A resonant DC-DC converter circuit comprising: a first input node, a second input node, and a third input node, each of the first, second and third input nodes (108, 110, 112) configured to receive one phase of a three phase input power; a three phase resonant circuit which comprises a delta circuit comprising a first leg connected between a first corner node and a second corner node, a second leg connected between the second corner node and a third corner node, and a third leg connected between the third corner node and the first corner node, wherein each phase comprises a resonant tank (114, 116, 118); and a first outer resonant device (RDoi) connected between the first input node and the first corner node; a second outer resonant device (RD O2) connected between the second input node and the second corner node; and a third outer resonant device (RD O3) connected between the third input node and the third corner node, wherein the outer resonant devices (RD O1, RD O2, RDO3) are located outside the delta circuit; wherein each leg (122, 124, 126) of the delta circuit consists of an inner resonant device (RD I1, RD I2 RD I3) connected in series with a corresponding transformer (T 1, T 2 T 3), wherein the inner resonant devices (RD I1, RD I2 RD I3) are located inside the delta circuit; characterized in that the first outer resonant device, the second outer resonant device, and the third outer resonant device comprises an inductor, and the first inner resonant device, the second inner resonant device, and the third inner resonant device comprises a capacitor; and wherein each resonant tank comprises an outer resonant device and an inner resonant device. a first input node, a second input node, and a third input node, each of the first, second and third input nodes (108, 110, 112) configured to receive one phase of a three phase input power; a three phase resonant circuit which comprises a delta circuit comprising a first leg connected between a first corner node and a second corner node, a second leg connected between the second corner node and a third corner node, and a third leg connected between the third corner node and the first corner node, wherein each phase comprises a resonant tank (114, 116, 118); and a first outer resonant device (RDoi) connected between the first input node and the first corner node; a second outer resonant device (RD O2) connected between the second input node and the second corner node; and a third outer resonant device (RD O3) connected between the third input node and the third corner node, wherein the outer resonant devices (RD O1, RD O2, RDO3) are located outside the delta circuit; wherein each leg (122, 124, 126) of the delta circuit consists of an inner resonant device (RD I1, RD I2 RD I3) connected in series with a corresponding transformer (T 1, T 2 T 3), wherein the inner resonant devices (RD I1, RD I2 RD I3) are located inside the delta circuit; characterized in that the first outer resonant device, the second outer resonant device, and the third outer resonant device comprises an inductor, and the first inner resonant device, the second inner resonant device, and the third inner resonant device comprises a capacitor; and wherein each resonant tank comprises an outer resonant device and an inner resonant device.