IEEE - Aerospace and Electronic Systems - January 2022 - 51

Bensaada et al.
Figure 9.
Maximum power point tracking circuitry.
A precision voltage reference diode has been added to
the MPP tracking. This should remove the requirement for
select on test components that comes with using the lower
precision 5V reference from the PWM IC. Linear array
tracking has also been introduced to improve tracking
across the full array temperature range. Due to the
response of the PT2000 thermistor on the solar arrays
used for array feedback, the heritage tracking results are
non-linear leading to a poorer result in the mid-range temperatures
in order to prevent the MPP tracking from entering
the margin at the extreme temperatures.
The BCR is a DC-DC switch mode converter of the
Buck (Step-down) type. The current design of the BCR is
based around the TL494ID Pulse Width Modulation control
IC (PWM). This IC incorporates a complete PWM, an
oscillator, two error amplifiers, a dead timer, and 5V reference.
The TL494ID also has under voltage lock out which
requires the addition ofa start-up circuit.
The main switching transistor of the circuit is a PChannel
International Rectifier HEXFET, namely the
IRF9540. The choice of a P-channel FET reflects the
need for the BCR to be 'hardened' to the radiation environment
in which it will be operating. N-channel FETs
characteristically have a lower RDS on (drain-source on
resistance), but their threshold voltage gradually
decreases in value, eventually moving through zero
volts and changing polarity, in a radiation environment.
The threshold voltage of a P-channel FET also shifts
with radiation, but the polarity stays the same, thus
making the design easier.
JANUARY 2022
The PWM output from the TL494ID drives the gate of
the FET via a driver circuit. The driver circuitry for the
FET has been designed to operate nominally in radiation
environments of up to 1Mrad. The gate drive circuit
includes a push-pull pair of bipolar transistors, which
decrease the rise and fall times of the switching waveform
and thus increasing the conversion efficiency. A capacitor
is used to level shift the square wave signal between the
source and the gate of the FET and an 18V zener diode
prevents the GS voltage exceeding the rate 20V. This
effectively references the switching waveform on the output
of the push-pull transistors with the source voltage, as
oppose to referencing the waveform to ground.
In a radiation environment, the threshold voltage of
the P-channel FET gradually decreases, becoming more
negative with respect to the source voltage. This means
that with increasing dose rate a higher (more negative)
voltage is needed to turn on the FET. At the BOL, the
threshold voltage of the FET is approximately -3 V. Since
the FET is biased via the bipolar transistors with approximately
12 V, this should provide enough leeway to compensate
for 1Mrad of radiation.
TEMPERATURE COMPENSATION METHOD
Thermistors are used by the BCR to sense, and compensate
accordingly, for changes in the solar panel temperatures.
In both instances, the temperature compensate
network will can be illustrated by Figure 10.
IEEE A&E SYSTEMS MAGAZINE
51
IEEE - Aerospace and Electronic Systems - January 2022

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