Method for determining a derating curve of an SMD resistor, and use of an SMD resistor

Abstract

The invention relates to a method for determining a derating curve of an SMD resistor (1), wherein the SMD resistor (1) comprises a non-conductive substrate (2) having a resistive layer (3) applied thereto, wherein a maximum permissible temperature is assigned to the SMD resistor (1), wherein the method comprises at least the following steps: - mounting the SMD resistor (1) on a test printed circuit board (4), - applying a temperature sensor (7) to the resistive layer (3), - applying a predefined current to the SMD resistor (1) and measuring the temperature of the resistive layer (3) by means of the temperature sensor (7) during application of the predefined current, wherein the predefined current is correlated with a rated power of the SMD resistor (1) which is above a maximum rated power specified by the manufacturer for the SMD resistor (1), - calculating a thermal resistance of the SMD resistor (1) on the basis of the measured temperature and the predefined current, - determining the derating curve of the SMD resistor (1) on the basis of the predefined current, the calculated thermal resistance, and the maximum permissible temperature.

Description

[0001] Method for determining a derating curve of an SMD resistor and use of an SMD resistor

[0002] The invention relates to a method for determining a derating curve of an SMD resistor and a use of an SMD resistor.

[0003] Field devices in process and automation technology serve to monitor and / or determine at least one process parameter of a medium, for example, a chemical or physical parameter. For the purposes of this application, in principle, all measuring instruments used close to the process and which provide or process process-relevant information are referred to as field devices. A large number of such field devices are manufactured and distributed by companies within the Endress+Hauser Group.

[0004] The process variable to be determined by the field device can be the fill level, flow rate, pressure, temperature, pH value, redox potential, or conductivity of the respective medium. The various possible measurement principles underlying the determination of the process variable are known from the prior art and will not be discussed further here. Field devices for measuring fill level are designed, in particular, as microwave level gauges, ultrasonic level gauges, time-domain reflectometric level gauges (TDR), radiometric level gauges, capacitive level gauges, conductive level gauges, and vibronic level gauges. Field devices for measuring flow rate, on the other hand, operate, for example, according to the Coriolis, ultrasonic, vortex, thermal, and / or magnetic-inductive measurement principles.Pressure measuring devices are preferably absolute, relative, or differential pressure devices. In addition to the aforementioned measuring devices and actuators, field devices also include remote I / Os, radio adapters, and generally any devices located at the field level.

[0005] A field device typically comprises a sensor that comes into contact with the process at least partially and / or temporarily, and an electronics unit used, for example, for signal acquisition, signal processing, and / or signal power supply. The electronics unit of the field device is typically housed in a casing and also has at least one connection element for connecting the electronics unit to the sensor and / or an external unit and for transmitting data and / or power. The connection element can be any type of connection, including wireless. The electronics unit and the sensor of the field device can be designed as separate units with separate casings or as a single unit within a casing. The field device generally includes at least one printed circuit board with components mounted on it.Printed circuit boards (PCBs) consist of an electrically insulating material with conductive traces attached to it, typically made of copper. Multilayer PCBs have conductive traces on both sides. Among other components, surface-mount devices (SMDs) resistors are usually mounted on the PCB. These SMD components are generally supplied with a datasheet in which the manufacturer specifies the maximum power rating of the SMD resistors. If the SMD resistors are to be used in potentially explosive atmospheres, they may only be operated at two-thirds of their maximum power rating, according to IEC standards, specifically IEC 60079-11.

[0006] It is therefore an object of the present invention to specify a method and a use in which a higher utilization of the SMD resistor is made possible.

[0007] The problem is solved according to the invention by a method according to claim 1, a method according to claim 10 and a use according to claim 12.

[0008] With regard to the method, the problem is solved according to the invention by a method for determining a derating curve of an SMD resistor, wherein the SMD resistor has a non-conductive substrate with a resistive layer applied thereto, wherein the SMD resistor is assigned a maximum permissible temperature, and wherein the method comprises at least the following steps:

[0009] Applying the SMD resistor to a test circuit board, applying a temperature sensor to the resistive layer, applying a predetermined current to the SMD resistor and measuring the temperature of the resistive layer using the temperature sensor while applying the predetermined current, wherein the predetermined current correlates with a rated power of the SMD resistor that is above a maximum rated power specified by the manufacturer for the SMD resistor, calculating the thermal resistance of the SMD resistor based on the measured temperature and the predetermined current, determining the derating curve of the SMD resistor based on the predetermined current, the calculated thermal resistance and the maximum permissible temperature.

[0010] Using the method according to the invention, the derating curve of the SMD resistor is determined at a rated power that is higher than the maximum rated power specified by the manufacturer. Typically, the datasheets for SMD resistors specify a maximum rated power that is significantly lower than the actual utilization of the SMD resistor. This is because the actual utilization and heat generation at the SMD resistor depend heavily on the printed circuit board (PCB) on which the SMD resistor is mounted and the surrounding environment of the SMD resistor on the PCB. The test PCB can therefore be designed to have less or at most the same thermal conductivity as the PCB on which the SMD resistor will later be used. For example, the test PCB can have less or at most the same amount of copper as the PCB on which the SMD resistor will later be used.The current applied to the SMD resistor can correspond to a rated power of the SMD resistor that is greater than a maximum rated power specified by the manufacturer.

[0011] It is evident that some SMD resistors cannot handle the maximum power rating specified by the manufacturer and will malfunction or become defective when this rating is exceeded. In such cases, further execution of the procedure is terminated. However, if the SMD resistor can be operated at the specified current and, in particular, without malfunction or defect, its temperature can be determined using the temperature sensor. Subsequently, the thermal resistance of the SMD resistor can be calculated, and the derating curve can be determined.

[0012] The derating curve describes the rated power of an SMD resistor as a function of its temperature. At the maximum permissible temperature, the rated power is zero. The derating curve can be designed so that the rated power decreases linearly from a certain temperature threshold to the maximum permissible temperature. Based on the determined derating curve, it is possible to see at which temperatures the SMD resistor can be operated at which rated power. The determined derating curve therefore makes it possible to operate the SMD resistor even at rated powers above the maximum rated power specified by the manufacturer.

[0013] The maximum permissible temperature of the SMD resistor is a temperature that the SMD resistor must not exceed, as otherwise a malfunction or defect of the SMD resistor will occur. The SMD resistor can be soldered to a contact pad on the test circuit board during installation.

[0014] In one embodiment, a thermocouple is used as the temperature sensor, for example a PT100.

[0015] In one embodiment, the temperature sensor is glued onto the resistive layer.

[0016] The adhesive can be designed to have high thermal conductivity. The adhesive can be designed to retain its adhesive properties at least up to the permissible temperature of the SMD resistor.

[0017] In one embodiment, the temperature sensor is applied to the center of the resistive layer. This enables precise measurement of the temperature of the resistive layer.

[0018] In one embodiment, a limit temperature is calculated based on the maximum permissible temperature, the specified current, and the calculated thermal resistance. If this limit temperature is exceeded, the rated power at the SMD resistor is reduced. The limit temperature can be represented in a graph of rated power versus temperature as the intersection of the rated power (correlated with the specified current) and a line extending from the permissible temperature towards lower temperatures with a slope equal to the calculated thermal resistance.

[0019] In one embodiment, the specified current correlates with a rated power of the SMD resistor, which is 101% to 500% of the maximum rated power specified by the manufacturer for the SMD resistor. 100% corresponds to the maximum rated power. In particular, the specified current correlates with a rated power of the SMD resistor, which is 110% to 500%, especially 120% to 500%, especially 120% to 400%, of the maximum rated power specified by the manufacturer for the SMD resistor.

[0020] In one embodiment, the SMD resistor is intended to meet the intrinsic safety protection standard. This intrinsic safety protection standard can stipulate that the SMD resistor is only utilized to two-thirds of its capacity. For this purpose, the derating curve can be adjusted so that the calculated thermal resistance is only factored in at two-thirds of its value.

[0021] In one embodiment, a defined number of identical SMD resistors are provided, and each SMD resistor is mounted on at least one test circuit board. A temperature sensor is mounted on each resistive layer, a predetermined current is applied to each SMD resistor, and the temperature of the resistive layer is determined. The thermal resistance of each SMD resistor is calculated, and the derating curve is determined based on the highest calculated thermal resistance value. Specifically, the derating curve is determined based on the highest calculated thermal resistance value, the predetermined current, and the maximum permissible temperature. By calculating the thermal resistances of several SMD resistors of the same type, instead of a single SMD resistor, and using these values ​​to determine the derating curve, the derating curve becomes more accurate.For example, different amounts of adhesive may be used when applying the temperature sensor to the SMD resistor, which introduces a certain error into the calculated thermal resistance. In this case, the highest calculated thermal resistance value is used, i.e., the maximum value among the calculated thermal resistances. The defined number of SMD resistors can be, for example, 5, 10, or 20. Using a defined number of ten SMD resistors has the advantage of fulfilling a condition according to section 9.13 of IEC 60079-11, Ed. 7, January 2023, entitled "Determination of parameters of loosely specified components." Specifically, the SMD resistors are identical.

[0022] In one embodiment, several different SMD resistors are applied to at least one test circuit board. A temperature sensor is applied to each resistive layer, and a predetermined current is applied to each SMD resistor, its temperature is measured. The thermal resistance of each SMD resistor is then calculated, and the derating curve is determined based on the highest calculated thermal resistance value. Specifically, the derating curve is determined using the highest calculated thermal resistance value, the predetermined current, and the maximum permissible temperature. The previously described procedure is then performed for each of the different SMD resistors to determine its respective derating curve. The different SMD resistors can have different component sizes and maximum power ratings.This design can be combined with the previous one, so that a defined number of SMD resistors of each type are presented, a thermal resistance is calculated for each, and finally, the derating curve of the respective type of SMD resistor is determined based on the highest value of the calculated thermal resistances.

[0023] In a training course, the determined derating curves of SMD resistors are compared with a given application that requires a specific resistance at a given temperature. The SMD resistor whose determined derating curve at the given temperature matches the required resistance is then selected. The application may require the use of an SMD resistor on a printed circuit board and specify a particular resistance at a given temperature.

[0024] In a training course, when selecting the SMD resistor whose derating curve at a given temperature matches the specified resistance, the component size of the SMD resistor is taken into account. This ensures that, among several SMD resistors whose derating curves match the specified resistance at the given temperature, the one with the smallest component size is selected. This saves space on the circuit board and may allow for the use of a smaller circuit board.

[0025] In one embodiment, the calculated thermal resistance is divided by the percentage of a utilization of the SMD resistor specified for the intrinsic safety type of ignition protection, and the derating curve is adjusted accordingly.

[0026] Regarding its use, the problem is further solved by using an SMD resistor, whose derating curve was determined according to one of the previous embodiments, on a printed circuit board that, in particular, fulfills the intrinsic safety type of ignition protection. Specifically, the SMD resistor is operated at a rated power that is above the maximum rated power specified by the manufacturer.

[0027] The invention will now be explained in more detail with reference to the following figures, Figs. 1-4. They show:

[0028] Fig. 1 : a schematic representation of an SMD resistor on a test circuit board.

[0029] Fig. 2: a schematic representation of an SMD resistor in an application on a printed circuit board.

[0030] Fig. 3: an example of a derating curve based on the manufacturer's maximum rated power.

[0031] Fig. 4: an example of a derating curve determined using the method according to the invention.

[0032] Figure 1 shows an SMD resistor 1 on a test circuit board 4. The SMD resistor 1 has a non-conductive substrate 2 and a resistive layer 3 applied to the substrate 2. The SMD resistor 1 can further have one or more terminal areas 9. Preferably, the SMD resistor 1 has two terminal areas 9 in the two end regions of the resistive layer 3. The terminal areas 9 are electrically conductive and may optionally have an electrode. The resistive layer 3 can be arranged between the two terminal areas 9. A temperature sensor 7 can be applied to the resistive layer 3, for example by means of an adhesive 11. The temperature sensor 7 can be a thermocouple and / or be applied centrally to the resistive layer 3. The test circuit board 4 can have a contact pad 5. The SMD resistor 1 can be soldered to the test circuit board 4 by means of a solder joint 6.In particular, the SMD resistor 1 can be soldered to the contact pad 5 via its connection areas 9. The test circuit board 10 can have at least one conductor 10 made of a metal, such as copper. The test circuit board 10 can be designed to have a lower thermal conductivity than a circuit board 8 on which the SMD resistor is to be used. For example, the test circuit board 10 can have less or at most the same amount of metal, especially copper, as the circuit board 8.

[0033] After mounting the SMD resistor 1 onto the test circuit board 4 and the temperature sensor 7 onto the resistive layer 3, a current with a predetermined current value is applied to the SMD resistor 1. A conductor 10 on the test circuit board 4 can be used for this purpose. The temperature at the resistive layer 3 caused by the applied current can be measured using the temperature sensor 7. The predetermined current value can correlate with a power rating of the SMD resistor or correspond to a power rating that is greater than the maximum power rating specified by the manufacturer for the SMD resistor. Based on the measured temperature and the predetermined current value, the thermal resistance of the SMD resistor 1 is then calculated. Finally, the derating curve of the SMD resistor 1 can be determined using the predetermined current value, the calculated thermal resistance, and the maximum permissible temperature.

[0034] The described method can be performed for a plurality of identical and / or different SMD resistors 1, and the corresponding derating curve can be determined for each SMD resistor 1. In particular, if a derating curve has been determined for several different SMD resistors 1, then one SMD resistor suitable for a given application can be selected from among them. For example, the application might require a specific resistance at a specific temperature. In this case, the SMD resistor 1 whose determined derating curve exhibits the specified resistance at the specified temperature can be selected. If several SMD resistors 1 are available whose determined derating curves exhibit the specified resistance at the specified temperature, then the SMD resistor 1 with the smallest component size can be selected.

[0035] Fig. 2 shows an example of a top view of a printed circuit board 8 on which an SMD-

[0036] Resistor 1, whose derating curve was determined according to the inventive method, is arranged. The SMD resistor 1 can be part of a circuit comprising a plurality of components 12.

[0037] Figures 3 and 4 show two derating curves of an SMD resistor. Figure 3 shows the derating curve obtained according to the manufacturer's specifications. The thermal resistance, according to the manufacturer, is 450 K / W, and the permissible temperature is 155°C. At a rated power of 100% of the maximum rated power, a limiting temperature of approximately 110°C is obtained. From this limiting temperature, the rated power of the SMD resistor should decrease linearly until it finally reaches zero at the maximum permissible temperature. The slope of this linear decrease is determined by the thermal resistance, which is shown as a dashed line. Additionally, a lined area is shown, which indicates the rated power as a function of temperature when the SMD resistor is to be used according to the intrinsic safety type of protection.According to the intrinsic safety type of ignition protection, the SMD resistor may only be operated at two-thirds, i.e., about 67%, of its maximum rated power.

[0038] The derating curve of the same SMD resistor, obtained by the method according to the invention, is shown in Fig. 4. A current was selected that corresponds to a rated power of 200% of the maximum rated power, i.e., twice the maximum rated power specified by the manufacturer. The calculated thermal resistance is 170 K / W, which is significantly lower than the thermal resistance specified by the manufacturer. Due to the lower thermal resistance, the limiting temperature shifts to higher temperatures, namely to approximately 120°C. The limiting temperature can be calculated using the maximum permissible temperature, the specified current, and the calculated thermal resistance.

[0039] If the SMD resistor is to be used in a printed circuit board that is intended to meet the intrinsic safety protection standard, the calculated thermal resistance can be divided by two-thirds of the utilization factor specified by the intrinsic safety protection standard, resulting in a thermal resistance of approximately 255 K / W. The lined area shows the possible utilization factor of the SMD resistor according to the intrinsic safety protection standard. The SMD resistor can be operated at its rated power of 200% up to a limit temperature of approximately 110°C before the rated power must be linearly limited. Reference symbol

[0040] 1 SMD resistor

[0041] 2 Substrate 3 Resistance layer

[0042] 4 Test circuit board

[0043] 5 contact pads

[0044] 6 solder joints

[0045] 7 Temperature sensor 8 Circuit board

[0046] 9 Connection area

[0047] 10 conductor track

[0048] 11 glue

[0049] 12 components, 13 conductive substrate

Claims

Patent claims 1. Method for determining a derating curve of an SMD resistor (1), wherein the SMD resistor (1) has a non-conductive substrate (2) with a resistive layer (3) applied thereto, wherein the SMD resistor (1) is assigned a maximum permissible temperature, wherein the method comprises at least the following steps: Applying the SMD resistor (1) to a test circuit board (4), applying a temperature sensor (7) to the resistive layer (3), applying a predetermined current to the SMD resistor (1) and measuring the temperature of the resistive layer (3) using the temperature sensor (7) while applying the predetermined current, wherein the predetermined current correlates with a rated power of the SMD resistor (1) which is above a maximum rated power specified by the manufacturer for the SMD resistor (1), Calculating the thermal resistance of the SMD resistor (1) based on the measured temperature and the specified current, determining the derating curve of the SMD resistor (1) based on the specified current, the calculated thermal resistance and the maximum permissible temperature.

2. Method according to claim 1, wherein a thermocouple is used as the temperature sensor (7).

3. Method according to one of the preceding claims, wherein the temperature sensor (7) is glued onto the resistive layer (3).

4. Method according to one of the preceding claims, wherein the temperature sensor (7) is applied centrally to the resistive layer (3).

5. Method according to one of the preceding claims, wherein a limit temperature is calculated based on the maximum permissible temperature, the specified current and the calculated thermal resistance, above which the rated power at the SMD resistor (1) is reduced.

6. Method according to any of the preceding claims, wherein the specified current is correlated with a rated power of the SMD resistor (1) which is 101 to 500% of the maximum rated power specified by the manufacturer for the SMD resistor (1).

7. Method according to one of the preceding claims, wherein the specified current is correlated with a rated power of the SMD resistor (1) which is 120 to 400% of the maximum rated power specified by the manufacturer for the SMD resistor (1).

8. A method according to any of the preceding claims, wherein a defined number of identical SMD resistors (1) are provided and each SMD resistor (1) is applied to at least one test circuit board (4), wherein a temperature sensor (7) is applied to each resistive layer (3), wherein a predetermined current is applied to each SMD resistor (1) and the temperature of the resistive layer (3) is determined, wherein the thermal resistance is calculated for each SMD resistor (1), and the derating curve is determined based on the highest value of the calculated thermal resistances.

9. Method according to one of the preceding claims, wherein the SMD resistor (1) is to fulfill the type of ignition protection intrinsic safety.

10. Method according to one of the preceding claims, wherein several different SMD resistors (1) are applied to at least one test circuit board (4), wherein a temperature sensor (7) is applied to each resistive layer (3), wherein a predetermined current is applied to each SMD resistor (1) and the temperature of the resistive layer (3) is determined, wherein the thermal resistance is calculated for each SMD resistor (1) and the derating curve is determined based on the highest value of the calculated thermal resistances.

11. Method according to claim 10, wherein the determined derating curves of the SMD resistors (1) are compared with a predetermined application which requires a predetermined resistance at a predetermined temperature, wherein the SMD resistor (1) is selected whose determined derating curve at the predetermined temperature exhibits the predetermined resistance.

12. Method according to claim 11 , where, when selecting an SMD resistor (1) from among several SMD resistors (1) whose derating curve at the specified temperature exhibits the specified resistance, the SMD resistor (1) with the smallest component size is selected.

13. Use according to any of the preceding claims, wherein the calculated thermal resistance is divided by the percentage of a utilization of the SMD resistor (1) specified for the intrinsic safety type of ignition protection and the derating curve is adjusted accordingly.

14. Use of an SMD resistor (1) whose derating curve was determined according to one of claims 1-13, on a printed circuit board (8), wherein the SMD resistor is operated with a rated power which is above the maximum rated power specified by the manufacturer.

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