A method of controlling a direct-current-to-direct-current converter wherein a primary converter is coupled to a secondary converter via a transformer, the method comprising: determining a maximum power transfer from the primary converter to the secondary converter based on observed measurements and optionally on estimated parameters, the observed measurements comprising measurements or observations of an input voltage ( Vi n or V 1) to the primary converter, an output voltage ( V 2 or V out) of the secondary converter, a modulation frequency of the primary converter and the secondary converter, and primary inductance (16, 416) in series with a primary winding of the transformer during an initialization mode, whereby optionally the one or more observed measurements of the input voltage ( Vi n or V 1) to the primary converter and the output voltage ( V 2 or V out) of the secondary converter are replaced with estimated parameter values for the input voltage and the output voltage in accordance with design specifications of the converter, estimating an observed power transfer from the primary converter to the secondary converter based on or proportional to an observed input voltage (Vin-observed) to the primary converter, an observed output voltage (Vout-observed) of the secondary converter, the modulation frequency of the primary converter and the secondary converter, and a primary inductance in series with a primary winding of the transformer; determining an operational load ratio or operational load percentage between the observed power transfer and maximum power transfer, wherein the operational load ratio or load ratio percentage is the observed power transfer divided by the maximum power transfer; selecting a load curve based on the determined operational load ratio or percentage from a set of load curves stored in a look-up table, each of the load curves associated with a corresponding ratio or percentage between the observed power transfer and maximum power transfer; and adjusting or maintaining the modulation frequency of the primary converter and the secondary converter consistent with an operation point on the selected load curve, where the operation point minimizes the power loss, power difference between input power and output power, or thermal energy dissipated from the direct-current-to-direct current converter, wherein each load curve describes the variation of the power loss or of the power difference with respect to the modulation frequency for a corresponding load ratio. determining a maximum power transfer from the primary converter to the secondary converter based on observed measurements and optionally on estimated parameters, the observed measurements comprising measurements or observations of an input voltage ( Vi n or V 1) to the primary converter, an output voltage ( V 2 or V out) of the secondary converter, a modulation frequency of the primary converter and the secondary converter, and primary inductance (16, 416) in series with a primary winding of the transformer during an initialization mode, whereby optionally the one or more observed measurements of the input voltage ( Vi n or V 1) to the primary converter and the output voltage ( V 2 or V out) of the secondary converter are replaced with estimated parameter values for the input voltage and the output voltage in accordance with design specifications of the converter, estimating an observed power transfer from the primary converter to the secondary converter based on or proportional to an observed input voltage (Vin-observed) to the primary converter, an observed output voltage (Vout-observed) of the secondary converter, the modulation frequency of the primary converter and the secondary converter, and a primary inductance in series with a primary winding of the transformer; determining an operational load ratio or operational load percentage between the observed power transfer and maximum power transfer, wherein the operational load ratio or load ratio percentage is the observed power transfer divided by the maximum power transfer; selecting a load curve based on the determined operational load ratio or percentage from a set of load curves stored in a look-up table, each of the load curves associated with a corresponding ratio or percentage between the observed power transfer and maximum power transfer; and adjusting or maintaining the modulation frequency of the primary converter and the secondary converter consistent with an operation point on the selected load curve, where the operation point minimizes the power loss, power difference between input power and output power, or thermal energy dissipated from the direct-current-to-direct current converter, wherein each load curve describes the variation of the power loss or of the power difference with respect to the modulation frequency for a corresponding load ratio.