Load Considerations
This topic contains information you may find useful as you connect specific types of loads to a power supply or SMU.
Capacitive Loads
Generally, a power supply or SMU remains stable when driving a capacitive load. Occasionally, certain capacitive loads can cause ringing in the transient response of the device. When the output voltage is reprogrammed while capacitive loads are present, the device may temporarily move into Constant Current mode or unregulated mode.
The slew rate is the maximum rate of change of the output voltage as a function of time. When driving a capacitor, the slew rate is limited to the output current limit divided by the total load capacitance, as expressed in the following equation:
| (ΔV/Δt) = (I/C) | |
| where | ΔV is the change in the output voltage |
| Δt is the change in time | |
| I is the current limit | |
| C is the total capacitance across the load |
Series resistance or lead inductance from cabling can affect the stability of the device. In some situations, it might be necessary to increase the capacitive load or locally bypass the circuit or system being powered to stabilize the power supply or SMU.
Inductive Loads
In Constant Voltage mode, most inductive loads remain stable. However,when operating in Constant Current mode in higher current ranges, increasing output capacitance may help improve stability. You can select the output capacitance of some power supplies or SMUs using the niDCPower Output Capacitance property or the NIDCPOWER_ATTR_OUTPUT_CAPACITANCE attribute.
Pulse Loads
Load current can vary between a minimum and a maximum value in some applications. In the case of a varying load, or pulse load, the constant current circuit of the power supply or SMU limits the output current. Occasionally, a peak current may try to exceed the current limit and cause the power supply or SMU to temporarily move into Constant Current mode or unregulated mode.
To avoid pulse loads and remain within the power supply or SMU output specifications, use the niDCPower Configure Current Limit VI or the niDCPower_ConfigureCurrentLimit function to configure the current limit to a value greater than the expected peak current of the load. In extreme situations, it may be possible to parallel connect multiple power supply channels to provide higher peak currents. SMU output channels should not be placed in parallel because SMUs are four quadrant devices, and some combination of sourcing and sinking occurs if the output voltages of the channels are not exactly identical.
Reverse Current Loads
Occasionally, an active load may pass a reverse current to the power supply or SMU. To avoid reverse current loads, use a bleed-off load to preload the output of the device. Ideally, a bleed-off load should draw the same amount of current from the device that an active load may pass to the power supply or SMU.
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Caution Power supplies not designed for 4-quadrant operation may become damaged if reverse currents are applied to their output terminals. Reverse currents can cause the device to move into an unregulated mode and can damage the device. Refer to NI PXI-4110 or NI PXI-4130 for more information about device channel capabilities. |
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Note The sum of the bleed-off load current and the current supplied to the load must be less than the maximum current of the device. |