An improved structure of transformer includes a lead frame main body (1), a lead frame cover (2), a trouser-like flap (2), and two iron cores (4) where the lead frame main body has a penetration hole (11) with a bobbin (12) formed at the circumference there around. The trouser-like flap being made of metal has a hollow part (31) for slipping on a bobbin (121) of the lead frame main body also includes horizontally stretched thigh parts. The outer circumference of the trouser-like flap is furnished with a primary winding. The lead frame cover being a sheet-shaped body has a hollow hole (21) furnished at the center thereof. On the inner-side surface, each of the two iron core has a main core part (41) capable of stretching through the penetration hole of the lead frame main body where the main core part has a containing circumferential trench (43) capable of containing the lead frame main body. What is more, the containing circumferential trench also has a pair of oppositely disposed side openings (44, 45) for b
