Locating and measuring sources of EFD and ESD burst interference

Shen Xueqi

September 29, 2007

Shen Xueqi

This "Product How-To" article focuses how to use a certain product in an embedded system and is written by a company representative.

Most electronic products must pass the electrical fast transient (EFT) and electrostatic discharge (ESD) standard tests. EFT and ESD are two types of burst interference. The monopulse peak voltage of an EFT signal can reach 4kV with a rising edge of 5ns. The peak voltage of an ESD signal, on the other hand, can be as high as 8kV under a contact discharge test with a rise time of 1ns. Both types of burst interference have an outburst, high voltage and broad spectrum.

It is thus important to supervise the working mode of equipment simultaneously when coupling the interference pulse from outside to inside the equipment. If the equipment does not pass the standard test, the test itself cannot offer any feasible solutions.

To locate the position in which the equipment under test is susceptive to burst interference, a signal measurement should be made in advance. However, using an oscilloscope can cause changes in the internal interference. Connecting a metal conductor probe to the oscillograph may generate an extra path of interference current, affecting the test result and making it difficult to locate ESD- or EFT-related problems.

EFT/ESD interference
The relevant burst interference should be imposed on the power cord, signal line or casing when conducting an EFT/ESD immunity-to-interference test.

Figure 1: Connecting a metal conductor probe to the oscillograph may generate an extra path of interference current.

The interference current may flow into the EUT's internal circuit via cables or casing and weaken its performance. The interference-proof feature of electronic products depends on the IC's susceptibility and the PCB design. The position in which the circuit is susceptive to EFT/ESD signal can be accurately located. Sensitive spot formations may be caused by the shape of the GND/VCC or the type of IC used. EFT/ESD occurs when the interference current flows to the low-impedance power system.

The interference current can directly enter the GND system and couple out from another position via line connections - it can enter the GND system and couple out in electric field mode (field beam) via electric induction with a metal block (e.g. casing). As shown in Figure 2, below, the interference pulse electric current (I) penetrates the PCB via cables or capacitors.

The interference current generates electric field interference (strength of electric field E) or magnetic field interference (magnetic intensity B). Magnetic pulse field (B) or electric impulse field (E) primarily affects PCB. Sensitive spots are generally caused by either the magnetic field or the electric field only. I flows into the equipment. A part of the electric current (I_A) leaves the EUT due to the existence of a bypass capacitor and the reduction of the internal interference current (Ii).

The magnetic field B generated by the interference current Ii may affect some circuit modules within several centimeters around it. In general circuit modules, only a few signal lines are susceptive to B field. Note that the magnetic field is generated not only by the interference electric current from the power cord and rows of cables, but also by the current path of the bypass capacitor (C) and the electric current of the internal GND and V_CC, which can expand the interference range.

Figure 2: The interference pulse electric current penetrates the PCB via cables or capacitors.

The interference current flowing through the power system generates very intense frequency spectrum EMF, which can interfere with IC or signal lines within several centimeters around it. The presence of susceptive signal lines or devices such as reset signal, chip select signal or crystals around the interference current path may cause the system to become unstable.

Generally, there is only a limited amount of sensitive spots. You can detect them and improve the interference-free performance of products by applying some solutions.

To determine the reason behind an EUT's failure in an EFT/ESD test, we should first locate the current path of the burst interference in the system and find out which susceptive signal lines and devices (sensitive spot) exist around the path. We should then improve the earthed system to change the current path, or shift the position of the susceptive signal lines and devices to solve the EFT/ESD problems at the least possible cost.