IPM Dead Time and Propagation Delay Specifications
The HCPL-4506 series include a Propagation Delay Differ-
ence 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 24) are off. Any
overlap in Q1 and Q2 conduction will result in large cur-
rents 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 characteristics of the IPM IGBT gate
drive circuit can be analyzed in the same way) it is impor-
tant to know the minimum and maximum turn-on (tPHL)
and turn-off (tPLH) propagation delay specifications, pref-
erably over the desired operating temperature range.
The limiting case of zero dead time occurs when the in-
put to Q1 turns off at the same time that the input to
Q2 turns on. This case determines the minimum de-
lay between LED1 turn-off and LED2 turn-on, which
is related to the worst case optocoupler propagation
delay waveforms, as shown in Figure 25. A minimum
dead time of zero is achieved in Figure 25 when the
signal to turn on LED2 is delayed by (tPLH max - tPHL
min) from the LED1 turn off. Note that the propagation
delays used to calculate PDD are taken at equal tem-
peratures since the optocouplers under consideration
are typically mounted in close proximity to each other.
(Specifically, tPLH max and tPHL min in the previous equa-
tion are not the same as the tPLH max and tPHL min, over the
full operating temperature range, specified in the data
sheet.) This delay is the maximum value for the propaga
tion delay difference specification which is specified at
450 ns for the HCPL-4506 series over an operating tem-
perature range of ‑40°C to 100°C.
Delaying the LED signal by the maximum propagation
delay difference ensures that the minimum dead time
is zero, but it does not tell a designer what the maxi-
mum dead time will be. The maximum dead time oc-
curs in the highly unlikely case where one optocoupler
with the fastest tPLH and another with the slowest tPHL
are in the same inverter leg. The maximum dead
time in this case becomes the sum of the spread
in the tPLH and tPHL propagation delays as shown in Figure 26.
The maximum dead time is also equivalent to the differ-
ence between the maximum and minimum propagation
delay difference specifications. The maximum dead time
(due to the optocouplers) for the HCPL-4506 series is
600 ns (= 450 ns - (-150 ns) ) over an operating tempera-
ture range of ‑40°C to 100°C.
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Data subject to change. Copyright © 2005-2008 Avago Technologies Limited. All rights reserved. Obsoletes AV01-0551EN
AV02-1360EN - June 20, 2008