Below are listed some common conditions in which the protection circuitry of the modules may be triggered. Following the description of each is a recommended best practice to avoid the condition.
Hot Switching Channels
If an SMU is outputting a voltage and then a connection to another voltage is made, there is a likelihood of protection errors. This is often caused by one of the following conditions:
- While disconnected from the DUT, most commonly via a relay on the device interface board (DIB), the SMU is configured to source current. This will typically cause the channel to float to its positive or negative voltage limit. When then connected to the DUT or other resource, the change in voltage will cause a current spike which could cause a protection error.
- While disconnected from the DUT, most commonly via a relay on the DIB, the SMU is configured to source a voltage, or the output is disabled. When the SMU is then connected to the DUT or other resource, if there is a substantial voltage difference, a protection error is likely to result.
The SMU should always be connected to the DUT or DIB resource. If this is not possible (for example, if switching or external disconnect is required) ensure the SMU is always connected with a known voltage, ideally 0V, and the SMU is sourcing this voltage in its highest current range when connected.
High Load Variation
If the SMU is connected to a load that is suddenly, greatly increased, a protection error is likely to result. This generally occurs when the SMU channel is configured for a small current range (relative to the device’s capabilities), and the load requires a large instantaneous current.
When connected to a load with a large inrush current, use the highest current range of the SMU when enabling or turning ON the device. Once a steady state is reached, if needed for accuracy, change the SMU to a lower current range.
When the SMU is configured to source current and continually reaches the voltage limit, a protection error is more likely to occur.
Ensure that proper limits are set for each test to avoid conditions which put the SMU into voltage compliance.
Combined Channels & Voltage Compliance
When reconfiguring merged channels, it is possible to have multiple channels connected and configured differently (for example, different modes, voltages, current, etc.). This generally increases the likelihood for reaching compliance and therefore causing protection errors.
Configure merged channels simultaneously by applying settings based on HW or SW triggers. This will increase repeatability and minimize the time that channels are configured differently. In addition, use different current/voltage limits for each channel so only one channel will go in/out of compliance at a time.
Some SMUs (e.g. PXIe-4139) support channels being disconnected from the front connector of the instrument using the ‘Output Connected’ property. On other NI SMUs (e.g. PXIe-4143), the output is always connected. Channels that are unnecessarily connected have the potential to reach the voltage limit unintendedly which may cause a protection error.
Tip: Refer to the NI DC Power Supplies and SMUs Help: Supported Properties by Device to determine which devices support the ‘Output Connected’ Property.
If a channel needs to apply very little loading on the DUT, set the ‘Output Connected’ property to FALSE instead of sourcing 0A as this will let the output float instead of potentially railing at the voltage limit. If the SMU has a channel that cannot be disconnected, consider using an external relay to accomplish this.