TEST & MEASUREMENT
each SMU is forcing voltage and outputting near- maximum currents, and the DUT is in a high- impedance state, then all current will go to the SMU that is sourcing the slightly lower voltage, which will most likely damage that SMU. Therefore, when connecting SMUs in parallel to a single terminal of a DUT, only one SMU should be sourcing voltage. Refer to Figure 4 for both correct and incorrect approaches to connecting SMUs in parallel.
Mitigating dissipation due to contact failure
When you connect two SMUs with the same output capacity in parallel to a single node in the circuit, one SMU must be able to sink all of the current being output by the other SMU. This scenario can occur, for example, when one of the leads breaks contact with the device (such as when the lead is accidentally disconnected or a contact isn’t made properly).
That means there is a short period during which one SMU must sink all the current from the other instrument. However, when there are more than two SMUs connected in parallel at a single circuit node, a single SMU cannot sink all of the current coming from the other SMUs. The SMU that will be forced to sink current if there’s a break in contact with the device is the SMU at the lowest voltage or lowest impedance, most likely the one sourcing voltage. In order to protect the signal input of the SMU forcing voltage, a diode such as the 1N5820 can be used. A diode is preferable because a fuse would react too slowly to provide protection and a resistor will cause too large of a voltage drop across it. A diode offers a much faster response than a fuse and has a much smaller maximum voltage drop across it (typically around 1V) than a resistor.
Protecting the source
However, to be truly safe when using this method, a diode should be used to protect all the SMUs in the configuration. That’s because if the DUT goes into a high-impedance state, the current sources will try to force their current into the voltage- sourcing SMU, but that would not be possible because the voltage-sourcing SMU is protected by a diode. That would cause the current-sourcing SMUs to increase their output voltage until they reached their voltage limit. Once this occurred, the current sources would go into compliance and become voltage sources themselves. That would mean there were now multiple voltage sources in parallel. Even if their voltage limits were set to exactly the same value, their outputs would still likely be very slightly different and they would damage each other. It’s important to be aware that
Figure 4. Incorrect and correct configurations for connecting SMUs in parallel. a) This incorrect configuration could allow high currents to damage the SMU that is sourcing a slightly lower voltage than the other one. b) This quasi-Kelvin configuration, although it doesn’t run the same risk of instrument damage as the first configuration, introduces additional error into the measurement that must be accounted for in the system’s error budget and limits the maximum output of the SMU. c) This “hybrid” approach also prevents SMU damage and allows adding SMU current sources as needed to reach the application’s current sourcing requirements
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putting a diode on each and every SMU in the configuration has some consequences. First, the inclusion of any diodes in the configuration means this method can only be used to source power but not to sink it because the diodes will not allow current to pass into the SMU. The second consequence is that, in order to obtain maximum output voltage, you will need to use four-wire connections on the current sources around the diode because the voltage drop across diode may cause the current sources to reach compliance prematurely. At these current levels, the typical voltage drop across a diode is about 1V.
SMUs offer a simple, highly integrated approach to designing test and measurement systems for a wide range of electronic devices. For the growing number of test applications that demand the ability to source and/or measure higher currents, the techniques outlined in this article offer useful, cost- effective alternatives to combining separate sources and measurement instruments.
www.solar-pv-management.com Issue III 2010
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