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　　Explanation of front-stage open and closed loop circuits in high-frequency inverter power supplies

　　The high-frequency inverter power supply is different from the ordinary inverter power supply. It can convert low-voltage DC power into high-frequency low-voltage DC power, and then generate high-voltage DC power through boost processing. Among high-frequency inverter power supplies, the front-end circuit is a more important and problematic part.

　　This article will introduce the turns ratio design of the closed-loop front-stage and quasi-open-loop front-stage transformers in the front-end circuit, and give a more detailed introduction.

　　Design of turns ratio of closed-loop front-stage transformer

　　Whether the front stage of the inverter is open loop or closed loop, only the turns ratio of the transformer and the parameters of the feedback loop are different. For example, you need to design a high-frequency modified square wave inverter with an input of 12V, a variation range of 10.5-15V, and an output voltage of AC 220V50Hz. If the front stage adopts a closed-loop structure, it is better to stabilize the 12V boosted DC voltage at 270V. In order to output 270V when the input is 10.5V, the transformation ratio of the transformer is approximately (270+2VD)/(10.5-VDS )/D, where VD is the voltage drop of the high-voltage rectifier tube, VDS is the voltage drop of the front-stage MOS tube, and D is the maximum duty cycle. The calculated result is approximately 28.

　　Special attention should be paid to the fact that when the current stage works in a closed-loop state, for example, if the input voltage is relatively high, the peak value of the pulse obtained by rectifying the positive terminals of D1 and D3 will exceed 270V, and the duty cycle will be less than 1. L1 and C11 smoothing filtering is required, so L1 cannot be omitted, and it must be large enough, otherwise the MOS tube heat loss will be great. The specific calculation can be based on the calculation of the output filter inductance of the forward switching power supply, and will not be repeated here.

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