Chapter 4 - Electromagnetic Compatibility And Medical Devices: Susceptibility to Electrostatic Discharge

Susceptibility to Electrostatic Discharge

Do you know what is the potential difference between you and the doorknob before you
shout “ouch!” on those winter days that electrostatic charges seem to love? 6 kV! But there
are also occasional 15-kV discharges—the kind that make you hope that someone else will
open the door for you. Regardless of the specific number of kilovolts, such discharges
suffice to fry many static-sensitive ICs. However, the primary concern with ESD events is
the large amount of RF energy they convey over an extremely wide band of frequencies.
IEC-61000-4-2 considers ESD between a human being and a medical device as the primary
source of ESD-related failures. Testing is done by delivering 3-kV discharges
directly to a device’s exposed conductive components and 8-kV air discharges to parts that
may be recessed.

The human body model of Figure 4.18 represents the discharge from the fingertip of a
standing person delivered to a device, modeled here by a 150-pF capacitor discharged
through a switching component and a 330-Ω series resistor into the device under test. This
model, which hasn’t changed much since it was developed in the nineteenth century, was
originally used to investigate explosive gas mixtures in mines. An ESD simulator is not
more than an instrument implementing this model. A high-voltage power supply is used to
charge a 150-pF capacitor via a charging resistor. The capacitor’s charge is then delivered
to the device under test by way of a 330-Ω resistor. The switch may be a vacuum relay, a
high-voltage semiconductor switch, or a spark gap.

Construction of the ESD simulator should enable it to generate a discharge waveform
with the parameters shown in Table 4.7. Obviously, the most critical design consideration
is being able to generate the ESD with a rise time of 0.7 to 1 ns. The frequency content of
such an ESD waveform is flat to around 300 MHz before it begins to roll off, so it contains
significant energy at 1 GHz and above. Short rise time is so important because it is the
pulse’s dV/dt as well as the dI/dt that it causes which allow ESD to induce currents and
voltages in a device’s circuits which lead to failures. Suppose, for example, that a PCB
track within the medical device’s circuit has an inductance of 10 nH/cm. If current from an
ESD event is directly or indirectly coupled to that PCB track, the voltage induced along a
length l of that track will be given by

Figure 4.18 The human body model of ESD represents discharge delivered to a device from the fingertip of a standing person. It is modeled by a 150-pF capacitor discharged through a switching component and a 330-series resistor into the device under test.

TABLE 4.7 IEC-801-2 (1991) ESD Waveform Parameters

Since even a modest ESD event can develop a current of 10 A in 1 ns, the voltage induced
across 1 cm of PCB track will be as high as 100 V! Similarly, high currents can flow
through capacitances on and across circuit components. Current flowing through a capacitor
is given by

If an ESD event causes a change in potential of 1 kV within 1 ns, the current flowing
through an unprotected input with 10-pF capacitance would be as high as 10 A.

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Chapter 4 - Electromagnetic Compatibility And Medical Devices: Susceptibility to Electrostatic Discharge

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