Human-Body Model and Electrostatic Discharge (ESD) Tests

Foundations

Consider a spherical capacitor consisting of two concentric, conducive spheres separated by a dielectric, as shown in Figure 1.

The capacitance of this structure is, [1]

  (1)

If we let the outer sphere extend to infinity, i.e., , we obtain an isolated sphere of radius a. The capacitance of this isolated sphere then becomes

  (2)

This capacitance is often referred to as an absolute capacitance. In free space

  (3)

Substituting Eq. (3) into Eq. (2) we get

   (4)

or

   (5)

where the radius a is in meters. If we modeled the body as a sphere of radius 1m, its capacitance would equal 111 pF. A human body has a surface area approximately equivalent to an area of a 0.5-m radius sphere. Therefore, the absolute capacitance of the human body is

   (6)

Using this value we can now create the human-body model which serves as the basis for the ESD testing in EMC. We start with the absolute capacitance of 50 pF. In addition to this capacitance we have an additional capacitance between each foot and ground; 50 pF per foot (total of 100 pF). Because of the presence of the adjacent objects, an additional capacitance of 50 to 100 pF may also exist [2]. This is shown in Figure 2.

Thus, the human-body capacitance can vary from about 50pF to about 250 pF. The equivalent circuit of the human body for ESD is shown in Figure 3.

The body capacitance C is first charged up to a voltage V, and then it is discharged through the body resistance R. This body resistance limits the discharge current i. The body resistance can vary from about 500 Ω to 10 KΩ. The body capacitance limits the discharge current rate.

The most common circuit model of human body consists of 150 pF and 330 Ω (Standard EN 61000-4-2). Typical RC combinations are

   (7)

Application

Figure 4 shows an ESD gun together with an RC cartridge. Each cartridge used in testing is of a particular RC combination, specified maximum discharge voltage, and either a positive or negative polarity.

ESD test specifications define several terms related to the testing methods:

• Contact discharge method – a method of testing, in which the electrode of the test generator is held in contact with the EUT, and the discharge actuated by the discharge switch within the generator.

When applying the contact discharge method a contact discharge adaptor is attached to the cartridge, as shown in Figure 5.

• Air discharge method – a method of testing, in which the charged electrode of the test generator is brought close to the EUT, and the discharge actuated by a spark to the EUT.
• Direct application – application of the discharge directly to the EUT.
• Indirect application – application of the discharge to a vertical coupling plane in the vicinity of the EUT.

Such a plane is shown in Figure 6.

The details of the ESD testing requirements according to ISO 10605 and IEC 6100-4-2 are shown in Tables 1 and 2, respectively.

An example ESD test table-top setup in a screen room is shown in Figure 7.

The details of the ISO 10605 – Powered DUT – Direct ESD test setup are shown in Figure 8.

The details of the ISO 10605 – Powered DUT – Indirect ESD test setup are shown in Figure 9.

Packaging and handling ISO 10605 – test setup details are shown in Figure 10.

Finally, the details of the ISO 61000-4-2 test setup for table-top equipment are shown in Figure 11.

Final Remarks About ESD Testing

The testing should be performed by direct and indirect application of discharges to the EUT according to a test plan. This should include:

1. Representative operating conditions of the EUT;
2. Whether the EUT should be tested as table-top or floor-standing;
3. The points at which discharges are to be applied;
4. At each point, whether contact or air discharges are to be applied;
5. The test level to be applied;
6. The number of discharges to be applied at each point for compliance testing.

The test results should be classified on the basis of the operating conditions and the functional specifications of the EUT, as in the following, unless different specifications are given by the product committees or product specifications:

1. Normal performance within the specification limits;
2. Temporary degradation or loss of function or performance which is self-recoverable;
3. Temporary degradation or loss of function or performance which requires operator intervention or system reset;
4. Degradation or loss of function which is not recoverable due to damage to equipment (components) or software, or loss of data.

References

1. Bogdan Adamczyk, Foundations of Electromagnetic Compatibility with Practical Applications, Wiley, 2017.
2. Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.