A high voltage direct current power supply comprising: a primary high voltage direct current supply offering a primary output ; a floating secondary power supply floating with respect to the primary output and connected to the primary output ; and an output terminal at the floating secondary power supply for providing an output voltage; characterised in that the high voltage direct current power supply further comprises: a controller operative to detect a change in the output voltage at the output terminal and to generate a control signal responsive to the change in output voltage; and a controllable current source arranged to provide current at the floating secondary power supply responsive to the said control signal and whereby the said current is provided to reduce charging of a secondary output capacitance as the output voltage changes, wherein the secondary output capacitance corresponds to an output capacitance of the floating secondary power supply; and wherein the controllable current source comprises: a chain of power electronic devices configured to operate as a constant current sink; and wherein the controllable current source increases or decreases a magnitude of the constant current sink by varying a transconductance of the power electronic devices in the chain based on the control signal. a primary high voltage direct current supply offering a primary output ; a floating secondary power supply floating with respect to the primary output and connected to the primary output ; and an output terminal at the floating secondary power supply for providing an output voltage; characterised in that the high voltage direct current power supply further comprises: a controller operative to detect a change in the output voltage at the output terminal and to generate a control signal responsive to the change in output voltage; and a controllable current source arranged to provide current at the floating secondary power supply responsive to the said control signal and whereby the said current is provided to reduce charging of a secondary output capacitance as the output voltage changes, wherein the secondary output capacitance corresponds to an output capacitance of the floating secondary power supply; and wherein the controllable current source comprises: a chain of power electronic devices configured to operate as a constant current sink; and wherein the controllable current source increases or decreases a magnitude of the constant current sink by varying a transconductance of the power electronic devices in the chain based on the control signal. a controller operative to detect a change in the output voltage at the output terminal and to generate a control signal responsive to the change in output voltage; and a controllable current source arranged to provide current at the floating secondary power supply responsive to the said control signal and whereby the said current is provided to reduce charging of a secondary output capacitance as the output voltage changes, wherein the secondary output capacitance corresponds to an output capacitance of the floating secondary power supply; and wherein the controllable current source comprises: a chain of power electronic devices configured to operate as a constant current sink; and wherein the controllable current source increases or decreases a magnitude of the constant current sink by varying a transconductance of the power electronic devices in the chain based on the control signal. <EOS>