19
IPM Dead Time and Propagation Delay Specications
The HCPL-3150/315J includes a Propagation Delay Dif-
ference (PDD) specication 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 25)
are o. Any overlap in Q1 and Q2 conduction will result
in large currents owing through the power devices
from the high- to the low-voltage motor rails.
To minimize dead time in a given design, the turn on of
LED2 should be delayed (relative to the turn o of LED1)
so that under worst-case conditions, transistor Q1 has
just turned o when transistor Q2 turns on, as shown in
Figure 34. The amount of delay necessary to achieve this
condi tions is equal to the maximum value of the propa-
gation delay dierence specication, PDD
MAX
, which is
specied to be 350 ns over the operating temperature
range of -40°C to 100°C.
Delaying the LED signal by the maximum propaga-
tion delay dierence 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 is equivalent to the dierence between the
maximum and minimum propa ga tion delay dier-
ence specica tions as shown in Figure 35. The maxi-
mum dead time for the HCPL-3150/315J is 700 ns
(= 350 ns - (-350 ns)) over an operating temperature
range of -40°C to 100°C.
Note that the propagation delays used to calculate PDD
and dead time are taken at equal tempera tures and test
conditions since the optocouplers under consider ation
are typically mounted in close proximity to each other
and are switching identical IGBTs.
1
3
2
4
8
6
7
5
C
LEDP
C
LEDN
1
3
2
4
8
6
7
5
C
LEDP
C
LEDN
SHIELD
C
LEDO1
C
LEDO2
Rg
1
3
V
SAT
2
4
8
6
7
5
+
V
CM
I
LEDP
C
LEDP
C
LEDN
SHIELD
* THE ARROWS INDICATE THE DIRECTION
OF CURRENT FLOW DURING –dV
CM
/dt.
+5 V
+
–
V
CC
= 18 V
• • •
• • •
0.1
µF
+
–
–
Figure 29. Optocoupler Input to Output Capacitance Model for
Unshielded Optocouplers.
Figure 30. Optocoupler Input to Output Capacitance Model for
Shielded Optocouplers.
Figure 31. Equivalent Circuit for Figure 25 During Common Mode Transient.
