-shoots, and dips during dynamic load conditions.
1d. Operating Conditions & Input Capacitor
Enter the expected efficiency and the input capacitance, based on the input voltage ranges.
Step 2: Controller Selection
2a. Choose Controller + MOSFET
Enter the maximum drain-to-source voltage for the primary-side FET, and the maximum current flowing through it. The SSR tool calculates and lists the suitable ICs.
2b. Magnetizing Inductance, Vcc
Enter the number of turns of the primary side winding on the magnetic core. The SSR tool calculates the magnetizing inductance.
2c. Semiconductor Voltage Stresses
Using the parameters entered in 2b, the SSR tool calculates the voltage, current, and temperature rise for the switching components.
Step 3: Core & Turns
3a. Core Selection
3b. Bobbin Selection
Based on the core selected, the SSR tool recommends a bobbin.
Using the input and output voltages, the SSR tool calculates the transformer turns ratio.
3d. Core Gap & Losses
Step 4: Windings
The tool calculates the minimum and maximum sizes for the winding wire, along with the skin effect depth.
Figure 11: Windings: Primary Windings
4b. Primary Windings
4c. Main Output
4d and 4e are missing since only one output was selected. The 4d and 4e are for multiple outputs
4f. Auxiliary Winding
The tool calculates the parameters.
4g. Transformer Shielding
4h. Transformer Schematic
Electrical parameters are displayed on the schematic.
Step 5: Power Semiconductors
5a. Diode Derating
5b. Main Output Diode
5e. MOSFET Losses
Step 6: Output Filters
6a. Main Output Filter
Step 7: Control
7a. Power Stage Characteristics
7b. Feedback Compensation
7c. Loop Gain
Explain the criteria stability analysis for the flyback converter. Plot gain and phase plot for the design.
7d. Controller Setup
Step 8: Snubbers & Input
8a. Primary-Side RCD Snubber
8b. Secondary-Side RC Snubbers
8c. AC Input circuit
Step 9: Efficiency
9a. Efficiency Plot
The SSR tool plots the efficiency of the flyback converter, comparing the percentage load to the input line voltage. The SSR tool also generates loss structures for the converter.
Design Tools & Support