PC-Based System Results in Faster Testing of Electronic Components

Author(s):
V. Arunachalam - Soliton Automation Private Limited
S. B.. Rajnarayanan - Soliton Automation Private Limited

Industry:
Semiconductor

Products:
LabVIEW,

The Challenge:
To build a cost-effective, fast, and accurate system to plot and verify the dynamic characteristic curves of a three-terminal electronic device.

The Solution:
The system built using LabVIEW and NI hardware, was vastly superior in testing speed and cost-effectiveness, compared to the previous test system which used a traditional curve tracer.

Abstract
A very versatile Virtual Instrument was developed for the production test of three-terminal electronic components by measuring its V-I characteristics and analyzing the measured curves against pre-stored master curves. This Virtual Instrument replaced an earlier system that used a Tektronix curve tracer, external relays and a PC interfaced via GPIB to download and analyze the V-I curves. The Virtual Instrument reduced test time from 30 seconds to 12 seconds and slashed the system cost to less than a fourth of the original. Further, the new system is more compact, versatile, and easier to setup and use.

Introduction
A large tier-I supplier to the automotive industry, was increasing the production of an advanced idle speed controller, and wanted a new test system to handle the increased capacity. The crucial component in this controller is a three-terminal electronic device, which has to pass very stringent tolerance tests. The customer was using a Tektronix 370A curve tracer with GPIB control and PC based analysis for the test. We were called to investigate the possibility of replacing the Tektronix curve tracer with a PC based system to reduce the cost of the second test station. When we determined that it is possible to meet the requirements with a Virtual Instrument and that the cost will work out to less than a fourth of the Tektronix curve tracer based system, the customer was very excited.

System Description
The test involved the measurement of the dynamic V-I characteristics of the three-terminal device across pairs of terminals and comparison with master curves at specific checkpoints. The dynamic V-I characteristics are obtained by applying a voltage (full-bridge rectified 50Hz sine wave) across a pair of terminals and measuring the current. The voltage applied for some tests was as high as 50 volts and the current ranged upto 1 mA. The measurement resolution needed was 0.1 mV and 1 mA.

Choosing any two terminals out of the three gives three combinations (tests) and reversing the polarity of the voltage leads to three more tests. The applied voltage range went upto 2 volts for the forward characteristics and upto 50 volts for the reverse characteristics. We classified these as low-voltage and high-voltage tests.
We chose a 16-bit E-series DAQ card from National Instruments for the application. This card has two 16-bit analog outputs with a maximum voltage of 10 volts. For the low-voltage tests, the analog output could be directly given to the device since the current drawn is less than 1 mA. For the high-voltage tests we custom-developed a high-voltage amplifier (0 to 10 volts ® 0 to 100 volts) with built-in power supply. The second analog output of the DAQ card was fed to the input of the high-voltage amplifier.

For the measurement of the voltage and the current, the SCXI-1120 Isolation Amplifier with the SCXI-1327 High-Voltage Terminal Block proved ideal. This combination provided both the high voltage attenuation and the low voltage amplification required on a per channel basis. The current was measured by using a high-precision shunt resistor of 249W from the National Instruments’ SCXI process current resistor kit.
The other challenge was to come up with a good switching scheme so that the low-voltage and the high-voltage outputs could be switched to the appropriate pairs of terminals for the various tests. For this we used an SCXI-1127 configured as an 8x4 matrix using the SCXI-1332 terminal block. The use of the 8x4 matrix greatly simplified the switching arrangement and provided a very compact solution.

To measure the V-I characteristics, the voltage is tapped right at the terminals of the DUT and fed to the input of the 1120 isolation amplifier. This basic step ensures that the voltage drops across the switches and other components in the path are not included in the terminal voltage readings. To guard against the rare possibility of an internal short in the DUT, a current limiting resistor was included in the high-voltage path. In the low voltage path, the shunt resistor itself acts as a current limiting resistor.

The voltage to be applied to the DUT, is generated in software and sent to the analog output buffer to be played out at 50kS/s, while the terminal voltage and the current are read at 50kS/s. To determine the pass/fail condition, the software checks if the current at specific voltage checkpoints is within tolerance. The center values for the checkpoints are obtained from the master curve acquired during the setup from a reference device and the operator enters the tolerance values. During the test, the user interface screen shows the curves obtained and a box indicating the tolerance range. The curve should pass within the box for the test to pass.

System Performance
The Virtual Instrument developed for this automated test exceeded all expectations of the customer by beating the Tektronix curve tracer based system handily in speed and cost while meeting the accuracy requirements of the test. Table 1 shows the comparison of the two systems.

Conclusion
In this project we were able to conclusively demonstrate the advantages of using a custom developed Virtual Instrument over the use of an off-the-shelf traditional instrument with GPIB control. By using LabVIEW for the development we were able to complete the system development in just 6 weeks. We believe that this was possible only because we were able to get practically all the required components for the system from a single vendor (National Instruments) which worked seamlessly together making our integration task extremely quick and easy.

The Virtual Instrument developed can be used as a cost-effective replacement in the place of traditional curve-tracers in many situations. In this particular application, the new system cut testing time to one half and the capital cost to a fourth of the original system which was built using traditional instruments. The Virtual Instrumentation based system was also easier to use. The customer was delighted with the result and it made them wish that they had gone with a Virtual Instrument in the first place!

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