EV Onboard Charger Flyback Transformer Design Example (400VDC to 12V)

by

Modern electric vehicles contain dozens of low-voltage systems that must operate regardless of the traction battery voltage.

Control modules, battery management systems, contactors, sensors, cooling systems, and communications networks often require isolated low-voltage power supplies.

One of the most common methods of generating this auxiliary power is a flyback converter.

At the heart of the flyback converter is the flyback transformer.

In this example, we will walk through the engineering process used to design a flyback transformer for converting a 400VDC battery bus into an isolated 12V auxiliary supply.

Flyback transformer generating isolated 12V power from a 400V electric vehicle battery system, showing ferrite core construction, winding arrangement, magnetic flux paths, and automotive power conversion.
Flyback transformers are commonly used in electric vehicles to generate isolated low-voltage power from high-voltage battery systems while maintaining safety and reliability.

Design Requirements

Input Voltage:

400VDC Nominal

Output Voltage:

12VDC

Output Power:

60W

Switching Frequency:

100 kHz

Isolation Voltage:

2500 VAC

Maximum Temperature Rise:

40°C

Automotive Environment:

  • Wide temperature range
  • High reliability
  • Long service life

Why EV Auxiliary Supplies Matter

Many people assume EVs only contain a high-voltage battery.

In reality, modern EVs contain extensive low-voltage systems.

Examples include:

  • Battery Management Systems (BMS)
  • Vehicle control modules
  • Cooling pumps
  • Fans
  • Communication interfaces
  • Safety systems

These circuits require reliable isolated power.

Quick Design Estimate

Before detailed transformer calculations begin, engineers often evaluate candidate magnetic solutions.

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 Analysis

Step 1: Determine Output Current

Output current:

60W ÷ 12V

= 5A

This establishes the secondary current requirement.

Step 2: Determine Turns Ratio

Flyback transformers require an appropriate turns ratio to balance:

  • Duty cycle
  • Efficiency
  • Device voltage stress
  • Regulation capability

The turns ratio is one of the most important design decisions in the entire converter.

Step 3: Evaluate Energy Storage

Unlike conventional transformers, flyback transformers store energy.

This energy must be stored safely without driving the core into saturation.

Energy storage requirements often determine:

  • Core size
  • Air gap
  • Magnetizing inductance

Step 4: Select Core Family

Potential candidates include:

  • ETD29
  • ETD34
  • PQ26
  • PQ32

Engineers evaluate:

  • Window area
  • Core volume
  • Thermal capability
  • Manufacturability

before selecting a final candidate.

Step 5: Design the Air Gap

The air gap determines:

  • Energy storage capability
  • Saturation margin
  • Magnetizing inductance

Proper air gap sizing is critical to flyback performance.

👉 Related Guide: Air Gap Design in Power Inductors

Step 6: Verify Saturation Margin

Flyback transformers routinely experience significant peak currents.

The design must maintain adequate saturation margin under:

  • Maximum load
  • Cold start
  • Worst-case voltage conditions

Check Saturation Margin

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 Analysis

Step 7: Design the Windings

The winding arrangement affects:

  • Leakage inductance
  • Copper losses
  • EMI
  • Manufacturability

Common approaches include:

  • Layer windings
  • Sectioned windings
  • Interleaved windings

Each approach involves tradeoffs.

Step 8: Select Wire Size

The conductors must safely carry current while minimizing losses.

Engineers evaluate:

  • Current density
  • Fill factor
  • Temperature rise
  • Manufacturability

Evaluate Wire Options

Wire Current Density Calculator

Estimate required copper area and approximate AWG size from RMS current and target current density.

Total Copper Area Required: mm²

Area Per Conductor: mm²

Approximate Suggested AWG:

This is a first-pass estimate. Real winding design also requires insulation diameter, window fill, AC loss, bend radius, and thermal checks.

Want optimized winding and CAD output?

Start Design Analysis

Step 9: Estimate Transformer Losses

Losses originate from:

  • Copper losses
  • Core losses
  • Leakage inductance effects

Reducing losses improves:

  • Efficiency
  • Reliability
  • Thermal performance

Estimate Losses

Inductor Loss Estimator

Estimate copper loss, core loss, and total loss for a preliminary inductor design.

Copper Loss: W

Core Loss: W

Total Loss: W

Thermal Concern:

Need a thermal-checked design package?

Start Design Analysis

Step 10: Automotive Thermal Design

Automotive systems often experience:

  • High ambient temperatures
  • Limited airflow
  • Continuous operation

Thermal performance becomes a primary design consideration.

👉 Related Guide: How to Reduce Inductor Temperature Rise

Isolation Requirements

Unlike many industrial applications, automotive designs require careful consideration of:

  • Creepage distance
  • Clearance distance
  • Insulation systems
  • Safety standards

Transformer construction is often driven as much by isolation requirements as electrical performance.

Practical Optimization Process

A production design typically evaluates:

  • Multiple core sizes
  • Multiple air gap values
  • Multiple winding arrangements
  • Multiple conductor strategies

before selecting a final design.

Conclusion

Flyback transformers remain one of the most common methods of generating isolated auxiliary power in electric vehicles.

Successful designs balance:

  • Isolation
  • Efficiency
  • Saturation margin
  • Thermal performance
  • Manufacturability

while meeting demanding automotive reliability requirements.

Need Help Designing Custom Magnetics?

The SolidMagnetics platform helps engineers optimize:

  • Core selection
  • Air gap sizing
  • Saturation margin
  • Thermal performance
  • Manufacturability

while automatically generating CAD models, engineering drawings, BOMs, and production-ready manufacturing outputs.

Need a Manufacturable Design?

Generate a custom magnetic design package directly from your electrical requirements.

Generate Design
Name
Would you like to subscribe to our newsletter?