A p p l i c at i o n N o t e
AN3021
Renesas/CEL Optocoupler Thermal Calculation
CEL Staff Application Engineer, Opto Semiconductors
CEL Product Marketing Manager, Opto Semiconductors
Due to its unique construction for providing electrical
isolation one needs to observe both derating curves
provided in the data sheet. The PS2801-1-A is used as the
example in this app note however the same methodology is
used for all other optocouplers including PS23xx, PS25xx,
PS27xx, PS28xx, and PS29xx.
Introduction
Heat is the number one killer of all electronic devices.
The purpose of this Application Note is to “guide”, “teach”,
and “educate” engineers or end users on how:
1. To calculate the maximum case temperature that a
particular optocoupler can withstand: Tcase<<<Tg
(glass transition temperature of the resin with the
typical Tg=~150°C plus) and as a good practice,
the Tcase should be kept to less than or equal to
121°C, then it is acceptable.
Thermal Resistance
The ease or difficulty with which heat propagates is
called thermal resistance. Below is a table of the thermal
resistance from case to ambient, Rth(c-a) (°C/mW) of
Renesas optocouplers.
2. To calculate the Tj of the phototransistor and IRED
and make sure that the Tj<<125°C.
Part Number
Thermal Resistance Rth (c-a) case to ambient (°C/mW)
PS23xx
0.200
In this app note, three examples are given:
PS25xx
0.170
PS27xx
0.312
Example #1 shows how to calculate the maximum Tcase.
PS28xx
0.390
PS29xx
0.442
Example #2 shows how to calculate Tcase and Tj of IRED
and phototransistor based upon the “real” application in
the analog mode.
Let us examine the IRED and phototransistor derating
curves provided in the PS2801 data sheet as shown below
in figure 2.0.
Example #3 shows how to calculate Tcase and Tj of IRED
and phototransistor based upon the “real” application in
the digital mode.
DIODE POWER DISSIPATION vs.
AMBIENT TEMPERATURE
Diode Power Dissipation PD (mW)
100
Background
50
25
0
25
50
75
100
Temperature TA (°C)
Fig 1.0: Typical Construction of anAmbient
Optocoupler
50
TA = +100°C
25
50
75
150
100
0
Am
TRANSISTOR POWER DISSIPATION
FORWARD
CURRENT vs.
vs. AMBIENT
TEMPERATURE
FORWARD VOLTAGE
50
150
TA = +100°C
+60°C
+25°C
10
5
100
PS2801-1
PS2801-4
1
50
0.5
0.1
0
0.7
0.8
1.2 mW/°C 0°C
–25°C
–55°C
75
25
50
0.9 1.0 1.1 1.2 1.3 1.4
Ambient Temperature TA (°C)
Forward Voltage VF (V)
70
PS28
PS28
50
100
COLLEC
COLLEC
70
60
50
40
30
20
10
100
1.5
Fig
2.0: Derating
curves of the
COLLECTOR
CURRENT
vs. PS2801-1-A
COLLECTOR
TOTO
EMITTER
DARK
COLLECTOR
EMITTER
VOLTAGE
CURRENT vs. AMBIENT TEMPERATURE
FORWARD CURRENT vs.
FORWARD VOLTAGE
A)
100
25
200
100
0.6 mW/°C
LED
0.6 mW/°C
200
Ambient Temperature TA (°C)
0.8 mW/°C
PS2801-1
0.8 mW/°C
PS2801-1
50
0
PS2801-4
75
PS2801-4
75
Forward
Current
IF (mA) PC (mW)
Transistor
Power
Dissipation
Diode Power Dissipation PD (mW)
The maximum power dissipation rating for an optocoupler
is defined as the greatest amount of power that the
device can dissipate without exceeding safe operating
conditions. Optocouplers have a unique feature which is
not found in a typical semiconductor device in that there
are two electrically isolated components; an infrared
emitting diode (IRED) on one side of the lead frame and a
phototransistor on the other side. Both components are
DIODE POWER DISSIPATION vs.
encapsulated inside a plastic package
as shown
in the
AMBIENT
TEMPERATURE
figure 1.0 below.
100
Photo Detector
TRANSIS
vs. AMB
Transistor Power Dissipation PC (mW)
Van N. Tran
Larry Sisken
Collector Current IC (mA)
Authors:
0
2
Colle
Page 1
COLLECT
COLLECT
40
AN3021
First extend the derating curves so that the power
dissipation=0.
In this example both the IRED and phototransistor have
zero (0) power dissipation at TA=125°C. The interception of
the derating curves and the horizontal axis is the maximum
allowable junction temperature (TJ (max)) of both components.
RE
As mentioned above the thermal resistances are Rth(ja)=1.667°C/mW for the IRED and Rth (j-a)=0.833°C/mW for
the phototransistor.
By taking the inverse of the slope of the derating curves, the
thermal impedance of the IRED and the phototransistor can
be found. The thermal impedance of the IRED=1.667°C/mW
and that of the phototransistor=0.833°C/mW as shown in
figure 3.0.
DIODE POWER DISSIPATION vs.
AMBIENT TEMPERATURE
Transistor Power Dissipation PC (mW)
Diode Power Dissipation PD (mW)
PS2801-4
75
0.8 mW/°C
PS2801-1
50
25
25
50
75
100
Ambient Temperature TA (°C)
TRANSISTOR POWER DISSIPATION
FORWARD
CURRENT vs.
vs. AMBIENT
TEMPERATURE
FORWARD VOLTAGE
50
150
PS2801-1
PS2801-4
50
0.5
0.8
125°C
PS2801-1
PS2801-4
0
Keep the case temperature,
Tc<117.5°C
(</= 100°C as recommended)
1.2 mW/°C
25
75
50
100
Temperature
Ambient Temperature TA (°C)
Tj = 125°C (max) LED
limit by the
package material
COLLECTOR CURRENT vs.
Fig 4.0
COLLECTOR
TO EMITTER VOLTAGE
because of the
Tg
(glass transition
70
temperature of the
60
molding material
Calculate the case temp. and junction
≈150°C plus)
75
25
50
0.9 1.0 1.1 1.2 1.3 1.4
Ambient Temperature TA (°C)
Forward Voltage VF (V)
100
1.5
125°C
temp. of the IRED
and phototransistor (PD) in the analog mode (or Vce of the
mA Phototransistor is NOT in saturation).
50
A
50
40
m
20 mA
10 Using
PS2801-1-A with the following drive condition in the
collector configuration as shown below in figure 5.0
with the operating temperature, Ta=25°C to 60°C.
20
IFcommon
= 5 mA
10
0
2
40
Collector Current IC (mA)
60
Calculate
the maximum
case temperature based on the
10 000
VCE = 80 V
40
V
derating
10
50 curves.
24 V
10 V
5
A
In
casemwithin the
5 V safe operating areas, the power
50
A
m
dissipation
of the20IRED
30
100
mA and phototransistor is roughly 15mW
10
and 30mW
at
Ta=100°C
respectively. By summing 1these
20
IF = 5 mA
10
0.5
two numbers, the total power dissipation is Pt = 45mW
at
10
ambient temperature Ta=100°C.
6
IF 8
10
VCC
COLLECTOR CURRENT vs.
COLLECTOR SATURATION VOLTAGE
70
1 000
the40worst
4
Collector to Emitter Voltage VCE (V)
Fig 3.0: Typical
Construction
of an Optocoupler
COLLECTOR
CURRENT
vs.
COLLECTOR
TOTO
EMITTER
DARK
COLLECTOR
EMITTER
VOLTAGE
CURRENT
vs. AMBIENT TEMPERATURE
Example
#1:
Collector to Emitter Dark Current ICEO (nA)
Collector Current IC (mA)
100
Photo Detector Tj = 125°C (max)
Phototransistor
dissipation
30
(~30 mW)
1.2 mW/°C 0°C
–25°C
–55°C
1
0.1
0
0.7
3. Below is the summary diagram of the above example.
Example #2:
TA = +100°C
+60°C
+25°C
10
5
100
=100°C+30mW * 0.833°C/mW=125°C (max).
150
Collector Current IC (mA)
Forward
Current
IF (mA) PC (mW)
Transistor
Power
Dissipation
200
100
1. IRED Junction temperature , TJ IRED=100°C+15mW* 1.667°C/mW=125°C (max).
200
IRED power
dissipation
(~15 mW)50
0.6 mW/°C
0
By applying the formula junction temperature, Tj=Ta+Rth (ja) * P we get the following:
TRANSISTOR POWER DISSIPATION
2. Phototransistor junction temperature, Tj phototransistor
vs. AMBIENT TEMPERATURE
100
00
5
allowable case temperature, Tc=Ta+Pt* Rth(c-a)=100°C+45
mw *0.390°C/mW≈117.5°C. However, one should operate
the device lower than 117.5°C (110°C preferably) since the
calculated number is for reference only.
50 mA
20 mA
10 mA
5 mA
2 mA
Ic
VOUT
Rin
RL
Fig 5.0 Common Collector Amplifier
IF = 1 mA
IRED side: IF=5.0mA, VF=1.1V (typ) or the power
consumption, P_IRED=5.5mW.
Phototransistor side: Ic=10mA (CTR=200%), Vcc=3.3V and
1
0
2
4
6
8
10
0.1
Using the
Rth(c-a)=(Tc–Ta)/Pt
with Ta=100°C,
RL=100Ω
and
*100Ω=2.3V and
0.2
0.4
0.6
0.8 Vce=Vcc–Ic*RL=3.3V–10mA
1.0
25
50
0
–50 formula
–25
0
75
100
Collector to Emitter Voltage VCE (V)
Pt=45mW, Ambient
and Temperature
Rth(c-a)=0.390°C/mW,
the maximum
TA (°C)
Collector Saturation
Voltage
VCE (sat) (V)
power
consumption,
P_PD=10mA *2.3V=23mW.
COLLECTOR CURRENT vs.
COLLECTOR SATURATION VOLTAGE
40
50 mA
Page 2
25
0
25
50
AN3021
75
Transistor Po
Diode Power
0.6 mW/°C
50
100
0
Ambient Temperature TA (°C)
At Ta=25°C:
5
1
The issue becomes more complex and involved and the
following steps need to be taken:
Step #1: Examine how the temperature affects the forward
voltage of the IRED.
By looking at the below graph of forward voltage and current
versus ambient temperature (Figure 6.0) one can see that VF
≈1.05V @ IF=5.0mA and the power consumption, P_ IRED =
5.25mW.
40
30
20
10
1.0
0.9
1.1
1.3
1.2
1.4
1.5
0
Co
Forward Voltage VF (V)
CURR
COLLEC
FORW
COLLEC
NORMALIZED
TRANSFER
COLLECTOR
TO CURRENT
EMITTER DARK
RATIO
vs.
AMBIENT
TEMPERATURE
CURRENT vs. AMBIENT TEMPERATURE
Collector to Emitter Dark Current ICEO (nA)
Normalized Current Transfer Ratio CTR
At Ta=60°C:
0.8
0.7
1.2
10 000
1.0
VCE = 80 V
40 V
24 V
10 V
5V
0.8
1 000
0.6
100
0.4
Normalized to 1.0
at TA = 25°C,
IF = 5 mA, VCE = 5 V
10
0.2
1
0.0
–50
–50
0
–25
25
50
75
25
50
–25
0
75
Ambient Temperature TA (°C)
Ambient Temperature TA (°C)
40
300 VCE = 5
n=3
250
10
150
1
100
0.5
50
0.1
0.02
0
100
100
100
Step# 4: Calculate
the junction temperature of IRED and1 000
V = 5 V,
50 I = 2 mA,
phototransistor to make sure that they do not exceed Tj
CTR = 236%
t
100
(max)=125°C.
CC
C
f
tr
10
By applying 5the formula for the junction
temperature,
t
Tj=Ta+Rth(j-a) * P. The result is as follows:
Similarly, reviewing the graph of Normalized CTR vs. Ta as
shown below, the CTR≈0.85 * CTR at Ta=25°C.
1. IRED Junction
temperature , TJ IRED=60°C+5.25mW* 0.5
1.667°C/mW≈68.8°C.
In this example, the CTR (at Ta=60°C)=0.85 *200%=170% or
Ic=8.5 mA instead of 10.0mA at Ta=25°C.
2. Phototransistor
junction
Tj5 phototransistor
0.01
0.05 0.1 temperature,
10
0.5 1
Load Resistance R (kΩ)
=60°C+21mW * 0.833°C/mW=77.5°C.
As a result, Vce=3.3V-8.5mA*100Ω=2.45V and power
consumption, P_PD=8.5mA* 2.45V=21mW.
Based on the above calculation, the junction temperatures
FREQUENCY RESPONSE
of both the IRED and PD do not exceed the Tj (max)=125°C, 1.2
I = 5 mA,
so the drive condition of IF= 5.0mA, Vcc=3.3V,
V = 5 V and RL=100Ω
0
1.0
is acceptable.
The case temperature, Tc=70.2°C is less than 110°C as
recommended, so it is acceptable.
ts
1
VCC = 5
IF = 5 m
CTR = 2
10
1
0.1
0.1
L
LONG
F
CE
–5
CTR (Relative Value)
By applying the formula: Rth(c-a)=(Tc– Ta)/Pt at Ta=60°C,
Pt=26.25mW, and Rth(c-a)=0.390°C/mW, the case
temperature, Tc=Ta+Pt* Rth(c-a)=60°C+ 26.25mW *0.390°C/
mW≈70.2°C.
d
0.1
Normalized Gain Gv
Step #3: Calculate the case temperature, Tc.
0
0
Colle
Step #2: Examine how the temperature affects the CTR.
The total power consumption by the device is Pt=5.25mW+
21mW=26.25mW.
Sam
200
5
Fig 6.0 Forward Voltage and
Current vs. Ambient
SWITCHING
TIME vs.Temperature & CTR vs. Ta
LOAD RESISTANCE
Switching Time t (μ s)
In example #2, one can see that the Tj of the phototransistor
=44.2°C is a dominant factor that influences the case
temperature (Tc=36.1°C).
50
0.1
1. IRED Junction temperature , TJ IRED=25°C+5.5 mW* 1.667°C/mW=34.2°C.
2. Phototransistor junction temperature, Tj phototransistor
=25°C+23mW * 0.833°C/mW=44.2°C.
0°C
–25°C
–55°C
Collector
Current
IC (mA)
Current
Transfer
Ratio
CTR (%)
By applying the formula for the junction temperature, Tj=Ta +
Rth(j-a) * P we get the following result:
10
0.5
60
Switching Time t (μs)
The case temperature, Tc=Ta+Pt* Rth(c-a) =25°C+28.5mW
*0.390°C/mW=36.1°C.
70
TA = +100°C
+60°C
+25°C
50
Forward Current IF (mA)
By applying the formula: Rth(c-a)=(Tc–Ta)/Pt at Ta=25°C,
Pt=28.5mW, and Rth(c-a)=0.390°C/mW.
100
Collector Current IC (mA)
The total power consumption is, Pt=P_IRED+P_PD=5.5mW
+ 23mW=28.5mW.
COLLE
COLLE
FORWARD CURRENT vs.
FORWARD VOLTAGE
0.8
Again, the Tj of the phototransistor is a dominant factor that
–10case temperature, Tc.
0.6
influences the
RL = 1 kΩ
–15
100 Ω
–20
0.5
0.4
0.2
1
2
5
10 20
300 Ω
50 100 200 500
0.0
10
Frequency f (kHz)
Page 3
AN3021
Example #3:
Calculate the case temp. and junction temp. of the IRED
and phototransistor in the digital mode (or Vce of the
phototransistor is in saturation).
Using the same information from example #2 except that
load resistance is changed to 1.0KΩ: At Ta=25°C.
IRED side: IF=5.0 mA, VF=1.1V (typ) and the power
consumption, P_IRED=5.5mW.
Phototransistor side: Ic=10mA (CTR=200%), Vcc=3.3V and
RL=1KΩ.
To verify if the device is operating in digital mode with Vce
is in saturation, the multiplication of Ic and RL is taken, Ic
* RL=10.0V>>>Vcc=3.3V! so Ic=10mA is no longer valid.
Instead, the correct Ic=(Vcc-Vce(sat)*)/RL=(3.3V-0.3V)/1KΩ
=3.0 mA.
Note (*): Saturation voltage, Vce is provided in the data
sheet.
The power consumption, P_PD=3mA *0.3V=0.9mW.
The total power consumption, Pt=5.5 mW+0.9 mW=6.4mW.
By applying the formula: Rth(c-a)= (Tc–Ta)/Pt with Ta=25°C, Pt
=6.4mW, and Rth(c-a)=0.390°C/mW, the case temperature,
Tc=Ta+Pt* Rth(c-a)=25°C+6.4mW *0.39°C/mW≈27.5°C.
Step #2: Examine how the ambient temperature affects
the CTR.
As shown in example #2, the Ic=8.5mA and >Ic * RL=8.5V
>>>Vcc=3.3V! so Ic=8.5mA is no longer valid. Instead, the
correct Ic=(Vcc-Vce(sat)*)/RL=(3.3V -0.3V)/1KΩ=3.0mA.
Please look at the app. note, AN3020 for detailed info
regarding different approaches to operate in the digital
mode. The application note is posted at:
http://www.cel.com/appnotes.
do?command=showByType&group=2
Again, the power consumption, P_PD=3mA *0.3V=0.9mW.
The total power consumption, Pt=5.25mW+0.9mW=6.4mW.
By applying the formula: Rth(c-a)=(Tc–Ta)/Pt with Ta=60°C,
Pt=6.4mW, and Rth(c-a)=0.390°C/mW, the case temperature,
Tc=Ta + Pt* Rth(c-a)=60°C+6.4mW *0.390°C/mW=62.5°C.
The case temperature, Tc=62.5°C is less than 110°C as
recommended, so it is acceptable.
By applying the formula for the junction temperature, Tj=Ta
+Rth(j-a) * P where P is the power dissipation. The result is
as follows:
1. IRED Junction temperature , TJ IRED=60°C+5.25mW* 1.667°C/mW=68.8°C.
By applying the formula for the junction temperature, Tj=Ta +
Rth(j-a) * P. The result is as follows:
2. Phototransistor junction temperature, Tj phototransistor
=60°C+0.9mW * 0.833°C/mW=60.7°C.
1. IRED Junction temperature , TJ IRED=25°C+5.5 mW* 1.667°C/mW=34.2°C.
Based on the above calculation, the junction temperatures of
both the IRED and phototransistor do not exceed the Tj (max)
=125°C. Again, the Tj of the IRED is the dominant factor that
influences the case temperature.
2. Phototransistor junction temperature, Tj phototransistor
=25°C+0.9mW * 0.833°C/mW=25.7°C.
In general, when the device is operating in the digital mode
the IRED junction temperature is a dominant factor because
the phototransistor, PD will consume less power due to Vce
being in saturation.
At 60°C, here are the steps that need to be taken:
Step #1: Examine how the temperature affects the forward
voltage of the IRED.
Conclusion
The reliability and performance of optocouplers are
profoundly affected by thermal factors such as, operating
temperature, drive conditions and operating mode (analog
or digital). This application note describes methods for
calculating the junction temperatures and case temperature
to ensure that the device is used properly.
By looking at Figure 6.0: Forward voltage and current versus
ambient temperature, one can see that VF=~1.05V @
IF=5.0mA> Power consumption, P_IRED=5.25mW.
Information and data presented here is subject to change without notice. California
Eastern Laboratories assumes no responsibility for the use of any circuits described
herein and makes no representations or warranties, expressed or implied, that such
circuits are free from patent infringement.
© California Eastern Laboratories 01/29/14
4590 Patrick Henry Drive, Santa Clara, CA 95054-1817
Tel. 408-919-2500
FAX 408-988-0279 www.cel.com
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