An LLC high frequency transformer must be evaluated at light load as carefully as at rated power. When output demand falls, the controller may raise switching frequency, enter burst mode, skip pulses, or change its regulation method. These modes alter magnetizing current, core loss, zero-voltage-switching margin, acoustic behavior, and EMI.
The transformer is part of the resonant network rather than a separate ideal ratio element. Magnetizing inductance, leakage inductance, turns ratio, parasitic capacitance, and winding resistance all influence converter gain and operating range.
Magnetizing inductance sets important tradeoffs
Lower magnetizing inductance increases circulating current. That current can help maintain zero-voltage switching, but raises conduction loss and transformer copper loss. Higher magnetizing inductance reduces circulating current but may narrow soft-switching margin at light load. Its tolerance and temperature behavior therefore matter to both efficiency and control stability.
The drawing should state magnetizing inductance under a defined frequency, voltage, and connection condition. If leakage is intentionally integrated into the transformer, the measurement method must clearly distinguish resonant leakage from external inductance and fixture effects.
High switching frequency does not always mean low loss
At light load, frequency may rise above the nominal design point. Flux swing per cycle can fall, but core-loss frequency increases and gate-drive or switching losses may become more important. Winding AC resistance also rises with frequency. The net thermal result must be measured rather than assumed.
Burst mode creates packet-level behavior
Burst operation groups switching pulses with idle intervals. Average power is low, but the active packets can still carry substantial peak magnetizing and resonant current. Packet repetition may fall into the audible range, exciting windings, core halves, adhesives, or the PCB. Burst edges can also create low-frequency components that appear in conducted or radiated EMI tests.
Parasitics shift the practical operating range
Leakage inductance, winding capacitance, rectifier capacitance, and layout parasitics can form additional resonances. As switching frequency moves, these may affect gain, ringing, and voltage stress. Prototype waveforms should include primary current, resonant capacitor voltage, switch-node voltage, secondary rectifier voltage, and output ripple across the full load range.
Verification points
- Magnetizing and leakage inductance with documented connections
- Frequency range from no load through maximum load
- ZVS margin at high input voltage and light load
- Core and winding temperature in continuous and burst modes
- Audible noise at burst repetition frequencies
- Drain, resonant, and rectifier voltage stress
- Conducted and radiated EMI across mode transitions
When requesting a high frequency transformer for an LLC converter, provide resonant tank targets, gain requirements, controller frequency limits, burst strategy, and measured prototype waveforms. BaoHui Tech can then align the winding construction and magnetic tolerances with the actual control window.
Frequently asked questions
Is no-load operation thermally harmless?
No. Average output power is low, but circulating current, high switching frequency, burst packets, and core excitation can still create loss.
Can leakage inductance be used as the resonant inductance?
Yes in some designs, but its tolerance, measurement method, production repeatability, and sensitivity to winding placement must be controlled.
Why can an LLC supply become audible only at light load?
Burst repetition, pulse skipping, and changing magnetic force can enter the audible range and excite mechanical structures.