MACS Monthly Newsletter - 2021 - JAN5

Quarto Technical Services, LLC

nite control for how gradually liquid will get heated and permits more precise approximations on how quickly the liquid is
heated and to maintain a specified or commanded heat level.
Since PTC heating elements have a very low resistance when
cold (and can act as an NTC), PWM control of a heating element
would limit the Inrush Current to reduce heating element aging,
mitigate the Cabin Heater HV Fuse from failing, and ensuring
that the HV Bus Voltage is not affected during vehicle operation or HV Battery Pack charging. The following chart illustrates
the Ramping Current of a popular 6kW PTC heater. The control
strategy for this (and most PTC units) will include a " soft start "
feature to reduce Inrush Currents. Once the PTC temperature
has increased and Current has plateaued, control would then be
switched to learn control (i.e., drive transistor is commanded to
100% (no PWM) and continue heating the liquid until the commanded temperature has been attained.
The graph in Figure 11 is an example of a control strategy
that illustrates the limit of Inrush Currents by using PWM.
New power requests will not be transferred instantly to output
power but will be incremented at 260mS per percentage stage.
This means, to get from 0% to 100% PWM, the elapsed time will
be 26 seconds. As stated previously, this control feature is an
embedded function in the F/W, for the purposes of preventing
high Inrush Currents and mitigate abrupt changes on the HV
Bus that could cause DTCs to be logged.
One failure mode of a Cabin Heater is high Inrush Currents
causing abrupt changes in the HV Bus (voltage sag), that could
cause a ΔV DTC to be logged. Therefore, using a DC Current
Clamp (Figure 12) and a scope to monitor the Cabin Heater HV
power wires (i.e., monitor PTC heater currents on ramping by
PWM) would be of benefit during a diagnostic test process. The
same process could be used for determining/confirming why a
Cabin Heater Fuse (typically 30-40 Amps) is failing prematurely
and causing a DTC to log. A state-of-health test could also be
performed on vehicles with HV Cabin Heaters to determine if
there are irregularities on the ramping period for the purposes
of identifying PTC elements that are aging (high or erratic ramping current or extremely low currents) and close to failure.

Figure 10: Picture of a cabin heater power electronics circuit board.

January 2021

Eberspaecher

to its memory (i.e. memory allocations).
There is no operating system to alter the behavior of the microcontroller, although F/W can be updated. The F/W only
provides dedicated (non-alterable) commands for the microcontroller to execute. Microcontroller behavior can only be
changed by uploading a new set of commands. Depending on
inputs from the vehicle operator, that will be delivered via the
LSCAN or LIN Bus commands. For those interested in F/W
programming, the most common code languages are written in
C, C++ or Python and then sent to a compiler to change syntax
to machine language (i.e., 1's and 0's), and finally sent to controller memory locations. There are many other languages that
could be used, but the aforementioned are most common.
Once the LSCAN/LIN bus commands are received, the F/W
will command the MOSFET/IGBT transistor to a specific pulse
width modulation (PWM) % or linear controlled power. The
PWM is the amount of time a microcontroller signal transmitted to the transistor is high (i.e., 5V or 12V). The more PWM ON
time, the more electrical power will be transferred from the HV
Bus to the PTC heating elements, and the liquid will increase
temperature. An example of a linear controlled power system would be to command full power ON until a specific liquid
temperature is attained. Once the desired
temperature has been reached, PWM controlling of a lower PTC kW heating element could be used to maintain or incrementally change the liquid temperature.
Linear power control would be analogous
to commanding a relay ON to power a
device, so technically there is no " power
control " because, maximum power is always being delivered, and only the ON
time determines liquid temperature and
how quickly the temperature is achieved.
From a controls system viewpoint, a linear
power control is simpler, but the control
has less fidelity.
A PWM control strategy permits infiFigure 11: Graph of cabin heater inrush current.

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