Research On Transformer Magnetic Bias Of Asymmetrical Half Bridge

Soft switching technology has been widely developed and applied in recent years,an increasing number of high efficiency topologies are proposed. Asymmetrical halfbridge (AHB) convertor has many advantages. For examples, simple architecture, easyto control, symmetric magnetization performance in transformer core, high utilizationrate. Asymmetrical half bridge convertor is accepted by more and more designengineer for switching power. At the same time, asymmetrical half bridge converterhas a zere voltage switching(ZVS) capability, which can take advantage of resonanceoscillation in the dead zone time between the leakage inductance of transformer andthe parasitic capacitance of power MOSFET. This characteristic reduces loss duringthe switching process greatly, and improves the efficiency of the converter.The asymmetric duty cycle of asymmetrical half bridge converter, which leads theDC magnetic biasing. As the accumulation of dc magnetic flux, the magnetic inductorintensities in transformers will probably excess saturated value. Magnetic saturation issolved usually by cut-off air cap on core, meanwhile it will reduce magnetizinginductance, so the magnetizing current and the loss are increasing, which reduces theefficiency of the converter. Base on the detailed analysis on Zero Voltage Switchoperation as well as the reasons of the DC magnetic biasing about the topology ofasymmetrical half bridge, the operational precondition of Zero Voltage Switching isgiven in this thesis, and then analysed zero voltage switching implementations of loadcurrent system and magnetizing current system, Compared four ways in solving theDC magnetic biasing. The simulation confirms that changing the turns of secondarywindings, changing the equivalent series resistances of the inductor and changing thevalues of inductor can remove the DC magnetic biasing, but only in a certain dutycycle. however, a current-double-rectifier asymmetrical half bridge with seriescapacitor in secondary wingding can remove the DC magnetic biasing in any dutycycle.This thesis adopts the peak current mode integrated control chip L6591 designed specifically for the topology of asymmetrical half bridge. And then, a experimentalprototype was designed and built depending on the current-double-rectifierasymmetrical half bridge with series capacitor in secondary wingding. Theexperimental results show that this method obtains satisfactory results in inhibiting thedc magnetic biasing. In the cooperation of the load current system and magnetizingcurrent system, all the power mosfets can achieve zero voltage switching at the wholeload range.