The noise spikes observed occur when the input current commutates
between the MOSFET and the output diode of the SEPIC stage, i.e. at
the edges of the PWM signal. The commutating current causes the
circuit parasitics (lead inductance of MOSFET, diode plus junction
capacitance) to oscillate in the MHz range. The oscillation is damped
by snubber resistors and capacitors across the diode (not shown in
the block diagram of the power supply) but it cannot be completely
removed. The AutoBox/Tandem-AutoBox power supply contains an
L/C Pi‑filter at the input to attenuate the switching spikes. However,
this filter can also not completely eliminate RF noise on the input
voltage lines.
A further reduction in the noise at the input lines is difficult to achieve
due to the high currents involved. Every component in series with the
input has to withstand the 30 A input current. The series resistance
must be low since the voltage drop across the component affects the
minimum input voltage limit. High‑current components usually have
poor RF characteristics, so damping at high frequencies is low.
To electronic circuits (e.g. dSPACE I/O boards) mounted in the
AutoBox/Tandem-AutoBox, the switching noise appears as common
mode noise. It is not recognized.
Measures for reducing
noise
Experiments have shown that there is no visible (measurable)
performance difference between dSPACE I/O boards installed in a
expansion box or installed in the AutoBox/Tandem‑AutoBox.
However, a prerequisite for this result is that the advice given in
Grounding and Shielding on page 196 is followed.
Results of noise reducing
measures
Filter Circuit for AutoBox/Tandem-AutoBox Power Supply
The guidelines below apply to AutoBox through Version
3.0 and Tandem-AutoBox through Version 4.0. They are
not relevant for later versions.
In special cases the AutoBox/Tandem-AutoBox power supply can
cause conducted interferences to other electronics installed in the car.
However, only electronics that are sensitive to conductive
interferences (like measuring amplifiers) are affected.
Objective
s
Notes on Using AutoBox
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DS1103 Hardware Installation and Configuration November 2014
207
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