11
IPM Dead Time and Propagation Delay Speci cations
The ACPL-P456/W456 includes a Propagation Delay
Di erence speci cation 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 21)
are o . Any overlap in Q1 and Q2 conduction will result
in large currents owing 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 optocou-
pler as well as the characteristics of the IPM IGBT gate
drive circuit. Considering only the delay characteristics
of the optocoupler (the characteristics 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-o (t
PLH
) propagation delay speci cations,
preferably over the desired operating temperature range.
The limiting case of zero dead time occurs when the input
to Q1 turns o at the same time that the input to Q2 turns
on. This case determines the minimum delay between
LED1 turn-o and LED2 turn-on, which is related to the
worst case optocoupler propagation delay waveforms,
as shown in Figure 22. A minimum dead time of zero is
achieved in Figure 22 when the signal to turn on LED2
is delayed by (t
PLH
max - t
PHL
min) from the LED1 turn
Figure 21. Typical Application Circuit.
o . Note that the propagation delays used to calculate
PDD are taken at equal temperatures since the optocou-
plers under consideration are typically mounted in close
proximity to each other. (Speci cally, previous equation
are not the same as the t
PLH
max and t
PHL
min, over the
full operating temperature range, speci ed in the data
sheet.) This delay is the maximum value for the propaga-
tion delay di erence speci cation which is speci ed at
450 ns for the ACPL-P456/W456 over an operating tem-
perature range of -40°C to 100°C.
Delaying the LED signal by the maximum propagation
delay di erence ensures that the minimum 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 23. The maximum
dead time is also equivalent to the di erence between
the maximum and minimum propagation delay di er-
ence speci cations. The maximum dead time (due to the
optocouplers) for the ACPL-P456/W456 are 600 ns (= 450
ns - (-150 ns)) over an operating temperature range of -
40°C to 100°C.
M
IPM
+HV
-HV
310 Ω
+5 V
CMOS
0.1 μF
V
CC1
20 kΩ
V
OUT1
61
52
43
SHIELD
I
LED1
310 Ω
+5 V
CMOS
0.1 μF
V
CC2
20 kΩ
V
OUT2
61
52
43
SHIELD
I
LED2
ACPL-P/W456
ACPL-P/W456
ACPL-P/W456
ACPL-P/W456
ACPL-P/W456
MM
-
+5 V
1
2
3
I
LED1
+5 V
3
