| With the development of society,power electronic technology changes and the types of electronic equipment become more and more.It is self-evident that the power supply for any electronic equipment becomes more important and the requirements of performance for power supply become higher in the market.According to the requirements of performance of the charger and adapter of portable electronic products,this paper designs a kind of high precision constant voltage and constant current(CV/CC)flyback AC-DC converter.In this paper,a high precision CV/CC flyback converter is presented,which uses the structure of the primary side regulation(PSR).Compared with the conventional secondary side feedback converter,it can save the optical coupling device and so on which simplify the circuit structure and reduce the cost.In this paper,a basic CV/CC flyback converter small signal model is established,by a derivation of each part of transfer function,the phase margin and the bandwidth which could reflect the system stability and rapidity are obtained.Then through the changes of the external capacitor which have influence on the stability of the system,an appropriate value can be received,so the influence parameter of the stability of the system can be obtained from the model's derivation.When the control chip works in the output mode of constant current,the peak of primary current keep unchange,and the magnetic time can be detected through the auxiliary winding,then the switch cycle can be control about two times of the magnetic to realize the constant current output,which the effects of constant current precision without the primary inductance of transformer also can be ensured.The bus voltage can be reflected by the sampling of the auxiliary winding,then a compensation current which changes with the bus voltage is injected to the detection of primary resistance,so the peak value of the primary current can be regulated to improve the accuracy of output current.In the constant voltage output mode,the output voltage can be sampled by the voltage of the auxiliary winding,and an error voltage can be generated by the reference voltage through the error amplifier,then the switching frequency of the control signal is genetated by the error voltage and the exponential wave which the constant voltage output can be relized.The cable compensation module works by detecting the switching frequency to generate a compensation current which changes with the switching frequency to improve the precision of output.According to the analysis of the theory,the circuit design and the layout of the system are given.A control IC for the proposed CV/CC converter has been fabricated in MXIC 0.8um L80A1 process and the performance are verified by a 5V/1.2A prototype.The experimental results show that the standby power is lower than 110mW in a wide range of universal-input ac voltage from 85V to 264V and the lowest can reveive 40mW.In the constant current mode,the precision of output current is ±1.25%,the line regulation is within±1.17%and load regulation within ±0.67%,the minimum and the maximum of the conversion efficiency are 71.9%and 80.1%,respectively.In the constant voltage mode,the precision of output voltage was within ±1.5%,the line regulation was±0.58%and load regulation within ± 1.1%,the minimum and the maximum of the conversion efficiency are 71.9%and 82.41%.respectively.Therefore,the proposed control chip has a promising application in low power CV/CC flyback AC-DC converter. |
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