Bi-directional DC-DC converter comprising: a first terminal circuit, a second terminal circuit, a transformer circuit, a first high voltage side comprising a first bridge circuit and coupled to said first terminal circuit, and a second low voltage side coupled to said second terminal circuit; wherein said first high voltage side and said second low voltage side are coupled to each other by means of said transformer circuit, and said first high voltage side comprises a resonant tank circuit coupled between said first bridge circuit of said first high voltage side and a high voltage side of said transformer circuit; wherein said resonant tank circuit i comprises: a first branch comprising a first capacitance C r 1 and a first inductance L r 1 coupled in series with each other, characterised in that the resonant tank circuit further comprises: a second branch comprising a second inductance L r2 and second capacitance C r 2 coupled in series with each other, and a third branch comprising a third capacitance C r 3 and a third inductance L r 3 coupled in series with each other; wherein said first branch, second branch and third branch are coupled to a common node; wherein said second branch is coupled between said common node and a first terminal of said high voltage side of said transformer circuit; wherein said third branch is coupled between said common node and a second terminal of said high voltage side of said transformer circuit; and wherein the first branch is coupled in series with the second branch. a first terminal circuit, a second terminal circuit, a transformer circuit, a first high voltage side comprising a first bridge circuit and coupled to said first terminal circuit, and a second low voltage side coupled to said second terminal circuit; wherein said first high voltage side and said second low voltage side are coupled to each other by means of said transformer circuit, and said first high voltage side comprises a resonant tank circuit coupled between said first bridge circuit of said first high voltage side and a high voltage side of said transformer circuit; wherein said resonant tank circuit i comprises: a first branch comprising a first capacitance C r 1 and a first inductance L r 1 coupled in series with each other, characterised in that the resonant tank circuit further comprises: a second branch comprising a second inductance L r2 and second capacitance C r 2 coupled in series with each other, and a third branch comprising a third capacitance C r 3 and a third inductance L r 3 coupled in series with each other; wherein said first branch, second branch and third branch are coupled to a common node; wherein said second branch is coupled between said common node and a first terminal of said high voltage side of said transformer circuit; wherein said third branch is coupled between said common node and a second terminal of said high voltage side of said transformer circuit; and wherein the first branch is coupled in series with the second branch. a first branch comprising a first capacitance C r 1 and a first inductance L r 1 coupled in series with each other, characterised in that the resonant tank circuit further comprises: a second branch comprising a second inductance L r2 and second capacitance C r 2 coupled in series with each other, and a third branch comprising a third capacitance C r 3 and a third inductance L r 3 coupled in series with each other; wherein said first branch, second branch and third branch are coupled to a common node; wherein said second branch is coupled between said common node and a first terminal of said high voltage side of said transformer circuit; wherein said third branch is coupled between said common node and a second terminal of said high voltage side of said transformer circuit; and wherein the first branch is coupled in series with the second branch. a second branch comprising a second inductance L r2 and second capacitance C r 2 coupled in series with each other, and a third branch comprising a third capacitance C r 3 and a third inductance L r 3 coupled in series with each other; wherein said first branch, second branch and third branch are coupled to a common node; wherein said second branch is coupled between said common node and a first terminal of said high voltage side of said transformer circuit; wherein said third branch is coupled between said common node and a second terminal of said high voltage side of said transformer circuit; and wherein the first branch is coupled in series with the second branch.