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EPS Stromversorgung GmbH
Alter Postweg 101 • 86159 Augsburg
Germany
Fon: +49 821 / 570451-0
Fax: +49 821 / 570451-25
www.eps-germany.de
PSI 9000 2U Series
3.2.2
Current regulation / constant current / current limiting
Current regulation is also known as current limiting or constant current mode (CC).
The DC output current is held constant by the power supply, once the output current to the load reaches the adjusted
limit. The current flowing from the power supply is determined by the output voltage and the load’s true resistance.
As long as the output current is lower than the adjusted current limit, the device will be either in constant voltage
or constant power mode. If, however, the power consumption reaches the set maximum power value, the device
will switch automatically to power limiting and sets the output current according to I
MAX
= P
SET
/ U
IN
, even if the
maximum current value is higher. The current set value, as determined by the user, is always an upper limit only.
While the DC output is switched on and constant current mode is active, then the condition “CC mode active” will
be indicated on the graphic display by the abbreviation CC and this message will be passed as a signal to the
analog interface, as well stored as status which can also be read as a status message via digital interface.
3.2.3
Power regulation / constant power / power limiting
Power regulation, also known as power limiting or constant power (CP), keeps the
DC output power of a power supply constant if the current flowing to the load in
relation to the output voltage and the resistance of load reaches the adjusted value
according to P = U * I resp. P = U² / R. The power limiting then regulates the output
current according to I = sqr(P / R), where R is the load’s resistance.
Power limiting operates according to the auto-range principle such that at lower
output voltages higher current flows and vice versa in order to maintain constant
power within the range P
N
(see diagram to the right).
While the DC output is switched on and constant power mode is active, then the
condition “CP mode active” will be shown on the graphic display by the abbrevia-
tion CP, as well stored as status which can also be read as a status message via
digital interface.
When using remote sensing, the power supply can deliver a higher voltage on the DC output
than adjusted, which results in additional power and may also cause the device to enter power
limitation without explicitly indicating “CP” in the display.
3.2.3.1
Power derating
Due to fusing and cross sections of conductors and the extended input voltage range, power supply models as
from 1500 W have a fixed derating. It becomes active below a certain input voltage level (for value see
) and will derate the maximum available output power for a 1500 W model down to 1000 W
and for a 3000 W model down to 2500 W. The derating only affects the available output power, thus the full range
for power set value adjustment remains, though the device will switch earlier to constant power operation. This
situation of constant power operation won’t be indicated by status “CP”. Active derating can then only be detected
by reading the actual values of voltage and current and by calculating the power.
3.2.4
Internal resistance regulation
Internal resistance control (abbr. CR) of power supplies is the simulation of a virtual internal resistor which is in
series to the voltage source and thus also in series to the load. According to Ohm’s law, this causes a voltage
drop, which will result in a difference between adjusted output voltage and actual output voltage. This will work in
constant current mode as well as in constant power mode, but here the output voltage will differ even more from
the adjusted voltage, because then constant voltage is not active.
The adjustable resistance range of a particular model is given in the technical specifications. The voltage setting
in dependency of the resistance set value and the output current is done by calculation of the microcontroller and
thus will be significantly slower the other controllers inside the control circuit. Clarification:
U
Act
= U
Set
- I
Act
* R
Set
P
Set
, I
Set
P
Ri
= (U
Set
- U
Act
) * I
Act
With resistance mode being activated the function generator will be unavailable and the actual
power value provided by the device does not include the simulated power dissipation of Ri.
