AN12.14

AN12.14
Remote Thermal Sensing Diode Selection Guide
Author:
OVERVIEW
Wayne Little
Microchip Technology Inc.
INTRODUCTION
This is a practical approach for selecting a remote
diode-connected transistor to use with a thermal
sensor, as illustrated in Figure 1.
This application note is aimed at designers who build
systems that use thermal sensors with remote diodes;
specifically, remote diodes that are discrete bipolar
junction transistors (BJTs).
Discussions of the semiconductor parameters of the
transistor that affect the accuracy of temperature measurement are included here as the requisite feature of
a remote thermal sensing diode.
Information presented here organizes important criteria
for selecting the remote sensing diode to use with
Microchip's high accuracy, low cost remote diode
thermal sensors.
A short table of qualified discrete 2N3904 NPN transistors is provided here. It lists devices from other manufacturers that have been tested and met established
standards of accuracy.
Microchip does produce temperature sensors that are
designed to work specifically with CPU thermal diodes.
So, these discussions are about selecting an appropriate BJT, as well as providing a list of acceptable BJTs,
several are mentioned.
Throughout this application note, the phrase “remote
diode-connected transistor” refers to a discrete, diodeconnected (Base-Collector junction shorted) BJT.
This application note assumes that the reader has
working knowledge of temperature sensing that uses
diode-connected transistors.
System
Controller
with
SMBus
Interface
Remote
Diode-Connected
Transistor
Remote
Sensor
DP
SMBus
Interface
SMBus
DN
FIGURE 1:
Block Diagram of a Typical Temperature Sensing System.
 2014 Microchip Technology Inc.
DS00001838A-page 1
AN12.14
These three semiconductor parameters are the primary
factors when considering diode-connected transistors
in temperature sensing applications.
• Ideality Factor (η)
• Forward Current Gain (beta or hFE)
• Series Resistance (RS)
Ideality Factor (η)
The ideality factor is a parameter in the diode currentvoltage relationship. It approaches a value of 1.0 when
the carrier diffusion dominates the current flow, and
approaches 2.0 when the recombination current dominates the current flow. This term is a constant on any
particular device, though it can vary among individual
devices.
Temperature sensors are calibrated during the final test
to provide accurate readings with a diode that has a
typical ideality factor. For the purposes of this document, the typical ideality factor value is expressed as
ηASSUMED and the ideality factor value of the user’s
diode-connected transistor is expressed as ηREAL.
The temperature indicated by a temperature sensor will
include an error from the real temperature, as defined
by the equation in Equation 1. To use this equation, the
temperature values must be converted to the Kelvin
scale. The result will be incorrect if the values used
reflect the Celsius or Fahrenheit scale.
EQUATION 1:
TEMPERATURE ERROR
DUE TO IDEALITY FACTOR
MISMATCH
 REAL
T MEASURED = --------------------------------  T REAL
 ASSUMED
Generally, a 2N3904 transistor is the preferred remote
diode. Several samples of each of the transistors listed
in Table 1 were evaluated and their ideality factor was
determined to be ~1.004. (Typically, the ideality factor
is not be stated in the data sheet for a transistor.) While
transistor devices other than the ones cited here could
be used; to be confident of accurate operation, they
should be qualified before use.
Note:
Qualification of these devices is ideally
performed by obtaining data, on the
parameters described in this application
note, from the device manufacturer. Precision thermal equipment is required to
measure the parameters. Contact your
Microchip Field Applications Engineer for
additional support.
DS00001838A-page 2
TABLE 1:
TYPICAL IDEALITY FACTOR
VALUES FOR 2N3904 DIODECONNECTED TRANSISTORS
Manufacturer
Typical Ideality Factor
ROHM Semiconductor
1.0038
Diodes® Incorporated
1.0044
NXP®
1.0049
STMicroelectronics
1.0045
ON Semiconductor®
1.0046
Chenmko CO., LTD.
1.0040
Infineon® Technologies AG
1.0044
Fairchild Semiconductor®
1.0046
National Semiconductor
1.0037
In Equation 1, the ideality factor value that the temperature sensor is calibrated for is ηASSUMED and the actual
ideality factor value of the diode-connected transistor is
ηREAL. In this equation, the temperature measurement
error is not a constant offset, but increases as TREAL,
the temperature of the remote diode-connected
transistor, increases.
Figure 2 shows the temperature-measurement error
that is induced solely from the differences between
ηASSUMED and ηREAL. In this figure, ηASSUMED is 1.004,
a typical ideality factor value for a 2N3904 NPN diodeconnected transistor. Temperature sensors are typically calibrated in the range of the 2N3904 (1.004)
because this is also very similar to the ideality factor of
the majority of substrate diode-connected transistors
that are found on CPUs and GPUs.
2.5
Measured Temperature Error
(°C)
DIODE PARAMETERS
2
1.5
1
0.5
0
-0.5
-1
-1.5
1.01
1.004
-2
0
1.008
1.002
20
1.006
1
40
60
80
100
Real Temperature (°C)
FIGURE 2:
Temperature Error vs.
Ideality Factor of Diode (with IC trimmed to 1.004).
Figure 2 also shows why true 2-terminal discrete
diodes are not used in temperature sensing applications instead of 3-terminal devices such as the 2N3904.
A discrete 2-terminal diode, ideally, would perform in
temperature sensing applications as well as a thermal
diode would. However, characterization in the labs
determined that discrete 2-terminal diodes typically
have an ideality factor much higher (1.2–1.5) than
ηASSUMED of 1.004. This discrepancy (between
ηASSUMED and ηREAL) would cause unacceptable
temperature measurement errors at all temperatures.
 2014 Microchip Technology Inc.
AN12.14
Forward Current Gain (beta or hFE)
A typical temperature sensor forces two fixed currents
(IF1 and IF2) into the thermal diode to measure
temperature, as shown in Figure 3.
Temp Sensor IC
VDD
IF2
Voltage to
Temperature
Conversion
IF1
IB
IC
VBE
Remote Diode
(2N3904)
IE
FIGURE 3:
Two Current Sources.
The temperature sensor measures the voltage, VBE,
which is developed based on the collector current; not
the emitter current.
EQUATION 2:
IDEAL DIODE
 VBE2 – V BE1   q
T = ------------------------------------------------- I C2
 k  1n  ---------
 I C1
The forward current gain (beta) of a transistor is not a
constant over all operating conditions, but varies over
temperature and as a function of IC. The variation in
beta over temperature does not induce temperature
measurement error. However, if the transistor has a
large variation in beta as a function of IC, the
temperature reading can be inaccurate, due to betainduced error.
Equation 3 shows the error induced from the
non-constant value of beta at the two currents. βF1 represents the beta of the transistor at the current value IF1
while βF2 represents the beta at the current value IF2.
‘N’ represents the fixed ratio of the two forced (IE1 and
IE2) currents. If beta is constant over the range of the
two currents (βF1 = βF2), then there is no temperature
measurement error induced because of beta variation.
EQUATION 3:
TEMPERATURE ERROR
DUE TO BETA VARIATION
   F2   1 +  F1  
- 
 1n  ----------------------------------------   F1   1 +  F2  
 TERROR = T REAL   ------------------------------------------------------
1n  N 






If the value of beta is relatively constant over the range
of forced emitter currents, then the ratio of IC2:IC1
remains equal to the ratio of the two forced emitter
currents and induces no error. It only becomes a
problem when the beta variation causes a mismatch
between the IC2:IC1 ratio and the IE2:IE1 ratio.
 2014 Microchip Technology Inc.
DS00001838A-page 3
AN12.14
Figure 4 presents a plot of allowable beta variation over
the sensor’s sourced current range (10 – 400 µA) to be
able to still maintain at least 1 degree accuracy at 70°C.
The beta of the transistor must reside between the two
lines in the plot, over the extremes of the current range
of the temperature sensor, in order to maintain 1°C
accuracy with the selected diode-connected transistor.
The x-axis represents the beta of the diode-connected
transistor at IF1, while the y-axis is for the beta at IF2.
varies over the sensor’s sourced current range.
1000
Beta 2
100
10
1
1
10
100
1000
Beta 1
FIGURE 4:
Allowed Beta Variations for 1 Degree Accuracy at 70°C.
Figure 5 shows typical values of transistor beta for a
limited sample of these devices. These devices were
characterized in Microchip characterization labs. This
data should not to be used as a guaranteed value for
the specific transistor, only a typical representation for
the limited quantity tested by Microchip.
Typical Beta Values for 2N3904 Transistors at 23C
500
400
Rohm
Diodes_Inc.
NXP
STMicro
ON_Semi
Chenmko
Infineon
Fairchild
National
300
)
Ib
/
c
(I
ta
e
B 200
100
0
1.E-06
1.E-05
1.E-04
1.E-03
Collector Current
FIGURE 5:
DS00001838A-page 4
Typical Beta Values for 2N3904 Transistors at 23°C.
 2014 Microchip Technology Inc.
AN12.14
The conclusion to draw from Figure 4, Figure 5 and
Table 2 is that for the set of 2N3904 transistors tested
by Microchip, the beta was consistently high and flat.
The measured value of beta easily resides inside the 2
lines of Figure 4, over the entire temperature sensor’s
sourced current range.
Table 2 quantifies the error induced from beta variation
using the 2N3904s that were tested. As demonstrated
through the tested devices, beta variation has a very
small affect on temperature measurement accuracy.
TABLE 2:
Table 3 quantifies some typical values of series resistance found for a sample of different 2N3904 devices.
This value of series resistance for the set of 2N3904s
tested was found to have a positive temperature coefficient and as a “rule-of-thumb”, typically increased
approximately 5% per +10°C increase.
Note:
TEMPERATURE ERROR DUE
TO 2N3904 BETA VARIATION
AT 70°C
Manufacturer
Temperature Error (°C)
ROHM Semiconductor
+0.07
Diodes Incorporated
+0.00
NXP
+0.04
STMicroelectronics
+0.03
ON Semiconductor
+0.01
Chenmko CO., LTD.
+0.15
Infineon Technologies AG
+0.03
Fairchild Semiconductor
+0.00
National Semiconductor
+0.00
Series Resistance (RS)
Series resistance is another parameter that affects
temperature measurement accuracy. Series resistance
causes the temperature sensor to report the temperature higher than the actual temperature of the thermal
diode. The relationship between temperature offset
and series resistance is displayed in the following
equation.
EQUATION 4:
TEMPERATURE OFFSET
ERROR DUE TO SERIES
RESISTANCE
Table 3 should not be used as a guideline
for offsetting the temperature reported by
an Microchip temperature sensor.
Microchip temperature sensors are
typically calibrated using a 2N3904 diodeconnected transistor which already
compensates for this series resistance
error term.
Table 3 is presented as a reference to
help thermal designers understand the
possible effects of non-idealities in
temperature measurement
TABLE 3:
TYPICAL VALUES OF SERIES
RESISTANCE FOR 2N3904
DIODE CONNECTED
TRANSISTORS
Manufacturer
Series Resistance (RS)
@70°C
ROHM Semiconductor
0.68
Diodes Incorporated
0.65
NXP
0.72
STMicroelectronics
0.58
ON Semiconductor
0.90
Chenmko CO., LTD.
0.73
Infineon Technologies AG
0.57
Fairchild Semiconductor
0.60
National Semiconductor
0.51
q  I F2 – IF1 RS
T offset =  ------ -----------------------------------  k
 IF2
1n  ---------
 IF1
The temperature error induced by series resistance is
a constant offset for all temperatures. When using a
typical Microchip temperature sensor, the magnitudes
of IF2 and IF1 induce approximately +0.67°C error per
Ohm of series resistance. For different 2N3904 devices
characterized by Microchip, the RS was found to be
less than 1Ω. This does not include the series resistance due to PCB traces connecting the sensor and
remote diode; this only represents the series resistance
found in the characterized 2N3904 devices.
 2014 Microchip Technology Inc.
DS00001838A-page 5
AN12.14
TESTED DIODE LIST
CONCLUSION
This table lists a limited selection of 2N3904 NPN transistors that have been characterized found to meet the
specifications to obtain 1°C accurate measurements.
In conclusion, while differences were seen between the
various manufacturer’s versions of 2N3904 BJTs, the
results, when using them with Microchip temperature
sensors, were very consistent. For all typical 2N3904
devices tested, temperature never varied more than
±0.2 °C from the true temperature. The 2N3904
devices listed in Table 4 (or any BJT/diode with equivalent parameters) will yield accurate temperature measurement results when used with Microchip
temperature sensors.
TABLE 4:
TESTED DIODES FOR
TEMPERATURE SENSING
APPLICATIONS
Manufacturer
Model Number
ROHM Semiconductor
UMT3904
Diodes Incorporated
MMBT3904-7
NXP
MMBT3904
STMicroelectronics
MMBT3904
ON Semiconductor
MMBT3904LT1
Chenmko CO., LTD.
MMBT3904
Infineon Technologies AG
SMBT3904E6327
Fairchild Semiconductor
MMBT3904FSCT
National Semiconductor
MMBT3904N623
DS00001838A-page 6
Microchip supplies a family of temperature sensors for
many applications. Several special functions, such as
resistance error correction and ideality configuration
are available. In addition, some devices are designed
to work specifically with CPU thermal diodes. Please
consult your Microchip representative or visit the
Microchip website for additional information at:
www.microchip.com.
 2014 Microchip Technology Inc.
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