Ferrite Cores for LLC Resonant Converters

The LLC converter is popular in applications where efficiency has to stay high across a wide load range and where the switching devices should be protected from hard transitions. Instead of chopping the input directly, it drives a resonant tank and lets the tank shape the current waveform. That shift in operating principle changes what the magnetic components have to do.

The consequence for core selection is significant. In an LLC design the magnetics are not simply storing and releasing energy each cycle; they are part of a tuned network, which makes the stability and tolerance of inductance a first-order design input rather than a secondary consideration.

Where the magnetics sit in an LLC converter

An LLC stage has a resonant inductor and a resonant capacitor forming the tank, a transformer providing isolation and voltage transformation, and output rectification with filtering. In many designs the resonant inductance is not a separate component at all — it is integrated into the transformer as leakage inductance, which reduces part count and cost at the price of tighter manufacturing tolerance.

Either way, the transformer core and any separate resonant inductor core are the elements that set the tank behavior.

Why the tank needs a defined inductance

The converter regulates output by varying switching frequency relative to the tank resonance. If the actual inductance differs from the design value, the operating point shifts: gain changes, the switching frequency range moves, and the soft-switching behavior the topology was chosen for can degrade.

That is why LLC magnetics are usually specified with a tighter inductance tolerance than a comparable non-resonant converter, and why a material whose permeability drifts with temperature or bias is a poor fit for the tank.

Why gapped ferrite dominates here

Ferrite has high permeability, which would normally produce very high inductance for a modest number of turns. In a resonant tank that is not desirable, so the core is deliberately gapped to bring inductance down to the target value and to store the energy the tank requires. The gap is a design feature, not a compromise.

• High permeability material allows the target inductance with fewer turns
• A defined gap sets the inductance precisely and repeatably
• Low core loss suits the elevated switching frequency typical of LLC stages
• Manganese-zinc grades are the usual choice for power conversion frequencies
• Gap geometry can be adjusted to tune inductance without changing core size

Where powder cores differ

Powder cores already contain a distributed gap, so they naturally provide lower, bias-stable permeability without a machined gap. That makes them attractive for high-current energy-storage inductors. In a resonant tank, however, the requirement is precise and stable inductance rather than high stored energy, and the permeability of ferrite combined with a controlled gap generally gives the tighter and more repeatable result.

The two families are therefore solving different problems. Choosing between them follows from whether the component is storing energy under heavy bias or defining a resonant frequency.

Engineering requirements to define

• Resonant frequency and the intended switching frequency range
• Required tank inductance and its acceptable tolerance
• Whether resonant inductance is discrete or integrated as leakage inductance
• Primary and secondary current levels and turns ratio
• Core loss at the operating frequency and flux swing
• Temperature rise at worst-case ambient
• Isolation and creepage requirements for the transformer function

Design considerations

Gap placement affects more than inductance. A gap in the center leg keeps fringing flux away from the windings, which reduces eddy-current loss in the copper; a gap adjacent to the winding can heat the wire noticeably. Gap length also interacts with the achievable tolerance, because a short gap is proportionally harder to hold precisely than a longer one.

Winding construction matters for the same reason. Litz wire reduces skin and proximity effect at elevated frequency, and the arrangement of primary and secondary windings affects both leakage inductance and interwinding capacitance — two quantities that feed directly back into converter behavior.

Application examples

• On-board chargers and DC-DC converters in electric vehicles
• Server and telecom power supplies with wide load range requirements
• Industrial power supplies where efficiency at partial load matters
• Flat-panel display and lighting drivers with height constraints
• Battery chargers requiring wide output voltage range

Selection checklist

• Confirm whether the tank inductance is discrete or integrated into the transformer
• Specify inductance with the tolerance the topology actually needs
• Evaluate core loss at the real frequency and flux swing, not a nominal point
• Check how gap placement affects winding loss
• Verify the resonant frequency holds across the temperature range
• Validate the prototype at full load, including soft-switching behavior

Custom development for resonant converters

LLC magnetics are usually application-specific, because the tank is tuned to the converter rather than selected from a catalogue. Development covers material grade, core shape and gap, winding arrangement and the resulting leakage inductance, followed by prototype evaluation and qualification for the target market.

See Ferrite Cores for High-Frequency Transformers for material grade selection, MnZn vs NiZn Ferrite for the frequency split between grades, and the Ferrite Core Guide for available shapes.

Frequently Asked Questions

Why do LLC converters use gapped ferrite cores?

Ferrite has high permeability, so a modest number of turns would produce more inductance than the resonant tank needs. Introducing a defined gap brings inductance down to the target value precisely and repeatably, and also allows the core to store the energy the tank requires.

Are ferrite cores suitable for LLC converters?

Yes. Gapped ferrite cores are the conventional choice for the resonant inductor and the transformer in LLC designs, because they combine low core loss at elevated frequency with high permeability that allows the target inductance in fewer turns.

Should the resonant inductor be a separate component?

It depends on the design. Many LLC converters integrate the resonant inductance into the transformer as leakage inductance, which reduces part count but demands tighter manufacturing tolerance. Others use a discrete resonant inductor for more direct control of the tank value.

What magnetic core is used in high-frequency transformers?

Manganese-zinc ferrite grades are the usual choice for power conversion transformers operating in the tens to hundreds of kilohertz. Nickel-zinc grades become relevant at higher frequencies and lower power levels, where their higher resistivity reduces eddy-current loss.

What should be specified when sourcing LLC converter cores?

Resonant frequency and switching range, required tank inductance and acceptable tolerance, whether inductance is discrete or integrated, primary and secondary currents, turns ratio, core loss at the operating point, temperature rise at worst-case ambient, and isolation requirements.