How to Choose a Transformer for an EV Charger?

ElecSuppliers Electronics GuideAug 12, 2026

A transformer can be an important part of an EV charger, particularly in isolated AC-DC and DC-DC power conversion stages. The right transformer needs to match the charger's power level, voltage requirements, switching frequency, insulation system and thermal conditions.

For high-power EV charging equipment, transformer selection should be considered together with the complete power conversion circuit rather than treated as a separate component.

Determine the Charger Power

Start with the charger's rated output power.

An EV charger may operate at relatively low power for residential applications or much higher power in commercial and fast-charging systems.

The transformer needs to handle the required power continuously without excessive temperature rise.

Peak operating conditions should also be considered if the charger experiences temporary increases in load.

Check the Input and Output Voltage

The required voltage ratio is one of the main transformer parameters.

Check the charger's input voltage and the voltage required by the downstream power conversion stage.

For an isolated DC-DC converter, the transformer ratio must be compatible with the switching topology and desired output voltage.

The actual operating voltage range should be considered rather than using only the nominal voltage.

Consider the Operating Frequency

Transformer selection depends heavily on operating frequency.

Traditional power-frequency transformers operate at 50 Hz or 60 Hz, while transformers used in switching power supplies operate at much higher frequencies.

High-frequency transformers can be much smaller than traditional transformers for the same power level, but the design must account for core losses and high-frequency winding effects.

Choose the Right Core Material

Core material affects transformer efficiency, size and thermal performance.

For high-frequency EV charger applications, ferrite and other suitable magnetic materials are commonly considered.

The core should be selected according to operating frequency, power level and magnetic flux density.

Using a core that is not suitable for the switching frequency can result in excessive core losses and heat.

Check Isolation Requirements

Electrical isolation is an important consideration in many EV charging systems.

The transformer may provide galvanic isolation between different parts of the power conversion system.

When isolation is required, the design should consider insulation resistance, dielectric strength, creepage and clearance.

The insulation system also needs to match the relevant safety requirements of the charger.

Consider Switching Topology

The transformer should be designed around the converter topology.

Depending on the charger architecture, the transformer may be used in an LLC, phase-shifted full-bridge, dual-active-bridge or another isolated converter.

Each topology produces different voltage and current waveforms.

This affects the required turns ratio, magnetizing inductance, leakage inductance and winding structure.

Check Current and Winding Design

High-power EV chargers can involve substantial current.

The transformer winding must carry the required current without excessive copper losses or temperature rise.

The manufacturer may use thicker copper wire, multiple parallel conductors or copper foil depending on the design.

At high frequencies, winding arrangement also affects AC resistance and parasitic effects.

Consider Leakage Inductance

Leakage inductance can have a significant effect on switching power supplies.

In some converter topologies, controlled leakage inductance is part of the circuit design.

In other applications, excessive leakage inductance can reduce efficiency and increase voltage spikes.

The required leakage inductance should therefore be considered during transformer design instead of treating it as an incidental parameter.

Check Magnetizing Inductance

Magnetizing inductance is another important transformer parameter in high-frequency converters.

Its value affects the current flowing through the primary winding and the operation of the switching circuit.

For resonant converters, the transformer parameters may be closely related to the resonant network.

The transformer should therefore be designed according to the complete converter rather than selected only by turns ratio.

Consider Efficiency

Efficiency is particularly important in EV charging equipment.

A charger may operate for many hours and process large amounts of electrical energy.

Transformer losses become heat and can increase the cooling requirements of the complete system.

Core loss and copper loss should both be evaluated when selecting the transformer.

Check Thermal Performance

The transformer must operate within its permitted temperature range.

Temperature rise depends on power, winding resistance, core losses, switching frequency and cooling conditions.

For high-power EV chargers, forced-air or liquid cooling may be used elsewhere in the system.

The transformer should be evaluated under the actual thermal environment in which it will operate.

Consider Power Density

Fast chargers need to deliver high power while keeping the equipment reasonably compact.

A smaller high-frequency transformer can help increase power density.

However, reducing transformer size too aggressively may increase temperature rise or reduce efficiency.

The design should balance size, power, thermal performance and reliability.

Consider EMI Performance

EV chargers contain high-power switching circuits that can generate electromagnetic interference.

Transformer construction can affect both conducted and radiated noise.

Winding arrangement, shielding, insulation structure and leakage characteristics should be considered as part of the overall EMI design.

Additional filtering components may also be required.

Consider Operating Environment

An EV charger may be installed indoors, outdoors or in industrial environments.

The transformer should be suitable for the expected temperature, humidity, vibration and other environmental conditions.

Outdoor fast chargers may require more attention to thermal management and protection from environmental factors.

Check Protection and Reliability

The transformer itself does not normally provide electronic protection, but its design must work with the converter's protection system.

The complete power stage may include overcurrent, overvoltage and overtemperature protection.

For commercial charging equipment, long service life and stable operation are important because failures can result in significant downtime.

Consider Custom Transformer Design

A standard transformer may not provide the required combination of power, voltage, frequency, size and thermal performance.

A custom EV charger transformer can be designed around the actual converter.

The manufacturer can optimize:

Core material

Turns ratio

Winding structure

Conductor size

Leakage inductance

Magnetizing inductance

Insulation

Dimensions

This is particularly useful for high-power DC fast chargers and specialized charging equipment.

Test the Transformer

Before moving to mass production, test transformer samples under realistic operating conditions.

Important tests may include:

Turns ratio

Inductance

DCR

Leakage inductance

Insulation resistance

Withstand voltage

Temperature rise

Efficiency

The transformer should also be tested in the actual charger circuit because its performance depends on the complete switching system.

How to Choose an EV Charger Transformer Supplier?

Look for a supplier with experience in high-frequency transformers and power electronics.

Check the manufacturer's core materials, winding technology, testing equipment and production capacity.

For custom transformers, engineering support is particularly important because the transformer needs to match the charger's topology and operating conditions.

A supplier with experience in EV charging, energy storage or other high-power applications may be better prepared to handle these requirements.

Choosing the Right Transformer for an EV Charger

The right EV charger transformer should match the charger's power, voltage, frequency, topology and isolation requirements.

Core material, winding design, leakage inductance, magnetizing inductance, efficiency and thermal performance should also be evaluated.

For high-power EV chargers, transformer design has a direct effect on power density, efficiency and reliability. Selecting the transformer together with the overall converter design can help create a more stable and efficient charging system.