Flyback converters are among the most widely used isolated power supply topologies in industrial and consumer electronics.
This example demonstrates the engineering process involved in designing a flyback transformer for a 400VDC to 24VDC power supply.
The goal is to illustrate the design workflow rather than produce a final production-ready magnetic component.
Design Requirements
Input Voltage:
400VDC
Output Voltage:
24VDC
Output Power:
120W
Switching Frequency:
100 kHz
Isolation:
1500 VAC
Maximum Temperature Rise:
40°C
Inductor Saturation Risk Checker
Estimate flux density from inductance, peak current, turns, and effective core area.
Estimated Flux Density: T
Risk Level:
Approximation: B ≈ L × Ipk / (N × Ae). Final design should use actual core data, gap, material Bsat, and temperature limits.
Need a full saturation and gap-checked design?
Start Design AnalysisStep 1: Determine Output Current
Output current:
120W ÷ 24V
= 5A
This establishes the secondary current requirement.
Step 2: Estimate Turns Ratio
A preliminary turns ratio is selected to achieve:
- Acceptable duty cycle
- Reasonable winding count
- Adequate efficiency
👉 Related Guide: Transformer Turns Ratio Explained
Step 3: Select Core Family
Potential core families include:
- ETD34
- ETD39
- PQ35
- EE35
The final choice depends on:
- Power level
- Thermal requirements
- Available winding area
Step 4: Determine Energy Storage
Unlike conventional transformers, flyback transformers store energy.
Stored energy must support the required output power without saturation.
👉 Related Guide: How to Calculate Inductor Energy Storage
Step 5: Air Gap Design
The air gap determines:
- Energy storage capability
- Saturation margin
- Magnetizing inductance
Proper air gap design is critical.
👉 Related Guide: Air Gap Design in Power Inductors
Inductor Quick Feasibility Checker
Use this quick estimator to check peak current, stored energy, and preliminary design difficulty.
Peak Current: A
Ripple Current: A p-p
Stored Energy: mJ
Preliminary Difficulty:
Likely Core Direction:
This is a quick educational estimate only. Final design requires core geometry, gap, winding, loss, fill factor, and thermal checks.
Need a manufacturable design package?
Run the full SolidMagnetics designer to generate optimized candidates, CAD files, BOM data, and design deliverables.
Start Design AnalysisStep 6: Saturation Analysis
Peak flux density must remain below the material saturation limit.
👉 Related Guide: Understanding Magnetic Saturation
Adequate design margin is required under worst-case operating conditions.
Step 7: Winding Design
The winding arrangement affects:
- Leakage inductance
- Copper losses
- EMI
- Manufacturability
👉 Related Guide: Transformer Leakage Inductance Explained
Step 8: Thermal Evaluation
Losses originate from:
- Copper losses
- Core losses
- Leakage energy
👉 Related Guide: Inductor Temperature Rise Explained
The design target is less than 40°C temperature rise.
Practical Design Optimization
Engineers typically evaluate multiple combinations of:
- Core size
- Air gap
- Wire size
- Turns ratio
- Winding structure
before selecting a final design.
Conclusion
Flyback transformer design requires balancing:
- Isolation
- Energy storage
- Saturation margin
- Thermal performance
- Manufacturability
By evaluating these factors together, engineers can create efficient and reliable isolated power supplies.
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