Protection at full power

DES PLAINES, IL—Protecting circuits may not be at the forefront of every design engineer’s mind, but it is paramount in the minds of Littelfuse engineers. The 80-year-old company manufactures fuses, gas-plasma arresters (also called gas-plasma tubes or GDTs), varistors, thyristors, electrostatic discharge (ESD) suppressors, positive temperature coefficient (PTC) devices, and diodes that protect sensitive electronic circuits, building wiring, and everything in between. While Littelfuse has some 27 facilities worldwide, product evaluations take place at technical centers in Des Plaines; Irving, TX; and Wuxi, China. At these facilities, engineers subject components to excessive current, lightning, ESD, overvoltage, and extreme temperatures. The electrical stresses range from a few volts and amperes to the thousands of both, depending on a product’s specifications.

Todd Marcucci oversees test labs in Illinois, Texas, and China.

In Irving, engineers who test the company’s semiconductor products perform environmental tests, semiconductor parametric tests, ESD tests, and lightning tests. They also measure high-frequency characteristics such as S-parameters, bit-error rate, capacitance, and impedance, and they perform time-domain reflectometry. In China, engineers perform semi-conductor parametric testing, environmental testing, ESD testing, and failure analysis.

The Des Plaines facility houses a high-power lab, a medium-power lab, and a product-evaluation lab in an area covering 6000 ft². The high-power lab boasts its own electrical generator that can deliver more than 11,500 V and 50,000 A to large fuses and other devices. Because the devices may explode when subjected to that much energy, they reside in a separate room during the tests to keep engineers and technicians safe.

Figure 1.  The high-power lab has a generator and resistor/inductor banks that generate enough power to cause a fuse to explode.

Engineers, including power lab supervisor Roberto Marquez, perform overload tests and short-circuit tests on fuses. After selecting the desired resistors and inductors for a test circuit, Marquez and his team of engineers and technicians will perform an open-circuit test to verify that the device under test (DUT) will receive the correct voltage. Fiber-optic transmitters from LeCroy produce light with an intensity that is proportional to voltage, and they deliver that signal to a Nicolet data-acquisition system in the control room. From there, digitized signals go to computers for waveform analysis and storage.

Roberto Marquez supervises engineers and technicians in the high-power lab.

The engineers perform an overload test by first running current through the fuse under test for up to several minutes. This current is supplied from another source, because using the generator will cause the resistors and inductors in the bank to overheat and become damaged.

For this test, engineers apply 200% of rated current through the fuse, but at a low voltage. “We monitor for voltage across the fuse, and when the voltage starts to increase, indicating that the fuse is starting to open, we apply the fuse’s rated voltage across it,” said Marquez. “A switch disconnects the current from the low-voltage current source and connects the high voltage, high current from the generator to the fuse under test.” The amount of current through the fuse is the same both before and after the switchover, but the voltage greatly increases.

To give me an idea of how much energy is involved, a technician connected a 10-gauge solid wire across the test terminals and applied 32,000 A at 600 V through the wire. The result: The wire vaporized with an explosion followed by a puff of smoke. (Click to watch a video of this event.)

The high-power lab houses a surge generator that emulates lightning surges on AC mains lines and telephone lines. A Hipotronics high-voltage power supply generates 100 kV at 50 mA DC. It also generates pulses of 100 kA at 75 kV. The 100-kA, 75-kV pulse has a rise time of 8 µs and a fall time of 20 µs (from 10% to 90% of peak current). The power supply contains a bank of twenty-eight 1-µF charging capacitors, each the size of a cinder block, that discharge through a switch. A grounded silver ball sits atop each capacitor to prevent arcing among the capacitors at the high voltages. A 1-Ω resistor, 18 in. long, completes the circuit and produces the desired wave shape.

“We calibrate the surge tester by measuring the open-circuit voltage and short-circuit current for each discharge,” said Marcucci. For the short-circuit current measurements, engineers measure current with a current probe from Pearson Electronics that connects to a Tektronix oscilloscope.

In the medium-power lab, engineers test devices such as automotive fuses, GDTs, varistors, PTCs, and electronic fuses. “Medium power” at Littelfuse is high power at most companies. Engineers can test devices with up to 500 A at voltages from 10 V to 480 V. At 60 A, engineers can apply up to 600 V to a device. To test the devices, engineers mount them on printed-circuit boards (PCBs) that connect the devices in series. A power supply then provides power to the devices. Parts that don’t mount on PCBs may be connected with wires.

Dave Shuemaker manages engineers and technicians in the product-evaluation lab, where parts undergo a wide range of functional and environmental tests.

High-power and medium-power tests let engineers evaluate products for their circuit-protection abilities, which relates to product safety. Many Littelfuse customers require this testing to obtain safety certifications from Underwriters Laboratories (UL). UL engineers often come to Littelfuse to witness tests. “We have a very good relationship with UL,” said product-evaluation lab manager Dave Shuemaker. “UL is here at least once a week,” added Marquez.

UL engineers may also witness tests in the product-evaluation lab, where engineers perform numerous electrical and environmental tests. Constant-current DC power supplies (built in-house) provide current ranging from 2 A at 150 V to 1000 A at 20 V. Engineers use these supplies to run overload (opening time) tests, current-carrying capacity (life) tests, temperature-rise tests, and voltage-drop tests. “Typically, half of the products in an evaluation lot receive destructive testing, while the other half receive evaluation testing,” said Shuemaker. “Design engineers come to the lab after they’ve completed initial testing.”

To run a life test on an automotive fuse (Figure 2), a technician will connect 12 or 24 parts in series, then pump 110% of the fuse’s rated current through the device. (The customer specifies if the test should use 12 or 24 fuses.) This test will run for 100 hr, after which engineers will evaluate the fuses for resistance. On some electronic fuses, engineers will run long-term reliability tests at 75% of rated current for one year.

Figure 2.  Automotive fuses, such as these in blade packages, undergo life tests that last 100 hr.

Environmental chambers from Thermotron, Blue M, Webber, and Espec let engineers run elevated ambient-life tests at temperatures ranging from –73°C to 343°C and let them perform humidity, moisture-resistance, accelerated aging, and whisker tests (for lead-free components) on devices from 10% RH to 98% RH at temperatures up to 180°C. The engineers also use the chambers to perform thermal-shock tests (per MIL-STD-202) from –70°C to 200°C.

“A typical thermal-shock test is –65°C to 125°C,” said Shuemaker. “We typically run that test for 15- or 30-minute cycles, while changing the devices temperature in just a few seconds. Dampers in the chamber alternately blow hot and cold air on the devices.”

Littelfuse also has thermal chambers that move devices among room-temperature, hot, and cold zones. For semiconductor devices, the company’s engineers may simply alternate parts between baths of ice water and boiling water.

Shuemaker mentioned that a military customer recently requested that devices be monitored for continuity during a thermal-shock test. For this test, the chamber that uses dampers to blow in hot or cold air works best because the parts don’t move. “If continuity breaks,” he said, “we stop the test. It’s purpose is to prove that our parts don’t have cold solder joints.” An Agilent data-acquisition system measures resistance of the DUTs while they are in the chamber.

A technician connects all of the 144 fuses in a series using fixtures that hold up to 10 parts. A digital multimeter (DMM) measures the continuity of each fixture. “At first, the customer wanted us to monitor every part,” said Shuemaker, “but that takes a technician a long time to connect wire to the parts.”

After Littelfuse gained the confidence of the customer, engineers changed to monitoring each fixture as a whole rather than wiring every part. Shuemaker noted that after six months of testing the parts, only one loss of continuity was detected, and it was because of a wire break in the test fixture. No parts have failed.

Because so many electronic components are now made with lead-free solder, Littelfuse engineers must evaluate the effects of lead-free solder reflow techniques on their components. Reflow ovens in Des Plaines subject PCBs containing Littelfuse components to air temperatures up to 310°C (board temperature to 265°C).

Littelfuse components must also withstand shock and vibration tests. The company’s engineers subject components to half-sine and sawtooth vibration waveforms and 100-Hz sinusoidal vibration per MIL-STD-202. Shock tests can inflict up to 1500 g of force on a part.

Figure 3.  An ESD simulator tests ESD suppressors in an automated tester that moves the parts into place. View the tester in operation.

Littelfuse also manufactures ESD suppressors in surface-mount packages with sizes as small as 0.04x0.02 in. (called “0402” packages). The product-evaluation lab contains a room where engineers such as Pete Pytlik evaluate ESD suppressors to see how well they withstand repeated pulses.

Pytlik developed an automated ESD tester that consists of a Schaffner ESD simulator. Using air discharges, the automated tester tests devices mounted on PCBs, up to 10 parts per board. The system can hold as many as 24 loaded boards. A motorized handler aligns each device to the tip of the ESD simulator where pneumatics move the ESD simulator tip to the device (Figure 3). (Click to view a video of the ESD tester in operation.)

Figure 4.  A shielded RF enclosure prevents electromagnetic (EM) fields generated by the ESD simulator from affecting test equipment.

The ESD test system also consists of a 4-GHz Agilent oscilloscope and a QuadTech megohmmeter/insulation-resistance tester. The oscilloscope monitors the ESD waveform while the megohmmeter measures the DUT’s resistance after the ESD simulator subjects the DUT to pulses. Resistance data on each part is stored in a SQL database. RF switches connect the 100X oscilloscope probes to the DUT.

Both the oscilloscope and megohmmeter reside in a Lindgren RF enclosure that acts as a Faraday cage (Figure 4), shielding test equipment from the broadband electromagnetic waves produced by the ESD pulses. “I don’t understand how anyone can do ESD testing without a Faraday cage,” commented Pytlik. “The pulses create EM fields that can affect the instruments. You can see the signal when you open the chamber door.”

Figure 5.  A custom tester simulates the rotations of a small engine for testing SCRs and thyristors.

The Des Plaines testing labs, and those in Texas and China, let engineers and technicians subject Littelfuse devices to a wide range of electrical and environmental stresses. Because Littelfuse devices protect circuits of all sizes, engineers must evaluate the parts using a wide range of electrical conditions that range from just a few volts and amps to thousands. All that testing produces parts that safeguard your circuits.