AN10137 Temperature sensing using TrenchPLUS devices Rev. 01 — 8 June 2009 Application note Document information Info Content Keywords Accurate temperature sensing, temperature control, sensor theoretical accuracy, forward voltage, temperature coefficient, trip temperature, trip temperature error Abstract As the automotive industry moves towards driving higher powered motors in Electronic Power-Assisted Steering (EPAS) and Integrated Starter Alternator (ISA) applications, the need for accurate sensing of temperature and current becomes paramount. This document considers some of the protection strategies available using NXP TrenchPLUS temperature sensing devices. AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices Revision history Rev Date Description 01 20090608 Updated to meet NXP Semiconductors house style and rewritten. Contact information For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: [email protected] AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 2 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices 1. Introduction The market-leader in the field of temperature sensing devices is the BUK9107-40ATC. An N-channel power MOSFET with monolithically-integrated temperature sensing and clamping diodes that internally monitor the temperature of the MOSFET chip. Designed for high current applications, the device has a typical RDS(ON) of 5.8 mΩ at 25 °C with a gate drive of 5 V. It is relatively simple to incorporate temperature sensing into your temperature protection strategy. Traditionally, a system can be protected against overtemperature using a comparator and a few passive components which directly measure the chip temperature. This application note demonstrates that a more accurate method of temperature control can be achieved using a suitable microcontroller. 2. Example of a temperature sensing device application A typical temperature sensing circuit is shown in Figure 1 which shows the MOSFET controlled by a microcontroller. The output from the MOSFET temperature sensor is connected to the analog-to-digital input of the microcontroller. The resistor values of RA and RG define the current in the sense diode and hence its forward voltage (VF) and gate switching time. MICROCONTROLLER LOAD BUKxxxx I/O RA 2 RG 1 3 4 A/D 5 001aak168 Fig 1. Typical temperature sensing circuit During normal operation, the VF of the diode is monitored and a reference level (VF(ref)) is chosen, below which the device switches off. The value that is chosen for VF(ref) depends on the VF temperature coefficient (SF) and the temperature at which the MOSFET should be switched off. The VF characteristic is linear over the full temperature range, which enables numerous overtemperature protection strategies to be implemented; see Figure 2. By continually monitoring VF, the microcontroller can provide an early warning of overtemperature conditions, and can also determine the rate-of-change of temperature. Depending on the microcontroller used, there are various possible ways to respond to the information provided by the temperature sensor. The information could be used to trigger a latched shutdown, shutdown and cyclic retries, or simply used as a diagnostic tool for the application. AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 3 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices 0.700 VF (V) 0.650 0.600 0.550 0.500 0.450 VF(ref) 0.400 0.350 0 50 100 150 175 200 T (°C) 001aak169 Fig 2. Temperature sense diode forward voltage as a function of temperature 2.1 Theoretical estimation of temperature sensor accuracy The theoretical accuracy of the temperature sensor depends on three factors: • Uncertainty of VF • Uncertainty of SF • Chosen value of VF(ref) The effect of the above factors is shown more clearly in Figure 3. Any inherent variability in VF adds a fixed offset to the trip temperature (Ttrip). Any variation in SF is shown by a change in the gradient. A lower value of SF causes the device to trip at a higher temperature Ttrip(u). Depending on the value of Ttrip this may lead to devices operating above their maximum operating temperature which may reduce their life expectancy. Conversely, a higher value of SF causes erroneous nuisance tripping below the desired set point. Both these factors have implications for the overtemperature protection strategy employed. AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 4 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices VF ∆VF SF(max) SF SF(min) VF(ref) Ttrip(l) Ttrip Ttrip(u) T (°C) 001aak170 Fig 3. Variation of Ttrip with changes in VF and SF The total error in temperature sensor accuracy is the sum of all contributions from the uncertainty of both VF and SF. The values for VF and SF of the temperature sensing diode given in the data sheet for the BUK9107-40ATC are shown in Table 1. Table 1. Temperature sense diode characteristics for BUK9107-40ATC Symbol Parameter Min Typ Max Unit VF forward voltage 648 658 668 mV SF temperature coefficient −1.4 −1.54 −1.68 mV/K The BUK9107-40ATC has a very tight VF tolerance of 10 mV, and the variation in SF is also correspondingly tight. However, if the device is used in the circuit shown in Figure 1 without calibration, then the total error in Ttrip due to errors in both VF and SF becomes significant. The maximum error will occur if the VF, at Tj = 25 °C, is at its highest value and SF is at its lowest value. If Ttrip is set to 150 °C and VF and SF are not measured, then the Ttrip error is given by Equation 1: ( V F – V F ( ref ) ) T trip = ---------------------------------- – ( T trip – 25 ) SF (1) where the average reference forward voltage V F ( ref ) is given by Equation 2: V F ( ref ) = V F – ( T trip – 25 ) × S F = 465mV (2) Substituting the values given in the data sheet into Equation 2 gives the following results: T trip ( u ) = 150 + 19.6 °C T trip ( l ) = 150 – 16.4 °C In practice, the results are much better than that shown. AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 5 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices Figure 4 shows Ttrip as a function of VF for where a number of devices have been measured across the temperature range, and the actual Ttrip value has been determined for each using a reference voltage of 465.5 mV. 180.0 Ttrip (°C) 170.0 160.0 Ttrip Limits 150.0 140.0 130.0 120.0 640.0 645.0 650.0 655.0 660.0 665.0 670.0 675.0 VF (mV) 001aak171 Tj = 25 °C Fig 4. Ttrip as a function of VF without error correction The box in Figure 4 defines the theoretical limits of Ttrip. The data clearly lies within ±10 °C of the target temperature. 2.2 Improving accuracy The accuracy of the device can be vastly improved if its VF is measured at room temperature. Using this value, VF(ref) can be reset to eliminate the error in VF. This situation is shown in Figure 5. AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 6 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices SF(max) VF SF SF(min) VF(ref) Ttrip(l) Ttrip Ttrip(u) T 001aak172 Fig 5. Ttrip error decreases when the value of VF at room temperature is known In this case there is still an error associated with SF shown by the variation in gradient. As before, the upper values of SF are taken from the maximum values given in the data sheet shown in Table 1. The total error at Ttrip is now given by Equation 3: dS F 1 d ( ∆T ) = – ∆V F ------------2- + ------ d ( ∆V F ) S F ( SF ) (3) where ∆VF is the voltage drop required to trip at 150 °C from 25 °C, and dSF is the variation in SF given in the data sheet (1.68 − 1.54 = 0.14). By adjusting VF(ref), the term on the right-hand side of Equation 3 becomes zero. Substituting the remaining values gives the following results: T trip = 150 ± 11°C Again, in practice, the results are better than this. Figure 6 shows the effect when an estimate of Ttrip is made for the same device using a corrected VF(ref). AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 7 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices 170.0 Ttrip (°C) 165.0 160.0 155.0 Ttrip Limits 150.0 145.0 140.0 135.0 130.0 640.0 645.0 650.0 655.0 660.0 665.0 670.0 675.0 VF (mV) Fig 6. 001aak173 Ttrip as a function of VF with VF(ref) corrected As in Figure 4, the outer box defines the theoretical limit which now gives the following results: T trip = 150 ± 11°C The measured values now lie within ±5 °C of the target. It is clear that significant improvements in accuracy are possible by measuring the value of VF at room temperature. A further theoretical improvement can be made because a relationship exists between SF and VF measured at 25 °C. The accuracy will be increased if VF (at 25 °C) is measured, and SF is calculated using the expression given in Equation 4. Using the graph, if VF (at 25 °C) is measured, the value of SF will lie in the range given in Equation 4: S F ( calc ) = { [ – 0.0041 × V F ( 25°C ) ] + 4.2387 } ± 0.099 (4) The error in SF of 0.099 represents 5 standard deviations from the mean. If we again assume that Ttrip is set to 150 °C, the accuracy now becomes: T trip = 150 ± 8°C . The greatest accuracy can be achieved if both VF and SF are measured for every device. In this case, VF (at 25 °C) and VF (at 150 °C) are measured, and SF is calculated and stored using Equation 5. V F ( 25 ) – V F ( 150 ) S F = -------------------------------------150 – 25 (5) Again, VF(ref) must be redefined as in Equation 3. In this way, Ttrip will be limited only by the accuracy of the voltmeter used, and an accuracy of ±1 °C can be readily achieved. This could be integrated into the module build to provide excellent temperature control in your system. AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 8 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices 3. Summary There are four methods of using the temperature sensing diode with increasing theoretical accuracy for each subsequent method. The two simplest methods have been measured and compared with theory. A summary of all the results is given in Table 2. Table 2. Summary of results Accuracy method Result Theory Experiment VF not measured T trip = 150 ± 19 °C T trip = 150 ± 10 °C VF measured (at 25 °C) and use SF = 1.40 − 1.68 T trip = 150 ± 11 °C T trip = 150 ± 5 °C VF measured (at 25 °C) and calculate SF T trip = 150 ± 8 °C Measure VF and SF T trip = 150 ± 1 °C AN10137_1 Application note © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 9 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices 4. Legal information 4.1 Definitions Draft — The document is a draft version only. The content is still under internal review and subject to formal approval, which may result in modifications or additions. NXP Semiconductors does not give any representations or warranties as to the accuracy or completeness of information included herein and shall have no liability for the consequences of use of such information. 4.2 Suitability for use — NXP Semiconductors products are not designed, authorized or warranted to be suitable for use in medical, military, aircraft, space or life support equipment, nor in applications where failure or malfunction of an NXP Semiconductors product can reasonably be expected to result in personal injury, death or severe property or environmental damage. NXP Semiconductors accepts no liability for inclusion and/or use of NXP Semiconductors products in such equipment or applications and therefore such inclusion and/or use is at the customer’s own risk. Applications — Applications that are described herein for any of these products are for illustrative purposes only. NXP Semiconductors makes no representation or warranty that such applications will be suitable for the specified use without further testing or modification. Disclaimers General — Information in this document is believed to be accurate and reliable. However, NXP Semiconductors does not give any representations or warranties, expressed or implied, as to the accuracy or completeness of such information and shall have no liability for the consequences of use of such information. Right to make changes — NXP Semiconductors reserves the right to make changes to information published in this document, including without limitation specifications and product descriptions, at any time and without notice. This document supersedes and replaces all information supplied prior to the publication hereof. Export control — This document as well as the item(s) described herein may be subject to export control regulations. Export might require a prior authorization from national authorities. 4.3 Notice: All referenced brands, product names, service names and trademarks are the property of their respective owners. TrenchMOS — is a trademark of NXP B.V. AN10137_1 Application note Trademarks © NXP B.V. 2009. All rights reserved. Rev. 01 — 8 June 2009 10 of 11 AN10137 NXP Semiconductors Temperature sensing using TrenchPLUS devices 5. Tables Table 1. Temperature sense diode characteristics for BUK9107-40ATC . . . . . . . . . . . . . . . . . . . . . . . .5 Table 2. Summary of results . . . . . . . . . . . . . . . . . . . . . . 9 6. Figures Fig 1. Fig 2. Fig 3. Typical temperature sensing circuit . . . . . . . . . . . .3 Temperature sense diode forward voltage as a function of temperature . . . . . . . . . . . . . . . . . . . .4 Variation of Ttrip with changes in VF and SF . . . . . .5 Fig 4. Fig 5. Fig 6. Ttrip as a function of VF without error correction . . 6 Ttrip error decreases when the value of VF at room temperature is known . . . . . . . . . . . . . . . . 7 Ttrip as a function of VF with VF(ref) corrected. . . . . 8 7. Contents 1 2 2.1 2.2 3 4 4.1 4.2 4.3 5 6 7 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Example of a temperature sensing device application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Theoretical estimation of temperature sensor accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Improving accuracy. . . . . . . . . . . . . . . . . . . . . . 6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Legal information. . . . . . . . . . . . . . . . . . . . . . . 10 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Trademarks . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Please be aware that important notices concerning this document and the product(s) described herein, have been included in section ‘Legal information’. © NXP B.V. 2009. All rights reserved. For more information, please visit: http://www.nxp.com For sales office addresses, please send an email to: [email protected] Date of release: 8 June 2009 Document identifier: AN10137_1