Balun transformer, mounting structure of balun transformer, and electronic apparatus having built-in mounting structure

1. A balun transformer, comprising:

first to fourth layer coils stacked in that order in respective layers and coupled together magnetically,

wherein,

each of the layer coils has a first end and second end, the first end being the innermost end of the coil,

the first end of each coil of the first to fourth layer coils is grounded,

the second and third layer coils are connected in parallel,

an unbalanced signal can be input or output or both to or from, respectively a common terminal of the second and third layer coils,

a first balanced signal can be input or output or both to or from, respectively, the second end of the first layer coil, and

a second balanced signal can be input or output or both to or from, respectively, the second end of the fourth layer coil.

2. The balun transformer according to claim 1 , wherein the first to third layer coils are all wound in a first direction, and the fourth layer coil is wound in second direction opposite that of the first direction.

3. The balun transformer according to claim 1 , wherein:

the each coil is stacked via a dielectric layer or a magnetic layer, and

the one end of the each coil is electrically connected via a via hole provided to the dielectric layer or the magnetic layer.

4. The balun transformer according to claim 1 , further comprising a first magnetic layer facing or in contact with an outermost coil of the each coil.

5. The balun transformer according to claim 4 , further comprising a dielectric layer or a magnetic layer interposed between the first magnetic layer and the outermost coil.

6. The balun transformer according to claim 1 , further comprising a second magnetic layer facing or in contact with an innermost coil of the each coil.

7. The balun transformer according to claim 6 , wherein at least part of the second magnetic layer is formed separately, and a wiring of the innermost coil passes through the separated part of the second magnetic layer.

8. The balun transformer according to claim 6 , wherein a dielectric layer or a magnetic layer is interposed between the second magnetic layer and the innermost coil.

9. The balun transformer according to claim 1 , further comprising first to fourth conductive layers formed surrounding outer peripheries of the first to fourth layer coils, respectively, and the first to fourth conductive layers are grounded.

10. The balun transformer according to claim 1 , wherein:

first and second coil sets each includes the first to fourth layer coils,

the first coil set stacks the first to fourth layer coils in this order in a first direction,

the second coil set stacks the first to fourth layer coils in this order in a second direction opposite to the first direction, and

the first layer coil of the first coil set is stacked facing the first layer coil of the second coil set.

11. A balun transformer mounting structure, wherein:

the balun transformer according to any one of claims 1 to 10 is mounted on a mounting substrate, and

the one end of the each coil of the balun transformer is connected to a ground wiring formed on the mounting substrate.

12. The balun transformer mounting structure according to claim 11 , wherein insulating resin is embedded between the each coil of the balun transformer and the ground wiring.

13. An electronic apparatus having the balun transformer mounting structure according to claim 11 .

14. An electronic apparatus having the balun transformer mounting structure according to claim 12 .