Troubleshooting EFT, Part 1

Introduction

Electrically fast transient/Burst (EFT/B) is a test that simulates transients due to arcing switches or motor startup and stop. These transients can couple to mains or longer data cables. For commercial products, the appropriate standard is IEC 61000-4-4. The simulated transient is a train of bursts as shown in Figure 1. Each burst is 15 ms in length and comprised of 5 x 50 ns spikes. Each burst occurs at 300 ms intervals (a 5 kHz rate). The most recent standard suggests an optional 100 kHz burst rate.

The EFT/B generator and EUT are bonded to a ground plane. Test voltages of 500, 1000, and 2000 in both polarities are either directly injected via capacitors to each wire under test or capacitively coupled to mains and data cables (longer than 3 m). Usually, long data cables like Ethernet, RS-232, RS-485, longer USB, etc., are tested. Figure 2 shows a typical test setup.

Benchtop Testing of EFT/B

Because performing a formal EFT/B test requires specialized equipment and test setups, it’s difficult to perform this test on the bench. In addition, the test voltages are potentially dangerous.

For this reason, a number of safer, less complicated alternatives have been developed. Henry Ott suggested the test setup in Figure 3 [1]. By inductively coupling the ground wire of a conventional ESD simulator to the cable under test for a 1 m distance, he empirically determined that the test voltage coupled to the cable was about half that of the ESD simulator. For example, with the ESD simulator set to 2 kV, the coupled voltage would be close to 1 kV. The ESD simulator was set to 10 or 20 pulses per second.

A better method for benchtop testing is using Langer EMV Technik’s “P1 Set” EFT/B injectors [2]. This set comes with three injectors, each having a unique coupling method at the tip (Figure 4). The price is $2,300 through the U.S. distributor, Saelig Electronics [3].

The red injector has an H-field loop that is designed to couple more widely to PC board circuitry. The tip of the yellow one also injects an H-field, but is designed to couple to individual traces. The tip of the blue one has an E-field tip and is also designed to couple into individual traces. Figure 5 illustrates how the fields couple.

The injectors require a single AA battery. A push button on the back initiates pulses. Each injector has a pulse width control (blue knob) that adjusts the rise/fall times from 2 to 8 ns (MIN to MAX). The pulse repetition rate is set to 5 kHz. There is a switch at the back that injects a single pulse per button press or a train of pulses. A switch on the probe tip end controls positive or negative pulses. A typical pulse with minimum width is shown in Figure 6 as measured using a Rhode & Schwarz MXO38 oscilloscope. When switched to a pulse train, each single pulse is 5 kHz apart.

Using the P1 Set Injectors

Because EFT/B transients can couple throughout circuitry on PC boards, I will use each to inject into portions of the circuit. I usually start with the red H-field injector because it injects a larger field. If I see soft or hard failures, I’ll switch over to the higher-resolution yellow (H-field tip) or blue (E-field tip) to locate individual traces that are susceptible (Figure 7).

A recommended set of steps would be:

1. Using the red injector, set the pulse intensity to Max with a pulse sequence of 5 kHz.
2. Sweep the circuitry about 5 cm from the surface, pointing the injector downwards for maximum coupling.
3. Switch the polarity over and repeat the sweep.
4. Gradually bring the injector closer to the board and repeat.
5. If no functional failures occur, then place the tip right on the board and sweep.
6. Once susceptibilities are located, use the other two injectors to determine sensitivities to either H- or E-fields by coupling to circuit traces or IC pins.

Typical susceptibilities at the board level could include poor layout, gaps in the return plane, insufficient return plane, or a lack of needed filtering.

Areas with large return planes are usually less susceptible. Another way to look at this is that a lack of an adjacent return plane allows better coupling of the pulses into circuitry.

Single-ended traces with low-impedance drivers (standard IC outputs) are more sensitive to H-fields, and high impedance circuits, such as quartz crystals, analog circuitry, or pull-up circuits, are more susceptible to E-fields.

Single-shot pulses are used to determine edge-sensitive signal traces or IC pins. One pulse is normally enough to trigger a fault.

Both the yellow (H-field) and blue (E-field) injectors are relatively high-resolution and can be coupled to individual traces or IC pins.

Summary

With today’s smaller, more crowded boards and more portable devices, it seems EFT/B failures have started to increase dramatically. While my clients have not had as many issues with EFT/B, a recent poll I made on LinkedIn with a couple of hundred designers contributing showed EFT/B failures were 10 to 12% of all EMC test failures, with ESD and radiated immunity both about the same percentage.

This was a surprise to me, so I decided to add additional bench test methods that may be performed in-house by product designers. Characterizing and resolving design issues prior to formal compliance testing saves both time and cost.

Next time, I’ll show a more robust test method for controlling EFT/B pulses into specific areas of a circuit board or whole system.

References

1. Ott, Electromagnetic Compatibility Engineering, 2nd edition (2009)
2. Langer EMV Technik
3. Saelig Electronics