Customizing the Transient Response of the PXIe-4051

The PXIe-4051 features transient response that can be customized to the application and setup. This allows systems using the PXIe-4051 as a load to have transient response to be tuned to the best possible performance for you. You can control the balance of response time, bandwidth and overshoots that is acceptable for you.

For example, in a particular system perhaps you can live with a slight overshoot in order to get fast setpoint rise and fall. Perhaps you wanted to have transient response signature that is different for the rising and falling edge of the sequence of setpoints. All this becomes possible once you master how to setup a finely tuned system of the PXIe-4051 and your DUT using the custom transient response feature.

NI Source-Adapt optimizes system transient response and interconnects for the following conditions:

• Optimized transient response time
• Controllable overshoots or undershoots
• Reduced Ringing

Long cables and high inductance between the DUT and the electronic load can lead to an unstable or oscillatory system. The following graph is an example of how SourceAdapt can be used to optimize a system with long cable length or high cable inductance and high required current setpoints.

Basic Considerations

NI Source-Adapt optimizes system transient response and interconnects for the following conditions:

• Optimized transient response time
• Controllable overshoots or undershoots
• Reduced Ringing

Long cables and high inductance between the DUT and the electronic load can lead to an unstable or oscillatory system. 

In the table 1 are the parameters used to effectively tune the loop gain of a system, using a PXIe-4051,

Table 1. Compensation Parameters

The Concept of Loop Gain

Depending on the length and thickness of the cables, these will show up as unwanted inductance to the system. Inductance slows down the system response as it will naturally limits the maximum slew rate of the whole system. To achieve the fastest (shortest) transient response time, use as short and as thick a cable as possible. The PXIe-4051 has connectors to use two parallel cables, use this to effectively increase the cable thickness.

For example, when using constant current mode (CC Mode), the PXIe-4051 will then attempt to regulate the current thru it by measuring the current and adjusting it continuously in a feedback loop. When the feedback loop has some inductance (unwanted or otherwise), for example, due to long or thin cables, the inductance will show up as a peak in what is known as the loop-gain of the feedback system.

Cable or system inductance as shown in figure 1, shows up as a peak (figure 2) in the system loop gain. When the ‘peak’ of the inductance as shown in figure 2, rises above the 0dB line the system will likely to be come unstable. In fact as the peak approaches the 0dB line the system will become more oscillatory with which the effects can be noticed in time domain as a over-shoots, ringing and then straight oscillation.

Figure 1. Loop Gain in a system where a Voltage Source (DUT) is connected with cables to PXIe-4051

Figure 2. Loop gain with various inductance, cable length

The effect of tuning the gain-bandwidth parameters (GBW) relative to the inductive peak and overall loop gain can be shown in Figure 3. Increasing GBW will make the system ‘faster’ however with a system with inductive peaks or any other peaks due to the device under test (DUT) would make the peak cross the 0dB even more leading to more instability. In the system in figure 3, in the normal configuration, the peak is already above 0dB, so the correct way of making the system stable would be to reduce GBW instead and bring the peak below the 0dB crossing and stabilizing the system.

Figure 3: GBW adjustments effects on loop-gain and cable inductance ‘peak’

In all the cases of attempting to stabilize the system, the pole-zero ratio settings of the source-adapt should be set to 1. This effectively disable the pole zero settings. Always tune with GBW only first to stabilize the system before using the pole zero settings to attempt to get better transient performance. Also, for optimum response when tuning for step/transient response for your system, ensure that the slew-rate setting is set to maximum. After tuning is done you can set the slew-rate to slow down the slew rate (rise and fall) of your system. However, you cannot use the slew-rate setting to set to a faster slew rate that the system can maximally achieved via the source-adapt tuning as this is the physical limit of the whole system.

At the stabilization stage, transient response may exhibit overshoots or undershoots but these can be alleviated later via the pole zero tuning. The important goal at this stage is to use the GBW settings to obtain a DC stable system at all the load points that you need the DUT to operate at.

It is critical that with the GBW setting that you found at this point to vary the load current to all the required operating points to make sure that they are stable. This is especially true at the highest current and at any short circuit (or protection limit) current that you are intending to test. Typically you should tune for optimal GBW at the highest current you intend to test the DUT with.

One good guidance to tuning is to adjust to the highest GBW that still results in a stable system (no ringing or oscillatory step response) and then back off for some margins to account for system/setup/DUT tolerances.

Once you have a stable system, then you can move to tuning the pole zero ratio to optimize on the transient response of the system.

Tuning Transient Response: Pole-Zero Ratio and Compensation Frequency

Pole zero Ratio and Compensation Frequency settings allows further tune for the transient or step response shape of your system. Typically we will aim for a fast slew rate without too much overshoot or alternatively tune for a slow 1^st order response depending on the tests.

After optimizing for GBW in the steps above. Typically you will end up with a time domain step response that may have either on overshoot or a (too) slow first order response as shown in the figure 4 and figure 5 below. If the response is ringing like figure 6 then stop here and reduce your GBW until ringing reduces to just a simple overshoot that quickly damped in a single cycle (figure 4).

Figure 4. Step response with overshoot, no ringing or oscillatory

Figure 5. Step response with no overshoot, perhaps too slow first order shape

Figure 6. Ringing or oscillatory step response

Start by placing the compensation frequency = GBW and then depending on whether your starting point is Figure 4 or 5:

• Response with overshoot, figure 4:
  1. Increase your pole-zero ratio slowly from 1 in small increments and you should observe the overshoot start to reduce.
  2. If increasing the pole zero ratio does not reduce overshoot further or the system starts to oscillate, then stop and back off to the last setting before the oscillations or before the point of diminishing gains of overshoot reduction with the increment of the pole-zero ratio.
  3. If the above steps results in a satisfactory response then it is ready.
  4. If needed you can also start to reduce the compensation frequency in small increments to further reduce the overshoot to what it is required.
  5. It is recommended to have a smaller pole-zero ratio and reduce the compensation frequency to achieve a good stable transient response. Large pole-zero ratio can make system marginally unstable. Pole zero ratio of > 3 may be a bit aggressive, you will typically achieve a more easily tuned system by having a small pole zero ratio and then decreasing the compensation frequency to reduce overshoots.
  6. Do not make pole-zero ratio < 1 as this reverses the effect of the compensation and may make your system unstable. Pole-zero ratio < 1 can be used for systems that needs to be tuned to the extreme but will require experience and good judgement as effectively you are reducing what is known as phase-margins to achieve fast rise-times at the extreme of GBWs for the system.
  7. If you are unable to reduce the overshoot with the pole-zero ratio and compensation frequency adjustments and system becomes unstable with aggressive settings, then it’s likely the GBW is too high to achieve a good response and needs to be reduced. In this case reduce GBW and start again.
  8. Allowing a small overshoot will typically give you a much faster rise time than aiming for zero overshoot in your transient step response.

• Response that is ‘too slow’, figure 5:
  • Typically, in this case you would increase the GBW slightly until you either get the response you needed or end up with some overshoot (figure 4) and then use the step above (from figure 4) to reduce the overshoot to what you needed or achieve the transient response type that you require.

At this point the system is at the fastest or optimum transient response possible considering the coupling and interaction of the DUT, wiring, switchgears and PXIe-4051. In all cases, before you start tuning for transient response it is important to make sure that you DUT is stable at the output currents that you plan to test in the step response. An unstable DUT may result in oscillations and will likely trip the PXIe-4051 with an error and/or damage your DUT permanently.

Always tune the step response in the range that you are going to use it – in the case of CC mode choose the 4A or 40A range. It is not recommended that you switch range while using the same custom settings. Also, it is not recommended that you switch in between range during a transient response test. For example, it is not recommendable to start in the 4A range sinking 1A and the switch to the 40A range sinking 20A in a transient step. In this case it is recommended to use the highest range that will work with the highest current in your step(s) and do not switch between ranges, in this case the 40A range. When using a custom transient response, a system tuned and stabilized in, for example, 4A current range, with the same transient response setup may become different or even unstable in the 40A range. If it is necessary to switch between ranges using custom transient response, you will need to change to the transient response that is stabilized for each of the range and current levels while changing the range (perhaps using advanced sequence or something equivalent).

Example Tuning

Here is an example on how a typical system may be tuned for optimum transient response. Here we have a system with a DUT that outputs 7V and we wanted to tune for an optimized 1A to 10A step response in the 40A range.

We start with a conservative setting,

GBW = 1kHz, Compensation Frequency = 1kHz, Pole-Zero Ratio = 1

Figure 7. Transient Response with GBW = 1kHz, Compensation Frequency = 1kHz, Pole-Zero Ratio = 1

The step response in figure 7 shows a stable system, however the response is a bit slower than what we like. So the first thing is to increase the GBW in small steps until the system seems faster and we get the fastest response we can get with maybe a slight overshoot.

Figure 8.  Transient Response with GBW = 25kHz, Compensation Frequency = 1kHz, Pole-Zero Ratio = 1

With the GBW adjusted to 25kHz, we end up with the system in figure 8, that has a much steeper/faster step response but with a slight overshoot. If this amount of overshoot is not acceptable for us, we can proceed to tune the pole-zero compensation to attempt to reduce it.

Figure 9. Transient Response with GBW = 25kHz, Compensation Frequency = 25kHz, Pole-Zero Ratio = 3

By putting the compensation frequency = GBW and slowing increasing the pole-zero ratio in small steps we end up with the settings in figure 9 which shows a marked reducing in overshoot. Further increments in pole-zero ratio at this point does not yield further reduction in overshoot and in-fact results in oscillations.

To reduce the overshoot further, we can slowly reduce the compensation frequency from 25kHz. In figure 10, compensation frequency has been reduced to 15kHz which results in a good optimum response for this system without any visible overshoots.

Figure 10. Transient Response with GBW = 25kHz, Compensation Frequency = 15kHz, Pole-Zero Ratio = 3

Slew Rate Settings

Once the optimum transient/step response is achieved, if required a slower slew rate can be programmed in the slew rate settings to reduce the slope of the rising and falling in the step response. Starting from the optimized tuned system in figure 10, in figure 11 below we programmed a rising slew rate of 0.025A/μs, resulting in a linear rise at the specific slew rate. Slew rate cannot be made faster that that optimized on figure 10 as this is setup by the source-adapt tuning as well as the physical setup, parasitic and limitations of the system and DUT. Note that both rising and falling slew rates can be programmed independently.

Figure 11. Transient Response with GBW = 25kHz, Compensation Frequency = 15kHz, Pole-Zero Ratio = 3, Slew Rate = 0.025A/μs

Figure 10: Transient Response with GBW = 25kHz, Compensation Frequency = 15kHz, Pole-Zero Ratio = 3, Slew Rate = 0.015A/μs