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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