Load Regulation

Load regulation is a measure of the ability of an output channel to remain constant given changes in the load.

Depending on the control mode enabled on the output channel, the load regulation specification can be expressed in one of two ways:

  • In constant voltage mode, variations in output current result in changes in the output voltage. This variation is expressed as a percentage of output voltage range per amp of current change, or as a change in voltage per amp of current change, and is synonymous with a series resistance.
    • When Sense - is connected to -, and Sense + is connected to + at the terminal block, the load regulation specification defines how close the output series resistance is to 0 Ω—the series resistance of an ideal voltage source. Many supplies have protection circuitry at the output that slightly increases the output series resistance.
    • The PXIe-4112 requires the niDCPower Sense property or NIDCPOWER_ATTR_SENSE attribute to be configured for Remote, even while maintaining a 2-wire configuration. Configure the channel for remote sense and connect the sense terminals externally to their respective output terminals (connect Sense - to the - terminal, and Sense + to the + terminal).
  • In constant current mode, variations in load voltage result in changes to the output current. This variation is typically expressed as a percentage of output current range per volt of output change, and is synonymous with a resistance in parallel with the output channel terminals. In constant current mode, the load regulation specification defines how close the output shunt resistance is to infinity—the parallel resistance of an ideal current source. In fact, when load regulation is specified in constant current mode, parallel resistance is expressed as 1/load regulation.

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.

Capacitive Loads

Generally, a power supply remains stable when driving a capacitive load. Occasionally, certain capacitive loads can cause ringing in the transient response of the instrument. The instrument may temporarily move into constant current mode or unregulated mode when the output voltage is reprogrammed while capacitive loads are present.

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 and lead inductance from cabling can affect the stability of the device. In some situations, you may need to increase the capacitive load or locally bypass the circuit or system being powered to stabilize the power supply.

Transient Response

In reference to power supplies and SMUs, transient response describes how a supply responds to a sudden change in load.

Changes in load current, such as a current pulse, can cause large voltage transients. The transient response specifies how long it takes before the transients recover. The following figure shows how the transient behavior is typically specified. The transient response time specifies how quickly the supply can recover to within a certain voltage (ΔV) when a specific change in load (ΔI) occurs. Some power supplies also specify a maximum transient voltage dip under the same load conditions.

Figure 20. Transient Response

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There is a trade-off between transient response and the stability of the supply under a wide variety of loads. To achieve the fastest transient response, an instrument should have a high gain-bandwidth (GBW) product, but the higher GBW is, the more likely it is that the instrument will become unstable with certain loads. Thus, most instruments compromise performance to achieve stability under most conditions. Other instruments allow a degree of customization to enable optimization of performance under different circumstances.

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 limits the output current.

Occasionally, a peak current may come close to exceeding the current limit and cause the power supply to temporarily move into constant current mode or unregulated mode.

To remain within the power supply output specifications with pulsed loads, use niDCPower Configure Current Limit to configure the current limit to a value greater than the expected peak current of the load.

In extreme situations, you may be able to parallel-connect multiple power supply channels to provide higher peak currents.

Reverse Current Loads

Occasionally, an active load may pass a reverse current to the power supply.

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.

Caution Power supplies not designed for four-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 instrument. Refer to the PXIe-4112 Specifications for more information about channel capabilities.
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 instrument.