9
common mode failure will occur.
Even if the LED momentarily
turns on, the 100 pF capacitor
from pins 6-5 will keep the
output from dipping below the
threshold. The recommended
LED drive circuit (Figure 13)
provides about 10 V of margin
between the lowest optocoupler
output voltage and a 3 V IPM
threshold during a 10 kV/
μ
s
transient with V
CM
= 1000 V.
Additional margin can be
obtained by adding a diode in
parallel with the resistor, as
shown by the dashed line connec-
tion in Figure 18, to clamp the
voltage across the LED below
V
F(OFF)
.
Since the open collector drive
circuit, shown in Figure 19,
cannot keep the LED off during a
+dV
CM/dt
transient, it is not
desirable for applications
requiring ultra high CMR
H
performance. Figure 20 is the AC
equivalent circuit for Figure 16
during common mode transients.
Essentially all the current flowing
through C
LEDN
during a +dV
CM/dt
transient must be supplied by the
LED. CMR
H
failures can occur at
dv/dt rates where the current
through the LED and C
LEDN
exceeds the input threshold.
Figure 21 is an alternative drive
circuit which does achieve ultra
high CMR performance by
shunting the LED in the off state.
IPM Dead Time and
Propagation Delay
Specifications
These devices include a
Propagation Delay Difference
specification intended to help
designers minimize “dead time” in
their power inverter designs.
Dead time is the time period
during which both the high and
low side power transistors (Q1
and Q2 in Figure 22) are off. Any
overlap in Q1 and Q2 conduction
will result in large currents
flowing through the power
devices between the high and low
voltage motor rails.
To minimize dead time the
designer must consider the
propagation delay characteristics
of the optocoupler as well as the
characteristics of the IPM IGBT
gate drive circuit. Considering
only the delay characteristics of
the optocoupler (the character-
istics of the IPM IGBT gate drive
circuit can be analyzed in the
same way) it is important to
know the minimum and maximum
turn-on (t
PHL
) and turn-off (t
PLH
)
propagation delay specifications,
preferably over the desired
operating temperature range.
The limiting case of zero dead
time occurs when the input to Q1
turns off at the same time that the
input to Q2 turns on. This case
determines the minimum delay
between LED1 turn-off and LED2
turn-on, which is related to the
worst case optocoupler propaga-
tion delay waveforms, as shown
in Figure 23. A minimum dead
time of zero is achieved in Figure
23 when the signal to turn on
LED2 is delayed by (t
PLH max
-
t
PHL min
) from the LED1 turn off.
This delay is the maximum value
for the propagation delay
difference specification which is
specified at 500 ns for the
HCPL-530X over an operating
temperature range of -55
°
C to
+125
°
C.
Delaying the LED signal by the
maximum propagation delay
difference ensures that the mini-
mum dead time is zero, but it
does not tell a designer what the
maximum dead time will be. The
maximum dead time occurs in the
highly unlikely case where one
optocoupler with the fastest t
PLH
and another with the slowest t
PHL
are in the same inverter leg. The
maximum dead time in this case
becomes the sum of the spread in
the t
PLH
and t
PHL
propagation
delays as shown in Figure 24. The
maximum dead time is also
equivalent to the difference
between the maximum and
minimum propagation delay
difference specifications. The
maximum dead time (due to the
optocouplers) for the HCPL-530X
is 670 ns (= 500 ns - (-170 ns))
over an operating temperature
range of -55
°
C to +125
°
C.