Flexible chargers for batteries (especially in the field of e-mobility), fuel cell applications or flexible photovoltaic battery combinations all contain switch-mode power supplies with DC converters, DC-DC converters in short. Usually the voltage ratio between output and input voltage of galvanically isolated DC-DC converters can only be varied in the range of 1:2. This is not the case with the new bidirectional, galvanically isolated resonant DC-DC converter: Here the voltage ratio can be variably adjusted in the range 1:4. With the resonant converter, the switching load is always balanced to achieve high efficiency. The principle is based on the series-resonant converter: By inserting primary or secondary side freewheels for whole resonant half oscillations, energy is supplied to the resonance circuit or selectively extracted from it, so that various control objectives, in particular a variable output voltage, are achieved. A short-circuit-proof SRC was also presented. The special feature of the boost and buck-boost modulation method used here for series-resonant converters: Parts of the excitation oscillations are omitted in the input circuit, which allows a lower energy level in the resonant circuit and therefore also a lower voltage at the output circuit (buck operation). If energy is not taken from the output circuit in each period of the oscillation, a higher energy level in the oscillating circuit and therefore a higher output voltage is obtained (boost operation). The resonant converter can be operated bidirectionally with the appropriate circuitry. A method for operating the DC-DC converter was also developed.