Load Regulation
- Updated2025-06-10
- 6 minute(s) read
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 using local sense in constant voltage mode, 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.
- You can use remote sense to improve the load regulation performance, 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 LO to Output LO, and Sense HI to Output HI).
- 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 or SMU 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 or SMU.
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.
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.
Configuring Transient Response
Use niDCPower Transient Response to set the transient response.
The following table lists the PXIe-4162 transient response settings available in NI-DCPower.
| Transient Response | Description |
|---|---|
| Slow | Increases stability while decreasing the speed of the device. Select Slow if connecting a DUT causes the device to exhibit symptoms of instability, such as unstable readings or excessive noise. |
| Normal (default) | Balances stability and the speed of the device. It is the default transient response setting and is appropriate for most situations. |
| Fast | Increases the speed of the device for improved transient response with benign loads. Select Fast if you need faster response and if doing so does not cause the instrument to exhibit symptoms of instability, such as unstable readings or excessive noise. |
| Custom | Allows freedom to adjust compensation for specific loads. Select Custom if you need to optimize the speed/stability tradeoff. |
Customizing Compensation
Set the niDCPower Transient Response property to Custom or the NIDCPOWER_ATTR_TRANSIENT_RESPONSE attribute to NIDCPOWER_VAL_CUSTOM to customize compensation on the device.
The following table lists the compensation parameter settings for the PXIe-4162. You can independently set the parameters for constant voltage mode and constant current mode. There are three compensation parameters for constant voltage mode and three compensation parameters for constant current mode, for a total of six writable and readable parameters.
| Compensation Parameter | Mode | Details |
|---|---|---|
| Gain Bandwidth (GBW) | Constant Voltage Mode | Set the GBW of the instrument. Higher values give faster response
but poorer stability. 10 Hz to 20 MHz |
| Constant Current Mode | Set the GBW of the instrument. Higher values give faster response
but poorer stability. 10 Hz to 20 MHz |
|
| Compensation Frequency | Both | Set the geometric mean of the pole frequency and the zero frequency.
It is the frequency of maximum phase shift caused by the pole-zero pair. 100 Hz to 300 kHz |
| Pole-Zero Ratio | Both | Set the ratio of the pole frequency to the zero frequency. A lag compensator has a pole-zero ratio set to a value less than 1.0, and a lead compensator has a pole-zero ratio set to a value greater than 1.0. If the pole-zero ratio is set to exactly 1.0, the pole and zero cancel each other and have no effect. You can set the pole-zero ratio to any value between 0.125 and 8.0. |
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 come close to exceeding the current limit and cause the power supply or SMU to temporarily move into constant current mode or unregulated mode.
To remain within the power supply or SMU 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. Generally, instrument output channels should not be placed in parallel because these instruments are four-quadrant devices, and some combination of sourcing and sinking occurs if the output voltages of the channels are not identical. Refer to Connecting Multiple NI Source Measure Unit Channels in Parallel at ni.com/r/smuparallel for more information.
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.