Avago Technologies ACPL-224-500E - Manual (Page 9)

AV02-4387EN 7 Avago Technologies
Figure 9. Half-Bridge Example.
Figure 10. Half-Bridge Inverter Waveforms.
High electrical noise levels can also
contribute to common-mode prob-
lems. A significant amount of electri-
cal noise is found in industrial envi-
ronments as a result of the starting
and operating of electric motors.
When a large motor first turns on,
it normally requires a large in-rush
current to reach operating speed.
This large current spike can gener-
ate a significant amount of electrical
noise in its own and nearby systems.
Even the electric motors in a typical
household environ ment vary in size
from fractional to low integral horse-
power units and are often noisy
ac-operated or brushed dc-motors.
Other sources of electrical noise
include microwave ovens, welding
equipment, and automobile igni-
tions.
trate what happens when Q1 turns
back on. As Q1 starts to turn on, the
current through D2 begins to de-
crease. The current through D2 con-
tinues to decrease and actually goes
negative for a short time due to the
storage of minority carrier charge in
its junction. It is when this charge
has been depleted that D2 begins to
turn off and V
CM
begins to increase.
If D2 turns off very quickly, V
CM
can
also rise very quickly, generating
a large common-mode transient
signal.
For the particular case of driving the
gate of an IGBT or power MOSFET
in a power inverter, the HCPL-3120
IGBT/MOSFET gate drive optical iso-
lator is an effective solution for com-
mon-mode problems, providing
protec tion against common-mode
transients with slew rates as fast as
15 kV/µs at V
CM
as high as 1500 V.
inverter shown in Figure 9; these
diodes are often referred to as “free-
wheeling” diodes. If the inverter is
driving an inductive load, such as a
motor winding, these diodes may
become forward biased during the
normal operation of the inverter. For
example, assume that Q1 of Figure
9 is turned on, Q2 is off, and current
is flowing through Q1 and into the
inductive load. When Q1 turns off,
voltage V
CM
swings in the negative
direction until diode D2 becomes
forward biased and conducts the
load current.
It is when Q1 turns back on that very
high rates of rise can be generated.
In extreme cases, when Q1 turns
on again, the rate of rise of voltage
V
CM
is deter mined by how quickly
diode D2 recovers from forward
conduc tion. The voltage and current
waveforms shown in Figure 10 illus-
Common-Mode Transient Rejection
MOTOR
+250 V
Q1
D1
-250 V
+250 V
∆∆∆∆V
∆∆∆∆t
500 V
0.1 µµµµs
5000 V
µµµµs
= =
V = 500 V CM
dV
dt
V
µµµµs
= 5000 CM
-250 V
100 ns
V CM
Q2
D2
V CM V CM
V
CM
I
D2
I
D2
I
Q1