Datasheet

14
Step 2: Check the ACPL-312T Power Dissipation and Increase Rg if
Necessary.
The ACPL-312T total power dissipation (P
T
) is equal to the
sum of the emitter power (P
E
) and the output power (P
O
):
P
T
= P
E
+ P
O
P
E
= I
F
.V
F
.Duty Cycle
P
O
= P
O(BIAS)
+ P
O (SWITCHING)
= I
CC
.(V
CC
- V
EE
) + E
SW
(R
G
, Q
G
).f
For the circuit in Figure 26 with I
F
(worst case) = 16 mA, Rg
= 8 Ω, Max Duty Cycle = 80%, Qg = 500 nC, f = 20 kHz and
TA max = 85C:
P
E
= 16 mA.1.8 V.0.8 = 23 mW
P
O
= 4.25 mA . 20 V + 5.2 μJ . 20 kHz
= 85 mW + 104 mW
= 189 mW
> 178 mW (P
O(MAX
) @ 85C
= 250 mW-15C*4.8 mW/C)
The value of 4.25 mA for I
CC
in the previous equation was
obtained by derating the I
CC
max of 5 mA (which occurs
at -40°C) to I
CC
max at 125C (see Figure 7). Since P
O
for
this case is greater than P
O(MAX)
, R
g
must be increased to
reduce the ACPL-312T power dissipation.
P
O(SWITCHING MAX)
= P
O(MAX)
- P
O(BIAS)
= 178 mW - 85 mW
= 93 mW
f
AXSWITCHINGMPO
MAXESW
)(
)( =
kHz
mW
20
93
=
= 4.65 µW
For Qg = 500 nC, from Figure 27, a value of ESW = 4.65 μW
gives a Rg = 10.3 Ω.
Esw – ENERGY PER SWITCHING CYCLE – µJ
0
0
Rg – GATE RESISTANCE –
50
6
10
14
20
4
30 40
12
Qg = 100 nC
Qg = 500 nC
Qg = 1000 nC
10
8
2
V
CC
= 19 V
V
EE
= -9 V
Ω
Figure 27. Energy dissipated in the ACPL-312T for each IGBT switching cycle.
Thermal Model
The steady state thermal model for the ACPL-312T is
shown in Figure 28. The thermal resistance values given
in this model can be used to calculate the temperatures
at each node for a given operating condition. As shown
by the model, all heat generated ows through q
CA
which
raises the case temperature TC accordingly. The value of
q
CA
depends on the conditions of the board design and
is, therefore, determined by the designer. The value of q
CA
= 83°C/W was obtained from thermal measurements us-
ing a 2.5 x 2.5 inch PC board, with small traces (no ground
plane), a single ACPL-312T soldered into the center of the
board and still air. The absolute maximum power dissipa-
tion de-rating specications assume a q
CA
value of 83°C/
W. From the thermal mode in Figure 28 the LED and detec-
tor IC junction temperatures can be expressed as:
P
E
Parameter Description
I
F
LED Current
V
F
LED On Voltage
Duty Cycle Maximum LED Duty Cycle
P
0
Parameter Description
I
CC
Supply Current
V
CC
Positive Supply Voltage
V
EE
Energy Dissipated in the
ACPL-312T for each IGBT
Switching Cycle
(See Figure 27)
E
SW
(Rg,Qg)
f Switching Frequency