Structures of planar transformer and balanced-to-unbalanced transformer

1. A planar transformer structure comprising:

a first planar coil comprising a first ring structure, a second ring structure and a connecting section, wherein the connecting section connects the first ring structure and the second ring structure;

a second symmetric planar coil having at least two turns, wherein a range of the second symmetric planar coil partially or entirely overlaps a range of the first ring structure, and an outmost turn of the first ring structure is arranged inside an outmost turn of the second symmetric planar coil; and

a third symmetric planar coil having at least two turns, wherein a range of the third symmetric planar coil partially or entirely overlaps a range of the second ring structure, and an outmost turn of the second ring structure is arranged inside an outmost turn of the third symmetric planar coil;

wherein, a transformer is constituted by the first planar coil and the second planar coil or by the first planar coil and the third planar coil;

wherein a current entering the first ring structure flows through all metal segments of the first ring structure before leaving the first ring structure;

wherein the current flowing in the first ring structure is opposite in direction to a current flowing in the second ring structure.

2. The planar transformer structure of claim 1 , wherein an outermost turn of the first planar coil is arranged outside the second planar coil or the third planar coil.

3. The planar transformer structure of claim 1 , wherein the first planar coil comprises a first node and a second node, which are directly connected to the connecting section.

4. The planar transformer structure of claim 1 , wherein the first planar coil comprises a first node and a second node, which locate in a middle part of the first planar coil with the first node directly connected to the first ring structure and the second node directly connected to the second ring structure.

5. The planar transformer structure of claim 1 , wherein the second planar coil comprises a first node and a second node, which form a first port, and the third planar coil comprises a third node and a fourth node, which form a second port, and the second planar coil and the third planar coil are respectively symmetric with respect to an axis of symmetry passing the first port and the second port.

6. The planar transformer structure of claim 1 , wherein the second planar coil and the third planar coil are at the same layer of the first planar coil.

7. The planar transformer structure of claim 1 , wherein at least one of the second planar coil and the third planar coil is at a different layer from the first planar coil.

8. The planar transformer structure of claim 1 further comprising:

a bridging metal segment, two ends of which connect first metal segments of one of the first ring structure, the second ring structure, the second planar coil and the third planar coil, and a middle part of which overlaps a second metal segment which is not connected with the first metal segments.

9. The planar transformer structure of claim 1 , wherein at least two metal segments of one of the first ring structure, the second ring structure, the second planar coil and the third planar coil are arranged side by side and no other metal segments are located between the two metal segments.

10. The planar transformer structure of claim 1 further comprising:

a substrate; and

a pattern-ground shielding layer implemented between the substrate and the first planar coil, the second planar coil and the third planar coil.