Driver circuits (1) for driving load circuits comprise transformer circuits (21) with primary side windings to be coupled to source circuits and with secondary side windings to be coupled to the load circuits. By providing the driver circuits (1) with determination circuits (22) for determining secondary side currents as functions of primary side currents, primary side voltages, primary side inductances and transformer ratios, the secondary side currents can be determined solely at primary sides of the transformer circuits (21). Functions may define secondary side currents to be proportional to differences between first signals proportional to integrals of primary side voltages divided by primary side inductances and second signals proportional to primary side currents, the differences being multiplied by the transformer ratios. The primary side inductances may be inductances measured at primary sides and the transformer ratios may be voltage ratios of the transformer circuits (21), both for open secondary sides of the transformer circuits (21).