Transformer and circuit board comprising same

A transformer comprising:

• a core unit comprising a lower core and an upper core;
• a bobbin unit at least partially disposed inside the core unit;
• and
• a coil unit at least partially disposed inside the bobbin unit, the coil unit comprising a primary coil and a secondary coil disposed so as to be spaced apart from each other in a second direction intersecting a first direction oriented from the upper core to the lower core,
• wherein the lower core comprises:
• a closed area overlapping the upper core in the first direction;
• an open area extending from the closed area in a third direction intersecting the first direction and the second direction so as to be spaced apart from the upper core;
• a pair of first lower outer legs and a first lower center leg disposed in the closed area, the first lower center leg being disposed between the pair of first lower outer legs;
• and
• a pair of second lower outer legs and a second lower center leg disposed in the open area, the second lower center leg being disposed between the pair of second lower outer legs,
• wherein the first lower center leg and the second lower center leg are disposed so as to be spaced apart from each other,
• wherein one of the primary coil and the secondary coil is at least partially disposed between the first lower center leg and the second lower center leg, and
• wherein the primary coil, the secondary coil, and the first lower center leg at least partially overlap each other in the second direction.