13

Figure 27. Energy Dissipated in the HCPL-5150

for Each IGBT Switching Cycle

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 28.

The HCPL- 5150 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

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

10 kV/

component complexity.

Techniques to keep the LED in

the proper state are discussed

in the next two sections.

Figure 28. Optocoupler Input to Output Capaci-

tance Model for Unshielded Optocouplers

Figure 29. Optocoupler Input to Output Capaci-

tance Model for Shielded Optocouplers

Figure 26. Typical Application Circuit with Negative IGBT Gate Drive

+ HVDC

3-PHASE

AC

- HVDC

0.1 µF

V CC = 15 V

1

3

+

2

4

8

6

7

5

Rg

Q1

Q2

V EE = -5 V

+

270

Ω

+5 V

CONTROL

INPUT

74XXX

OPEN

COLLECTOR

_

_

LED Current

LED On Voltage

Duty Cycle

Maximum LED

Duty Cycle

Supply Current

Positive Supply

Voltage

Negative Supply

Voltage

Energy Dissipation in

the HCPL-5150 for

each IGBT Switching

Cycle (See Figure 27)

f

Switching Frequency

0

0

Rg - GATE RESISTANCE -

Ω

100

3

20

7

40

2

60

80

6

Qg = 100 nC

Qg = 250 nC

Qg = 500 nC

5

4

1

1

3

2

4

8

6

7

5

1

3

2

4

8

6

7

5

SHIELD