Pursuing efficiency from wafer to system

Power electronic devices are proliferating, finding use in applications ranging from PV and wind farms to high-brightness, efficient lighting. To make sure these devices will perform efficiently and reliably, designers and applications engineers will need ways to accurately characterize and test them all the way from the wafer level through the circuit-board (Figure 1) and subsystem level to final product.

Figure 1. Power measurements on a circuit-board assembly
Courtesy of Yokogawa

Bill Gatheridge, product manager at Yokogawa, summarized the classes of instruments his company makes for power test. Power analyzers, he said, can perform power-input/output efficiency tests during validation and characterization stages to meet Energy Star requirements, for example.

“Digital oscilloscopes are used throughout the design and development process,” he added. “They also are very useful in the circuit debug operations.” Further, he said, “Function generators can be used to generate basic and application-specific arbitrary waveforms for circuit testing during the design and development process. During the debug operation, the function generator can be used to create noise that is applied to the device being designed or tested.”

And finally, he said, source-measure units (SMUs) can be used as accurate DC voltage or current sources during the design and development process, and some can be used as curve tracers to characterize semiconductors.

Device vs. system test

According to Ken Johnson, director of marketing and product architect, and Jeff Krauss, applications engineer, at Teledyne LeCroy, “We would consider ‘power electronics test’ to consist of two different test segments:

power semiconductor device testing—involving a single IGBT or FET, and
power conversion system test—involving multiple IGBTs or FETs in a system that performs a higher level function.”

Power semiconductor device testing, they explained, consists of measuring power loss during turn-on, turn-off, and conduction operating zones to gain insight into the performance of the device; performing safe-operating-area tests to verify that the switching device is operating within its maximum specified voltage and current limits; measuring Rds(on) to determine the conducting resistance of the device; and measuring dv/dt or di/dt to understand the rate of change of the voltage or current through the device.

They added, “Power conversion systems can consist of simple single- or dual-device systems or more complicated half-bridge (four-device, single-phase) or cascade H-bridge (six-device, three-phase) systems.”

Johnson and Krauss cited LED drivers or switch-mode power supplies as examples of simple (single- or dual-device) systems. “These systems typically require device testing and control-loop response analysis to provide insight to understand a control loop’s response to critical events such as a power supply’s soft start or step response to line and load changes. Line-power analysis tools provide insight to power consumption as well as enable quick precompliance testing to EN 61000-3-2.”

They added, “These device, control-loop-response, and line-power testing capabilities are all supported in our PWR software option. The PWR software option can be installed on a wide range of our oscilloscopes, including the eight-channel HDO8000 Series and MDA800 Series. It provides quick and easy setup of voltage and current inputs and makes measurements as simple as the push of a button. Tools are provided to help reduce sources of measurement errors; measurement parameters include details of single cycle or average value over multiple cycles.”

They continued, “More complex power conversion systems (H-bridges or cascaded H-bridges) provide two-way power-flow control of single- or three-phase outputs.” Examples include solar PV inverters and variable-frequency (motor) drives. Typical applications involve measuring single device losses, switching/timing of various devices under controlled conditions, and input/output operational voltage, current, power, and distortion behaviors.

“For these types of applications,” they added, “our HDO8000 oscilloscope and MDA800 Motor Drive Analyzer provide eight analog input channels at 12-bit resolution,” with 16 digital channels optional.

The MDA800 Motor Drive Analyzer offers further analysis capability by providing complete motor-drive system debug and validation in one instrument. “Until launching the MDA,” they pointed out, “it required multiple instruments to accomplish this task (typically a power analyzer instrument and an oscilloscope). Now, one instrument acquires any signal from high-speed embedded control signals to low-speed mechanical signals and the power-system signals in between.”

Device-level characterization

According to Lee Stauffer, senior staff technologist at Keithley Instruments, “Developing and using MOSFETs, IGBTs, diodes, and other high-power devices require comprehensive device-level characterization such as breakdown voltage, on-state current, and capacitance measurements. Keithley’s high-power Parametric Curve Tracer (PCT) configurations support the full spectrum of device types and test parameters, including those fabricated from compound semiconductors like silicon carbide and gallium nitride.”

He explained that PCT measurement channels are configured from Keithley Series 2600B and Series 2650A SourceMeter SMUs and an optional Model PCT-CVU multifrequency capacitance-voltage (C-V) meter. The C-V meter, he added, is designed for C-V measurements on two-, three-, or four-terminal devices.

Figure 2. Model 8020 high-power interface panel
Courtesy of Keithley Instruments

Stauffer added that Keithley has developed a set of precision cables to connect PCT instrumentation to either the Model 8010 high-power device test fixture for package part testing or the Model 8020 high-power interface panel for wafer-level testing (Figure 2). Automatic characterization suite (ACS) Basic Edition software is designed to take advantage of the capabilities of Keithley instrumentation and includes several parametric test libraries for performing common high-power device tests. “This software allows the user almost unlimited flexibility in configuring all of the measurement channels to create tests far beyond what a traditional curve tracer could achieve,” he said. ACS Basic Edition also supplies the tools users need to develop their own test algorithms.

Also for power-device applications, Keithley offers the Series 2290 high-voltage power supplies. These supplies, said Robert Green, senior market development manager, “… provide the voltages necessary to perform breakdown testing and leakage current test on high-voltage devices and materials.” The Model 2290-5 5-kV supply has a 5-mA current capacity. The Model 2290-10 10-kV supply has a 1-mA current capacity. All Series 2290 supplies feature high-voltage interlocks for safety. The optional Model 2290-PM-200 module protects low-voltage instrumentation from voltages greater than 200 V when the device-under-test breaks down. “Thus,” he said, “SMUs can be used to measure low-level leakage currents while being protected from damage during DUT breakdown.”

Power device analyzer

For testing power semiconductor devices, Keysight offers the B1505A Power Device Analyzer/Curve Tracer and B1506A Power Device Analyzer for Circuit Design, according to Alan Wadsworth, market development manager for semiconductor test.

“The B1505A is focused on the needs of power semiconductor device manufacturers,” Wadsworth said. “It is modular and has various options that allow it to test both packaged and on-wafer devices up to 1,500 A and 10 kV.”

He added, “The B1506A is focused on the needs of power-circuit designers who need reliable datasheet information on the power semiconductor devices that they want to use in their circuits. It has a simplified user interface and can test packaged devices up to 1,500 A and 3 kV.”

Figure 3. PA2201A IntegraVision power analyzer
Courtesy of Keysight Technologies

For testing power semiconductors and power converters (for example, a solar inverter, automotive DC/DC converter, or a variable-frequency drive) Keysight offers the IntegraVision power analyzer in single-phase (Figure 3) and three-phase versions, according to Mike Hawes, energy and automotive program manager at Keysight’s Power and Energy Division. “The IntegraVision power analyzers are used in research and design, debug, characterization, validation, and troubleshooting,” he said.

Keysight also offers the N8900 Series 5-kW, 10-kW, and 15-kW autoranging DC power supplies and the N6900 and N7900 Series 1-kW and 2-kW Advanced Power System supplies. All these supplies, Hawes said, can be used across all stages of the development and manufacturing process.

Design and troubleshooting

Tektronix addresses system-level test and compliance as well as power-converter design troubleshooting. For the latter, explained Scott Davidson, product marketing manager for oscilloscopes, several power semiconductor devices might be integrated into larger modules (switches, rectifiers, filters, and transformers, for instance) within a power conversion system. “Power quality needs to be monitored at every step within such systems,” he said, “and oscilloscopes are effective troubleshooting tools for these applications.” He specifically cited the MSO/DPO5000B mixed-signal touchscreen oscilloscopes paired with DPOPWR power analysis software (Figure 4) to enable quick and easy power analysis.

Figure 4. Power-analysis software displaying switching losses
Courtesy of Tektronix

As for compliance, Seshank Malap, applications engineer for power analyzers at Tektronix, explained, “Most power-electronics designs need system-level testing where it’s critical to analyze system efficiency including power in and power out. For most AC/DC converter designs that use switching topologies, input currents can be highly distorted, leading to poor input power quality. It is important to test input power factor, crest factor, and harmonic content to make sure power-factor correction and other control circuits are working adequately. Many times, for attaining CE or Energy Star compliance, designs have to meet requirements of the standards like IEC 61000-3-2 for current harmonics and IEC 62301 for standby power. Regulatory standards also dictate stringent efficiency requirements for various load conditions and require power-analysis equipment with very high accuracy specifications.”

To assist with system test and compliance, Malap said, “The Tektronix PA1000 power analyzer provides research and development engineers with highly accurate (0.04% reading and range accuracy) power measurements on single-phase power-electronics products. A high-resolution, high-speed sampling (1-MS/s) system, along with two precision current shunts, maintains high measurement accuracy even for applications such as standby power with very low-level signals. Special test modes plus PWRVIEW software simplify compliance to key regulatory standards such as Energy Star, 80 PLUS, and various IEC standards.”

Function generators, scopes, and SMUs

Gatheridge at Yokogawa provided details on some of the specific instruments the company makes for power test. The FG400 Series function generator is available as a one- or two-channel instrument, with output channels totally isolated, providing a benefit to the design engineer for use in the development of floating power electronic circuits. He noted that noise signals acquired and saved with a DL850E ScopeCorder or DLM4000 eight-channel digital oscilloscope can be played back using the FG400.

Yokogawa offers its Model 765670 Curve Tracer Software, which is a high-speed, high-accuracy real-time I/V curve tracer that works with the GS Series SMUs. It is particularly well-suited to DC parametric tests of small-signal devices. The connection between the GS SMU and PC is via USB.

As for details on the SMUs themselves, Yokogawa offers the single-channel wide-range Model GS610 (+110 V and +3.2 A). The GS820 is a two-channel unit offering 18-V and 1.2-A or 50-V and 0.6-A ranges. The GS200 is a single-channel unit providing ±32 V @ 200 mA with 5½-digit resolution.

National Instruments also offers SMUs, but in the PXI Express form factor. The NI PXIe-4139 SMU employs the company’s SourceAdapt technology for fast and stable measurements. It provides 60 V or 3 A within a 20-W limit and offers extended range pulsing up to 10 A with 500-W pulse power. A company application note¹ describes two benefits of pulsing. First, you can use a single SMU to test at high instantaneous power. Second, a DUT undergoing pulse testing exhibits minimal self-heating, which otherwise could damage the DUT or alter its characteristics, leading to skewed IV data. The application note provides pulse testing of a high-brightness LED as an example.

Thermal test

Mentor Graphics announced in May its next-generation MicReD Industrial Power Tester 1500A product, which provides electronic-component power-cycling and thermal testing for up to 12 devices simultaneously. (The previous version offered three channels.)

“Advanced thermal management is crucial for the design of power modules and power electronics packages,” said Chang-Sheng Chen, manager, power devices packaging technology department, Advanced Package Technology Division, Electronics and Opto-Electronics Research Laboratories, at the Industrial Technology Research Institute at the time of the product’s release.² “Our partners use thinner and thermally more conductive TIMs, new substrate materials, and new interconnect technologies to decrease thermal resistance and increase the lifetime of their products. Power-cycling tests are required to gather information about the lifetime and possible failure modes of these new devices; however, in order to have reliable data, statistical analysis is important. The new MicReD Power Tester 1500A with 12 channels can increase the throughput of our power tester by four times, allowing us to finish our projects significantly faster.”

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