Symmetric split transformer for EMI reduction
Disclosed herein is a symmetric transformer in the context of a DC-DC isolated converter. The symmetric transformer reduces or eliminates asymmetry in the distribution of parasitic capacitance across the isolation barrier going from one end to another end of a primary coil, and as a result, undesirable electromagnetic interference (EMI) due to common mode dipole emission across the isolation barrier may be reduced. In some embodiments, a primary winding is split into separate first and second coils, with a serial impedance connected in between the first and second coils. The transformer is symmetric in the sense that a capacitive coupling of the first coil to a secondary winding is the same as a capacitive coupling of the second coil to the secondary winding, such that common mode EMI may be reduced.
1 . An isolated DC-DC converter, comprising:
a driver circuit coupled to a DC power source, the driver circuit to drive an AC signal based on the DC power source;
a primary winding coupled to the driver circuit to receive the driven AC signal, the primary winding having a first coil in series with a second coil, the first coil coupled between first and second terminals, the second coil coupled between third and fourth terminals, wherein at least one coil portion of each of the first coil and the second coil is disposed in a first plane;
a secondary winding coupled between fifth and sixth terminals, wherein a capacitance of the first coil to the secondary winding equals a capacitance of the second coil to the secondary winding;
an impedance comprising a single capacitor, the impedance coupled between the second and third terminals and connected in series with the first and second coils in a current path from the first terminal to the fourth terminal; and
a rectifier circuit to convert signals received in the secondary winding to generate an output DC signal,
wherein the driver circuit is configured to drive the AC signal to cause a resonant operation with the impedance and an inductance of the primary winding forming an LLC resonant network.
2 . The isolated DC-DC converter of claim 1 , further comprising a substrate, wherein
the second coil comprises a plurality of coil portions disposed in the first plane, and
the secondary winding comprises a plurality of coil portions disposed in a second plane parallel to and separated from the first plane by the substrate.
3 . The isolated DC-DC converter of claim 2 , wherein the second coil further comprises at least one bridge connecting a pair of adjacent coil portions in the second coil, wherein
the at least one bridge is disposed in a third plane parallel to and offset from the first plane.
4 . The isolated DC-DC converter of claim 3 , wherein the impedance is coupled between a pair of conductive structures, each of the pair of conductive structures coupled to the third and fourth terminals, respectively, and wherein
the pair of conductive structures are disposed in the third plane.
5 . The isolated DC-DC converter of claim 4 , wherein the pair of conductive structures comprise a pair of pads.
6 . The isolated DC-DC converter of claim 5 , wherein the pair of conductive structures further comprise a pair of traces.
7 . The isolated DC-DC converter of claim 4 , further comprising at least one bond wire coupled to the pair of conductive structures.
8 . The isolated DC-DC converter of claim 2 , wherein at least one coil portion in the second coil is aligned with at least one coil portion in the secondary winding along a vertical direction perpendicular to the first plane.
9 . The isolated DC-DC converter of claim 3 , wherein the fifth and sixth terminals are each connected to a pair of pads through a pair of vias, and wherein
the pair of pads are disposed in the third plane.
10 . The isolated DC-DC converter of claim 2 , wherein the secondary winding comprises at least one bridge connecting a third coil portion to a fourth coil portion in the secondary winding, wherein
the at least one bridge connecting the third coil portion to the fourth coil portion is disposed in a fourth plane parallel to and offset from the first plane.
11 . An isolated DC-DC converter, comprising:
a driver circuit coupled to a DC power source, the driver circuit to drive an AC signal based on the DC power source;
a primary winding coupled to the driver circuit to receive the driven AC signal, the primary winding comprising a first coil and a second coil, wherein at least one coil portion of each of the first coil and the second coil is disposed in a first plane;
a secondary winding isolated from the primary winding, wherein a capacitance of the first coil to the secondary winding equals a capacitance of the second coil to the secondary winding;
a current path serially through the first coil;
an impedance provided in the current path including a single capacitor between the first and second coils; and
a rectifier circuit to convert signals received in the secondary winding to generate an output DC signal,
wherein the driver circuit is configured to drive the AC signal to cause a resonant operation with the impedance and an inductance of the primary winding forming an LLC resonant network.
12 . The isolated DC-DC converter of claim 11 , further comprising a substrate, wherein
the secondary winding comprises a plurality of coil portions disposed in a second plane parallel to and separated from the first plane by the substrate.
13 . The isolated DC-DC converter of claim 12 , wherein the second coil comprises a plurality of coil portions, and at least one bridge connecting a pair of adjacent coil portions in the second coil, wherein
the at least one bridge is disposed in a third plane parallel to and offset from the first plane.
14 . The isolated DC-DC converter of claim 13 , wherein the impedance is coupled between a pair of pads, each of the pair of pads coupled to the first coil and the second coil, respectively, and wherein
the pair of pads are disposed in the third plane.