A Practical Automatic Resonant Frequency Tracking Method Based on Instant Transformer Voltage in Unregulated LLC Converters

An unregulated LLC circuit is often used in two-stage converters for electrical isolation and fixed-ratio voltage transformation. When the resonance parameters deviate from the design, the preset switching frequency deviates from the actual resonant frequency, decreasing the efficiency. By analyzing the time-based waveforms of different LLC circuit operation modes, we found that above a certain power load, the instant transformer voltage at the edge of the primary bridge voltage is unity above the resonant frequency and is distinctly smaller than unity below the resonant frequency. Accordingly, a new automatic resonant frequency tracking method based on the instant transformer voltage is proposed. When the instant transformer secondary voltage sampled at a specific time is smaller than the output voltage, the switching frequency increases by a minor step. Otherwise, the switching frequency decreases by a minor step. The switching frequency gradually converges to the resonant frequency. Meanwhile, the applicable load power and inductor ratio ranges of the method are discussed in detail. The method is digitally implemented using the DSP with practical issues, including parasitic capacitors and parameter deviations of the control circuit considered. The proposed method is verified on a 1.5 kW 48 V prototype.