A CLLC converter is a bidirectional resonant topology used in battery storage, EV charging, DC microgrids, and isolated DC-DC conversion. Its high frequency transformer provides galvanic isolation and voltage conversion, but its magnetizing inductance, leakage inductance, winding capacitance, and loss also shape the resonant network. Successful CLLC resonant transformer design therefore requires a converter-level model rather than a turns-ratio calculation followed by a generic magnetic design.
Start with bidirectional gain requirements
Define both bus-voltage ranges, power flow in each direction, nominal conversion ratio, switching-frequency range, startup conditions, and allowable circulating current. A turns ratio that is ideal in forward power flow may force an undesirable gain range in reverse. Control strategy, synchronous rectification, phase relationship, and frequency modulation all influence the required operating region.
The ratio should include semiconductor voltage drops, dead time, device capacitance, and resonant gain, not only nominal DC voltages.
Magnetizing inductance is part of the tank
Magnetizing inductance affects gain shape, circulating current, soft-switching range, light-load efficiency, and short-circuit response. A lower value provides more current for zero-voltage switching but raises conduction loss. A higher value reduces magnetizing current but can narrow soft-switching margin.
Specify magnetizing inductance with test frequency, voltage amplitude, temperature, core assembly, and tolerance. Ferrite permeability, gap, mating surfaces, clamp force, and production variation all contribute. A small-signal LCR value may not represent large-signal operation.
Leakage can be intentional or parasitic
Some CLLC designs use transformer leakage as part of one or both resonant inductances. Others use separate inductors for tighter control. Integrated leakage can reduce component count, but winding position, tape, margins, lead routing, and bobbin tolerance make it harder to hold a narrow value.
Interleaving reduces leakage but usually raises primary-secondary capacitance. Separating windings increases leakage and can reduce capacitance, but may increase AC copper loss or window usage. BaoHui Tech treats primary-referred and secondary-referred leakage targets, test terminal conditions, and tolerance as explicit requirements.
Winding capacitance and common-mode current
Fast bridge transitions drive displacement current through interwinding capacitance. This current can flow through chassis, battery, heatsinks, cables, and control references. Bidirectional systems may have different common-mode behavior in charge and discharge directions.
Winding sequence, conductor overlap, insulation thickness, electrostatic shields, and core grounding affect capacitance. A shield can redirect current but adds capacitance to its reference and must remain open around the magnetic path.
Core and winding loss
Switching frequency can vary widely with voltage and load. Core loss depends on flux waveform, frequency, material, temperature, and minor loops. Copper loss depends on RMS current and harmonics, which may be dominated by circulating resonant current rather than output power.
Litz wire, foil, PCB windings, or parallel strands should be selected from the complete current spectrum and field distribution. Termination resistance and current sharing matter at high current.
Validation checklist
- Measure gain and tank current in both power-flow directions.
- Verify magnetizing and leakage inductance with defined fixtures and temperature.
- Capture winding voltage and integrate it to check flux balance.
- Measure loss and hot spots across frequency and load.
- Check common-mode current, EMI, and soft-switching margin.
- Test startup, direction change, burst, overload, short circuit, and recovery.
BaoHui Tech needs both bus ranges, power, resonant values, switching range, bridge waveforms, current spectrum, target inductances, capacitance limit, insulation, cooling, and package.
Frequently asked questions
Can all resonant inductance be built into the transformer?
Sometimes, but tolerance, capacitance, winding loss, and manufacturing repeatability may favor separate inductors.
Is loss identical in both directions?
No. Current, frequency, gain, commutation, and winding utilization can differ.