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Impact of standards on test impulses

Posted: 11 May 2012 ?? ?Print Version ?Bookmark and Share

Keywords:International Electrotechnical Commission? IEC 60060-1? Coupling-Decoupling Network?

The popular combination waveform impulse tester is defined in IEC 61000-4-5, and both the voltage and current waveforms are defined. As noted above, the standard defines only a 2- tester, but the standard notes other output impedances as well. The impedance of a surge tester is defined as the ratio of the peak open-circuit voltage to the peak short-circuit current. These values will change when a device is being tested.

Coupling-decoupling networks
When conducting a powered test in accordance with IEC 61000-4-5, use a Coupling-Decoupling Network (CDN) to present a high back impedance to the surge waveform toward the supply. That still lets the impulse flow to the DUT. The standard presents circuits for CDNs used for various tests, and the CDNs have to be designed to provide a waveform which is within tolerances specified in IEC 61000-4-5. Because of these performance requirements, CDNs have to be designed for specific waveforms, voltages and currents, or there is a chance the CDN may not perform to the specifications. Always use a CDN when performing mains-powered tests. IEC 61000-4-5 has various CDN illustrative schematics for use with other ports as well. A CDN isn't required for unpowered tests because there's no power supply to protect.

Depending on which of these two standards your end-use standard references, the tester you select can vary greatly. If the scope of your end-use standard uses IEC 61000-4-5 as the referee document for impulse waveforms, then the pulse will be defined without the device being tested in place. That is, the tester's voltage pulse will be evaluated when delivered into an open circuit, and the current pulse will be evaluated into a direct short. If, however, IEC 60060-1 is the referee document, then the pulse will be evaluated with the device being tested as part of the circuit. This can cause huge differences in the pulse if the device being tested presents anything other than an open circuit to the impulse tester in its tested configuration. Although IEC 60060-1 does allow waveforms to vary when the load is capacitive, these variances are limited. In these cases, the limiting factor may not be impulse tester power, but the lead and internal tester impedance, which may be difficult to decrease.

Waveform verification
If the end-use standard covering the equipment being tested specifies either IEC 61000-4-5 or IEC 60060-1 as the referee document, then you can verify correct operation by using an oscilloscope and comparing the waveform to the standard. Some waveforms in end-use standards, however, are defined by specifying the input voltage and circuit components. These pulses have undefined output peak voltage. While this is a valid method for defining a pulse, it is difficult to verify correct pulse delivery unless the source voltage can be accessed, and component tolerances can be checked.

Some end-use standards use proprietary circuits that include the impulse tester and a CDN. The tester controls component values and input voltages. In these testers, however, you may have difficulty finding the correct output as well. In these cases, comparison of the waveform to a waveform record taken at the last equipment calibration may be the best verification method.

There are many definitions of impulse waveforms, and different referee documents are very different in the way they treat the application of the impulse to the device being tested. Some waveforms are used to evaluate insulation systems, or determine behavior at an expected breakdown voltage, or to evaluate systems when presented with a mains transient.

Impulse waveforms can be defined differently, and the definition scheme may make the waveform difficult to verify that it was delivered correctly. If the source voltage is the defining value, comparison of the waveform to calibration records may be the best way to evaluate the delivered waveform.

Check the scope of your end-use standard to see which standard defines the impulse waveforms used. Be sure to read that standard to make sure your test will be in compliance of al required standards.

About the author
Jeffrey D. Lind has over 33 years of electrical engineering experience. He launched his career working at Underwriters Laboratories (UL) and then for Atari and Sega Gremlin. In 1997, Lind started Compliance West. He received his Bachelors of Science in electronic engineering from Cal Poly San Luis Obispo.

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