Circuitry for use in a DC/DC converter that has a power stage circuit that includes an input, an output, and a transformer with a core, and that is configured to operate in a power transfer mode during which power is transferred from the input to the output and a reset mode during which flux density in the core of the transformer is reduced, and a pulse generator circuit configured to generate pulses that regulate the output of the power stage circuit, the circuitry comprising: a flux density monitor circuit configured to: generate flux density information indicative of the flux density of the core of the transformer during both the power transfer mode and the reset mode; preserve a representation of the flux density in the core of the transformer at the end of the reset mode; and modify the representation of the flux density in the core of the transformer to track changes in the flux density in the core of the transformer during both the power transfer mode and the reset mode; and a power stage control logic configured to: regulate the output of the power stage circuit based on the pulses and to prevent the core of the transformer from saturating based on the flux density information, including to terminate the power transfer mode when the representation of the flux density in the core of the transformer reaches or exceeds a positive pre-determined threshold; and prevent the core from saturating during the reset mode by causing the reset mode to terminate when the flux density information indicates that the flux density has reached or exceeded a negative pre-determined threshold, wherein the flux density monitor circuit includes a capacitance and two electronic switches (243, 245) and is configured to cause a voltage across the capacitance to track changes in the flux density of the core of the transformer during both the power transfer mode and the reset mode, wherein the electronic switches (243, 245) are controlled by signals from an output of the power stage control logic. a flux density monitor circuit configured to: generate flux density information indicative of the flux density of the core of the transformer during both the power transfer mode and the reset mode; preserve a representation of the flux density in the core of the transformer at the end of the reset mode; and modify the representation of the flux density in the core of the transformer to track changes in the flux density in the core of the transformer during both the power transfer mode and the reset mode; and generate flux density information indicative of the flux density of the core of the transformer during both the power transfer mode and the reset mode; preserve a representation of the flux density in the core of the transformer at the end of the reset mode; and modify the representation of the flux density in the core of the transformer to track changes in the flux density in the core of the transformer during both the power transfer mode and the reset mode; and a power stage control logic configured to: regulate the output of the power stage circuit based on the pulses and to prevent the core of the transformer from saturating based on the flux density information, including to terminate the power transfer mode when the representation of the flux density in the core of the transformer reaches or exceeds a positive pre-determined threshold; and prevent the core from saturating during the reset mode by causing the reset mode to terminate when the flux density information indicates that the flux density has reached or exceeded a negative pre-determined threshold, regulate the output of the power stage circuit based on the pulses and to prevent the core of the transformer from saturating based on the flux density information, including to terminate the power transfer mode when the representation of the flux density in the core of the transformer reaches or exceeds a positive pre-determined threshold; and prevent the core from saturating during the reset mode by causing the reset mode to terminate when the flux density information indicates that the flux density has reached or exceeded a negative pre-determined threshold,