+5 V
1
2
3
4
8
V
= 15 V
CC
+ HVDC
270 Ω
0.1 µF
+
_
7
6
5
Rg
Q1
= -5 V
EE
3-PHASE
AC
CONTROL
INPUT
V
_
+
74XXX
OPEN
COLLECTOR
Q2
- HVDC
Figure 26. Typical Application Circuit with Negative IGBT Gate Drive
LED Drive Circuit Considerations for
Ultra High CMR Performance.
P Parameter Description
E
1
2
3
4
8
7
6
5
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 28. The HCPL-
5120 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. However, this shield
does not eliminate the
capacitive coupling between the
LED and optocoupler pins 5-8
as shown in Figure 29. This
capacitive coupling causes
I
LED Current
F
CLEDP
V
LED On Voltage
F
Duty Cycle
Maximum LED
Duty Cycle
CLEDN
P Parameter Description
O
Figure 28. Optocoupler Input to Output Capaci-
tance Model for Unshielded Optocouplers.
I
Supply Current
CC
V
Positive Supply
Voltage
CC
CLEDO1
1
2
3
4
8
7
6
5
V
Negative Supply
Voltage
EE
CLEDP
CLEDO2
E
(Rg, Qg) Energy Dissipation in
the HCPL-5120 for
SW
CLEDN
each IGBT Switching
Cycle (See Figure 27)
perturbations in the LED
SHIELD
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
10 kV/µs CMR while minimizing
component complexity.
f
Switching Frequency
Figure 29. Optocoupler Input to Output Capaci-
tance Model for Shielded Optocouplers.
7
Qg = 100 nC
6
Qg = 250 nC
Qg = 500 nC
5
VCC = 19 V
4
3
2
VEE = -9 V
1
0
Techniques to keep the LED in
the proper state are discussed
in the next two sections.
0
20
40
60
80
100
Rg - GATE RESISTANCE - Ω
Figure 27. Energy Dissipated in the HCPL-5120
for Each IGBT Switching Cycle
13
HP [ HEWLETT-PACKARD ]
