When interfacing with a DUT, it is often important to limit the voltage or current that the device will provide or draw in order to prevent damage to the DUT. There are important differences between the way limit functions work on SMUs and PPMUs. For SMUs, limiting voltage and current is completely programmable. (Note- there are some exceptions to this. Namely, the NI PXI-4130 for SMUs; however, these will not be discussed in the scope of this paper.) On the other hand, PPMUs have the ability to programmatically limit voltage, but current is regulated by the selected current range of the device.
Figure 2 below shows a basic battery test example which we will use to demonstrate the main differences in limiting between a SMU and a PPMU. Everything inside the dotted line represents the device in question. Vsrc is the voltage source; Vmeter is the onboard voltage meter; and Ameter is the onboard current meter. Src Hi and Src Lo are connected to the DUT to supply voltage or current while Sense Hi and Sense Lo are used for 4 wire voltage measurements. Bear in mind that this is not an entirely accurate depiction of a device’s block diagram and is used only for example purposes.
In the simplest case, we can force 5 volts across a 1 kohm DUT which will result in a 5 mA current. We can set a current limit of 9 mA to protect our DUT, and this scenario works fine without any issues.
Figure 2: Limiting
However, if we halve the resistance of our DUT, then the current that flows into the DUT now doubles. Now, the 10 mA current is higher than the 9 mA limit we set to protect our DUT. Because we’ve exceeded our current limit, the SMU will realize something is not right. However, it is not possible to force the current down to 9 mA while simultaneously forcing 5 V across a .5 kohm resistance. Ohm’s Law dictates that if the resistance is set, we only have control of either the voltage or the current. Because of this, the SMU will actually switch to a current source and force 9 mA, which will give us a resulting 4.5 V potential across the DUT as shown below in Figure 3. This will protect our DUT from being damaged even if a higher than anticipated voltage difference forces a large current.
Figure 3: Limiting, SMU
For PPMU, we can set the desired current range we want the device to operate in, but this is not the same as setting a limit. This current range defines the accuracy at which the device will source, but it must be known beforehand as the PPMU will not automatically choose a range. The NI PXIe-6556 has selectable current ranges of ±2 mA, ±8 mA, ±32 mA, ±128 mA, ±512 mA, ±2 mA, ±8 mA and ±32 mA. Setting the current range to the smallest range guarantees a current resolution of 60 pA provided the actual current sourced falls in that range. If the current sourced is outside of the preselected range, then the accuracy is no longer guaranteed and a new range must be manually selected.
As previously mentioned, setting the current range is not the same as setting a limit, and if the DUT requires more current (again, Ohm’s Law dictates we can only control one aspect) then the PPMU will provide it. However, setting the current range does provide a general guideline for how much current the device will source or sink. An example of this is shown below in Figure 4. Note that the actual current provided exceeds the current limit set, but not by too much. The current range will regulate the current provided but will not provide a hard cap. Because of this, it is important to know how much current your DUT can source or sink and to not force a voltage that will result in a current higher than that. If need be, PPMUs do provide fully programmable voltage limiting.
Figure 4: Limiting, PPMU