XBOX 360 power supply problem

I have bought an used xbox 360 S from japan. It's power supply input is required 100 v-127 v and 50/60 Hz. But in my country 220 V 50 Hz. Then I bought step down transformer 220 v 50 Hz to 100 V, 20 W . Now, when I have connected my x box power supply unit with this step down transformer 220 v 50 Hz to 100 V, 20 W, x box power supply orange lights (stand by) glows. When I push power of x box, it turns to green ( good power) lights and x box system switched on. But within 3 sec, x box console shuts down and power supply box turn to red light. I don't understand why my xbox console is not operating with this converted power supply.