Tech Briefs Magazine - December 2023 - BET-2

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Power semiconductor devices (e.g., IGBTs, SiC-MOSFETs) are used in various fields which tend to
employ high power and large currents for power conversion and control circuits, and isolators (e.g.,
optocouplers, digital isolators) are used in environments where the voltage difference between two
sides - i.e., primary side and secondary side - needs to be isolated. Because higher voltage differences
or potential differences appear across these components, it is very important to ensure that
these components can maintain a good voltage insulation under normal operating conditions and
have no continuous partial discharge (PD) that can cause the insulation to degrade. In this interview,
Kenny Ni, Product Manager at Chroma ATE, discusses best practices to ensure voltage insulation.
Battery & Electrification Technology: Can you explain in
simple terms what is partial discharge?
Kenny Ni: When there are multiple
insulation materials in series between
the electrodes of the applied voltage
(e.g., epoxy resin + void, or ceramics
+ air gap), partial breakdown of the
insulation (e.g., of the void or air
gap) occurs while at least one insulation
material (e.g., the epoxy resin
or ceramics) still maintains a normal
insulation state. This discharge phenomenon
is called " partial discharge " (PD). For example, when
there are abnormal voids inside the insulation material of the
device under test (DUT), due to the fact that when the volume
of the void is the same as the insulation material, the capacitance
of air is lower than that of the insulating material and
when the distance of the void is the same as the insulation
material, the breakdown voltage of air is lower than that of
the insulation material, therefore, the abnormal void will have
a relatively high proportion of voltage and be prone to partial
discharge. By applying a sufficient test voltage to the device
under test (DUT), the charge quantity of the partial discharge
can be measured by using PD measurement to check whether
the insulation material of the DUT has a potential risk of
abnormal insulation quality. Therefore, a test voltage slightly
higher than the maximum rated working voltage of the component
is applied for the PD test. This ensures the quality
and reliability of long-term operation under normal operating
voltage for the component.
BET: Why do insulation quality defects occur in power
components?
Ni: IGBT and SiC-MOSFET, power semiconductor devices, are
used in various fields such as electronic products, industrial
equipment, aerospace, military equipment, railway systems, renewable
energy applications, smart grids, and electric vehicles.
They are frequently used in high-power conversion, large-current
power conversion and power control circuits, which may
have operating voltages of several kilo-volts. Because they will
be switched ON and OFF very frequently, there is a PWM (PulseWidth
Modulation) high voltage difference across the gate and
collector or between the gate and drain of the module, as well
as between the module and heatsink. When the high voltage
runs across insulation materials that contain voids, air gaps, or
cracks, there is a higher likelihood of partial discharge occurring.
After long-term operation, the insulation material will be
gradually degraded, which will eventually lead to failure of the
insulation material and cause product damage.
BET: Can you please explain this further?
Ni: The operating bias voltage (threshold voltage) between
the Gate and Emitter or between the Gate and Source of each
module may be provided by individual transformers. And these
transformers may have a high voltage difference with high
frequency across their primary and secondary sides. When
the insulation capability between the primary and secondary
sides of a transformer is insufficient, the surges of continuous
abnormal discharge may interfere with the digital control operations
and result in transistor failure. The wires used in the
transformers may have sufficient withstand voltage capabilities
(e.g., 3000V) by themselves, and when the coils/wires on
the primary and secondary sides are very close to each other,
it may appear that the wires can withstand a relatively high
voltage (e.g., 6000V). However, after operating for some time
under the typical voltage (e.g., 1000V), insulation failure may
occur. Because the permittivity of general wire's insulation
layer is much greater than that of air, the voltage division ratio
across the air gaps between the wires is relatively high. When
the voltage across the air gap between the wires reaches
>350V (the discharge inception voltage for the shortest air
distance at 1 atm), partial discharge begins to occur on part of
the surface between the wires. Because the insulation layer of
the wires does not deteriorate or get damaged immediately,
the insulation layer gradually carbonizes after continuous use
over a period of time, ultimately leading to a short circuit on
the primary and secondary sides of the transformer.
BET: Can you explain this with an example?
Ni: Photocouplers and digital isolators are applied in various
environments that require isolation. When isolation voltage
is applied across insulation materials that contain voids
or cracks, the voltage across the voids or cracks may be
sufficiently high to cause partial discharge. After long-term
degradation, the voltage isolation fails due to failure of the
insulation material.
For more information, visit www.chromaate.com.
2
Battery & Electrification Technology, December 2023
http://info.hotims.com/84486-710
Tech Briefs Magazine - December 2023

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