Voltage Mode/Current Mode. Why a power supply can only regulate only one of these parameters at a time

Most of Spellman’s high voltage power supplies offer Voltage Mode and Current Mode with automatic crossover, dependent upon settings and load conditions. This requires the power supply to have two regulating loops: voltage mode and current mode. Additionally, each loop requires a programing signal provided by the user so the power supply can regulate and limit accordingly. 

Voltage Mode

Running the power supply in voltage mode is the way most customers use our power supplies. When the power supply is operated in voltage mode it behaves as a voltage source. Here the power supply will actively regulate the output voltage from 0 to 100% of the rated output voltage, dependent upon the selected setting.

In this situation the output current is determined by the magnitude of the output voltage and the impedance of the load placed upon the power supply. Most users set the Current Mode to maximum. In this situation if a short circuit were placed upon the output of the power supply it would automatically cross over from voltage mode to current mode, regulating the current at 100% of the maximum rated current.

Current Mode

Running the power supply in current mode is the less frequent way customers use our power supplies. When the power supply is operated in Current Mode it behaves as a current source. Here the power supply will actively regulate the output current anywhere from 0 to 100% of the rated output current, dependent upon the selected setting.

In this situation the output voltage is determined by the magnitude of the output current and the impedance of the load placed upon the power supply. Most users set the Voltage Mode to maximum. In this situation if an open circuit were placed upon the power supply it would automatically cross over from current mode to voltage mode, regulating the voltage at 100% of the maximum rated voltage.

Voltage Mode/Current Mode Programmability

In the above situations the voltage and current loops were set to 100% of rated output, but these signals are typically programable from 0 to 100% of rated output, as required by the customers application.

Only One Mode at a Time

As outlined above, the typical high voltage power supply can only regulate one parameter (be it voltage or current) at a time. If you run in Voltage Mode, then you have regulated voltage and current compliance. If you run in Current Mode, you have regulated current and voltage compliance. The main point here is the power supply cannot regulate both voltage and current at the same time. If operated in Voltage Mode (as most do) the power supply will regulate the output voltage, but the current drawn from the power supply is dependent upon the voltage setting and load impedance placed on the output of the power supply in question.