Avago Technologies ACPL-224-500E - Manual (Page 49)

AV02-4387EN 46 Avago Technologies
IPM Dead Time and Propagation Delay Specifications
Figure 3. Propagation Delay and Dead Time
analyzed in the same way) it is im-
portant to know the minimum and
maximum turn-on (t
PHL
) and turn-
off (t
PLH
) propagation delay specifi-
cations, 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 LED
1
turn-off and LED
2
turn-on, which is
related to the worst case optocou-
pler propagation delay waveforms,
as shown in Figure 3. A minimum
dead time of zero is achieved in
Figure 3 when the signal to turn on
LED
2
is delayed by (t
PLH max
- t
PHL min
)
from the LED
1
turn off. Note that the
propagation delays used to calcu-
late PDD are taken at equal temper-
atures since the optocouplers under
Motor Control Applications
Many of Avago’s gate drive and IPM
interface optocouplers include a
Propagation Delay Difference (PDD)
specification intended to help de-
signers minimize “dead time” in their
power inverter designs. Dead time is
the time periods during which both
the high and low side power transis-
tors (Q
1
and Q
2
) of a power module
are off. Any overlap in Q
1
and Q
2
con-
duction will result in large currents
flowing through the power devices
between the high and low voltage
motor rails.
To minimize dead time the design-
er must consider the propagation
delay characteristics of the optocou-
pler as well as the characteristics of
the IGBT gate drive circuit. Consider-
ing only the delay characteristics of
the optocoupler (the characteristics
of the IGBT gate drive circuit can be
consideration are typically mounted
in close proximity to each other.
(Specifically, t
PLH max
and t
PHLmin
in
the Figure 3 equations are not the
same as the t
PLH max
and t
PHL min
,
over the full operating temperature
range, specified in the data sheet.).
This delay is the maximum value
for the propagation delay differ-
ence specification that is specified
at 450 ns for the HCPL-4506 over
an operating temperature range of
-40°C to +100°C. Delaying the LED
signal by the maximum propaga-
tion delay difference 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 op-
tocoupler 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 3.
The maximum dead time is also
equivalent to the difference between
the maximum and minimum propa-
gation delay difference specifica-
tions. The maximum dead time
(due to the optocouplers) for the
HCPL-4506 is 600 ns over an operat-
ing temperature range of -40°C to
+100°C.
t PHL
MIN.
I
LED2
I LED1
V OUT1
V OUT2
t PLH
MIN.
t PLH
MAX.
t PHL
MAX.
PDD*
MAX.
MAX.
DEAD TIME
Q1 ON
Q2 OFF
Q2 ON
Q1 OFF
MAXIMUM DEAD TIME (DUE TO OPTOCOUPLER)
= (t
PLH MAX. - t PLH MIN. ) + (t PHL MAX. - t PHL MIN. )
= (t PLH MAX. - t PHL MIN. ) + (t PLH MIN. - t PHL MAX. )
= PDD* MAX. - PDD* MIN.
PROPAGATION DELAY DIFFERENCE (PPD) MAX.
= (t
PLH - t PHL ) MAX. = (t PLH MAX. - t PHL MIN. )
NOTE: THE PROPAGATION DELAYS USED TO CALCULATE THE
PDD AND MAXIMUM DEAD TIME ARE TAKEN AT EQUAL TEMPERATURES.