TEST & MEASUREMENT
Going Beyond the Spec
The end goal of any solar or PV product is to produce energy. The very electricity that is channelled can also cause damage to devices if not correctly managed. Keithley Instruments’
Applications Engineering Staff discuss sourcing
and measuring higher currents to ensure efficient and safe devices.
management devices, high-brightness LEDs, and RF power transistors, often require high currents, sometimes as much as 40A or even higher for power MOSFETs and insulated gate bipolar transistors (IGBTs), which can require currents in excess of 100A. However, the maximum DC current that a single supply is specified to handle is typically limited. This spec limit is typically dependent on the power supply’s design, the safe operating area of the discrete components used in the instrument, the spacing of the metal lines on the internal printed circuit board, etc. If you need to increase the amount of current sourced and you use an SMU (source-measure unit), you can employ several test modes and multiple channels.
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Although DC current sources typically don’t let you pulse their outputs, you can build pulse circuits yourself. Pulsed sources are often essential for testing power devices such as MOSFETs or IGBTs because DC test currents would skew the DUTs’ resistance values due to Joule heating. Although high-power pulse generators are available, they have no built-in measurement capabilities, so they require synchronizing the operation of a separate ammeter with the pulsed test signal.
Pulsed sweeps for higher power
You can substitute a pulsed sweep for a DC sweep to achieve higher power I-V sweeps with little impact on results. However, with some DUTs, such as capacitors, pulsed sweeps may not correlate adequately with DC sweeps because large displacement currents, which can be generated at the sharp edges of the voltage pulse, may change these devices’ electrical properties. On the other hand, pulsed I-V testing is essential for other device types, such as high-power RF power amplifiers and even low-power nanoscale devices, to obtain optimal results. During high power continuous wave (CW) testing, the semiconductor material itself will start to dissipate the applied power as heat. As the material in the device heats, the conduction current decreases because the carriers have more collisions with the vibrating lattice (phonon scattering). Therefore, the measured current will be erroneously low due to self-heating effects. Given that these types of devices typically run in pulsed mode (i.e., intermittently rather than continuously), the erroneously low DC-measured currents won’t accurately reflect their performance. In these circumstances, pulsed testing must be used.
igh-power test applications, such as characterizing solar cells, power
www.solar-pv-management.com Issue III 2010
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