Research On The Analysis Of LLC Resonant Converter Control Characteristics And Its Improvement

Recently, there has been a growing interest in the research of high-power battery withthe fast development of Electrical Vehicle(EV). LLC resonant converter has high efficiencyand high power density, which performs well as the rear-end DC-DC converter in two-stageAC-DC-DC charge system. Excellent performance of charger will be beneficial for thebattery’s state of health(SOH), playing an significant role in improving the chargingefficiency and cycle life. This paper investigated LLC resonant converter and do someresearch to improve its output quality.Firstly, this paper investigated the small-signal model of LLC resonant converter basedon equivalent circuit approach. The small-signal disturbance of frequency was analyzed,deriving steady-state equivalent circuit and small-signal equivalent circuit of different partsin the circuit. Small-signal dynamic characteristic of LLC resonant converter driftsubstantially as the operating conditions altered. This paper calculated small-signal modelsin two classic different working points and designed control parameters based on differentdesign principle.Two stage AC-DC-DC converters are widly used in battery charge systems. While inthe two stage AC-DC-DC converter system with a front-end single phase rectifier, thepulsating power which is the twice of the grid fundamental frequency will affect the DCbus voltage, which will eventually deteriorate the output voltage quality of the rear-endDC-DC converter. This paper proposed a quantitative method to analyze low frequencyvoltage ripple propagation in LLC DC-DC resonant converter based on signal modulationand demodulation theory. With this analytical method, the specific mathematicalrelationship between the amplitude of input and output low frequency voltage ripple isobtained. PIR control strategy is applied to reduce the output low frequency voltage ripple,which could decrease the volume and cost of the system and improve the reliability.Finally, this paper investigated four different digital control implementation of LLCresonant converter. Combined with the gain-frequency characteristic of LLC resonantconverter, the accuracy of DSP and L6599were calculated in details. Also, the four digitalcontrol implementation were compared based on their performance, complexity, cost andexpansibility, which could serve as useful reference in different applications.