Patent ID: 8044754

Claim:
A transformer comprising: a bobbin; a magnetic core assembly partially sleeved by said bobbin; a first primary winding coiled around said bobbin and including a first winding portion and a second winding portion, wherein said first winding portion has larger electromagnetic interference (EMI) comparing to said second winding portion; a secondary winding coiled on said first primary winding; a second primary winding coiled on said secondary winding and including a third winding portion and a fourth winding portion; a first shielded element disposed between said first primary winding and said secondary winding for disconnecting the EMI transmission from said first primary winding to said secondary winding, wherein said first winding portion of said first primary winding is adjacently disposed to said magnetic core assembly for shielding the EMI of said first primary winding by using said magnetic core assembly, said second winding portion is coiled on said first winding portion and adjacently disposed to said secondary winding for increasing the electromagnetic coupling rate of said first primary winding and said secondary winding; and a second shielded element disposed between said second primary winding and said secondary winding, for preventing EMI of said second primary winding from transmitting to said secondary winding; and a first auxiliary winding coiled on said second primary winding and disposed on the most outer layer of said transformer; wherein the EMI of said third winding portion is smaller than that of said fourth winding portion, said third winding portion coiled on said second shielded element is adjacently disposed to said secondary winding, and said fourth winding portion is coiled on said third winding portion, for increasing the electromagnetic coupling rate between said second primary winding and said secondary winding.