Surge Testing of Digital Isolators
Many applications require isolation of hazardous voltages in order to meet
international safety standards. To ensure the safety of equipment and operators, these standards often require that isolating components, such as digital
isolators or optocouplers, survive high voltage surges in excess of 10 kV
(peak). The ability to test the surge capability of an isolator is, therefore, an
essential part of developing safe and reliable components.
The International Electrotechnical Commission (IEC) and Verband der
Elektrotechnik (VDE) are two organizations that publish standards governing
system and component level application of isolation technologies for medical,
industrial, consumer, and automotive systems, among others. To ensure the
safety of people and equipment in the presence of a high voltage surge, these
standards specify different surge ratings depending on the class, or level, of
isolation required for a given application.
discharge the circuit after each pulse has been applied. An oscilloscope
with a 1000:1 high voltage probe is used to monitor the pulses. The gun
is set at the lowest voltage called for in the test plan, and the oscilloscope
is set for single triggering. Ten pulses are applied at that voltage level, and
each pulse is monitored with the oscilloscope. A breach in the isolation
barrier is manifested by a precipitous decrease in the pulse amplitude
(decaying to 50% in much less than 50 μs). If the part survives ten
pulses, then the gun voltage is increased, and ten more pulses are
applied. This continues until the barrier fails or the maximum test voltage
has been reached.
ISOLATION
BARRIER
There are three common classes of isolation: functional, basic, and reinforced.
Functional isolation has few safety requirements because it is used typically
only where separation of ground references is required to allow a circuit to
operate properly. Safety, and, therefore, surge performance, is not a primary
consideration for functional isolation.
Safety is, however, the key consideration for basic and reinforced isolation,
so surge levels are critical in defining the quality of isolation. Basic isolation
provides protection from shock for users of the end equipment, and reinforced
isolation is a single isolation system that provides protection equal to two
redundant single, or basic, isolation systems.
Medical and industrial applications commonly require reinforced insulation to
protect patients or end users from potentially lethal shocks. VDE’s standard
for reinforced isolation for digital isolators, VDE 0884-10, specifies a minimum
surge voltage (VIOSM) rating of 10 kV, in addition to working voltage (VIORM)
and withstand voltage (VISO) requirements.
The surge voltage rating of a digital isolator defines survivability after a repetitive series of short duration high voltage pulses. Figure 1 illustrates the timing
characteristics of a surge waveform according to IEC 61000-4-5.
GND1
VDD1
VDD2
I/O1
I/O2
GND1 GND2
GND2
100k𝛀
1000k𝛀
V
1000pF
Figure 2. Surge test setup.
The ability to pass this test is primarily determined by the insulation thickness
(also known as distance through insulation, or DTI) and the quality of the
insulating material. The applied electric field tends to concentrate at defect
points within the insulator, so lower defect densities generally lead to higher
breakdown ratings. Thicker materials are more resistant to breakdown since
the field strength is inversely proportional to the distance between the
conductors on either side of the insulation.
Figure 1. Surge voltage waveform.
Optocouplers commonly pass 10 kV surge testing because the insulation is
very thick (typically 400 µm), which reduces the impact of insulation quality
on the breakdown characteristics. Simply put, the insulation is so thick that
a high quality material is not required to pass the 10 kV test. Transformerbased isolators use a high quality 20 µm to 32 μm polyimide layer deposited
in a clean room environment. Since this material has a much lower defect
level than the injection molded epoxies used in optocouplers, a much thinner
layer can still meet the 10 kV requirement. Capacitive isolators also use a
high quality insulating layer, in this case silicon dioxide (SiO2) deposited
during wafer fabrication. Silicon dioxide has a high dielectric strength but
typically can’t be deposited in very thick layers without creating mechanical
stress within the film. Thicker SiO2 also reduces the capacitance, which,
in turn, reduces the coupling efficiency across the barrier. For this reason,
capacitive isolators typically will not pass the 10 kV surge test and, therefore, cannot currently be certified by VDE as reinforced insulation.
The test is carried out by placing the device on a test board and shorting
all pins on both sides of the isolation barrier (see Figure 2). A high voltage
pulse generator is connected to one side of the isolation barrier through a
1000 Ω/1000 pF network. The generator return is connected to the other side
of the barrier. A 100 kΩ, 2.5 W resistor is placed across the barrier to
Protection from a 10 kV surge is required in applications requiring reinforced
isolation, where people and equipment are being protected. Surge testing
is a critical step in establishing the safety level of isolating components in
such applications. Analog Devices offers a large portfolio of iCoupler® and
isoPower® products that meet this requirement.
VOLTAGE
VIOSM
90% VIOSM
50% VIOSM
10% VIOSM
1.2𝛍s
50𝛍s
TIME
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