A high voltage direct current power supply comprising: a primary high voltage direct current supply offering a primary output; a floating secondary power supply (103, 104, 107, 203, 204) 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 (701, 802, 803, 804, 805) 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 (801, 804) arranged to provide current at the floating secondary power supply (103, 104, 107, 203, 204) responsive to the said control signal and whereby the said current is provided to reduce charging of a secondary output capacitance (205, 305) 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 (103, 104, 107, 203, 204) 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 (701, 802, 803, 804, 805) 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 (801, 804) arranged to provide current at the floating secondary power supply (103, 104, 107, 203, 204) responsive to the said control signal and whereby the said current is provided to reduce charging of a secondary output capacitance (205, 305) 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 (701, 802, 803, 804, 805) 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 (801, 804) arranged to provide current at the floating secondary power supply (103, 104, 107, 203, 204) responsive to the said control signal and whereby the said current is provided to reduce charging of a secondary output capacitance (205, 305) 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.