Green-Power Push Leads To High-Voltage SMU

March 21, 2012
Next-generation power semiconductors based on SiC and GaN pose measurement challenges. Keithley Instruments' Model 2657A high-power source-measure unit (SMU) tackles them head-on.

Power efficiency is a critical design goal of late, especially in high-power applications like uninterruptible power supplies (UPSs), motor controls, electric vehicles, alternative power-generation schemes, and mass transportation systems. We want power generated, transmitted, and consumed more efficiently. To a great degree, this comes down to power semiconductor devices, which must operate at higher levels of voltage and/or current, power, and frequency.

Thus, there continues to be substantial growth in power devices such as IGBTs, MOSFETs, and diodes, with voltage ratings from 600 V to 5 kV and more. Peak currents through these components can be as much as 200 A. In addition, there is an ongoing shift toward silicon carbide (SiC) and gallium nitride (GaN) power devices due to their greater power density, smaller size, and improved high-temperature performance.

Leakage current in the new generation of SiC/GaN power semis is an order of magnitude lower than silicon-based devices. So is drain-source on-resistance (RDS(ON)). All of the above leads to new measurement challenges concerning power semiconductors. These challenges span the gamut from R&D to production test to quality assurance.

To meet this challenge, Keithley Instruments’ Model 2657A high-power System SourceMeter is optimized for the combination of high voltage, fast response, and precise measurement of voltages and currents (see the figure). Last year, Keithley rolled out its high-current Model 2651A, which handles either 50 A or 100 A. The 2657A complements that earlier source-measure unit (SMU) and is specifically designed for the test and characterization of high-voltage devices and circuits.

Keithley Instruments’ Model 2657A high-power System SourceMeter is optimized for the combination of high voltage, fast response, and precise measurement of voltages and currents.

The instrument sources or sinks up to 3 kV at up to 180 W, enabling it to capture parametric data that competing instruments (some of which are long obsolete, such as curve tracers) cannot. The 2657A delivers its full 180 W in either continuous or pulsed modes. It generates a 3-kV pulse in <15 ms and a 500-V pulse in <2 ms. Not only that, but it’s also capable of four-quadrant operation. It’s one thing to charge a DUT to 3 kV but another to discharge it; it can take a power supply up to 10 minutes for that discharge cycle to complete. The 2657A, by being able to operate in the two “off” quadrants, can discharge a full 3 kW in just 7 ms.

Moreover, it provides 1-fA current resolution, which means it can characterize the extremely low leakage of next-generation SiC/GaN power semiconductors. The 2657A achieves its accuracy by virtue of two sets of analog-to-digital converters (ADCs), which precisely characterize transient and steady-state behavior, including rapidly changing thermal effects. In digitizing measurement mode, the instrument’s 18-bit ADCs support 1-µs sampling, fast enough to avoid averaging of multiple readings to characterize transients. In integrating measurement mode, 22-bit ADCs bring high accuracy and resolution. In each mode, one set of ADCs is devoted to digitizing current and the other to voltage; the sets run simultaneously to maintain high test throughput.

Thanks to Keithley’s TSP-Link architecture, the model 2657A is fully expandable into a multi-channel, tightly synchronized system. TSP-Link technology amounts to a virtual backplane for communication between instruments. Additionally, the company’s TSP Express LXI-based test software utility makes test development with a PC much easier.

An important accessory for the Model 2657A is the Model 8010 high-power test fixture, which supports both lab and bench applications in making the instrument safe for operators, equipment, and DUTs alike. The fixture sports replaceable socket modules for different package types as well as ports for scope and thermal probes. With full safety interlock features, the fixture offers integrated protection circuits to protect the inputs of lower-voltage Series 2600A SMUs from damage in the event of a fault.

Pricing for the Model 2657A high-power SMU starts at $17,900 with shipments beginning in May, 2012. The Model 8010 test fixture costs $6500.

Keithley Instruments

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