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Answering a Call to Power

Thu, 12/15/2011 - 2:39am
Paul Livingstone

As the semiconductor industry is moving toward higher-voltage applications, the test industry must balance power with accuracy.

R&D December Test & Measurement Keithley

Keithley Instruments' 2651A SourceMeter source-measure unit tests a wide range of power semiconductor devices, handling current sourcing and measurement from 1 pA to 50 A. Image: Keithley Instruments

The power electronics industry has long relied on instrumentation that would allow researchers to test their innovations quickly and reliably by both supplying power and measuring its performance within the device under test. As the semiconductor industry expanded, it became one of the most significant customers of source-measure units (SMUs).

A source-measure unit is almost like a traditional power supply, but has the added complication of being asked to function as both a power source and a test device. As a result, most SMUs are created to fulfill a specific test need. Some are geared toward high-power applications that support high-voltage or high-wattage electronics. Others are designed specifically to detect very little current, often less than 1 pA, to determine power behavior in semiconductor devices. Because SMUs offer fast read times and high repeatability, they are a useful tool in automated test systems.

Semiconductor technologies have been constantly improving, and these devices, which can include battery assemblies, current converters, or power-line voltage-switching equipment, can increasingly handle levels of power that current SMUs have trouble testing. Keithley Instruments, Cleveland, has addressed this need with the 2651 SMU, the first in a new line of high-power test equipment that is benchmarked at 200 W direct current (DC).

With the 2651, says Mark Cejer, marketing director for Keithley, the innovation now is the ability to handle large current levels—up to 50 A—without losing measurement accuracy. Launched earlier this year, the 2651, and its variant 2651A, is the first model of the company’s fourth generation of high-power electronics test equipment to hit the market.

"We've made some investments in high-power technology. What that means is that levels of current and voltage are roughly 10 times than what we had done previously," says Cejer.

Responding to market changes
The technology packaged in the new 2651 initially was developed in response to the rapidly changing power semiconductor market. Two areas in particular focused Keithley's engineering efforts: energy grid and light-emitting diodes (LEDs).

Thanks to optimization of materials and improved semiconductor processes, high brightness LEDs have finally emerged in the general-purpose and industrial applications areas. High brightness (HB) LEDs are beginning to appear in automobiles, street lamps, and residential settings.

Along the same lines, the desire for efficiency is driving improvements to the energy grid. Increasingly, these improvements rely on electronic products that can handle high-voltage switching. Because these high-power devices are semiconductor-based, they act as switches. There is a potential for electron leakage, and manufacturers will need test equipment that can both handle the high-power levels but also detect low-power losses.

"Customers are turning to better materials to help get silicon to operate more efficiently," says Cejere. Materials such as silicon carbide and gallium nitride, for example, are used to deal with heat more effectively, reducing the size of the heat sink and overall semiconductor package. The result is ever higher power maximums.

The main difference in the 2651 is the level of available DC power: 200 W. Keithley's previous lineup of rack-mounted SMUs handled up to 20 W.

A number of design considerations bring the SMU up to such high current levels without disrupting the accuracy or the target power bandwidth. To supply power, engineers chose field-emission transistors (FETs) with a unique combination of materials and design that would permit minimal heat-sinking requirements. This was one of the most difficult design considerations because the 2651 was designed to fit into a 19-in 2U rack enclosure and the typical heat sinks wouldn't allow this form factor.

"They spent a lot of time on the material that goes between the power FETs and the heat sink itself to facilitate heat transfer," says Cejer.

Heat can greatly complicate SMU design because the devices must both shed heat and keep the environment consistent. Changes in airflow and rapid temperature shifts work against precise measurement.

Besides heat-handling issues, the other major design consideration for this SMU was vibration. Fans help greatly in cooling a power supply or SMU, but for a test device nthe introduction of vibrations or noises can hurt low-power measurement. The team spent time balancing the fan and heat-sink design.

"If you put a fan inside an instrument like this, it will produce noise. When measuring a pico-amp, you’ve got a problem," says Cejer.

In addition to characterizing semiconductor devices, the 2651 retains functionality common to other SMUs, including four-quadrant voltage supply, waveform and pulse generation, current source, electronic load, and digital multimeter modes.

Meeting industry needs
To meet the needs of the semiconductor industry, Keithley engineers made further design adjustments. A pulse mode was developed to allow the 2651 to supply up to 2,000 W at 40 V in a pulse-width modulated interval. The purpose of this scheme is to allow HB LED developers to maximize their test results. These type of LEDs actually cycle on and off, faster than the human eye can detect, to provide consistent brightness and color.

"LEDs get brighter with more current, but they heat up and change color. That ends up being a problem," says Cejer, especially when consistency is desired.

With the 2651, the user can control pulse width and duty cycle, and create the exact waveform needed to test on a production line. High-power handling is crucial to such tests, says Cejer, because many manufacturers produce LEDs in parallel or series, creating high current levels. Complicating matters is the nonlinear nature of such of the more exotic materials used to create these LEDs, such as galllium nitride.

Another addition made to the 2651 is a high-speed 18-bit analog-to-digital (ADD) converter operating at 1 MS/Pt. Typically, a user would have to buy an oscilloscope or data acquisition card separately. Even then, says Cejer, they would only be sampling at 8-bits.

With the 18-bit ADD, the 2651 can measure a 300-µs pulse at up to 50 A without the need for additional test equipment.

By mixing the high-power pulsing capability with this high-speed digitizing measurement, Keithley has attracted some interest from a potentially new type of customer who wants to test high-current magnetic coils. Users can also opt to use the digitizer as a traditional integrating 22-bit ADD.

Because the 2651 is capable of up to 1 million readings per second, Keithley realized that traditional PCI or PCIe backplanes used for communicating with a host system would be inadequate for the data throughput. A script processor was developed to operate on the company's TSP-Link high-speed, low-latency bus. The faster bus speed can be maintained on up to 32 interconnected SMUs by using CAT5e crossover cables to daisy-chain the instruments.

Applications are growing
According to Cejer, about a third of the 2651s in use now are in the power semiconductor industry, a third in the optical (LED and laser diode) industry, and a third in other types of devices and materials. The SMU is being used to test a new generation of laser diode for high-speed fiber-optic communications.

"Most of the applications for [the 2651] are for high-current precision measurement in the semiconductor field. High power, high current to low current, and pulsing are new technologies for us. We haven’t pushed the envelope with this instrument yet," says Cejer.

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