Avago Technologies ACPL-224-500E - Manual (Page 8)

AV02-4387EN 6 Avago Technologies
Figure 8. Full-Bridge Power Switch Configuration.
(A1, B1) is attached to the drain of
a second set of transistors (A2, B2).
When transistor set A turns on, set
B turns off. Current flows from the
positive supply, through transistor
A1, through the load, and through
transistor A2. When set B turns on,
set A turns off, and the polarity of
the current through the inductive
load is reversed.
How does this operation create
a common-mode problem? The
input of each gate drive circuitry
is referenced to the ground of the
digital control circuitry; the output
common, on the other hand, is float-
ing and referenced to the source
of its associated power transistor.
The floating commons of the upper
gate drive circuits rapidly switch
between the positive and negative
power supplies. This rapid switching
creates a large voltage swing across
the input to output of the gate
drive circuitry. As an example, a half
bridge circuit that switches between
As long as the amplitude V
CM
and
value of dV
CM
/dt are less than the
ratings for the optocoupler being
used, V
OH
will remain above 2 V
(maxi mum TTL V
IH
) and V
OL
will
remain below 0.8 V (minimum TTL
V
IL
). Note that the slight perturba-
tions in output voltage occur some-
time after the input pulse which
causes them, due to the non-zero re-
sponse time of the output transistor
to the “perturbation signal.”
Common-mode signals can origi-
nate from several different sources.
A full bridge power inverter, shown
in Figure 8, is a good example of an
application that can exhibit large
amounts of common-mode noise.
Full-bridge inverters are commonly
found in motor-speed control and
switching power supply applica-
tions. The power inverter is generally
used to produce an ac output from
a dc input. In a full-bridge inverter
application like that shown in Figure
8, the source of one set of transistors
+250 V and -250 V in 100 ns creates
a common-mode transient signal
of 5000 V/µs with an amplitude of
500 V (see Figure 9). The device that
carries the control infor mation to
each MOSFET must be able to with-
stand this level of common-mode
interference. Although this example
may seem extreme, it is a fact that
engineers continue to use faster-
switching transistors to increase
motor efficiency. Power MOSFETs,
for example, are commonly used in
power inverter applications because
they are capable of high frequen-
cy, high power switching. The fast
switching speeds of the transistors,
however, can generate common-
mode signals with very high rates of
change (dV
CM
/dt).
The common-mode signal rate
of rise can also be affected by the
reverse recovery characteristics of
diodes D1 and D2 in the power
Common-Mode Transient Rejection
ISOLATION
CONTROL
CIRCUITRY
GATE
DRIVE
GATE
DRIVE
GATE
DRIVE
ISOLATION
1 B 1 A
A2 B2
MOTOR
HV +
GATE
DRIVE
HV -
1
2