PRODUCTS & SERVICES
Temperature in control
A new generation of durable inverters that are able to measure and control
temperature changes are enabling the expansion of electrified vehicle development
In the wake of the
electrification of the
passenger car, a multitude of other
applications have started to adopt
the notion of zero emissions.
However, a variety in applications
also means a variety in requirements
for the respective motor and
controller systems.
Solutions for automotive or
industrial full-electric driving, P0-P4
mild-hybrid systems or hydraulic
pumps for example have vastly
di erent requirements, which cannot
simply be expressed by an overall
kW value. The inverter-controller
functionality must match the overall
vehicle-control structure, powerratings
and lifetime vehicle
operating conditions.
Temperature is key to understand,
predict, control and balance power
and lifetime – especially the
repetition of temperature changes of
the power-switches, which have the
strongest impact on thermal stress
and thus define product lifetime.
The dependency of being long
lasting in spite of temperature
changes is very strong and highly
nonlinear. Errors in measurement
and estimation of temperatures of
a few degrees will result in very
large di erences between the real
and estimated lifetime. Leading to
either oversized and costly inverters
or overstressed systems not
reaching their planned lifetimes.
Accurate thermal models that can
simulate temperatures in a system
are therefore necessary in the
design-phase to optimize the
integration of the inverter in the
overall vehicle system – balancing
output power against lifetime and
cooling. The same models are also
needed later on in the regular
operation to calculate critical system
temperatures based on the
measurable temperatures and also
load conditions.
For the SKAI3 LV, a thermal
model has been derived that can
predict temperatures of the power
switches and the capacitors,
depending on cooling conditions
and load conditions. Even with rapid
steps in the output current as shown
in the diagram, an accuracy of more
than 2°C after 400 seconds
compared to the measurement is
achieved. This allows the evaluation
of required ambient and cooling
conditions and a prediction of the
achievable lifetime for any kind of
load cycle.
With the same basic model,
in-operation temperature calculation
is possible even for highly dynamic
conditions, such as load steps,
where reference temperatures from
a separate sensor would be too slow
and inaccurate.
A Foster-model approach is used
as mathematical formalism, making
it easy for the user to implement a
176 // July 2019 // www.electrichybridvehicletechnology.com
The SKAI3 low-voltage inverter is
designed for battery voltages ranging
from 48V to 144V DC
precise thermal calculation in the
system control-loop.
The upcoming SKAI3 LV, the third
generation of Mosfet-based
inverters, o ers high power in a very
small volume, while leaving
maximum flexibility for system
integration. Supported by models
for power-loss and temperature
calculation, a wide range of
applications can be addressed.
The SKAI3 LV is a compact
powercore, which needs a
control-board to be completed. With
a power-density of more than
25kVA/l and a total volume of less
than 1.8l, the design already fits into
many applications with its standard
case. For designs with special
requirements regarding space,
cooling or power-connectors, the
design suits as a perfect starting
point for a customer specific design.
The interface between customer
controller and gate-driver has an
easy-to-use structure and requires
only a single 13V supply. Fed back
control signals are already
voltage-scaled, while current and
temperature sensors are routed to
a separate connector to maintain
electrical isolation.
To simplify controller and
software design, a dedicated
housing is available, providing easy
access to the controller-PCB in
design, while already using the
power section in its full functionality
and performance.
The SKAI3 LV product family is
designed for battery voltages with
a wide range of input voltages,
ranging from nominal 48V up to
144V DC covering all typical
batteries in industrial applications
and other vehicle applications.
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