18
below the threshold during a
transient. A minimum LED cur-
rent of 10 mA provides adequate
margin over the maximum IFLH
of 5 mA to achieve 15 kV/µs
CMR.
CMR with the LED Off (CMRL)
A high CMR LED drive circuit
must keep the LED off
(VF≤VF(OFF)) during common
mode transients. For example,
during a -dVCM/dt transient in
Figure 31, the current flowing
through CLEDP also flows
through the RSAT and VSAT of
the logic gate. As long as the low
state voltage developed across
the logic gate is less than
VF(OFF), the LED will remain off
and no common mode failure
will occur.
The open collector drive circuit,
shown in Figure 32, cannot keep
the LED off during a +dVCM/dt
transient, since all the current
flowing through CLEDN must be
supplied by the LED, and it is
Figure 27. Energy Dissipated in the HCPL-
3150 for Each IGBT Switching Cycle.
Esw – ENERGY PER SWITCHING CYCLE – µJ
0
0
Rg – GATE RESISTANCE – Ω
100
3
20
7
40
2
60 80
6Qg = 100 nC
Qg = 250 nC
Qg = 500 nC
5
4
1
V
CC
= 19 V
V
EE
= -9 V
LED Drive Circuit Considerations for
Ultra High CMR Performance
Without a detector shield, the
dominant cause of optocoupler
CMR failure is capacitive
coupling from the input side of
the optocoupler, through the
package, to the detector IC as
shown in Figure 29. The HCPL-
3150/315J improves CMR
performance by using a detector
IC with an optically transparent
Faraday shield, which diverts
the capacitively coupled current
away from the sensitive IC
circuitry. How ever, this shield
does not eliminate the capacitive
coupling between the LED and
optocoupler pins 5-8 as shown
in Figure 30. This capacitive
coupling causes perturbations in
the LED current during common
mode transients and becomes
the major source of CMR failures
for a shielded optocoupler. The
main design objective of a high
CMR LED drive circuit becomes
keeping the LED in the proper
state (on or off) during common
mode transients. For example,
the recommended application
circuit (Figure 25), can achieve
15 kV/µs CMR while minimizing
component complexity.
Techniques to keep the LED in
the proper state are discussed in
the next two sections.
not recommended for applica-
tions requiring ultra high CMRL
performance. Figure 33 is an
alternative drive circuit which,
like the recommended
application circuit (Figure 25),
does achieve ultra high CMR
performance by shunting the
LED in the off state.
Under Voltage Lockout Feature
The HCPL-3150/315J contains
an under voltage lockout (UVLO)
feature that is designed to
protect the IGBT under fault
conditions which cause the
HCPL-3150/315J supply voltage
(equivalent to the fully-charged
IGBT gate voltage) to drop below
a level necessary to keep the
IGBT in a low resistance state.
When the HCPL-3150/315J
output is in the high state and
the supply voltage drops below
the HCPL-3150/315J VUVLO-
threshold (9.5 <VUVLO- <12.0),
the optocoupler output will go
into the low state with a typical
delay, UVLO Turn Off Delay, of
0.6 µs. When the HCPL-3150/
315J output is in the low state
and the supply voltage rises
above the HCPL-3150/315J
VUVLO+ threshold
(11.0 < VUVLO+ < 13.5), the
optocoupler will go into the high
state (assuming LED is “ON”)
with a typical delay, UVLO
TURN On Delay, of 0.8 µs.
IPM Dead Time and Propagation
Delay Specifications
The HCPL-3150/315J includes a
Propagation Delay Difference
(PDD) specification intended to
help designers minimize “dead
time” in their power inverter
CMR with the LED On (CMRH)
A high CMR LED drive circuit
must keep the LED on during
common mode transients. This is
achieved by overdriving the LED
current beyond the input
threshold so that it is not pulled