Third Level Double Forward Converter

Double Forward Converter(DFC) has many advantages, such as inherent anti-break-though capability, auto clamping the switch voltage etc. Regarding the excellent reliability, DFC is one the most widely used topology in the industrial applications, especially for low output voltage area, such as telecommunication energy systems and welding power supply. However, there are some shortcomings in DFC. One is that the operation duty cycle can not be larger than 50% because of the requirement for the transformer resetting. As a result, the voltage stress of the secondary diode is high, which limits the usage of DFC in high output voltage application. Another is that DFC is also not suitable for high input voltage application, such as following the output of three phase power factor correction circuit, for the voltage stress of switch in DFC is equal to the input voltage.In this paper, a Three-Level Double Forward Converter(TDFC) is proposed to meet the requirement for high input voltage and high output voltage application. TDFC is mainly combined by two DFCs whose input ports are connected in series and the output ports of which are connected in parallel communing the same output filter. So the voltage stress of switch can be halved by utilizing this circuit structure. In addition, a way of by using the series of circuits instead of series of devices is adapted to halve the voltage stress of the secondary diodes. The operation principle of TDFC is analyzed and the imbalance problem of input capacitor voltage is studied in this paper. Also the small signal model of TDFC is established, furthermore, which is verified by the frequency characteristic of TDFC measured by HP4195A Network/Spectrum analyzer. Final a control method is designed for the TDFC. And the method to measure the frequency characteristic is discussed too. A 1.5KW experiment model is made to verify the analyze above.