Feature Test & Measurement
Keeping a finger on the test pulse E
Jennifer Cheney, Senior Applications Engineer at Keithley Instruments, explores how High-Speed A/D converters can ensure greater flexibility in pulse sourcing and measurement
ngineers are constantly seeking to design more efficient semiconduc- tor devices, as part of improving the efficiency of end products such as switching power supplies and power inverters. The push for LEDs in commer- cial and residential applications drives the use of pulse width modulation as a light-dimming technique.
For these inherently pulsed applica- tions, accurate testing under pulsed conditions of the discrete components that make up the end product is critical. The use of a pulsed test stimulus, demands instruments capable of high- speed measurements. Keithley has enhanced its Model 2651A High Power System SourceMeter instru- ment for characterising high-power electronics in response to this need. This source/measure unit (SMU) is particularly useful for pulsed testing applications because it includes two high-speed A/D converters (ADCs), one for voltage and one for current, in addi- tion to the two integrating ADCs common to all Series 2600A System SourceMeter instruments. These high- speed ADCs allow users to digitise fast pulses with 18-bit resolution (the inte- grating ADCs provide 22 bits resolution). The instrument offers users the flex- ibility to examine raw data; the prod- uct's high-speed ADCs offer a 1MHz burst sampling rate; these readings can be periodically stored in an on-board non-volatile reading buffer before transfer to the system controller for close analysis.
The availability of two high-speed ADCs allows for simultaneous meas- urement with true 1MHz sampling for
both voltage and current; there's no degradation in sample rate when the application requires sampling both signals at once. It also allows the reali- sation of a current waveform without the use of an oscilloscope and also a current probe.
Still more importantly, the instru- ment doesn't limit users to source- delay-measure sequencing; both the high-speed and integrating ADCs sup- port asynchronous measurement, so users can set the pulse measurement trigger wherever they desire in relation to the pulse. This greater flexibility allows users to trigger the measure- ment before the pulse, during the pulse, after, or across the pulse.
Ways of taking advantage of trigger model flexibility The high-speed ADCs can digitise the top of the pulse when the measure- ments are made synchronously with the source. The ability to characterise the slope of the measured voltage at the top of the pulse is important for evaluating pulse amplitude flatness.
Keithley’s enhanced Model 2651A High Power System SourceMeter instrument for characterising a variety of high- power electronics
For LEDs, the slope of the measured forward voltage (VF) indicates whether there is significant device self-heating. Analysis software is often used to average sampled data to improve accu- racy. The Model instruments averaging and median filters can be applied to readings from the high-speed ADC to obtain spot mean measurements. Applications include the study of the thermal impedance in power semi- conductor devices.
For example, changes in the VF of the source-drain diode, along with external temperature measurements of the device package and surrounding environment, indicate how the semi- conductor junction temperature changes whenever a high power pulse passes through it, and how the heat the device generates is transferred out of the packaging.
For a variety of applications, it is useful to characterise how a pulse is transmitted through a device or system; this characterisation requires digitising the entire pulse, including the rising and falling edges, a measurement that is enabled by using the high-speed ADCs to measure asynchronously with the source operation.
Pre-pulse characterisation involves triggering measurements before the pulse occurs and is another useful sourcing and measurement mode. Pulses are sometimes employed to provide power stresses to the device. It is important to note the device state before the stress is applied.
Jennifer Cheney is Senior Applications Engineer at Keithley
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ompliance 5 emission and immunity test software has been released by TESEQ. Compliance 5 is a single, integrated test platform capable of performing all RF EMC testing needs. The upgraded software is simple to use with a Commercial Application Package included that aids in speeding set-up and makes compliance testing quick and easy. Other application packages for military and aerospace are available as options. Further new packages for reverberation chambers and automotive testing will be available in the next few months. Embedded into each test package are hardware configuration templates. Simple drag and drop tools allow the operator to customise the configuration to match the equipment in use. Selections for any test need only to be chosen once and are then saved for continued use. TESEQ:
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This can be done by programming a pulse with a non-zero idle level and triggering the measurements before the pulse. The instrument allows users to specify how long before the pulse the measurements should be made. Post-pulse characterisation requires triggering measurements after the pulse occurs. When using pulse test- ing to stress a device, the device must also be characterised after the stress is applied. This is typically done by sourcing a pre-defined test voltage or current after the pulse.
The test level is chosen so as not to cause any additional thermal or elec- trical stress to the device. The meas- urement can be made by sourcing a pulse with a non-zero idle level and using the high-speed ADCs to perform the measurement. The results from the high-speed ADCs indicate how the device recovers from the stress. Keithley Instruments
www.keithley.com Enter 204
JULY 2011 Electronics
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