TEST & MEASUREMENT
You must take two factors into account when making the change from the use of a DC sweep to a pulsed sweep. The pulse must be wide enough to allow sufficient time for the device transients, cabling, and other interfacing circuitry to settle so the system can make a stable, repeatable measurement. At the same time, however, the pulse cannot be so wide that it exceeds the test instruments’ maximum pulse width and duty cycle limits, which would violate the instrument’s allowed power duty cycle. Pulses that are too wide can also create the same device self-heating problems that can occur with DC sweeps.
Combining multiple SMU channels to achieve higher DC current
The most common method of combining SMU channels to achieve higher DC currents is to connect the current sources in parallel across the DUT, as shown in Figure 1.
This test setup takes advantage of a well-known electrical principle (Kirchhoff’s current law), which states that two current sources connected to the same circuit node in parallel will have their currents added together. Both SMUs source current and measure voltage. All of the LO impedance terminals (FORCE and SENSE) of both SMUs are tied to earth ground. Table 1 provides an overview of the characteristics of this particular configuration. You should set the output currents for SMU1 and SMU2 to the same polarity to obtain maximum output. Whenever possible, one SMU should be in a fixed source configuration and the other SMU performs the sweep. This is preferable to having both sweeping simultaneously. If both SMUs are sweeping, their output impedances are naturally changing, for example, as the meter autoranges up and down. The DUT’s output impedance may also be changing significantly, such as from a high-resistance off-state to a low- resistance on-state. With so many of the impedance elements in the circuit changing, this could increase overall circuit settling time at each bias point. Although this is a transient effect that damps out, fixing one SMU’s source and sweeping the other usually results in more stable and faster- settling transient measurements, for higher test throughput.
Merging pulse sweeps
New SMU architectures simplify merging the power-enhancement advantages of the pulse sweep method with multiple SMU channels operated in parallel. For example, some dual-
channel SMUs allow increasing the number of SMU channels from two to four. Using pulse sweep and multi-channel capabilities in tandem allows sourcing far higher currents than using a single SMU with DC sweeps. Obviously, implementing this test method demands the exercise of extraordinary caution to ensure personnel safety. For safety, it is critical to insulate or install barriers to prevent user contact with live circuits. Additional protection techniques are needed to prevent damage to the test setup or the DUT. The multiple pulses must be tightly synchronized (with nanosecond precision) so that one piece of equipment is not applying power and damaging units that are not yet turned on.
Figure 2 shows the results of an experiment in which Keithley engineers used a single SMU to generate a 10A pulse; the results were observed on an oscilloscope. A high power precision resistor (0.01W, ±0.25%, KRL R-3274) was used as the test DUT with a pulse width of 300 microseconds. The oscilloscope showed a nearly square waveform of
Figure 1. The most common method of combining SMU channels to achieve higher DC currents
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Figure 2. Results of an experiment using a single SMU to generate a 10A pulse
www.solar-pv-management.com Issue III 2010
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