ON-OFF Controlled Very-High-Frequency Class E DC-DC Converter

The development of power electronics has been continuously motivating the efforts to achieve lightweight and miniaturization of switching-mode power supply.Increasing the switching frequency is an effective way to reduce the size of passive components in switching-mode power supply and thus improve the power density.Recently,very-high-frequency power converters,with the switching frequency up to dozens of megahertz,have attracted much more attentions in low-power conversion applications.To reduce the switching loss in such a high switching frequency,the Class E dc-dc converter,which can achieve zero-voltage-switching(ZVS),has been extensively investigated.The ON-OFF control,featured with simple implementation and fixed switching frequency,is beneficial to the optimization of the converter parameters.This dissertation is dedicated to the optimized parameter design and efficiency improvement of the ON-OFF controlled very-high-frequency Class E dc-dc converter.In this dissertation,the operating principle of the ON-OFF controlled Class E dc-dc converter is analyzed.The mode transition between ON mode and OFF mode is discussed,and it is found that there exists transition loss during the mode transition.It is preferred to reduce the modulation frequency for saving the transition loss and improving the conversion efficiency.When designing the ON-OFF controlled Class E dc-dc converter,two key requirements should be satisfied.One is the input power during ON mode should be higher than the rated output power,and the other is to achieve ZVS for the power switch.The input power during ON mode and the ZVS performance with respect to the input voltage are investigated,and it is pointed out that the Class E dc-dc converter should be designed at the minimum input voltage for satisfying the two requirements.The voltage/current stresses of the power devices and passive components are also analyzed,and it is revealed that both the voltage and current stresses increase with the rise of the input voltage.Furthermore,the ON-OFF control with output current limiting is proposed by incorporating a output current closed-loop,and thus the output current limiting is realized under over-load and even shor-circuit conditions.The simulation results are provided to verify the validity of the theoretical analysis and the effectiveness of the ON-OFF control scheme with output current limiting.A parameter design approach is proposed for the ON-OFF controlled Class E dc-dc converter in this dissertation.Firstly,the ZVS condition are derived at the minimum input voltage by checking if the switch voltage could resonate back to zero.Then,the time instant when the switch voltage resonates back to zero is optimized for minimizing the switch voltage stress,switch root-mean-square(RMS)current,and switch voltage harmonic components.After that,a step-by-step parameter design approach is proposed for the case when the input inductor is quite large and the input current is nearly a dc current.For the case of small input inductor,a parallel capacitor compensation approach is proposed.In this approach,an extra compensation capacitor is added to be paralleled with the power switch according to the reduced inductance of the input inductor.In this way,the converter performance with reduced input inductor is almost the same as that with large input inductor.The analysis shows that,with the increase of the input voltage,the switch voltage will resonate back to zero earlier,and the power switch will conduct reversely accordingly.In order to avoid the reverse conduction of the power switch,an adaptive duty-cycle adjustment scheme is proposed,in which the duty cycle is increased with the rise of the input voltage,leading to an improved conversion efficiency.A prototype of20-MHz,9-18-V input,5-V/10-W Class E dc–dc converter is fabricated and tested in the lab,and the experimental results are presented to verify the effectiveness of the proposed optimized parameter design approach and the adaptive duty-cycle adjustment scheme.For the ON-OFF controlled Class E dc-dc converter,if the switching frequency is constant,the input power during ON mode increases with the rise of the input voltage,leading to a higher modulation frequency and higher RMS current of the converter,and thus degrading the efficiency.It is found that the input power decreases with the increase of the switching frequency.With this discovery,a variable switching frequency(VSF)ON-OFF control is proposed,which slightly increases the switching frequency when the input voltage increases,and the input power during ON mode is reduced,and thus highly improving the conversion efficiency.The guideline of switching frequency selection and the implementation of the VSF ON-OFF control scheme are presented.A prototype of 9–18-V input,5-V/10-W Class E dc–dc converter has been fabricated and tested in the lab.The experimental results show that the proposed VSF ON-OFF control could greatly improve the conversion efficiency.No matter the switching frequency is constant or variable,the load power of the Class E dc-dc converter is synthesized by the input power in ON mode and zero power in OFF mode.Since the input power in ON mode is far higher than the load power at light load,and the zero power in OFF mode is far lower than the load power at heavy load,it is difficult to achieve high efficiency over the entire load range.Based on the principle that the input power in ON mode decreases with the increase of the switching frequency,a multi-power-level(MPL)hysteresis control is proposed.In the MPL hysteresis control,multiple switching frequencies are selected for obtaining multiple input power levels,and the two adjacent input powers closest to the load power are chosen to synthesize the load power.The parameters design and the switching frequencies selection for the Class E dc–dc converter with the MPL hysteresis control are presented.The performance comparison of the MPL hysteresis control with the VSF ON-OFF control and the constant switching frequency(CSF)ON-OFF control are given,and it is shown that the MPL hysteresis control could reduce the modulation frequency and the RMS current in the converter,and thus the conversion efficiency is improved.A prototype of 5-V/10-W Class E dc–dc converter with 9–18-V input is fabricated and tested in the lab,and the experimental results are provided to verify the effectiveness of the proposed MPL hysteresis control scheme.