The value of 4.25 mA for ICC in
the previous equation was
obtained by derating the ICC max
of 5 mA (which occurs at -40°C)
to ICC max at 90°C (see Figure 7).
shown in Figure 29. The HCPL-
3150 improves CMR performance
by using a detector IC with an
optically transparent Faraday
shield, which diverts the capaci-
tively coupled current away from
From the thermal mode in Figure
28 the LED and detector IC
junction temperatures can be
expressed as:
•
TJE = P (θLC||(θLD + θDC) + θCA)
E
Since PO for this case is greater
than PO(MAX), Rg must be
increased to reduce the HCPL-
3150 power dissipation.
the sensitive IC circuitry. How
•
θLC θDC
•
+ PD
(
–––––––––––––––– + θCA
)
+ TA ever, this shield does not
θLC + θDC + θLD
eliminate the capacitive coupling
between the LED and optocoup-
ler pins 5-8 as shown in
•
θ
θDC
TJD = PE
(
–––––L–C––––––––– + θCA
)
θLC + θDC + θLD
PO(SWITCHING MAX)
= PO(MAX) - PO(BIAS)
= 154 mW - 85 mW
= 69 mW
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.
+ P (θDC||(θLD + θLC) + θCA) + T
D
A
Inserting the values for θLC and
θDC shown in Figure 28 gives:
PO(SWITCHINGMAX)
ESW(MAX) = –––––––––––––––
f
•
69 mW
TJE = PE (230°C/W + θCA)
= ––––––– = 3.45 µJ
•
+ P (49°C/W + θCA) + TA
D
20 kHz
•
TJD = P (49°C/W + θCA)
E
•
+ P (104°C/W + θCA) + TA
D
For Qg = 500 nC, from Figure
27, a value of ESW = 3.45 µJ
gives a Rg = 41 Ω.
For example, given PE = 45 mW,
PO = 250 mW, TA = 70°C and θCA
= 83°C/W:
Thermal Model
The steady state thermal model
for the HCPL-3150 is shown in
Figure 28. The thermal resistance
values given in this model can be
used to calculate the tempera-
tures at each node for a given
operating condition. As shown by
the model, all heat generated
flows through θCA which raises
the case temperature TC
accordingly. The value of θCA
depends on the conditions of the
board design and is, therefore,
determined by the designer. The
value of θCA = 83°C/W was
obtained from thermal measure-
ments using a 2.5 x 2.5 inch PC
board, with small traces (no
ground plane), a single HCPL-
3150 soldered into the center of
the board and still air. The
absolute maximum power
dissipation derating specifications
assume a θCAvalue of 83°C/W.
•
•
TJE = PE 313°C/W + PD 132°C/W + T
A
Techniques to keep the LED in
the proper state are discussed in
the next two sections.
= 45 mW• 313°C/W + 250 mW
• 132°C/W + 70°C = 117°C
•
•
TJD = PE 132°C/W + PD 187°C/W + T
A
= 45 mW• 132C/W + 250 mW
• 187°C/W + 70°C = 123°C
7
Qg = 100 nC
6
5
Qg = 250 nC
Qg = 500 nC
TJE and TJD should be limited to
125°C based on the board layout
and part placement (θCA) specific
to the application.
V
V
= 19 V
= -9 V
CC
EE
4
3
2
LED Drive Circuit
Considerations for Ultra
1
0
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
0
20
40
60
80
100
Rg – GATE RESISTANCE – Ω
Figure 27. Energy Dissipated in the
HCPL-3150 for Each IGBT Switching
Cycle.
1-208
HP [ HEWLETT-PACKARD ]
