Constant power drive scheme and thermal flow sensor

Abstract

The application provides a constant power driving scheme and a thermal flow sensor. In the constant power driving scheme, a sampling resistor is connected in series with a circuit through which working current of a heating element passes. A voltage drop on the sampling resistor is collected by a sampling circuit. The voltage drop is converted into a sampling voltage positively correlated with the voltage drop. The sampling voltage is compared with a reference voltage determined according to a target heating power required by the heating element and an actual resistance of the heating element. When the sampling voltage is equal to the reference voltage, the heating power of the heating element is kept at the target heating power. When the sampling voltage is higher than the reference voltage, a DC voltage converter in a control circuit reduces an output voltage provided to the heating element. Conversely, the DC voltage converter increases the output voltage provided to the heating element. The scheme of the application can keep the heating power of the heating element in the thermal flow sensor at the target heating power, and improve the detection accuracy of the thermal flow sensor.

Description

Technical Field

[0001] This application relates to the field of circuit technology, and in particular to a constant power drive scheme and a thermal flow sensor. Background Technology

[0002] Taking a respiratory humidifier as an example, a respiratory humidifier provides heating and humidification of the breathing gas supplied to the patient. This is necessary to maintain the normal clearance function of the mucociliary system on the airway surface and the normal contraction and diffusion characteristics of the alveolar epithelium, thus reducing airway dryness and inflammatory symptoms. Therefore, a respiratory humidifier needs to accurately monitor the flow rate of the breathing gas supplied to the patient. Based on this flow rate, the liquid in the tank is heated to bring the humidity of the breathing gas to a stable set value. Simultaneously, the heating power of the tubing is controlled according to this flow rate to ensure the breathing gas reaches the set temperature. Currently, most mainstream respiratory humidifiers use thermal flow sensors for flow detection. Their working principle is as follows: a constant current is supplied to the heating element inside the sensor to maintain it at a stable temperature. When gas flows through, it carries away the heat from the heating element, causing its temperature to drop. The magnitude of the temperature drop is positively correlated with the gas flow rate. Therefore, the gas flow rate can be calculated by monitoring the temperature change of the heating element (usually converted into an electrical signal).

[0003] In existing technologies, the mainstream approach to providing driving energy to the heating element of a thermal flow sensor is to use a fixed constant current source circuit with a fixed output current. This circuit is typically built entirely in hardware, and its output current value is set to a constant value during circuit design. During operation, this fixed constant current source directly outputs a constant current to the heating element, which serves as the reference for both heating and flow detection.

[0004] The actual heating power generated by the heating element follows the formula ,in For driving current, This refers to the resistance of the heating element. A fixed constant current source can only guarantee the current. The resistance of the heating element is constant; however, due to unavoidable manufacturing errors in the actual production of the heating element, the resistance at a constant current... Differences in resistance can cause deviations in the heating power of the heating element. Since thermal flow sensors rely on the heating power of the heating element as a benchmark for flow detection, deviations in heating power can distort the flow detection signal. This is especially true at low flow rates, where even small power deviations can be amplified, leading to a decrease in flow detection accuracy and failing to meet the precision requirements of high-precision medical equipment. Summary of the Invention

[0005] This application provides a constant power drive scheme and a thermal flow sensor to solve the problem that in the prior art, when a fixed output current is provided to the heating element of a thermal flow sensor, the heating power of the heating element will be incorrect under a fixed output current due to unavoidable process errors in the resistance of the heating element during actual manufacturing, which in turn leads to a decrease in the detection accuracy of the thermal flow sensor.

[0006] In a first aspect, this application provides a constant power drive scheme, which includes a controllable constant current source circuit and a software control module. The controllable constant current source circuit includes a DC-DC voltage converter, a sampling resistor, a current sampling circuit, a comparator, and an output voltage control circuit.

[0007] The positive terminal of the DC power supply is connected to the voltage input terminal of the DC voltage converter, and the negative terminal of the DC power supply is connected to the ground terminal of the DC voltage converter.

[0008] The voltage output terminal of the DC voltage converter is connected to one end of the heating element, and the other end of the heating element is connected to the negative terminal. The DC voltage converter is used to convert the DC input voltage of the DC power supply into a DC output voltage output to the heating element.

[0009] The sampling resistor is connected in series in the circuit through which the working current flowing through the heating element passes. The current sampling circuit is connected to both ends of the sampling resistor and the first inverting input terminal of the comparator. The current sampling circuit is used to convert the voltage drop across the sampling resistor into a sampling voltage that is positively correlated with the voltage drop, and input the sampling voltage to the first inverting input terminal.

[0010] The output terminal of the comparator is connected to the feedback voltage input terminal of the DC voltage converter through the output voltage control circuit. After the software control module determines the reference voltage based on the target heating power of the heating element and the actual resistance of the heating element, it outputs the reference voltage to the first non-inverting input terminal of the comparator. When the sampling voltage is equal to the reference voltage, the actual heating power of the heating element is the target heating power.

[0011] The comparator is used to: when the sampled voltage is greater than the reference voltage, output a first level signal through the output terminal of the comparator to control the DC voltage converter to reduce the DC output voltage; and when the sampled voltage is less than the reference voltage, output a second level signal through the output terminal of the comparator to control the DC voltage converter to increase the DC output voltage.

[0012] Optionally, the sampling resistor is connected in series in the circuit connecting the voltage output terminal and the heating element.

[0013] Optionally, the sampling resistor is connected in series in the circuit connecting the heating element and the negative terminal.

[0014] Optionally, the current sampling circuit is specifically used to, after detecting the voltage drop, amplify the voltage drop to an amplified voltage proportional to the voltage drop, and then superimpose the amplified voltage with a preset forward bias voltage to generate the sampling voltage;

[0015] The magnitude of the preset positive bias voltage is greater than the absolute value of the input offset voltage of the comparator.

[0016] Optionally, the current sampling circuit includes a first current detection amplifier and a first DC voltage source;

[0017] The positive and negative input terminals of the first current detection amplifier are respectively connected to the two ends of the sampling resistor; the first signal output terminal of the first current detection amplifier is connected to the first inverting input terminal.

[0018] The reference voltage input terminal of the first current sense amplifier is connected to the first DC voltage source, and the reference voltage input terminal is used to set the reference output level of the first current sense amplifier;

[0019] The first DC voltage source is used to input the preset positive bias voltage to the reference voltage input terminal;

[0020] The first circuit detection amplifier is used to detect the voltage drop, amplify the voltage drop to the amplified voltage, superimpose the amplified voltage with the preset forward bias voltage to generate the sampling voltage, and output the sampling voltage to the first inverting input terminal through the first signal output terminal.

[0021] Optionally, the current sampling circuit includes an operational amplifier, a second current detection amplifier, and a second DC voltage source;

[0022] The positive and negative input terminals of the second current detection amplifier are respectively connected to the two ends of the sampling resistor; the second signal output terminal of the second current detection amplifier is connected to the second non-inverting input terminal of the operational amplifier through the first input resistor. The second current detection amplifier is used to detect the voltage drop, amplify the voltage drop to the amplified voltage, and output the amplified voltage to the second non-inverting input terminal through the second signal output terminal.

[0023] The second inverting input terminal of the operational amplifier is grounded through a grounding resistor, and the second non-inverting input terminal is connected to the second DC voltage source through a second input resistor. The second DC voltage source is used to input the preset positive bias voltage to the second non-inverting input terminal.

[0024] The third signal output terminal of the operational amplifier is connected to the second inverting input terminal through a feedback resistor, and the third signal output terminal is connected to the first inverting input terminal.

[0025] The operational amplifier is used to superimpose the amplified voltage with the preset forward bias voltage to generate the sampling voltage, and input the sampling voltage to the first inverting input terminal through the third signal output terminal.

[0026] Optionally, the software control module includes a controller, a voltage sampling element, and a reference voltage setting element; the controller is communicatively connected to the voltage sampling element and the reference voltage setting element, and the reference voltage setting element is connected to the first in-phase input terminal;

[0027] The voltage sampling element is used to collect the actual voltage across the heating element.

[0028] The controller is used for:

[0029] Obtain the current value of the operating current, and obtain the actual voltage from the voltage sampling element;

[0030] Calculate the actual resistance based on the actual voltage and the current value;

[0031] Based on the target heating power and the actual resistance, calculate the target value of the operating current when the actual heating power of the heating element is equal to the target heating power;

[0032] The target voltage value is calculated by multiplying the target value and the resistance value of the sampling resistor. The reference voltage is obtained by superimposing the target voltage value with the preset forward bias voltage.

[0033] Based on the reference voltage, a digital instruction is generated and sent to the reference voltage setting element to control the reference voltage setting element to input the reference voltage to the first in-phase input terminal.

[0034] Optionally, the voltage sampling element includes an analog-to-digital converter;

[0035] and / or,

[0036] The reference voltage setting element includes a digital-to-analog converter.

[0037] Optionally, the output voltage control circuit includes a voltage divider circuit connected in parallel between the voltage output terminal and the negative terminal. The voltage divider circuit includes a first voltage divider resistor and a second voltage divider resistor connected in series. One end of the first voltage divider resistor is connected to the voltage output terminal, and the other end of the first voltage divider resistor is connected to an intermediate node. One end of the second voltage divider resistor is connected to the intermediate node, and the other end of the second voltage divider resistor is connected to the negative terminal. The intermediate node is connected to the feedback voltage input terminal.

[0038] The constant power drive scheme also includes an N-type field-effect transistor, the gate of which is connected to the output terminal of the comparator, the drain of which is connected to the intermediate node, and the source of which is connected to the negative terminal.

[0039] The field-effect transistor is configured as follows:

[0040] When the gate receives the first level signal, it controls the drain and the source to be cut off, thereby controlling the DC voltage converter to reduce the DC output voltage;

[0041] When the gate receives the second level signal, it controls the conduction between the drain and the source to control the DC voltage converter to increase the DC output voltage.

[0042] Secondly, this application provides a thermal flow sensor, the sensor including the constant power drive scheme as described in the first aspect;

[0043] The sensor also includes:

[0044] Heating elements are installed in the fluid flow path;

[0045] At least one temperature sensing element disposed in the fluid flow path, the temperature sensing element being used to detect the temperature difference of the fluid before and after flowing through the heating element;

[0046] The controllable constant current source circuit in the constant power drive scheme is connected to the heating element. The constant power drive scheme provides drive current to the heating element through the controllable constant current source circuit and maintains the heating power of the heating element at the target heating power.

[0047] The constant power drive scheme and thermal flow sensor provided in this application have the following technical advantages:

[0048] This scheme connects a sampling resistor in series in the circuit through which the operating current flowing through the heating element passes. The voltage drop across the sampling resistor is positively correlated with the magnitude of the operating current. By configuring the sampling circuit, a sampling voltage that is positively correlated with the magnitude of the operating current can be obtained in real time as a reference for subsequent control.

[0049] Furthermore, based on the target heating power of the heating element and its actual resistance value, a corresponding reference voltage is calculated and set. The comparator continuously compares the sampled voltage, reflecting the actual operating current, with this reference voltage. This comparison process essentially compares the real-time heating power of the heating element with the target heating power. When the sampled voltage is greater than the reference voltage (meaning the actual operating current is too high, potentially causing the actual heating power to exceed the target heating power), the comparator outputs a first-level signal, which controls the DC-DC converter to reduce its DC output voltage. Conversely, when the sampled voltage is less than the reference voltage (meaning the actual operating current is too low, resulting in insufficient heating power), the comparator outputs a second-level signal, controlling the DC-DC converter to increase its DC output voltage. The change in the DC-DC converter's output voltage causes a corresponding change in the operating current flowing through the heating element. Through this continuous and dynamic adjustment, the sampled voltage can be made to approach the reference voltage infinitely, thus ensuring that the operating current flowing through the heating element is always corrected to the precise value that enables the heating element to generate the target heating power. While ensuring that the heating element maintains its target heating power, the detection accuracy of the thermal flow sensor can be improved. Attached Figure Description

[0050] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0051] Figure 1 Schematic diagram of the constant power drive scheme architecture provided in the embodiments of this application Figure 1 ;

[0052] Figure 2 Schematic diagram of the constant power drive scheme architecture provided in the embodiments of this application Figure 2 ;

[0053] Figure 3 A circuit diagram of the constant power drive scheme provided in the embodiments of this application;

[0054] Figure 4 This is a diagram illustrating the output current error of the constant power drive scheme provided in the embodiments of this application.

[0055] Figure 5 This diagram illustrates the output current error of the heating element drive circuit in the prior art.

[0056] Figure label:

[0057] 10 - Voltage input terminal;

[0058] 11-Grounding terminal;

[0059] 12 - Voltage output terminal;

[0060] 13 - Feedback voltage input terminal;

[0061] 14 - First non-inverting input;

[0062] 15 - First inverting input terminal;

[0063] 16 - The output of the comparator;

[0064] 17-Voltage divider circuit;

[0065] 18 - First voltage divider resistor;

[0066] 19 - Second voltage divider resistor;

[0067] 20 - Intermediate node.

[0068] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0069] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments matching this application. Rather, they are merely examples of apparatuses and methods matching some aspects of this application as detailed in the appended claims.

[0070] In existing technologies, a fixed output current is provided to the heating element of a thermal flow sensor. However, due to unavoidable manufacturing errors in the resistance of the heating element, the heating power of the heating element varies under a fixed output current, leading to a decrease in the detection accuracy of the thermal flow sensor. The technical concept of this application is as follows:

[0071] In the existing circuit supplying power to the heating element, a sampling resistor is connected in series in the circuit through which the operating current flowing through the heating element passes. A current sampling circuit collects the voltage drop across this resistor and converts it into a sampling voltage that is positively correlated with the voltage drop. This sampling voltage is compared with a reference voltage, which is determined based on the target heating power that the heating element needs to maintain and its actual resistance. This ensures that when the sampling voltage equals the reference voltage, the heating power of the heating element is maintained at the target heating power. When the sampling voltage is higher than the reference voltage, the DC-DC voltage converter in the control circuit reduces the output voltage supplied to the heating element; conversely, when it is lower, the DC-DC voltage converter increases the output voltage supplied to the heating element.

[0072] The technical solution of this application and how the technical solution of this application solves the above-mentioned technical problems are described in detail below with specific embodiments. These specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments. The embodiments of this application will now be described with reference to the accompanying drawings.

[0073] Example 1

[0074] Figure 1 Schematic diagram of the constant power drive scheme architecture provided in the embodiments of this application Figure 1 ,like Figure 1 As shown, the constant power drive scheme includes a controllable constant current source circuit and a software control module. The controllable constant current source circuit includes a DC-DC voltage converter, a sampling resistor, a current sampling circuit, a comparator, and an output voltage control circuit.

[0075] The positive terminal of the DC power supply is connected to the voltage input terminal 10 of the DC voltage converter, and the negative terminal of the DC power supply is connected to the ground terminal 11 of the DC voltage converter. Figure 1 In the middle, the positive terminal of the DC power supply (corresponding to...) Figure 1 The "power supply voltage +" terminal is electrically connected to the voltage input terminal 10 of the DC voltage converter, and the negative terminal of the DC power supply corresponds to... Figure 1 The “power supply voltage-” in the circuit is electrically connected to the ground terminal 11 of the DC voltage converter, thus forming the basic power supply circuit.

[0076] The voltage output terminal 12 of the DC voltage converter is connected to one end of the heating element, and the other end of the heating element is connected to the negative terminal (system ground). The DC voltage converter is used to convert the DC input voltage of the DC power supply into the DC output voltage output to the heating element. Figure 1 In the diagram, "System Output Voltage +" and "System Output Voltage -" are connected to the two ends of the heating element.

[0077] The sampling resistor is connected in series with the circuit through which the operating current flowing through the heating element passes. Figure 1 In this circuit, the sampling resistor is connected in series between the voltage output terminal 12 and the heating element, that is, in series between the voltage output terminal 12 of the DC voltage converter and the connection node of the heating element. Figure 2 Schematic diagram of the constant power drive scheme architecture provided in the embodiments of this application Figure 2 ,like Figure 2 As shown, the sampling resistor can also be connected in series in the circuit that connects the heating element and the negative terminal.

[0078] The current sampling circuit is connected to both ends of the sampling resistor and the first inverting input terminal 15 of the comparator. The current sampling circuit is used to convert the voltage drop across the sampling resistor into a sampling voltage that is positively correlated with the voltage drop, and input the sampling voltage to the first inverting input terminal 15.

[0079] The output terminal 16 of the comparator is connected to the feedback voltage input terminal 13 of the DC voltage converter through the output voltage control circuit. The first non-inverting input terminal 14 of the comparator is input with a reference voltage. The reference voltage is determined according to the target heating power of the heating element and the actual resistance of the heating element, so that when the sampling voltage is equal to the reference voltage, the actual heating power of the heating element is the target heating power. Specifically, after the software control module determines the reference voltage according to the target heating power of the heating element and the actual resistance of the heating element, it outputs the reference voltage to the first non-inverting input terminal of the comparator.

[0080] The comparator is used to: when the sampled voltage is greater than the reference voltage, output a first level signal through the output terminal 16 of the comparator to control the DC voltage converter to reduce the DC output voltage; when the sampled voltage is less than the reference voltage, output a second level signal through the output terminal 16 of the comparator to control the DC voltage converter to increase the DC output voltage.

[0081] The working principle and process of this circuit will be explained in detail below.

[0082] After the DC power supply is connected to the circuit, it establishes an electrical connection with the DC voltage converter through its positive and negative terminals. The DC voltage converter, acting as an energy conversion hub, receives the input voltage from the DC power supply and, through its internal high-frequency switching and energy conversion mechanism, outputs a controlled DC output voltage to the external load side. In this embodiment, a switching DC-DC converter (switching regulator) can be selected as the DC voltage converter. Switching regulators regulate energy transfer through high-frequency switching operations, resulting in extremely low power consumption, high efficiency, and low heat generation, thus avoiding excessive heat interference with temperature-sensitive thermal flow sensors. When the DC output voltage supplied to the heating element is lower than the input voltage of the DC power supply, a buck DC voltage converter can be selected; if the DC output voltage supplied to the heating element may be higher than the input voltage of the DC power supply, a boost or buck-boost DC voltage converter can be used.

[0083] In the main power circuit, the output voltage is applied across the heating element. Specifically, the voltage output terminal 12 of the DC-DC converter is directly connected to one end of the heating element, while the other end of the heating element is directly connected back to the negative terminal of the DC power supply, thus forming a complete closed current loop. The operating current flows from the voltage output terminal 12, through the heating element, and finally returns to the negative terminal of the power supply, enabling the heating element to generate initial heat based on the Joule effect.

[0084] Because the actual resistance of the heating element has errors, a current sampling mechanism is introduced to accurately control the actual heating power of the heating element. Specifically:

[0085] The sampling resistor is connected in series in the path of the operating current in the main power circuit. When the operating current flows through the sampling resistor, according to Ohm's law, a very small voltage drop will be generated across the resistor. The magnitude of this voltage drop is strictly proportional to the operating current; therefore, this voltage drop can be used as a physical quantity to indirectly characterize the magnitude of the operating current. The sampling resistor can be a metal film resistor, a manganese copper shunt resistor, or a surface-mount alloy shunt resistor. Low-temperature drift four-terminal precision resistors or high-power soldered resistors can also be used. The material of the sampling resistor can be manganese copper, nickel-chromium alloy, thick-film alloy, or metal foil structure to reduce temperature drift and improve long-term stability. The resistance value of the sampling resistor is typically in the milliohm to ohm range, and the specific value can be matched according to the operating current and allowable power consumption.

[0086] The current sampling circuit is connected to both ends of the sampling resistor and is responsible for sensing the voltage drop. To effectively detect this voltage drop, the current sampling circuit performs the following signal conditioning operations: First, a differential amplifier circuit with a high common-mode rejection ratio extracts the small differential voltage under a high potential background; then, a high-precision gain amplifier converts this weak voltage into a single-ended sampling voltage with a standard level and strong anti-interference capability. Finally, the processed sampling voltage is sent to the first inverting input terminal 15 of the comparator. Optionally, the current sampling circuit can be a differential amplifier sampling structure, a Hall sampling structure, or a current mirror sampling structure. It can also be implemented using a voltage amplification and filtering circuit composed of operational amplifiers, or an integrated current sensing amplifier chip. The components of the current sampling circuit may include operational amplifiers, precision resistors, capacitors, filter elements, and semiconductor sensing devices.

[0087] The comparator acts as the decision-making center, with its first non-inverting input terminal 14 connected to a preset reference voltage and its first inverting input terminal 15 receiving the sampling voltage. Optionally, the comparator can be an operational amplifier comparator chip, a dedicated high-speed comparator chip, or an analog front-end device with integrated comparison functions.

[0088] It should be noted that the amplitude of this reference voltage is set based on the target heating power of the heating element and its actual resistance, to ensure that when the sampling voltage equals the reference voltage, the actual heating power of the heating element can be maintained at the target heating power. The actual resistance can be determined using an initial calibration method. Specifically, this initial calibration is performed during system power-on initialization, before constant power heating begins. The software control module allows a small and precise known test current to flow through the heating element, and then a high-precision ADC measures the voltage across the heating element at this time. According to Ohm's law, the actual resistance of the heating element at ambient temperature can be calculated. Furthermore, based on the target heating power and the actual resistance of the heating element, the target value of the operating current flowing through the heating element can be calculated. The target value of the voltage drop across the sampling resistor can be obtained by multiplying the target value of the operating current by the resistance of the sampling resistor. Then, based on the target value of the voltage drop across the sampling resistor and the conversion relationship between the voltage drop and the sampling voltage, the target value of the sampling voltage (i.e., the reference voltage value) can be calculated.

[0089] After determining the reference voltage value, the reference voltage can be generated using a microcontroller and digital-to-analog converter (DAC) in the software control module. The DAC is connected to the first non-inverting input terminal 14. The microcontroller generates a corresponding digital instruction based on the reference voltage value and sends it to the DAC. The DAC responds to the digital instruction, generates the actual analog voltage (reference voltage), and inputs the reference voltage to the first non-inverting input terminal 14. Alternatively, an adjustable three-terminal voltage regulator IC or a low-dropout linear regulator can be used. Through an externally connected resistor feedback network, the output voltage of the regulator is set to the reference voltage, and the reference voltage is input to the first non-inverting input terminal 14 through the regulator.

[0090] like Figure 1 As shown, the constant power drive scheme also includes a voltage divider circuit 17, which is connected in parallel between the voltage output terminal 12 and the negative terminal. The voltage divider circuit 17 includes a first voltage divider resistor 18 and a second voltage divider resistor 19 connected in series. One end of the first voltage divider resistor 18 is connected to the voltage output terminal 12, and the other end of the first voltage divider resistor 18 is connected to the intermediate node 20. One end of the second voltage divider resistor 19 is connected to the intermediate node 20, and the other end of the second voltage divider resistor 19 is connected to the negative terminal. The intermediate node 20 is connected to the feedback voltage input terminal (13).

[0091] The constant power drive scheme also includes an N-type field-effect transistor. The gate of the N-type field-effect transistor is connected to the output terminal 16 of the comparator, the drain of the N-type field-effect transistor is connected to the intermediate node 20, and the source of the N-type field-effect transistor is connected to the negative terminal.

[0092] The field-effect transistor is configured as follows:

[0093] When the gate receives the first level signal, it controls the cutoff between the drain and the source to control the DC voltage converter to reduce the DC output voltage;

[0094] When the gate receives the second level signal, it controls the conduction between the drain and the source to control the DC voltage converter to increase the DC output voltage.

[0095] Specifically, the voltage divider circuit 17 is composed of a first voltage divider resistor 18 and a second voltage divider resistor 19 connected in series, and directly connected in parallel between the voltage output terminal 12 of the DC-DC voltage converter and system ground. The connection point of the first voltage divider resistor 18 and the second voltage divider resistor 19 is an intermediate node 20, which is directly connected to the feedback voltage input terminal (13) of the DC-DC voltage converter (usually marked as the FB pin). The feedback voltage input terminal (13) is the input terminal of the error amplifier inside the DC-DC voltage converter, used to monitor the output voltage and adjust the switching duty cycle accordingly. The gate of the N-type field-effect transistor is directly controlled by the output terminal 16 of the comparator, its drain is connected to the intermediate node 20 of the voltage divider circuit 17, and its source is directly connected to system ground.

[0096] The current sampling circuit linearly converts the voltage drop across the sampling resistor into a sampling voltage, which is then sent to the first inverting input (15) of the comparator. The first non-inverting input of the comparator receives the reference voltage. Next, the comparator compares the sampling voltage with the reference voltage.

[0097] Scenario A: When the operating current is too high, the voltage drop across the sampling resistor increases, causing the sampling voltage to be higher than the reference voltage. At this time, the comparator output state flips and outputs a first level signal (low level voltage).

[0098] Scenario B: When the operating current on the heating element is low, the sampling voltage will be lower than the reference voltage. At this time, the comparator outputs a second level signal (high level voltage).

[0099] The comparator's output level directly controls the on / off state of the N-type field-effect transistor, thereby changing the voltage at the intermediate node 20 of the voltage divider circuit 17, and thus controlling the DC output voltage of the DC voltage converter.

[0100] In scenario A, when the comparator outputs a first-level signal (low level) to the gate of the field-effect transistor (FET), the FET is in the off state, and its drain and source exhibit high impedance. At this time, the voltage at the intermediate node 20 of the voltage divider circuit 17 is entirely determined by the voltage division ratio of the first voltage divider resistor 18 and the second voltage divider resistor 19. This voltage value reflects the actual output voltage of the DC-DC converter. For most DC-DC converters, when the voltage at its feedback pin (FB) is higher than its internal reference voltage (e.g., 1.2V), the internal control logic determines that the current output voltage is too high, thereby reducing the duty cycle of its internal switches and causing the DC output voltage to drop. The drop in DC output voltage will directly lead to a decrease in the operating current flowing through the heating element.

[0101] In scenario B, when the comparator outputs a second-level signal (high level) to the gate of the field-effect transistor (FET), the FET enters a saturated conduction state, and its drain and source are approximately short-circuited. This causes the intermediate node 20 of the voltage divider circuit 17 to be forcibly pulled low to near ground potential (0V). At this time, the feedback pin (FB) voltage of the DC-DC voltage converter is pulled low to a value far below its internal reference voltage. Based on this, the converter's internal control logic determines that the current output voltage is too low and immediately increases the duty cycle of the internal switch, causing the DC output voltage to rise. This increase in output voltage will drive an increase in the operating current flowing through the heating element.

[0102] It should be noted that in actual operation, the above adjustment process is continuous and high-speed. In scenario A, the decrease in DC output voltage reduces the operating current, resulting in a decrease in the sampling voltage, gradually approaching the reference voltage. In scenario B, the increase in DC output voltage increases the operating current, resulting in an increase in the sampling voltage, gradually approaching the reference voltage. Eventually, the system converges and stabilizes at a dynamic equilibrium point. At this equilibrium point, the voltages at the two input terminals of the comparator are infinitely close, and the sampling voltage is precisely locked to the reference voltage. Since the reference voltage is precisely set according to the target heating power, the operating current flowing through the heating element is precisely controlled, ensuring that its actual heating power is always equal to the target heating power.

[0103] In this embodiment, to provide sufficient driving voltage while also ensuring system fault protection, the voltage output terminal 12 of the DC voltage converter is connected to one end of the heating element via a current-limiting resistor, and the other end of the heating element is directly connected to the negative terminal (i.e., system ground) of the DC power supply, forming a power loop. The function of the current-limiting resistor is specifically as follows:

[0104] First, the current-limiting resistor serves as a transient overcurrent protection and soft-start mechanism. Specifically, since heating elements are typically made of materials whose resistance changes significantly with temperature, their initial temperature is low and their resistance is at its minimum at the moment the circuit is powered on. Without current limiting, the DC-DC converter may generate a very large transient inrush current during startup. This could easily trigger the overcurrent protection within the converter, leading to startup failure, or it could damage the heating element itself. The current-limiting resistor, utilizing its resistance characteristics, effectively limits this transient inrush current during the initial startup phase, providing a smooth soft-start process for the system.

[0105] Secondly, during long-term use of the thermal flow sensor, if the heating element or its lead-out line experiences a short circuit to ground, the current-limiting resistor can limit the magnitude of the short-circuit current, preventing the large current from burning out the power switching transistor inside the DC-DC voltage converter.

[0106] Example 2

[0107] The current sampling circuit is specifically used to detect a voltage drop, amplify the voltage drop to an amplified voltage that is proportional to the voltage drop, and then superimpose the amplified voltage with a preset forward bias voltage to generate a sampling voltage.

[0108] The magnitude of the preset forward bias voltage is greater than the absolute value of the comparator's input offset voltage.

[0109] Specifically, the current sampling circuit can include a high-precision differential amplifier and a precision voltage source. The two inputs of the differential amplifier are connected to the two ends of the sampling resistor to detect the small voltage drop across it, while the precision voltage source generates a stable, preset forward bias voltage, typically 0.3V. The function of the current sampling circuit is to first linearly amplify the voltage drop across the sampling resistor to obtain an amplified voltage; then, to algebraically add this amplified voltage to the preset forward bias voltage, ultimately generating a raised sampling voltage output.

[0110] It should be noted that the magnitude of the preset forward bias voltage must be greater than the absolute value of the selected comparator's input offset voltage. The comparator's offset voltage refers to the inherent error caused by internal transistor mismatch, resulting in a non-zero output even when the two input voltages are equal; it is typically in the millivolt range. By setting a forward bias voltage of several hundred millivolts (e.g., 0.3V, typically ranging from 0.1V to 0.5V), the comparator's operating point can be "raised" from near zero, making the offset voltage a very small proportion of the operating point voltage, thus essentially eliminating its effect.

[0111] Meanwhile, in this embodiment, the set value of the reference voltage is equal to the amplified voltage corresponding to the working current flowing through the sampling resistor when the heating element is working at the target heating power, plus the same preset forward bias voltage.

[0112] Optionally, the current sampling circuit includes a first current sensing amplifier and a first DC voltage source;

[0113] The positive and negative input terminals of the first current detection amplifier are connected to the two ends of the sampling resistor, respectively; the first signal output terminal of the first current detection amplifier is connected to the first inverting input terminal 15.

[0114] The reference voltage input terminal 10 of the first current sensing amplifier is connected to the first DC voltage source, and the reference voltage input terminal 10 is used to set the reference output level of the first current sensing amplifier.

[0115] The first DC voltage source is used to input a preset positive bias voltage to the reference voltage input terminal 10;

[0116] The first circuit detection amplifier is used to amplify the voltage drop to an amplified voltage after detecting the voltage drop, and to superimpose the amplified voltage with a preset forward bias voltage to generate a sampling voltage, and output the sampling voltage to the first inverting input terminal 15 through the first signal output terminal.

[0117] Figure 3 This is a circuit diagram of the constant power drive scheme provided in the embodiments of this application. The circuit corresponds to... Figure 1 The architecture shown. Specifically, as... Figure 3 As shown, the first current sensing amplifier can be the INA240AIPWR model of the current sampling circuit section, which has a dedicated reference voltage input terminal 10 (REF1 and REF2) for setting the reference output level.

[0118] like Figure 3 As shown, the first DC power supply can be implemented using a +5V DC power supply combined with a capacitor network consisting of C129 and C131. Alternatively, the first DC power supply can be implemented using a low-temperature drift, high-precision reference voltage source chip (such as a bandgap reference source).

[0119] like Figure 3 As shown, the positive input (IN+) and negative input (IN-) of the INA240AIPWR are connected across the sampling resistor to detect the small voltage drop across it. The output (OUT) of the INA240AIPWR serves as the first signal output and is connected to the first inverting input 15 of the comparator. The output of the first DC voltage source is connected to the reference voltage input 10 (REF pin) of the INA240AIPWR. This voltage is the preset forward bias voltage, used to set the reference level of the amplifier output.

[0120] When the operating current flows through the sampling resistor, a tiny voltage drop is generated. The INA240AIPWR, with its high precision and low offset characteristics, linearly amplifies this weak signal (e.g., by a factor of 20). Simultaneously, its internal gain network algebraically superimposes the preset forward bias voltage (e.g., 0.3V) provided by the first DC voltage source with the amplified voltage signal. Ultimately, the sampling voltage obtained from the amplifier output (OUT) is a voltage signal that has been boosted by 0.3V and is strictly proportional to the operating current.

[0121] exist Figure 3 Based on the circuit shown, Figure 4 This diagram illustrates the output current error of the constant power drive scheme provided in this embodiment. Figure 5 This diagram illustrates the output current error of the heating element drive circuit in the prior art.

[0122] Figure 4 The graph contains multiple colored line segments, each representing the trend of actual output error at a specific target current range (from 1mA to 15mA) as a function of sampling points. Observing the trends of each curve reveals that all lines fluctuate around the center line (zero line). Within the entire 0-15mA wide range, the vast majority of data points are closely distributed within a narrow band of ±0.01mA. Although the curves exhibit slight ups and downs, the overall amplitude of the peaks and troughs is extremely small, demonstrating high convergence. This indicates that the system maintains extremely high output accuracy at all current ranges, and the error consistency between different ranges is excellent, with no obvious periodic deviations.

[0123] Figure 5 This illustrates the error of existing technology when outputting a constant 10mA current. It's a two-dimensional coordinate scatter plot, with the horizontal axis representing the output voltage (i.e., the voltage across the load) and the vertical axis representing the absolute error of the output current. Each point in the plot represents the degree to which the actual output current deviates from 10mA at that specific output voltage.

[0124] visible, Figure 5 The blue scatter plots in the image show a very clear downward trend. As the output voltage gradually increases from 4V to 26V, the current error rapidly decreases from a relatively high positive value (maximum exceeding 0.15mA). Although the error is larger in the low voltage range (e.g., 4V-10V), the range of error values ​​is very large throughout the entire operating range, and the data points are quite discrete, failing to closely adhere to the ideal zero-error line. This indicates that the accuracy of the existing technical solution is greatly affected by changes in output voltage (or load impedance) and lacks good constant current characteristics.

[0125] Optionally, the current sampling circuit includes an operational amplifier, a second current sensing amplifier, and a second DC voltage source;

[0126] The positive and negative input terminals of the second current detection amplifier are connected to the two ends of the sampling resistor, respectively; the second signal output terminal of the second current detection amplifier is connected to the second non-inverting input terminal of the operational amplifier through the first input resistor. The second current detection amplifier is used to detect the voltage drop, amplify the voltage drop into an amplified voltage, and output the amplified voltage to the second non-inverting input terminal through the second signal output terminal.

[0127] The second inverting input terminal of the operational amplifier is grounded through a grounding resistor, and the second non-inverting input terminal is connected to a second DC voltage source through a second input resistor. The second DC voltage source is used to input a preset positive bias voltage to the second non-inverting input terminal.

[0128] The third signal output terminal of the operational amplifier is connected to the second inverting input terminal through a feedback resistor, and the third signal output terminal is connected to the first inverting input terminal 15.

[0129] The operational amplifier is used to superimpose the amplified voltage with a preset forward bias voltage to generate a sampling voltage, and input the sampling voltage to the first inverting input terminal 15 through the third signal output terminal.

[0130] Specifically, the second current sense amplifier can be an integrated circuit designed specifically for detecting small differential signals under high common-mode voltage. Its positive input terminal (IN+) and negative input terminal (IN-) are connected to the two ends of a sampling resistor, respectively, to measure the small voltage drop across the sampling resistor connected in series in the main power circuit. The second signal output terminal of the second current sense amplifier outputs an amplified voltage proportional to this voltage drop.

[0131] Furthermore, the output of the second current-sensing amplifier is connected to the second non-inverting input of the operational amplifier via a first input resistor. An independent second DC voltage source is also connected to the second non-inverting input of the operational amplifier via a second input resistor. This voltage source provides a stable and accurate preset forward bias voltage. The implementation of the second DC voltage source can refer to the aforementioned embodiments. The output of the operational amplifier (the third signal output) is connected back to its second inverting input via a feedback resistor, forming negative feedback. The second inverting input is also connected to system ground via a grounding resistor. Finally, the third signal output of the operational amplifier is directly connected to the first inverting input 15 of the comparator, outputting the final sampled voltage.

[0132] In the above scheme, the operational amplifier unit constitutes a non-inverting adder, and its non-inverting input terminal receives two current signals: one is the current from the amplified voltage through the first input resistor, and the other is the current from the preset forward bias voltage through the second input resistor.

[0133] Based on the operational amplifier's "virtual short" and "virtual open" principles, the voltage at its non-inverting input is determined by the two currents and the feedback network at the inverting input. By carefully matching the resistor values—for example, typically setting the first and second input resistors to be equal, and ensuring the grounding and feedback resistors meet a specific ratio—the operational amplifier's closed-loop gain is the same for both the amplified voltage and the preset forward bias voltage. Therefore, its output voltage sampling voltage is the sum of both amplified with the same gain: Sampling voltage = Operational amplifier closed-loop gain × (Amplified voltage + Preset forward bias voltage), where the operational amplifier's closed-loop gain is determined by the feedback resistor and the grounding resistor. Typically, the operational amplifier's closed-loop gain is designed to be 1, while simultaneously ensuring the first and second input resistors are equal. In this case, the superposition relationship is:

[0134] Sampling voltage = Amplified voltage + Preset forward bias voltage.

[0135] Example 3

[0136] Optionally, the software control module includes a controller, a voltage sampling element, and a reference voltage setting element; the controller is communicatively connected to the voltage sampling element and the reference voltage setting element, and the reference voltage setting element is connected to the first in-phase input terminal;

[0137] The voltage sampling element is used to collect the actual voltage across the heating element;

[0138] The controller is used for:

[0139] Obtain the current value of the operating current and the actual voltage from the self-voltage sampling element;

[0140] Calculate the actual resistance based on the actual voltage and current values;

[0141] Based on the target heating power and the actual resistance, calculate the target value of the operating current when the actual heating power of the heating element is equal to the target heating power.

[0142] The target voltage value is calculated by multiplying the target value and the resistance value of the sampling resistor. The reference voltage is obtained by superimposing the target voltage value with a preset forward bias voltage.

[0143] Based on the reference voltage, a digital instruction is generated and sent to the reference voltage setting element to control the reference voltage setting element to input the reference voltage to the first in-phase input terminal.

[0144] Specifically, the controller can be a microcontroller (MCU) or a digital signal processor (DSP), the voltage sampling element can be an analog-to-digital converter (ADC), and the reference voltage setting element can be a digital-to-analog converter (DAC). The controller acquires the actual voltage across the heating element via the ADC. In a thermal flow sensor, this actual voltage is directly related to the resistance change caused by the airflow cooling effect. Simultaneously, the controller sends data to the first inverting input 15 of the comparator via the DAC.

[0145] In thermal flow sensors, it's crucial to maintain a constant heating power from the heating element, rather than a constant operating current. This is because the temperature and resistance of the heating element change with airflow velocity. According to the power calculation formula, if the resistance increases, simply maintaining a constant operating current will lead to an increase in the heating power of the heating element. Therefore, the software control module needs to dynamically adjust the target current based on the real-time resistance changes of the heating element. In this embodiment, the specific workflow of the software control module is as follows:

[0146] Step 1:

[0147] The controller obtains the actual voltage across the heating element through the ADC. At the same time, the controller can obtain the operating current value of the heating element through an independent detection circuit (or obtain the voltage drop of the sampling resistor through a self-sampling circuit and then convert it). Then, the controller uses Ohm's law to calculate the current actual resistance value of the heating element in real time in the internal microprocessor.

[0148] Step Two:

[0149] By using the preset target heating power of the heating element and the calculated actual resistance of the heating element, the correct current value that should flow through the element at this time can be calculated, i.e., the target value of the operating current. It should be noted that this calculation process can be performed by the controller every fixed time interval (e.g., 10 milliseconds) to achieve real-time calculation.

[0150] Step 3:

[0151] Since the calculated target value of the operating current is a digital quantity, the controller can send the target value to the DAC. The DAC internally converts the digital current value into the corresponding analog voltage value, i.e., the reference voltage, according to the preset conversion coefficient (e.g., 1V corresponds to 100mA). The DAC then sends the reference voltage to the first inverting input terminal 15 of the comparator.

[0152] The solution in this embodiment has the following technical effects:

[0153] By calculating the actual resistance of the heating element in real time and dynamically adjusting the target value of the operating current, and then correcting the reference voltage based on the target value of the operating current, the system can automatically compensate for changes in resistance due to ambient temperature, self-heating temperature, manufacturing tolerance, or long-term aging. This ensures that the heating power of the heating element is strictly maintained at the target heating power, which can eliminate the influence of resistance error on the accuracy of flow detection in principle, and further improve the detection accuracy of thermal flow sensors.

[0154] Example 4

[0155] This embodiment provides a thermal flow sensor, which includes the constant power drive scheme described in the foregoing embodiments;

[0156] The sensor also includes:

[0157] Heating elements are installed in the fluid flow path;

[0158] At least one temperature sensing element is disposed in the fluid flow path, and the temperature sensing element is used to detect the temperature difference of the fluid before and after flowing through the heating element;

[0159] In the constant power drive scheme, the controllable constant current source circuit is connected to the heating element. The constant power drive scheme provides drive current to the heating element through the controllable constant current source circuit and maintains the heating power of the heating element at the target heating power.

[0160] Specifically, the thermal flow sensor includes a fluid flow path for guiding fluids (such as air, oxygen, and other gases) through.

[0161] The temperature sensing element includes at least two highly sensitive temperature sensing units. The first temperature sensing element is located upstream of the heating element and is used to detect the initial temperature of the fluid. The second temperature sensing element is located downstream of the heating element and is used to detect the temperature of the fluid after it has been heated by the heating element. These two temperature sensing elements together form a differential detection bridge, and the measured temperature difference reflects the amount of heat carried away by the fluid from the heating element, that is, it reflects the gas flow rate.

[0162] Constant power drive scheme: Its specific structure and operation are as described in the aforementioned embodiments, wherein the controllable constant current source circuit is electrically connected to the heating element. The control objective of this scheme is to ensure that the actual heating power of the heating element remains at the target heating power regardless of changes in the resistance of the heating element due to temperature, aging, or manufacturing tolerances.

[0163] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A constant power drive scheme, characterized in that, The constant power drive scheme includes a controllable constant current source circuit and a software control module. The controllable constant current source circuit includes a DC-DC voltage converter, a sampling resistor, a current sampling circuit, a comparator, and an output voltage control circuit. The positive terminal of the DC power supply is connected to the voltage input terminal (10) of the DC voltage converter, and the negative terminal of the DC power supply is connected to the ground terminal (11) of the DC voltage converter. The voltage output terminal (12) of the DC voltage converter is connected to one end of the heating element, and the other end of the heating element is connected to the negative terminal. The DC voltage converter is used to convert the DC input voltage of the DC power supply into a DC output voltage output to the heating element. The sampling resistor is connected in series in the circuit through which the working current of the heating element flows. The current sampling circuit is connected to both ends of the sampling resistor and the first inverting input terminal (15) of the comparator. The current sampling circuit is used to convert the voltage drop across the sampling resistor into a sampling voltage that is positively correlated with the voltage drop, and input the sampling voltage to the first inverting input terminal (15). The output terminal (16) of the comparator is connected to the feedback voltage input terminal (13) of the DC voltage converter through the output voltage control circuit. After the software control module determines the reference voltage based on the target heating power of the heating element and the actual resistance of the heating element, it outputs the reference voltage to the first non-inverting input terminal (14) of the comparator. When the sampling voltage is equal to the reference voltage, the actual heating power of the heating element is the target heating power. The comparator is used to: when the sampled voltage is greater than the reference voltage, output a first level signal through the output terminal (16) of the comparator to control the DC voltage converter to reduce the DC output voltage; when the sampled voltage is less than the reference voltage, output a second level signal through the output terminal (16) of the comparator to control the DC voltage converter to increase the DC output voltage.

2. The constant power drive scheme according to claim 1, characterized in that, The sampling resistor is connected in series with the circuit that connects the voltage output terminal (12) and the heating element.

3. The constant power drive scheme according to claim 1, characterized in that, The sampling resistor is connected in series in the circuit that connects the heating element and the negative terminal.

4. The constant power drive scheme according to claim 1, characterized in that, The current sampling circuit is specifically used to, after detecting the voltage drop, amplify the voltage drop to an amplified voltage proportional to the voltage drop, and then superimpose the amplified voltage with a preset forward bias voltage to generate the sampling voltage; The magnitude of the preset positive bias voltage is greater than the absolute value of the input offset voltage of the comparator.

5. The constant power drive scheme according to claim 4, characterized in that, The current sampling circuit includes a first current detection amplifier and a first DC voltage source; The positive and negative input terminals of the first current sensing amplifier are respectively connected to the two ends of the sampling resistor; The first signal output terminal of the first current detection amplifier is connected to the first inverting input terminal (15); The reference voltage input terminal (10) of the first current sensing amplifier is connected to the first DC voltage source, and the reference voltage input terminal (10) is used to set the reference output level of the first current sensing amplifier; The first DC voltage source is used to input the preset positive bias voltage to the reference voltage input terminal (10); The first circuit detection amplifier is used to detect the voltage drop, amplify the voltage drop to the amplified voltage, superimpose the amplified voltage with the preset forward bias voltage to generate the sampling voltage, and output the sampling voltage to the first inverting input terminal (15) through the first signal output terminal.

6. The constant power drive scheme according to claim 4, characterized in that, The current sampling circuit includes an operational amplifier, a second current detection amplifier, and a second DC voltage source; The positive and negative input terminals of the second current detection amplifier are respectively connected to the two ends of the sampling resistor; the second signal output terminal of the second current detection amplifier is connected to the second non-inverting input terminal of the operational amplifier through the first input resistor. The second current detection amplifier is used to detect the voltage drop, amplify the voltage drop to the amplified voltage, and output the amplified voltage to the second non-inverting input terminal through the second signal output terminal. The second inverting input terminal of the operational amplifier is grounded through a grounding resistor, and the second non-inverting input terminal is connected to the second DC voltage source through a second input resistor. The second DC voltage source is used to input the preset positive bias voltage to the second non-inverting input terminal. The third signal output terminal of the operational amplifier is connected to the second inverting input terminal through a feedback resistor, and the third signal output terminal is connected to the first inverting input terminal (15). The operational amplifier is used to superimpose the amplified voltage with the preset forward bias voltage to generate the sampling voltage, and input the sampling voltage to the first inverting input terminal (15) through the third signal output terminal.

7. The constant power drive scheme according to claim 4, characterized in that, The software control module includes a controller, a voltage sampling element, and a reference voltage setting element; the controller is communicatively connected to the voltage sampling element and the reference voltage setting element, and the reference voltage setting element is connected to the first in-phase input terminal (14); The voltage sampling element is used to collect the actual voltage across the heating element. The controller is used for: Obtain the current value of the operating current, and obtain the actual voltage from the voltage sampling element; Calculate the actual resistance based on the actual voltage and the current value; Based on the target heating power and the actual resistance, calculate the target value of the operating current when the actual heating power of the heating element is equal to the target heating power; The target voltage value is calculated by multiplying the target value and the resistance value of the sampling resistor. The reference voltage is obtained by superimposing the target voltage value with the preset forward bias voltage. A digital instruction is generated based on the reference voltage, and the digital instruction is sent to the reference voltage setting element to control the reference voltage setting element to input the reference voltage to the first in-phase input terminal (14).

8. The constant power drive scheme according to claim 7, characterized in that, The voltage sampling element includes an analog-to-digital converter; and / or, The reference voltage setting element includes a digital-to-analog converter.

9. The constant power drive scheme according to claim 1, characterized in that, The output voltage control circuit includes a voltage divider circuit (17), which is connected in parallel between the voltage output terminal (12) and the negative terminal. The voltage divider circuit (17) includes a first voltage divider resistor (18) and a second voltage divider resistor (19) connected in series. One end of the first voltage divider resistor (18) is connected to the voltage output terminal (12), and the other end of the first voltage divider resistor (18) is connected to the intermediate node (20). One end of the second voltage divider resistor (19) is connected to the intermediate node (20), and the other end of the second voltage divider resistor (19) is connected to the negative terminal. The intermediate node (20) is connected to the feedback voltage input terminal (13). The constant power drive scheme also includes an N-type field-effect transistor, the gate of which is connected to the output terminal (16) of the comparator, the drain of which is connected to the intermediate node (20), and the source of which is connected to the negative terminal. The field-effect transistor is configured as follows: When the gate receives the first level signal, it controls the drain and the source to be cut off, thereby controlling the DC voltage converter to reduce the DC output voltage; When the gate receives the second level signal, it controls the conduction between the drain and the source to control the DC voltage converter to increase the DC output voltage.

10. A thermal flow sensor, characterized in that, The sensor includes the constant power drive scheme according to any one of claims 1-9; The sensor also includes: Heating elements are installed in the fluid flow path; At least one temperature sensing element disposed in the fluid flow path, the temperature sensing element being used to detect the temperature difference of the fluid before and after flowing through the heating element; The controllable constant current source circuit in the constant power drive scheme is connected to the heating element. The constant power drive scheme provides drive current to the heating element through the controllable constant current source circuit and maintains the heating power of the heating element at the target heating power.

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