What is EFT Testing?
EFT or Burst testing is a conducted immunity test where an EUT is subjected to a series of fast rise time and duration pulses (5ns by 50ns respectively) to ensure compliance and meet product reliability requirements. This EMI event is typically coupled onto power lines through the use of a CDN, but testing can be required on communication, data, and signal lines as well.
What are Electrical Fast Transients (EFT)?
The image, a switch closing and arcing across, provides an easy to understand example of how this type of electromagnetic interference (EMI) can occur. It is most commonly tested in accordance with EN/IEC 61000-4-4 (Buy/Preview Standard) and often used to duplicate field failures. This standard falls under the basic EMC publication HF conducted disturbances.
The arc(s) associated with switching, is often several arcs and have the potential to vary based upon the distance and breakdown voltage of the air gap.(3)
This article is a basic overview of this occurrence and is by no means a complete explanation. Its purpose is to provide the ground work for the concepts and associated testing. For additional information, please view the resources section and associated standards.
Where It Occurs
This type of interference has the potential to occur in a large amount of electrical systems and devices. The interconnection of relays to power mains offers many situations for potential arcing and electrical transients. Commonly they are tested to and occur in:
- AC Power Mains
- Automotive DC Electrical Systems
- Ethernet & Data Lines
- Electrical Systems & Subsystems(2)
Each electromagnetic (EM) environment has an impact on how this commonly occurring transient materializes and is defined. Given the lower voltage automotive DC systems, EFT in this environment they're often considered "microbursts". As an electronics system associated with the occurrence changes, so does how they're defined and tested.
IEC 610000-4-4 Requirements
This common conducted immunity standard is used for testing electrical and electronics system's ability to function given the interference (often called stress) or a burst event. Immunity is commonly thought of as what interference or noise equipment is able to withstand given the electromagnetic environment in which it functions. These often involve several tests, the most common being surge, EFT, and electrostatic discharge (ESD) events.
The IEC version has been harmonized with EN 61000-4-4 and under the Electromagnetic Compatibility (EMC) Directive(3).The most recent version of this standard is the 2012 version and the International Electrotechnical Commission (IEC) offers a Reline version for comparing changes.
The table below, in Texas Instruments IEC 61000-4-x Tests for TI’s Protection Devices(4) provides associated test levels with corresponding line types. The associated test levels, 1-4 are defined by the standard and should be referenced accordingly.
What is the EFT waveform?
The waveforms associated with these types of transients are generally categorized by pulse time/duration (tp) and rise time (tr). For EFT a 5 nanoseconds rise time by 50 nanoseconds duration 5/50. These relatively quick pulses in nanosecond range and are specified as repeated pulses/bursts for 15 milliseconds duration(4).
The below image from the Teseq NSG 3040 manual provides the shape and associated timing. Keep in mind, the voltage will vary based upon associated test level, upwards of 4kV for level 4.
This standard offers both testing at both 5kHz and 100kHz frequency being applied every 300 milliseconds. The diagram to the right image was taken from the EM Test NX5 user manual and illustrates the different associated frequencies. You can notice both the 5 kHz and 100 kHz shown is to IEC 61000-4-4 requirements. Testing by the standard also specifies a 50Ohm source impedance, built into the test generator. Both frequency options are often provided, however typically the 100Khz option tends to be the most commonly used.
What is the frequency for EFT/Burst?
Comparing common EMI Pulse(s)
As mentioned above, the pulse time/duration, rise time, and test level as well as many other factors are determined by the associated standard. Pulses are often categorized by the rise time/duration, Burst 5/50ns, Surge 1.2/50μs, etc.
The associated voltage (open circuit) rise times based around common immunity tests are provided below.
EFT/Burst Test Equipment
EFT Pulse Generators
Burst or EFT test generators are one of the most common pieces of immunity test equipment and often pared with surge capabilities. The injection method will help determine what equipment is needed, with injection on data lines done with a capacitive coupling clamp (CCL) and power lines through the coupling decoupling network (CDN).
Many test systems have built-in coupling decoupling networks, often capable of handling upwards of 300 volts or 16 amps designed for single phase EUT/DUT.
What are EFT Generators?
High Voltage/Current 3-Phase
The power requirements of the equipment under test determine the need for external CDN’s. If the equipment requires more than 300v and/or draws more than 16a, an external CDN may be required. These external networks designed for EFT are often manual allowing for use with a variety of pulse generators. For combination wave surge testing to IEC 61000-4-5 line synchronization communication is required.
The associated diagram, from the CDN 3083-B100 manual demonstrates how the coupler provides 3-phase testing capabilities. It is crucial when setting up a manual CDN that a solid connection is made and correct gauge wire is used.
Coupling onto data or communications lines for burst and electrical fast transients is accomplished through the use of a capacitive coupling clamp. These pieces of test equipment are generally built in accordance with IEC 61000-4-4 and are usually not generator specific.
Most coupling clamps can be used with a variety of generators given that the proper connection to the generator is provided.
Communication with the generator isn't required for burst testing on data lines. It is possible to use a Teseq clamp with a Haefely EFT System, so long as the right connector is provided. An example of a EFT setup with a CCL (Teseq CDN 3425) can be seen below.
Given the quick risetimes and voltages associated with burst testing, grounding is especially important. The majority of generators and systems offer grounding cables ensuring that proper connection and paths are maintained.
When referencing setup requirements, spacing can play an important consideration given the potential for arcing. All safety precautions shown in the associated standard and with the generator should be followed to avoid potential health issues.
How to use a EFT Generator
While transient generators design can vary by manufacturer, most include EFT to at least 4.4 kV, a burst/EFT out connection, coupling decoupling network (CDN), as well as an interface to modify testing criteria.
The video for EFT/burst testing with the Haefely Axos 5 includes:
- Connections to front and back of unit
- Methods for connecting to EUT/DUT
- Capacitive coupling clamp (CCL) connection
- Modifying testing criteria & levels
- System interface and CDN
Testing through the CDN
The two methods for EFT testing are from the direct out with a capacitive coupling clamp, or through the CDN for power ports. The connections for the EUT supply input are typically located on the back of the generators with the front side connections providing output to EUT through the CDN.
You will notice from the image above, that the connections for the CDN are separated by line instead of the more common NEMA style plugs. This allows for both AC and DC testing and can require plugs be made to allow for connection to common low voltage local mains power.
The EUT supply voltage and current levels can be changed by using a power supply in lieu of local power mains. The voltage and current limitations of coupling decoupling networks determine what the max voltage and current they are able to operate at, limiting which EUTs can be used. While external CDNs can allow for EUTs upwards of 200 Amps, most built-in networks typically have a limit of 16 Amps with varying voltage levels.
How to Verify EFT Pulses
What equipment is required to verify EFT Pulses?
While formal ISO 17025 calibrations are typically done on an annual basis, it is best practice to verify that the transient generators are operational on a more regular basis. The below video provides a brief overview of how to conduct a basic verification of electrical fast transient pulses ensuring the generator is functioning correctly.
The video for EFT pulse verification with the EM Text Compact NX5 includes:
- Modifying test criteria on display for verification
- Verification both through CDN and Burst out
- EFT pulses through 50 Ohm adapter
- Evaluation of pulse width, duration, and max value
- Pulse criteria displayed via oscilloscope screen
Following general methods provided by IEC 61000-4-4 guidelines, it is necessary to use both a 50 Ohms and 1k Ohm resistors, the main components of an EFT verification kit (Teseq CAS 3025 in the video). Our analysis of the pulses from the test generator will act as a quick verification of the generator, ensuring correct pulses are being generated. While there are many factors that are evaluated for a formal calibration, we’re mainly concerned with the rise time, pulse width or duration, as well as peak value.
To simplify this further, the video only verifies at the 2 kV test level, allowing us to compare pulses though both resistors. During the video, outputs for both the direct out and CDN line one will be verified, the direct out with the 50 Ohm adapter and line one with the 1k Ohm. This same methodology can be used to verify a variety of different combinations and test levels.
EFT Test Setup
Table Top Setup Requirements
The generic setup for testing to IEC 61000-4-4 for table top equipment can be seen below from the user manual from the EM Test NX5. The guidelines from the associated test standard (as well as safety precautions) should be followed to ensure compliance and accurate results.
- Ground Reference Plane
- Pulse Generator
- CDN (CCL if applicable)
- Insulating Support
- Connection/Grounding Cabling
Ground Reference Plane - Usually made of aluminum, copper, brass or other metal. Often times the same ground plane can be used for IEC 61000-4-2 if EUT sizing requirements are met. Copper and aluminum work excellent for this requirement, are malleable, and commonly referenced in a variety of conducted immunity setups.
Insulating Support - Most commonly polystyrene (Styrofoam), polypropylene or insulating foam-based products are used. When selecting for compliance to a particular standard, reference relative permittivity εr. A table of the permittivity/dialectic constant of common materials can be found at this Website.
Grounding Strap/Cable - These are especially important given safety with this type of testing. They are often included with the generator, however for specific sizing requirements they can be purchased here as well.