A planar transformer, comprising a winding structure and a magnetic core structure , wherein the winding structure comprises a primary-side winding and a secondary-side winding, the magnetic core structure comprises a first magnet part , a second magnet part , and a plurality of magnetic cylinders , the plurality of magnetic cylinders are located between the first magnet part and the second magnet part , the primary-side winding is wound around M magnetic cylinders in the plurality of magnetic cylinders, M is a positive integer, M≥3, and a cross-sectional area of at least one of the M magnetic cylinders is different from a cross-sectional area of another magnetic cylinder; the secondary-side winding is wound around one of the plurality of magnetic cylinders; the planar transformer further comprises at least one secondary-side parallel winding, each secondary-side parallel winding is wound around one of the plurality of magnetic cylinders, the secondary-side winding and the at least one secondary-side parallel winding are connected in parallel; a total quantity of secondary-side windings and at least one secondary-side parallel winding is P, P is a positive integer, and P≥2; a ratio of cross-sectional areas of P magnetic cylinders corresponding to the P secondary-side windings and parallel windings is A1: A2:...: AP; and quantities of turns of the P secondary-side windings and secondary-side parallel windings around the P magnetic cylinders are respectively Ns1, Ns2, ..., and NsP, characterised in that at least one of cross-sectional areas A1, A2,..., AP of the magnetic cylinder having a corresponding secondary winding is different from another of cross-sectional areas A1, A2,..., AP and at least one of quantities of turns of the secondary winding Ns1, Ns2, ..., and NsP is different from another of quantities of turns Ns1, Ns2, ..., and NsP, and in that values of A1*Ns1, A2*Ns2, ..., and AP*NsP meet at least one of the following conditions: the values are equal, or a ratio between any two values is from 80% to 120%. the primary-side winding is wound around M magnetic cylinders in the plurality of magnetic cylinders, M is a positive integer, M≥3, and a cross-sectional area of at least one of the M magnetic cylinders is different from a cross-sectional area of another magnetic cylinder; the secondary-side winding is wound around one of the plurality of magnetic cylinders; the planar transformer further comprises at least one secondary-side parallel winding, each secondary-side parallel winding is wound around one of the plurality of magnetic cylinders, the secondary-side winding and the at least one secondary-side parallel winding are connected in parallel; a total quantity of secondary-side windings and at least one secondary-side parallel winding is P, P is a positive integer, and P≥2; a ratio of cross-sectional areas of P magnetic cylinders corresponding to the P secondary-side windings and parallel windings is A1: A2:...: AP; and quantities of turns of the P secondary-side windings and secondary-side parallel windings around the P magnetic cylinders are respectively Ns1, Ns2, ..., and NsP, characterised in that at least one of cross-sectional areas A1, A2,..., AP of the magnetic cylinder having a corresponding secondary winding is different from another of cross-sectional areas A1, A2,..., AP and at least one of quantities of turns of the secondary winding Ns1, Ns2, ..., and NsP is different from another of quantities of turns Ns1, Ns2, ..., and NsP, and in that values of A1*Ns1, A2*Ns2, ..., and AP*NsP meet at least one of the following conditions: the values are equal, or a ratio between any two values is from 80% to 120%. the values are equal, or a ratio between any two values is from 80% to 120%. <EOS>