Miniature flow sensor and on-chip thermostatic control circuit for flow sensor

Abstract

This invention discloses a miniature flow sensor and an on-chip constant temperature difference control circuit for the flow sensor. The circuit includes a first resistor, a second resistor, an ambient resistor, an adjustable resistor, a micro-heater, an operational amplifier, a start-up resistor, and a start-up power supply. The ambient resistor and the micro-heater have the same temperature coefficient. One end of the first resistor is connected to one end of the second resistor and the output terminal of the operational amplifier. The other end of the first resistor is connected to one end of the start-up resistor, one end of the adjustable resistor, and the positive input terminal of the operational amplifier. The other end of the second resistor is connected to one end of the micro-heater and the negative input terminal of the operational amplifier. The other end of the start-up resistor is connected to the positive terminal of the start-up power supply. This constant temperature difference control circuit effectively maintains a constant overheating temperature of the micro-heater, thereby achieving a wide measurement range, low power consumption operation, and suppressing drift caused by ambient temperature, significantly improving the accuracy of the measurement results.

Description

Technical Field

[0001] This invention relates to the field of sensor technology, and more particularly to a miniature flow sensor and an on-chip constant temperature difference control circuit for the flow sensor. Background Technology

[0002] Flow measurement plays a vital role in industrial production, transportation, process control, aerospace, military, and medicine. Among the many flow sensors based on different principles, thermal flow sensors have attracted widespread attention due to their small size, absence of moving parts, simple structure, high accuracy, ability to measure flow direction, low power consumption, high sensitivity, and low output signal drift.

[0003] Generally, the detection process of micro thermal flow sensors based on MEMS technology involves multi-physics coupling processes involving fluid mechanics, thermodynamics, and electricity, as well as energy domain conversion. Therefore, thermal flow sensors must be paired with dedicated integrated circuits (ICs) for temperature control, signal amplification, temperature compensation, analog-to-digital conversion, and other functions to convert fluid velocity signals into electrical signals, ultimately enabling interaction with the external environment. Furthermore, CMOS integration significantly shortens the signal path length between chips, reducing manufacturing costs and area, minimizing parasitic capacitance, and enhancing robustness.

[0004] From the perspective of the sensing principle, thermal flow sensors require an essential micro-heating unit (R). h The flow rate is measured by manipulating the micro-heater of the thermal flow sensor, which is based on changes in the heat transfer behavior of the micro-unit. For the micro-heater in a thermal flow sensor, control circuitry is essential to maintain its operating mode. Typically, thermal flow sensors operate in two modes: one uses a constant current or constant voltage to drive the micro-heater, and the other uses a feedback circuit to maintain the micro-heater at a constant temperature. While the isothermal mode is more complex to implement, it helps thermal flow sensors achieve better resolution, frequency response, and measurement range.

[0005] However, these advantages are based on the precise control of the microheater to operate under isothermal conditions. Current research still relies on empirical methods to configure subsequent off-chip circuitry after the microheater has been fabricated and thermally characterized. This significantly increases sensor cost and module size. In 2019, researchers fabricated a molybdenum-thermal flow sensor and equipped it with a isothermal control circuit using a 0.18μm CMOS process. For N2 flow, this flow sensor achieved a high sensitivity of 0.71mV / (m / s) and a wide bidirectional detection capability of -26–26m / s. However, the operational amplifier used in the isothermal control circuit of this study had low gain, and the driving capability of the isothermal control circuit was not optimized for the on-chip microheater. As the flow rate increased, the heating temperature of the microheater decreased by approximately 6K, leading to a decrease in the sensor's measurement range. Further research revealed that the higher the microheater's operating temperature, the narrower the measurable flow range of the sensor. On the other hand, the existing constant temperature mode design still has shortcomings. When the ambient temperature changes, the output of the thermal micro flow sensor in constant temperature mode will drift.

[0006] For applications requiring high flow measurement accuracy, such as respiratory monitoring in medicine, it is essential to ensure precise control of the micro-heater and resistance to environmental changes to prevent output drift in thermal micro-flow sensors. Therefore, existing control circuits for thermal micro-flow sensors suffer from limitations such as small measurement range and output drift. Summary of the Invention

[0007] This invention provides a miniature flow sensor and an on-chip constant temperature difference control circuit for the flow sensor, aiming to solve the problems of small range and output drift in the control circuits used for thermal miniature flow sensors in the prior art.

[0008] In a first aspect, embodiments of the present invention disclose an on-chip constant temperature difference control circuit for a flow sensor, the circuit comprising a first resistor, a second resistor, an ambient resistor, an adjustable resistor, a micro heater, an operational amplifier, a start-up resistor, and a start-up power supply; the ambient resistor has the same temperature coefficient as the micro heater;

[0009] One end of the first resistor is connected to one end of the second resistor and the output terminal of the operational amplifier; the other end of the first resistor is connected to one end of the start-up resistor, one end of the adjustable resistor, and the positive input terminal of the operational amplifier; the other end of the second resistor is connected to one end of the micro heater and the negative input terminal of the operational amplifier; the other end of the start-up resistor is connected to the positive terminal of the start-up power supply.

[0010] The other end of the adjustable resistor is connected to one end of the ambient resistor, and the other end of the ambient resistor is connected to the other end of the micro heater and the negative terminal of the starting power supply and grounded.

[0011] The on-chip constant temperature difference control circuit for the flow sensor, wherein the adjustable resistor is an adjustable resistor without a temperature coefficient.

[0012] The on-chip constant temperature difference control circuit for the flow sensor is described above, wherein the operational amplifier is a two-stage operational amplifier.

[0013] The on-chip constant temperature difference control circuit for the flow sensor includes an operational amplifier comprising a first MOS transistor, a second MOS transistor, a third MOS transistor, a fourth MOS transistor, a fifth MOS transistor, a sixth MOS transistor, a seventh MOS transistor, an eighth MOS transistor, a ninth MOS transistor, a tenth MOS transistor, an eleventh MOS transistor, a third resistor, and a fourth resistor.

[0014] The source of the first MOSFET is connected to the source of the second MOSFET and a current source; the gate of the first MOSFET serves as the negative input terminal of the operational amplifier, and the gate of the second MOSFET serves as the positive input terminal of the operational amplifier; the drain of the first MOSFET is connected to the source of the eighth MOSFET and the drain of the tenth MOSFET; the drain of the second MOSFET is connected to the source of the ninth MOSFET and the drain of the eleventh MOSFET; the gate of the eighth MOSFET is connected to the gate of the ninth MOSFET, the gate of the tenth MOSFET is connected to the gate of the eleventh MOSFET, and the source of the tenth MOSFET is connected to the source of the eleventh MOSFET and grounded;

[0015] The drain of the eighth MOS transistor is connected to one end of the third resistor, the gate of the fifth MOS transistor, and the gate of the sixth MOS transistor; the drain of the ninth MOS transistor is connected to one end of the fourth resistor.

[0016] The other end of the third resistor is connected to the drain of the fifth MOS transistor, the gate of the third MOS transistor, and the gate of the fourth MOS transistor; the other end of the fourth resistor is connected to the drain of the sixth MOS transistor and the gate of the seventh MOS transistor.

[0017] The source of the fifth MOS transistor is connected to the drain of the third MOS transistor, the source of the sixth MOS transistor is connected to the drain of the fourth MOS transistor, and the drain of the seventh MOS transistor serves as the output terminal of the operational amplifier.

[0018] The source of the third MOS transistor is connected to the source of the fourth MOS transistor, the source of the seventh MOS transistor, and the power supply voltage.

[0019] The on-chip constant temperature difference control circuit for the flow sensor is described above, wherein the first MOS transistor, the second MOS transistor, the third MOS transistor, the fourth MOS transistor, the fifth MOS transistor, the sixth MOS transistor, and the seventh MOS transistor are all P-MOS transistors.

[0020] In the on-chip constant temperature difference control circuit for the flow sensor, the eighth MOS transistor, the ninth MOS transistor, the tenth MOS transistor, and the eleventh MOS transistor are all N-MOS transistors.

[0021] The on-chip constant temperature difference control circuit for the flow sensor is described above, wherein the output terminal of the operational amplifier is connected to one end of the load resistor, and the other end of the load resistor is grounded.

[0022] The on-chip constant temperature difference control circuit for the flow sensor, wherein the power supply voltage is 3.3V.

[0023] In a second aspect, embodiments of the present invention also disclose a miniature flow sensor, wherein the miniature flow sensor is equipped with an on-chip constant temperature difference control circuit for a flow sensor as described in the first aspect above.

[0024] The miniature flow sensor includes a substrate, and a first microstrip line and a second microstrip line mounted on the substrate;

[0025] The first microstrip line is mounted on a cavity provided on the substrate, and the second microstrip line is disposed on the upper end surface of the substrate; the first microstrip line and the second microstrip line are located on the same plane and are spaced apart;

[0026] The first microstrip line serves as the microheater, and the two ends of the first microstrip line serve as the two connection terminals of the microheater; the second microstrip line serves as the ambient resistor, and the two ends of the first microstrip line serve as the two connection terminals of the ambient resistor.

[0027] The aforementioned miniature flow sensor, wherein the substrate is a silicon substrate.

[0028] This application discloses a miniature flow sensor and an on-chip constant temperature difference control circuit for the flow sensor. The circuit includes a first resistor, a second resistor, an ambient resistor, an adjustable resistor, a microheater, an operational amplifier, a start-up resistor, and a start-up power supply. The ambient resistor and the microheater have the same temperature coefficient. One end of the first resistor is connected to one end of the second resistor and the output terminal of the operational amplifier. The other end of the first resistor is connected to one end of the start-up resistor, one end of the adjustable resistor, and the positive input terminal of the operational amplifier. The other end of the second resistor is connected to one end of the microheater and the negative input terminal of the operational amplifier. The other end of the start-up resistor is connected to the positive terminal of the start-up power supply. The other end of the adjustable resistor is connected to one end of the ambient resistor, and the other end of the ambient resistor is connected to the other end of the microheater and the negative terminal of the start-up power supply, and grounded. This constant temperature difference control circuit effectively maintains the overheating temperature of the microheater constant, thereby achieving a wide measurement range, low power consumption operation, and suppressing drift caused by ambient temperature, significantly improving the accuracy of the measurement results. Attached Figure Description

[0029] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 A circuit structure diagram of the constant temperature difference control circuit provided in an embodiment of the present invention;

[0031] Figure 2 A partial circuit structure diagram of the constant temperature difference control circuit provided in an embodiment of the present invention;

[0032] Figure 3 Equivalent circuit diagram provided for embodiments of the present invention;

[0033] Figure 4 This is an overall structural diagram of the sensing unit in the miniature flow sensor provided in an embodiment of the present invention;

[0034] Figure 5 This is a side view of the sensing unit in the miniature flow sensor provided in an embodiment of the present invention;

[0035] Figure 6 This is a schematic diagram illustrating the effect of the constant temperature difference control circuit provided in an embodiment of the present invention;

[0036] Figure 7 This is another schematic diagram illustrating the effect of the constant temperature difference control circuit provided in an embodiment of the present invention;

[0037] Figure 8This is another schematic diagram illustrating the effect of the constant temperature difference control circuit provided in an embodiment of the present invention;

[0038] Figure 9 This is another schematic diagram illustrating the effect of the constant temperature difference control circuit provided in the embodiment of the present invention;

[0039] Figure 10 This is a schematic diagram illustrating the effect of the constant temperature difference control circuit provided in an embodiment of the present invention.

[0040] Reference numerals: R1, first resistor; R2, second resistor; R r Environmental resistance; R c Adjustable resistor; R h Microheaters; OP-AMP; operational amplifiers; R off Starting resistor; V off 1. Power supply; M1, first MOSFET; M2, second MOSFET; M3, third MOSFET; M4, fourth MOSFET; M5, fifth MOSFET; M6, sixth MOSFET; M7, seventh MOSFET; M8, eighth MOSFET; M9, ninth MOSFET; M 10 10th MOSFET; M 11 11. MOSFET; R3. Resistor 3; R4. Resistor 4; 10. Substrate; 11. First microstrip line; 12. Second microstrip line; 101. Cavity; 13. Basic control circuit; Ib. Current source. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0043] It should also be understood that the terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0044] It should also be further understood that the term "and / or" as used in this specification and the appended claims refers to any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0045] This invention discloses an on-chip constant temperature difference control circuit for a flow sensor, the circuit including a first resistor R1, a second resistor R2, and an ambient resistor R. r Adjustable resistor R c Micro heater R h Operational amplifier (OP-AMP), start-up resistor R off and starting power supply V off The environmental resistance R r With the micro heater R h The temperature coefficients are the same; one end of the first resistor R1 is connected to one end of the second resistor R2 and the output terminal of the operational amplifier OP-AMP; the other end of the first resistor R1 is connected to the start-up resistor R off One end of the adjustable resistor R c One end of the second resistor R2 is connected to the positive input terminal of the operational amplifier OP-AMP; the other end of the second resistor R2 is connected to the micro heater R. h One end of the operational amplifier (OP-AMP) and the negative input terminal of the OP-AMP; the start-up resistor R off The other end is connected to the starting power supply V. off The positive terminal; the adjustable resistor R c The other end is connected to the ambient resistor R r At one end, the environmental resistance R r The other end is connected to the micro heater R h The other end and the starting power supply V off The negative terminal is grounded. The starting resistor and starting power supply combination constitute the starting circuit, which is used to provide the initial starting current of the circuit.

[0046] The constant temperature difference control circuit in this application's technical method is specifically designed for MEMS (Micro-Electro-Mechanical System) thermal flow sensors. For example... Figure 1 As shown, the constant temperature difference control circuit includes an operational amplifier (OP-AMP) and a micro-heater (R). h The Wheatstone bridge, in which the micro-heater R h That is, the thermistor being measured, the adjustable resistor R c For adjustable resistor R without temperature coefficient c Its working principle can be simply summarized as follows: when the flow rate increases, the micro-heater R... hThe temperature of the heating element decreases due to the cooling effect, causing its resistance to decrease. This increases the output voltage of the operational amplifier (OP-AMP) due to the increased input voltage difference, generating more power to prevent the resistance of the heating element from decreasing, ultimately leading to circuit rebalancing. The micro-heater R... h The temperature remains the same as before. Set the ambient resistance R... r With micro heater R h Having the same temperature coefficient enables the circuit to have a constant temperature difference control function. Based on the constant temperature difference control, the thermal flow sensor can achieve the effects of low power consumption, low drift, and accurate measurement.

[0047] To achieve the coupling of the microheater R h The integration design and optimization of its interface circuit require the establishment of a micro heater R. h The analytical model was used to perform circuit performance analysis in the design process of the Wheatstone bridge and operational amplifier (OP-AMP) of the constant temperature difference control circuit.

[0048] Specifically, the power loss of the microheater in a MEMS flow sensor, which exchanges heat with the medium, is often described using the semi-empirical equation King's law. On the other hand, the resistance of the microheater is calculated using the temperature coefficient, as shown in formula (1).

[0049]

[0050] Among them, V h It is the voltage across the micro heater, I h This represents the current passing through the microheater. A, B, and n are constants, where A is the heat loss coefficient considering conduction, radiation, and free convection, and BU is the constant. n This indicates forced convection, where the exponent n depends on the sensor structure, and ΔT h This indicates the superheat temperature, which is the operating temperature of the microheater (T). h ) and ambient temperature (T) a The difference between ρ and ρ heater V heater C heater Let α and R represent the density, volume, and specific heat capacity of the microheater, respectively; τ represent time; and R represent the time. h0 These are the temperature coefficient and resistance of the heating element at the reference temperature T0, respectively.

[0051] According to formula (1), an equivalent circuit model can be established, and the equivalent circuit diagram is as follows. Figure 3 As shown, the actual performance of the control circuit can be evaluated by coupling analysis based on the equivalent circuit model of the microheater and its dedicated constant temperature difference control circuit. The calculation formula shown in equation (2) can be derived from the equivalent circuit model:

[0052]

[0053] To obtain the values ​​of A, B, and n, the relationship between the power consumption P of the microheater and the flow rate U needs to be established. The heat exchange between the microheater and other media in various directions is as follows: Figure 5 As shown. Figure 5 Figure (a) reveals the power consumption sources of the microheater in the vertical and horizontal directions from the cross-sectional view of A-A'. The heat exchange in the horizontal and vertical directions and below is mainly due to heat conduction, while the heat exchange above is mainly due to convective heat transfer and heat conduction between the surface of the microheater and the fluid. Figure 5 Figures (b) and (c) reveal the sources of power consumption in the front-to-back direction of the microheater from the cross-sectional view (B-B') and top view, respectively. The heat exchange in the front-to-back direction mainly originates from the heat conduction between the microheater and the support beam and the air. It is worth noting that the thermal conductivity of the front and rear support beams is different because there are metal wires in the front support beam used to connect the microheater to other circuits. According to the law of conservation of energy, the following formula (3) can be obtained:

[0054]

[0055] Where P represents power consumption, conv represents heat convection, cond represents heat conduction, f represents the fluid above the thin film (the airflow measurement film formed by wrapping the microheater, with the film located above the cavity), s represents the thin film, f1 represents the fluid below the thin film, and x, y, and z represent the three coordinate axes, with positive and negative signs indicating the direction of the coordinate axes.

[0056] For all the heat conduction terms in the above formula (3), formula (4) can be derived from the definition of thermal conductivity:

[0057]

[0058] Where, k s k f1 L represents the thermal conductivity of the thin film and the fluid beneath it, respectively. h 2W represents the length and width of the microheater, respectively; t represents the film thickness; 2L and h represent the length and height of the bottom cavity, respectively; k s1 k s2 W represents the thermal conductivity of the rear and front support beams, respectively. b and L b These represent the width and length of the supporting beam, respectively.

[0059] For the heat convection term in the x-axis direction, formula (5) can be derived according to the definition of the convective heat transfer coefficient:

[0060]

[0061] in, This represents the average convective heat transfer coefficient.

[0062] For the average convective heat transfer coefficient in formula (5) Formula (6) can be used for calculation.

[0063]

[0064] Among them, 2L c k represents the width of the chip. f ρ and C represent the thermal conductivity of the fluid above the thin film. p The density, specific heat capacity, and flow velocity of the fluid above the U-film are given by equations (3) to (6), where μ represents the dynamic viscosity. By simultaneously solving equations (3) to (6), the power consumption P of the microheater at different flow velocities U can be obtained, as shown below. Figure 6 As shown. The results show that the power consumption obtained through theoretical analysis using the microheater model is in good agreement with the test results. According to formula (1), the values ​​of A, B, and n can be obtained by fitting the relationship between the power consumption of the microheater and the flow rate.

[0065] The novel constant temperature difference control circuit described above is based on the optimization and improvement of the existing circuit structure, so that the difference between the working temperature of the micro heater and the ambient temperature remains constant, thereby avoiding the output of the thermal flow sensor from drifting due to changes in the ambient temperature. Figure 1 The aforementioned constant temperature difference control circuit includes a carefully designed operational amplifier and a Wheatstone bridge equipped with a micro-heater. The Wheatstone bridge is a fundamental circuit in electrical engineering, widely used in automatic sensor control, automatic detection, and many other fields due to its simple structure, high accuracy, and high sensitivity. Generally, a Wheatstone bridge has four arms arranged in a diamond shape, with resistance on each arm. Figure 1 The Wheatstone bridge in the constant temperature difference control circuit shows the relationship between the resistance of the micro heater and the temperature as shown in formula (7):

[0066] R h =R h0 [1+α(T h -T0)] (7);

[0067] Among them, R h0 It is R h The resistance at reference temperature T0, α is R h The temperature coefficient. R1 and R2 are used to determine the ratio of the left and right sides of the bridge, and their ratio is set to a fixed value as needed (generally, the larger the ratio, the lower the power consumption of the circuit):

[0068]

[0069] R rAs an environmental resistor, it is fabricated on the same system-on-a-chip. Due to the very high thermal conductivity of silicon, R... r The temperature is equal to the ambient temperature, thus serving to detect the ambient temperature. Additionally, R... r The materials used in its production and R h They are the same, both have the same temperature coefficient, and R r With R h The ratio at the reference temperature T0 is set to be equal to n, therefore:

[0070]

[0071] Among them, R r0 It is R r The resistance at reference temperature T0, T a The ambient temperature is used. When the control circuit is balanced, according to the virtual short and virtual open conditions, the input voltage of the operational amplifier (high amplification factor) satisfies Vin+ = Vin-. At this time:

[0072]

[0073] Substituting formulas (7) to (9) into formula (10), we obtain the superheat temperature as follows:

[0074]

[0075] In practice, R c Since it is an adjustable resistor without a temperature coefficient, it can be seen from equation (11) that by changing R... c Different values ​​of R can be used to obtain different superheat temperatures, once R... c Once the value is determined, the superheating temperature of the micro heater remains constant and is unaffected by the ambient temperature, meaning the thermal flow sensor operates in constant temperature difference mode.

[0076] The operational amplifier OP-AMP is a two-stage operational amplifier OP-AMP. The operational amplifier OP-AMP includes a first MOSFET M1, a second MOSFET M2, a third MOSFET M3, a fourth MOSFET M4, a fifth MOSFET M5, a sixth MOSFET M6, a seventh MOSFET M7, an eighth MOSFET M8, a ninth MOSFET M9, and a tenth MOSFET M1. 10 11th MOSFET M 11The first MOSFET M1 is connected to the source of the second MOSFET M2 and the current source Ib; the gate of the first MOSFET M1 serves as the negative input terminal of the operational amplifier OP-AMP, and the gate of the second MOSFET M2 serves as the positive input terminal of the operational amplifier OP-AMP; the drain of the first MOSFET M1 is connected to the source of the eighth MOSFET M8 and the tenth MOSFET M1. 10 The drain of the second MOS transistor M2 is connected to the source of the ninth MOS transistor M9 and the eleventh MOS transistor M1. 11 The drain of the eighth MOS transistor M8; the gate of the eighth MOS transistor M8 is connected to the gate of the ninth MOS transistor M9, and the tenth MOS transistor M... 10 The gate of the eleventh MOS transistor M is connected. 11 The gate of the tenth MOS transistor M 10 The source of the eleventh MOS transistor M is connected. 11 The source of the fifth MOSFET M8 is grounded; the drain of the eighth MOSFET M8 is connected to one end of the third resistor R3, the gate of the fifth MOSFET M5, and the gate of the sixth MOSFET M6; the drain of the ninth MOSFET M9 is connected to one end of the fourth resistor R4; the other end of the third resistor R3 is connected to the drain of the fifth MOSFET M5, the gate of the third MOSFET M3, and the gate of the fourth MOSFET M4; the other end of the fourth resistor R4 is connected to the drain of the sixth MOSFET M6 and the gate of the seventh MOSFET M7; the source of the fifth MOSFET M5 is connected to the drain of the third MOSFET M3; the source of the sixth MOSFET M6 is connected to the drain of the fourth MOSFET M4; the drain of the seventh MOSFET M7 serves as the output terminal of the operational amplifier OP-AMP; the source of the third MOSFET M3 is connected to the source of the fourth MOSFET M4, the source of the seventh MOSFET M7, and the power supply voltage.

[0077] Specifically, the first MOSFET M1, the second MOSFET M2, the third MOSFET M3, the fourth MOSFET M4, the fifth MOSFET M5, the sixth MOSFET M6, and the seventh MOSFET M7 are all P-MOSFETs. The eighth MOSFET M8, the ninth MOSFET M9, and the tenth MOSFET M1 are all P-MOSFETs. 10 and the eleventh MOS transistor M 11 All are N-MOS transistors.

[0078] In a more specific embodiment, the output of the operational amplifier (OP-AMP) is connected to one end of a load resistor, and the other end of the load resistor is grounded. The power supply voltage is 3.3V.

[0079] As can be seen from the principle of the Wheatstone bridge described above, one of the decisive conditions for the microheater's overheating temperature to reach the expected level and remain constant (i.e., precisely controlled) is that the operational amplifier is sufficient to ensure Vin+ = Vin-. Furthermore, the increase in the microheater's temperature requires a certain amount of power. Based on these two considerations, the operational amplifier used in the constant temperature difference control circuit not only needs to have high gain but also strong driving capability. Therefore, employing... Figure 2 The two-stage amplifier circuit shown constitutes an operational amplifier used in a constant temperature difference control circuit. The first stage is the gain stage, mainly responsible for signal amplification, and uses a folded cascode PMOS differential input pair amplifier circuit. The second stage is the output stage, mainly used to enhance the driving capability and increase the output swing. Since only the output current needs to drive the Wheatstone bridge in the constant temperature difference control of the thermal flow sensor, and there is no need to draw current from the outside, in order to reduce circuit complexity and additional power consumption, the commonly used push-pull output structure is simplified to a single PMOS. With a power supply voltage of 3.3V, the power consumption of this operational amplifier is as low as 0.13mW.

[0080] Figure 7 Figure (a) shows the AC test diagram of the operational amplifier. The AC test results of the operational amplifier with an input common-mode voltage of 1.65V and a load of 1.5kΩ are as follows. Figure 7 As shown in Figure (a), the operational amplifier has an open-loop gain of 108.6 dB, which is sufficient to reduce the difference between Vin+ and Vin- in the constant temperature difference control circuit to as low as tens of microvolts, thus meeting the gain requirements of the constant temperature difference control circuit. Furthermore, the operational amplifier has a phase margin of approximately 62.8°, which meets the stability requirements.

[0081] Figure 7 Figure (b) shows the theoretical test results of the operational amplifier's driving capability. For the operational amplifier's driving capability, the following parameters are used: Figure 7 The negative feedback circuit shown in Figure (b) was simulated. When the curve reaches an inflection point (i.e., when it enters the saturation region from the operating region), it indicates that the amplifier's output voltage or output current has reached saturation. The results show that for a 1.5kΩ load, the operational amplifier's output voltage can reach 2.9V and the output current can reach 1.93mA.

[0082] Additionally, because the operational amplifier uses a PMOS differential input pair, when the input voltage is too high (expected to exceed V), DD -V th,PMOS When the PMOS differential pair is cut off, the operational amplifier will not work. To avoid this situation, the ratio of the upper and lower resistors of the Wheatstone bridge must meet the following conditions:

[0083]

[0084] Among them, V DD This indicates the power supply voltage, which is 3.3V in this invention. th，pmos The threshold voltage of the PMOS differential pair is -589mV in this invention.

[0085] This application also discloses a miniature flow sensor, such as... Figure 4 As shown, the miniature flow sensor is equipped with the on-chip constant temperature difference control circuit for the flow sensor described in the above embodiments; the miniature flow sensor includes a substrate 10, and a first microstrip line 11 and a second microstrip line 12 mounted on the substrate 10; the first microstrip line 11 is mounted on a cavity 101 provided on the substrate 10, and the second microstrip line 12 is disposed on the upper end surface of the substrate 10; the first microstrip line 11 and the second microstrip line 12 are located on the same plane and are spaced apart; the first microstrip line 11 serves as the micro heater R. h The two ends of the first microstrip line 11 serve as the microheater R. h The two connection terminals; the second microstrip line 12 serves as the environmental resistor R. r The two ends of the second microstrip line 12 serve as the ambient resistance R. r The two connection ends. The substrate 10 is a silicon substrate.

[0086] To test the actual performance of the thermal flow sensor based on the aforementioned constant temperature difference control circuit in the technical method of this application, a corresponding monolithic integrated thermal miniature flow sensor can be manufactured. The sensing unit in the miniature flow sensor is as follows: Figure 4 As shown, the first microstrip line 11 is mounted on the cavity 101 of the substrate 10, and the second microstrip line 12 is disposed on the side of the first microstrip line 11. The second microstrip line 12 can be positioned upstream or downstream of the first microstrip line 11 along the airflow direction. In the constant temperature difference control circuit, besides the ambient resistance R... r With micro heater R h The other circuit components are combined to form the basic control circuit 13, which is located on one side of the cavity 101. The specific structure is as follows: Figure 4 As shown. Actual measurements were performed on the manufactured miniature flow sensor, where the operational amplifier's own power consumption was only 0.1254mW. Figure 8 The actual test results for the operational amplifier's driving capability show that, with a load of 1.5kΩ, the operational amplifier's output swing and current driving capability can reach 2.82V and 1.88mA, respectively. Compared with the theoretical results, the change in its operating range is less than 2.8%.

[0087] Multiple measurements were performed on the constant temperature difference control circuit at room temperature, and the results are as follows: Figure 9 As shown. Figure 9 Figure (a) shows the relationship between voltage and flow rate of the microheater. The errors between the three measurement data and the theoretical results are 1.73%, 1.87%, and 1.77%, respectively. This indicates that the theoretical results of the microheater analytical model can be in good agreement with the measurement results. Figure 7 Figure (b) in the figure evaluates the effect of flow rate on the superheating temperature ΔT of the microheater. h Due to the influence of the flow rate, the average temperature differences of the three measurements were 48.18℃, 48.99℃, and 49.31℃, with standard deviations of 0.54℃, 0.52℃, and 0.42℃, respectively. The errors from the theoretical results were 1.54%, 0.12%, and 0.77%, respectively. Despite the change in flow rate, the superheated temperature measured in multiple measurements fluctuated around the theoretical results of the microheater model. This indicates that the control circuit achieved constant temperature difference control of the microheater at room temperature, thanks to the high gain and strong drive design of the operational amplifier.

[0088] Figure 10 The results are from measurements taken on a constant temperature difference control circuit at ambient temperatures of 15℃, 25℃, and 35℃. Figure 10 Figure (a) shows the relationship between the voltage and flow rate of the microheater. The errors between the actual test data and the theoretical results of the microheater analytical model at different ambient temperatures are 0.43%, 1.63%, and 2.77%, respectively. This indicates that the theoretical results of the microheater analytical model can match the measurement results well. Figure 10 Figure (b) in the figure evaluates the effect of flow rate on the superheating temperature ΔT of the microheater at different ambient temperatures. h The effects of ambient temperature on the microheater were investigated. Under the control of the high gain and strong driving capability of the operational amplifier, the superheated temperatures of the microheater were 47.41℃, 46.93℃, and 48.27℃, with standard deviations of 0.38℃, 0.47℃, and 0.46℃, respectively. The errors from the theoretical results were 1.64%, 2.27%, and 0.88%, respectively. The measurement results show that the superheated temperatures of the microheater under different ambient temperatures are basically close to the theoretical results of the model, and the drift caused by ambient temperature is suppressed. That is, the control circuit has achieved precise constant temperature difference control of the microheater.

[0089] This invention presents a low-power, low-drift, and precise constant temperature difference control circuit for the microheater of a MEMS thermal flow sensor, involving an operational amplifier and a Wheatstone bridge. Actual testing shows that, thanks to the high gain and strong driving capability of the operational amplifier and the appropriate configuration of the Wheatstone bridge, the constant temperature difference control circuit can resist the influence of ambient temperature changes and effectively maintain the overheating temperature of the microheater at a constant level. Furthermore, to achieve integrated design and optimized adjustment of the constant temperature difference control circuit, this invention establishes an analytical model for the microheater, and the theoretical results of the model agree well with the actual test data.

[0090] This invention discloses a miniature flow sensor and an on-chip constant temperature difference control circuit for the flow sensor. The circuit includes a first resistor, a second resistor, an ambient resistor, an adjustable resistor, a micro-heater, an operational amplifier, a start-up resistor, and a start-up power supply. The ambient resistor and the micro-heater have the same temperature coefficient. One end of the first resistor is connected to one end of the second resistor and the output terminal of the operational amplifier. The other end of the first resistor is connected to one end of the start-up resistor, one end of the adjustable resistor, and the positive input terminal of the operational amplifier. The other end of the second resistor is connected to one end of the micro-heater and the negative input terminal of the operational amplifier. The other end of the start-up resistor is connected to the positive terminal of the start-up power supply. The other end of the adjustable resistor is connected to one end of the ambient resistor, and the other end of the ambient resistor is connected to the other end of the micro-heater and the negative terminal of the start-up power supply, and grounded. This constant temperature difference control circuit effectively maintains a constant overheating temperature of the micro-heater, thereby achieving a wider measurement range, low power consumption operation, and suppressing drift caused by ambient temperature, significantly improving the accuracy of the measurement results.

[0091] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An on-chip constant temperature difference control circuit for a flow sensor, characterized in that, The circuit includes a first resistor, a second resistor, an ambient resistor, an adjustable resistor, a microheater, an operational amplifier, a start-up resistor, and a start-up power supply; the ambient resistor has the same temperature coefficient as the microheater. One end of the first resistor is connected to one end of the second resistor and the output terminal of the operational amplifier; the other end of the first resistor is connected to one end of the start-up resistor, one end of the adjustable resistor, and the positive input terminal of the operational amplifier; the other end of the second resistor is connected to one end of the micro heater and the negative input terminal of the operational amplifier; the other end of the start-up resistor is connected to the positive terminal of the start-up power supply. The other end of the adjustable resistor is connected to one end of the ambient resistor, and the other end of the ambient resistor is connected to the other end of the micro heater and the negative terminal of the starting power supply and grounded. The operational amplifier includes a first MOSFET, a second MOSFET, a third MOSFET, a fourth MOSFET, a fifth MOSFET, a sixth MOSFET, a seventh MOSFET, an eighth MOSFET, a ninth MOSFET, a tenth MOSFET, an eleventh MOSFET, a third resistor, and a fourth resistor; The source of the first MOSFET is connected to the source of the second MOSFET and a current source; the gate of the first MOSFET serves as the negative input terminal of the operational amplifier, and the gate of the second MOSFET serves as the positive input terminal of the operational amplifier; the drain of the first MOSFET is connected to the source of the eighth MOSFET and the drain of the tenth MOSFET; the drain of the second MOSFET is connected to the source of the ninth MOSFET and the drain of the eleventh MOSFET; the gate of the eighth MOSFET is connected to the gate of the ninth MOSFET, the gate of the tenth MOSFET is connected to the gate of the eleventh MOSFET, and the source of the tenth MOSFET is connected to the source of the eleventh MOSFET and grounded; The drain of the eighth MOS transistor is connected to one end of the third resistor, the gate of the fifth MOS transistor, and the gate of the sixth MOS transistor; the drain of the ninth MOS transistor is connected to one end of the fourth resistor. The other end of the third resistor is connected to the drain of the fifth MOS transistor, the gate of the third MOS transistor, and the gate of the fourth MOS transistor; the other end of the fourth resistor is connected to the drain of the sixth MOS transistor and the gate of the seventh MOS transistor. The source of the fifth MOS transistor is connected to the drain of the third MOS transistor, the source of the sixth MOS transistor is connected to the drain of the fourth MOS transistor, and the drain of the seventh MOS transistor serves as the output terminal of the operational amplifier. The source of the third MOS transistor is connected to the source of the fourth MOS transistor, the source of the seventh MOS transistor, and the power supply voltage.

2. The on-chip constant temperature difference control circuit for a flow sensor according to claim 1, characterized in that, The adjustable resistor is an adjustable resistor without a temperature coefficient.

3. The on-chip constant temperature difference control circuit for a flow sensor according to claim 1, characterized in that, The operational amplifier is a two-stage operational amplifier.

4. The on-chip constant temperature difference control circuit for a flow sensor according to claim 1, characterized in that, The first MOS transistor, the second MOS transistor, the third MOS transistor, the fourth MOS transistor, the fifth MOS transistor, the sixth MOS transistor, and the seventh MOS transistor are all P-MOS transistors.

5. The on-chip constant temperature difference control circuit for a flow sensor according to claim 1, characterized in that, The eighth MOS transistor, the ninth MOS transistor, the tenth MOS transistor, and the eleventh MOS transistor are all N-MOS transistors.

6. The on-chip constant temperature difference control circuit for a flow sensor according to any one of claims 1-3, characterized in that, The output terminal of the operational amplifier is connected to one end of the load resistor, and the other end of the load resistor is grounded.

7. The on-chip constant temperature difference control circuit for a flow sensor according to claim 1, characterized in that, The power supply voltage is 3.3V.

8. A miniature flow sensor, characterized in that, The miniature flow sensor uses the on-chip thermostatic control circuit for flow sensors as described in any one of claims 1-7; The miniature flow sensor includes a substrate, and a first microstrip line and a second microstrip line mounted on the substrate; The first microstrip line is mounted on a cavity provided on the substrate, and the second microstrip line is disposed on the upper end surface of the substrate; The first microstrip line and the second microstrip line are located on the same plane and are spaced apart; The first microstrip line serves as the microheater, and the two ends of the first microstrip line serve as the two connection terminals of the microheater; the second microstrip line serves as the ambient resistor, and the two ends of the first microstrip line serve as the two connection terminals of the ambient resistor.

9. The miniature flow sensor according to claim 8, characterized in that, The substrate is a silicon substrate.

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