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Advanced Electronic Power Simulation
Figure 2. Examples of power simulation test scenarios
Vb
Vdc
t6
td
T (s)
13.5V Rampen
10.0V
von bis Dauer
+10V ... +8V 10 min
8.0V
6.0V
4.0V
+4V ... +2V
+8V ... +6V
+6V ... +4V
10 min
10 min
10 min
2.0V
Batteriespannung Ub nach jeder
Rampen ohne Unterbrechung
zurück zu +13.5V.
10min
+7.5V
U=13.5V
1V
1V
Frequenz 4Hz +/-1Hz
12V
1s
Vb
10t 3t 2t t
Vb
Vp [V]
VI [V]
V2 [V]
[Hz]
V3 [V]
0
t1 [s]
t2 [s]
t3 [s]
0
t
[rns]
t1
[s]
10s
0
t
[rns]
10t 3t 2t t
t1 [s]
9V/18V
250ms
5s
Vb
0
V2[V]
t0 [s] t1 [s] t2 [s] Int [s]
0
Vh [V]
750ms
VI [V]
th
tI [s]
[s]
5/10V
t1 [s]
t2
[s]
2s Vb
t1 [s] t2 [s]
VI [V]
VI [V]
V1[V]
v
0
t3 [s]
Int [s]
VB1
VB3
f1 f2 fn
0
t0 [s] t1 [s] t2 [s] Int [s]
VB2
t1 t2 t1 t2 t1
t
The need for fast analog control
Over the years many test scenarios have been created.
For every electrical failure a new simulation had to be
developed. Figure 2 shows some of the many examples of
test scenarios used by the different car manufactures.
A very practical method to reproduce these typical power
simulation waveforms is through an arbitrary waveform
generator connected to the analog input for the DC
output voltage control. Most switch-mode power supply
provides an analog control input. This is normally a 25-
pin connector allowing for many control functions, like:
set output voltage, set current limit, voltage read-back
monitoring and current read-back monitoring. Typically
a zero (0) to five (5) or ten (10) volts signal will drive
the DC output voltage from zero (0) to maximum rated
voltage. The benefits for the use of a general arbitrary
waveform generator is its extreme flexibility, combined
with its memory depth and sequencing capability it can
potentially simulate any power transient behavior for
now or the future. Again I classify this as a very practical
solution, because both the arbitrary waveform generator
and switch-mode power supply are very common and
relatively inexpensive pieces of equipment. Only two
attributes are really important to watch for: The analog
control input of the switch-mode DC power supply should
have a bandwidth of at least 1kHz and the arbitrary
waveform generator should be able to output at least
10Vpp in an open circuit.
Some switch-mode power supplies have an arbitrary
waveform generator built in. Of course this solution is
more compactly integrated. But mostly these built-in
arbitrary waveform sequence generators lack the flexibility
an external dedicated arbitrary waveform generator
can offer. In most cases the capability of the built-in
arbitrary waveform sequence generators are adequate
for production purposes, but for R&D more flexibility is
needed. In case of such a built-in arbitrary sequence
generator, a minimum requirement will be a voltage,
current or power ramp of 1ms or less and a sequence
programming resolution of 1ms or less.
The next power supply attribute to discuss is transient
response. Very high current demands are activated on and
off in a car. Figure 3 shows the same ISO 7637 standard,
but now with the typical corresponding current demands.
These large current demand changes cause
transients in the DC output voltage.
Figure 4 shows the transient effects on the DC output
voltage of a switch-mode power supply due to large
current changes. Important specifications are the size
of the overshoot and the time it takes to recover back to
the set output voltage. The internal voltage control-loop
regulates this. The faster the voltage control-loop the
higher the overshoot. The slower the voltage control-loop
the longer it takes for the power supply to recover to the
set voltage.
A large portion of automotive electronics testing relates
to breakers, fuses, relays, etc. To perform these tests
properly without damaging the devise under test due to
the test method alone, the voltage overshoot needs to be
kept to a minimum. To achieve this the current demand
step for the power supply needs to be reduced. A smaller
current demand step will cause smaller overshoots. A
simple way to avoid these overshoots, is to put a preload
in parallel with the devise under test (see figure
5). Imagine that 50% of the current travels through
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