Electronic medical devices, especially those that come into
contact with the human body, are at risk for electrostatic discharge (ESD.)
Inadequate protection may damage the IC or interfere with communications
critical to patient care. This article shows how to choose an ESD device that
will give the best chance of a successful first pass design.
ESD protection is vital for medical electronics. However, most
datasheets for ESD protective devices leave out the most important piece of
information: how much protection the device will actually provide.
Some basic ESD topologies and their typical “turn-on”
characteristics
(Note: The ESD threat/pulse is defined by the IEC61000-4-2
standard shown in Fig. 1.). We will focus on silicon devices because they tend
to give the best ESD clamping, but the points apply to just about any ESD
device.
The two most common silicon protection devices are TVS/Zener
diodes (Fig. 2) and Diode/Rail Clamps (Fig. 3). The main difference between
them is the amount of parasitic capacitance they add.
Each structure is intended to steer both positive and
negative ESD pulses away from the IC being protected. The TVS/Zener will
“turn-on” for positive transients when a voltage VZ (typically 6-8 V) is
reached and provide a resistive shunt to GND. Likewise, the diode array will
steer a positive current through the “upper” diode and into the internal TVS
device when a voltage VF+VZ is reached (typically 6-8 V). For negative ESD
pulses both structures will conduct when -VF (typically 0.6-0.8 V) is exceeded
or the bus being protected falls one diode drop below GND.
The electrical characteristics typically given on ESD device
datasheets:
Electrical Characteristics
The majority of silicon ESD devices are specified with four
main characteristics:
- ESD Level
- Reverse Standoff Voltage/Leakage
- Capacitance
- Breakdown Voltage.
(There is a fifth characteristic discussed later.)
The ESD level indicates the level of ESD the device can
withstand without damage. It gives no guarantee that the IC being protected
will survive. The breakdown voltage (usually between 6V-8 V at 1 mA or 10 mA)
gives a data point to ensure the ESD device remains inactive during normal
circuit operation, but does not indicate the shunt resistance or clamping
voltage to expect under an ESD strike. The other two characteristics relate to
the device’s “parasitics” and not its performance during an ESD transient.
Characteristic Plots
Some of the more common waveforms on an ESD device datasheet
are:
- Capacitance vs. Reverse Bias
- Power Derating Curve
- Insertion Loss (S21)
- ESD Response or Clamp
The first three give no information about the ESD device’s
ability to clamp an ESD pulse. The ESD Response plot appears to be helpful but
usually leaves out the conditions under which it was taken. However, there is a
parameter that can be universally applied to put all ESD devices on equal ground.
Dynamic Resistance
The job of a protection device is to provide the lowest
resistance shunt path to GND under an ESD event. Figure 4 shows the ESD
protection device as a variable resistor with high impedance (low leakage)
during normal circuit operation and low impedance during any Electrical Over
Stress or ESD event.
A good way to evaluate the effective resistance of an ESD
device during an ESD pulse is to use IEC61000-4-5, which defines a current
pulse (Fig. 5) with an 8 ?s risetime
instead of the 1 ns risetime of an ESD pulse. This tends to minimize the
effects of the test setup.
Most ESD vendors include the results of this test on their
datasheets, either in the electrical table or by means of a plot. But designers
tend to overlook these data points since they are either unfamiliar with the
“-4-5” test or believe that an “8/20 µs current pulse” is outside the scope of
their design.
No matter how the data is presented (electrical table or
plot), calculating the dynamic resistance is simple: Find two points that
include a clamping voltage and current level (usually the two lowest current
levels to minimize self-heating effects). The example below uses data from the
datasheet electrical table of a Littelfuse SP3001 series diode/rail clamp
array. In the table, a parameter called “Clamping Voltage” lists input currents
of 1 A and 2 A which (typically) gives a 9.5 V and 10.6 V clamp voltage,
respectively. Therefore:
This is essentially the shunt resistance the ESD device will
provide under an ESD pulse. This calculation makes it possible to effectively
evaluate all ESD devices on equal ground irrespective of datasheet differences.
Important Considerations
It is common to think of an ESD pulse in terms of current.
As shown in Figure 6, a simplified model of an ESD generator, the voltage on
the capacitor (i.e. 8 kV) is discharged through a 330 ? resistor into an ESD protection device. The ESD device is
typically <10 ?, so the “output” of
the ESD generator is more a current pulse than a voltage spike.
As seen in Figure 1, an ESD pulse reaches its highest
voltage in the first nanosecond and then dissipates in less than 100 ns. Table
1 shows the (IEC61000-4-2 specified) relation between the two.
The dynamic resistance can be used only in relative terms or
for comparison purposes. For example, take two TVS/Zener ESD devices with the
same breakdown voltage (VZ) but different dynamic resistances. Device A=1.0 ? and Device B=2.0 ?. Device B will clearly give a higher peak clamping voltage to
an 8 kV ESD strike.
These peak voltages exist for only a few nanoseconds, and
often the designer knows only what DC voltage he/she cannot exceed without
causing irreparable damage to the IC. Still it is apparent that Device A will
give a far better chance of achieving a successful first pass design.
Summary
An ESD protector safeguards the health of medical
electronics designs. Yet criteria such as parasitic capacitance, standoff
voltage, and ESD withstand are not enough to vet all the possible choices for a
suitable protection device. The sometimes overlooked “Clamping Voltage” or “tP
= 8/20 ?s” specification makes a wise
choice possible.