By inserting the secondary windings of the coupled inductors in series with the primary of the built-in transformer at the middle branch, a new concept is introduced for achieving ultra large voltage gain. The secondary and tertiary windings of the built-in transformer are inserted in the voltage multiplier cell. This configuration not only extends the voltage gain and minimises the voltage stress across power MOSFETs, but also gives a more flexibility to increase the voltage gain by the turns ratios of the built-in transformer and coupled inductors. Hence, high duty cycle is avoided and low voltage switches can be adopted which reduces the conduction losses. Also, the regenerative diode in each of the phases is connected to the other phase to improve equal current sharing performance. The energy of the leakage inductances is recycled by passive clamps and the high voltage spikes are avoided. Meanwhile, due to the leakage inductances of the magnetic means, the zero current switching of semiconductor devices is provided and the reverse recovery problem is alleviated. Finally, a 600 W prototype with 28-400 V voltage conversion and 97% conversion efficiency is built to demonstrate the performance of the converter.