# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 454

442
Fig. 12.26
Measuring theDC-bus voltageusinghigh-impedance separation
The identically constructed high-impedance resistance chains at the inputs of the
operational amplifier reduce the high voltage of the DC-bus to a safe level. These
resistance chains can bemade up of 100k
SMD resistors, for example. The resulting
total resistance R
1
should have a value of at least 1M
. The minimum number of
individual resistors derives not only from the arithmetical relationship but also from the
requirement for clearance and creepage distances, which are needed in order to be
able to realise a sufficiently high-impedance separationpath.
In a symmetrical constructionwith narrow-tolerance resistors, the amplification is set via
the resistance ratioR
1
toR
2
and this applies then for theoutput voltageU
out
:
(
)
+
− ⋅
=
DC
DC
1
2
out
U U
R
R U
Eq. 12.18
The separately implemented and also narrow-tolerance resistor pair R
3
is used tomake
independent adjustments to the commonmode control U
cm
:
(
)
+
+ + ⋅
=
DC
3 2
3 2 1
3 2
cm
U
RR RRR
RR
U
Eq. 12.19
Another example, this time implementing galvanic isolation
After theDC-bus
voltage division to values in the rangeof only a few volts, the signal is fed to a stage that
recognises when a threshold has been exceeded. The threshold limit can be set by the
two resistors R
S1
andR
S2
. Another path forms the route for theDC-bus voltage that has
been divided down to the input of a
Σ
/
-ADC, the role of which is to isolate the signal.
After the signal is amplified and displaced (zero point is set with e.g. R
L1
=R
L2
to 2.5V) it
is fed toamicrocontroller for processing.
Generally, when selecting the resistors for the voltage divider, the losses that can occur
at each resistor must be taken into account so that overload does not occur. Also, care
must be taken with regard to the voltage capability of each resistor when isolating the
path from U
DC+
to U
DC-
, so that it is impossible for a flashover to occur due to over-
voltage.
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