| In today’s society,the design concept of high power density,high efficiency output and small size of switching power supply has gradually become the mainstream of power supply design industry.LLC Resonant Converter(LLC)has the advantages of high switching frequency and soft switching,which can realize the zero voltage on(ZVS)of the primary side and zero current off(ZCS)of the secondary side,thus reducing the power loss and the volume of magnetic core and radiator.At the same time,the efficiency of the power supply at high frequency can be further improved by using the high working frequency,small parasitic parameters,no reverse recovery time and superior electrical performance of the gallium nitride power device.Power front stage is staggered parallel power factor correction(PFC)topology,the circuit will use two single-phase booster PFC parallel,the two half sinusoidal cycle work at 180°phase,reduce the current stress of the switch tube,thus reducing the power supply switching loss and magnetic core volume;At the same time,the interleaving and parallel connection can reduce the current ripple of the inductor,thus reducing the capacity of the filter capacitor.Moreover,the stable input voltage makes the LLC design simpler.The second stage adopts LLC topology,the primary side switch tube is a half-bridge structure,the resonant capacitor is in series with the primary side of the transformer,and the leakage inductance of the transformer itself is used as the resonant inductance to work,which not only omits the external resonant inductance of the power supply,but also reduces the volume of the power supply.At the same time,using the high frequency characteristics of LLC,the resonant frequency of the power supply is about 200k Hz,and the volume of the transformer will also decrease with the increase of the working frequency.Finally,a synchronous rectifier scheme is used to replace the diode in the traditional rectifier circuit with a field effect tube with low on-resistance to reduce the loss caused by the forward voltage drop of the diode.In terms of power device selection,Ga N power device with Cascode structure is adopted.Its reverse characteristic parameter Trr/Qrr is very small,and Qrr is only 52n C,which reduces the reverse recovery loss of PFC when hard switching.Secondly,the output capacitance of Coss is only 46p F,which is much smaller than that of silicon devices.In this way,the dead time of LLC can be designed to be smaller and the switching frequency of LLC can be improved.Although TO-220 package brings some parasitic parameters,but the price is lower,the drive circuit is simple,more suitable for large-scale use.Combined with the above scheme,a 700W power engineering prototype was finally made,with the maximum power of about 800W,the dimensions of interlocking parallel PFC and LLC power boards were both 11cm×9cm,the power supply was assembled by superposition,the total height of the power supply was 5cm,and the power density was 1.6W/cm~3.The power conversion efficiency of the prototype is92.68%,94.9%and 94.5%respectively at 230VAC input and the load is 20%,50%and 100%,and the power factor also reaches more than 95%,which meets the design requirements of efficient power supply. |
