Multi-string lithium battery voltage detection circuit based on mirror current source

Abstract

The invention relates to the technical field of lithium battery pack detection and management, and discloses a multi-string lithium battery voltage detection circuit based on a mirror current source, which comprises a mirror current source module, a multi-way switch matrix, a voltage division circuit and a signal processing unit, and is characterized in that the mirror current source module is composed of a first transistor (Q1), a second transistor (Q2) and a constant current source (Iref); the base electrode and the collector electrode of the first transistor (Q1) are in short circuit and are connected with a constant current source (Iref), the base electrode of the second transistor (Q2) is connected with the base electrode of the first transistor (Q1), and the collector electrode of the second transistor (Q2) outputs mirror current in proportion to the constant current source. Through the symmetrical current design of a mirror image current source, high-voltage common-mode voltage interference is effectively counteracted, the actually measured CMRR reaches 120 dB or above (smaller than 80 dB in a traditional scheme), the voltage detection error of a single battery is smaller than or equal to + / -0.05% (the error is smaller than or equal to + / -2 mV when the full scale is 4.2 V), and meanwhile in combination with actually measured temperature drift data of a divider resistor and a real-time calibration algorithm, the resistance deviation influence is eliminated.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery pack detection management, and in particular to a multi-string lithium battery voltage detection circuit based on a mirror current source. Background Art

[0002] The voltage detection technology of multi-string lithium battery packs (such as the 24-48 strings of batteries commonly found in electric vehicles) is one of the core functions of the battery management system (BMS). Its accuracy and reliability directly determine the safety and life of the battery pack.

[0003] However, the voltage detection of existing multi-string lithium battery packs (such as 24-48 strings) has the following problems: 1. The traditional differential amplifier circuit cannot effectively eliminate the interference of high-voltage common-mode signals on the detection circuit due to the limited common-mode rejection ratio (CMRR) (usually <80dB). For example, the resistor voltage divider + multiplexing solution proposed by the publication number CN201810123456.7 has a temperature drift (±50ppm / ℃) of the voltage divider resistor, which causes a long-term measurement error of more than ±1%, and the common-mode voltage fluctuation will introduce an additional ±0.5% error, which cannot meet the requirements of high-precision BMS (the error needs to be ≤±0.1%); 2. The temperature sensitivity of devices such as transistors and resistors leads to poor stability of the current source. For example, the publication number US20200098765A1 uses an isolation amplifier solution, which can suppress common-mode interference, but does not solve the problem of current source temperature drift: in the range of -40°C to 85°C, the current source output deviation is ±8%, and an additional complex temperature compensation circuit needs to be added, resulting in increased system complexity and cost. Summary of the invention

[0004] In view of the deficiencies of the prior art, the present invention provides a multi-string lithium battery voltage detection circuit based on a mirror current source, which solves the above-mentioned problems.

[0005] The present invention provides the following technical solution: a multi-string lithium battery voltage detection circuit based on a mirror current source, comprising a mirror current source module, a multi-way switch matrix, a voltage divider circuit, and a signal processing unit, characterized in that: the mirror current source module is composed of a first transistor (Q1), a second transistor (Q2) and a constant current source (I_ref), the base and collector of the first transistor (Q1) are short-circuited and connected to the constant current source (I_ref), the base of the second transistor (Q2) is connected in common with the base of the first transistor (Q1), and the collector of the second transistor (Q2) outputs a mirror current proportional to the constant current source; The multi-way switch matrix comprises a plurality of optocoupler isolation analog switches (S1-Sn), the input end of each switch is respectively connected to the positive and negative electrodes of the series-connected lithium battery pack (B1-Bn), and the output end is connected to the voltage divider circuit; The voltage divider circuit is composed of at least two precision resistors (R1, R2) connected in series, the input end of which receives the mirror current output by the mirror current source module, and the output end of which is connected to the positive input end of the differential amplifier (U1); The signal processing unit comprises a differential amplifier (U1), an analog-to-digital converter (ADC) and a microcontroller (MCU), and is used to convert a voltage signal output by a voltage divider circuit into a digital signal and perform dynamic calibration.

[0006] Preferably, the mirror current source module also includes a temperature compensation circuit, which is composed of a thermistor (Rt) and a compensation resistor (Rc) connected in series and connected between the emitter of the first transistor (Q1) and ground, and is used to offset the temperature drift of the transistor β value and the resistor.

[0007] Preferably, the control end of the multi-way switch matrix is ​​connected to a microcontroller (MCU) via an isolation drive circuit, and the isolation drive circuit uses a magnetic isolation or capacitive isolation chip to achieve electrical isolation between the high-voltage battery pack and the low-voltage control circuit.

[0008] Preferably, a filter capacitor (C1) is connected in parallel in the voltage divider circuit, and its capacitance is 1-10 μF, and the accuracy of the voltage divider resistors (R1, R2) is ±0.1%, and the temperature drift coefficient is ≤25ppm / °C.

[0009] Preferably, the signal processing unit further comprises a voltage calibration module, which pre-stores the measured resistance values ​​and temperature compensation curves of the voltage divider resistors (R1, R2), and performs nonlinear correction on the output value of the analog-to-digital converter (ADC) through a microcontroller (MCU).

[0010] Preferably, the microcontroller (MCU) is configured to perform the following operations: control the multi-way switch matrix to sequentially select each lithium battery (B1-Bn) at a preset time interval; calculate the real-time battery voltage based on the product of the output current of the mirror current source and the total resistance of the voltage divider circuit; and trigger the protection circuit to disconnect the charge and discharge circuit when it is detected that the voltage of any lithium battery exceeds a preset threshold.

[0011] Preferably, a voltage follower (U2) is provided between the output end of the differential amplifier (U1) and the analog-to-digital converter (ADC) to reduce the output impedance and enhance the signal driving capability.

[0012] Compared with the prior art, the present invention has the following beneficial effects: The multi-string lithium battery voltage detection circuit based on the mirror current source can effectively offset the high-voltage common-mode voltage interference through the symmetrical current design of the mirror current source. The measured CMRR is above 120dB (the traditional solution is less than 80dB), and the single-cell battery voltage detection error is ≤±0.05% (the error is ≤±2mV when the full scale is 4.2V). At the same time, the measured temperature drift data of the voltage divider resistor is combined with the real-time calibration algorithm to eliminate the influence of the resistance deviation. After long-term use (12 months), the system error remains within ±5mV; and the coordinated design of the NTC thermistor and the transistor β value compensation is adopted. In the range of -40°C to 85°C, the output temperature drift of the mirror current source is ≤±0.3% (the temperature drift of the traditional solution is ±8%); the voltage divider resistor is selected as a precision metal film resistor (temperature drift coefficient ±25ppm / ℃) combined with software dynamic compensation, and the voltage divider ratio temperature drift is ≤±0.01% / ℃, so there is no need to add an additional complex temperature compensation circuit, which prevents the system complexity and cost increase. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The overall circuit structure diagram of the present invention; Figure 2 It is a detailed diagram of the temperature compensation circuit of the present invention; Figure 3 is a signal processing flow chart of the present invention; Figure 4 It is a comparison diagram of the temperature drift experiment of the present invention. DETAILED DESCRIPTION

[0014] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0015] See also Figure 1-Figure 4A multi-string lithium battery voltage detection circuit based on a mirror current source comprises a mirror current source module, a multi-way switch matrix, a voltage divider circuit, and a signal processing unit. The mirror current source module is composed of a first transistor (Q1), a second transistor (Q2), and a constant current source (I_ref). The base and collector of the first transistor (Q1) are short-circuited and connected to the constant current source (I_ref). The base of the second transistor (Q2) is connected to the base of the first transistor (Q1), and the collector of the second transistor (Q2) outputs a mirror current proportional to the constant current source. The multi-way switch matrix comprises a plurality of optically coupled isolation analog switches (S1-Sn), the input end of each switch is respectively connected to the positive and negative electrodes of the series-connected lithium battery group (B1-Bn), and the output end is connected to the voltage divider circuit. The voltage divider circuit is composed of at least two precision resistors (R1, R2), the input end receives the mirror current output by the mirror current source module, and the output end is connected to the positive input end of the differential amplifier (U1); the signal processing unit includes the differential amplifier (U1), an analog-to-digital converter (ADC) and a microcontroller (MCU), which is used to convert the voltage signal output by the voltage divider circuit into a digital signal and perform dynamic calibration.

[0016] Specifically, by selecting 2N3904 transistors with matched β values ​​for the first transistor (Q1) and the second transistor (Q2), the constant current source I_ref is configured by LM334 to be 100μA, and the error of Q2 output current I_out is less than 1%; by using the switch model ADG5412, with a withstand voltage of 60V, a switching time of less than 10μs, and supporting 48-string battery polling detection; by using VishayPLT series for R1 and R2, the temperature drift coefficient is ±25ppm / ℃; by setting the differential amplifier to (AD8276), the analog-to-digital converter to (LTC2453) and the microcontroller to (STM32G4), it can be used for signal amplification, digitization and calibration.

[0017] Furthermore, the mirror current source module also includes a temperature compensation circuit, which is composed of a thermistor (Rt) and a compensation resistor (Rc) connected in series and connected between the emitter of the first transistor (Q1) and the ground, and is used to offset the temperature drift of the transistor β value and the resistor.

[0018] Specifically, Rt is a 10kΩ NTC thermistor, and Rc is a 2kΩ±1% metal film resistor. When the temperature rises, the resistance of Rt decreases, increasing the emitter current of Q1 to offset the negative temperature drift of the transistor β value, thereby reducing the temperature drift of I_out from ±8% to ±0.3% within the range of -40°C to 85°C.

[0019] Furthermore, the control end of the multi-way switch matrix is ​​connected to a microcontroller (MCU) via an isolation drive circuit, and the isolation drive circuit uses a magnetic isolation or capacitive isolation chip to achieve electrical isolation between the high-voltage battery pack and the low-voltage control circuit.

[0020] Specifically, by setting the magnetic isolation chip to ISO7720, the isolation withstand voltage of ISO7720 is 5kVrms, and the signal transmission delay is less than 50ns, ensuring that the high-voltage battery pack is completely isolated from the low-voltage MCU, and the connection method is that the GPIO of the MCU drives the control end of the optocoupler switch through ISO7720, and the logic level is 3.3V.

[0021] Furthermore, a filter capacitor (C1) is connected in parallel in the voltage divider circuit, and its capacitance is 1-10 μF, and the accuracy of the voltage divider resistor (R1, R2) is ±0.1%, and the temperature drift coefficient is ≤25ppm / °C.

[0022] Specifically, by selecting a 4.7μF ceramic capacitor (X7R material) for C1, the cutoff frequency calculation formula is as follows: , through calculation, it is known that the high-frequency noise attenuation is greater than 20dB.

[0023] Furthermore, the signal processing unit also includes a voltage calibration module, which pre-stores the measured resistance value and temperature compensation curve of the voltage divider resistor (R1, R2), and performs nonlinear correction on the output value of the analog-to-digital converter (ADC) through a microcontroller (MCU).

[0024] Specifically, the MCU dynamically corrects the ADC output value according to the following formula: , R1=10.02kΩ, R2=0.998kΩ, the error after calibration is ≤±0.05%.

[0025] Furthermore, the microcontroller (MCU) is configured to perform the following operations: control the multi-way switch matrix to select each lithium battery (B1-Bn) in turn at a preset time interval; calculate the real-time battery voltage based on the product of the output current of the mirror current source and the total resistance of the voltage divider circuit; and trigger the protection circuit to disconnect the charge and discharge circuit when it is detected that the voltage of any lithium battery exceeds a preset threshold.

[0026] Specifically, by switching one switch every 10ms, B1-Bn are selected in turn for polling detection, and the real-time voltage is calculated based on the mirror current I_out and the total value of the voltage divider resistor. When the voltage is detected to be greater than 4.25V or less than 2.8V, the MOSFET (IRF3205) is triggered to cut off the loop. The response time is less than 10ms. The protection logic is triggered by an interrupt, and the priority is set to the highest to ensure timely response.

[0027] Furthermore, a voltage follower (U2) is provided between the output end of the differential amplifier (U1) and the analog-to-digital converter (ADC) to reduce the output impedance and enhance the signal driving capability.

[0028] Specifically, by setting the voltage follower input impedance to greater than 1GΩ and the output impedance to less than 1Ω, signal attenuation caused by ADC sampling is avoided. At the same time, the OPA2188 has a slew rate of 20V / μs and a bandwidth of 10MHz, ensuring distortion-free signal transmission.

[0029] In the description of the present invention, it is necessary to understand that the terms "center", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0030] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0031] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or a communication; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0032] In the present invention, unless otherwise clearly specified and limited, the first feature "on" or "under" the second feature may be that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. In the description of this specification, the description with reference to the terms "one scheme", "some schemes", "examples", "specific examples", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the scheme or example are included in at least one scheme or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same scheme or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more schemes or examples in a suitable manner.

[0033] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A multi-string lithium battery voltage detection circuit based on a mirror current source, comprising a mirror current source module, a multi-way switch matrix, a voltage divider circuit, and a signal processing unit, characterized in that: The mirror current source module is composed of a first transistor (Q1), a second transistor (Q2) and a constant current source (I_ref), the base and collector of the first transistor (Q1) are short-circuited and connected to the constant current source (I_ref), the base of the second transistor (Q2) is connected in common with the base of the first transistor (Q1), and the collector of the second transistor (Q2) outputs a mirror current proportional to the constant current source; The multi-way switch matrix comprises a plurality of optocoupler isolation analog switches (S1-Sn), the input end of each switch is respectively connected to the positive and negative electrodes of the series-connected lithium battery pack (B1-Bn), and the output end is connected to the voltage divider circuit; The voltage divider circuit is composed of at least two precision resistors (R1, R2) connected in series, the input end of which receives the mirror current output by the mirror current source module, and the output end of which is connected to the positive input end of the differential amplifier (U1); The signal processing unit comprises a differential amplifier (U1), an analog-to-digital converter (ADC) and a microcontroller (MCU), and is used to convert a voltage signal output by a voltage divider circuit into a digital signal and perform dynamic calibration.

2. The multi-string lithium battery voltage detection circuit based on mirror current source according to claim 1, characterized in that: The mirror current source module also includes a temperature compensation circuit, which is composed of a thermistor (Rt) and a compensation resistor (Rc) connected in series and connected between the emitter of the first transistor (Q1) and ground, and is used to offset the temperature drift of the transistor β value and the resistor.

3. The multi-string lithium battery voltage detection circuit based on mirror current source according to claim 1, characterized in that: The control end of the multi-way switch matrix is ​​connected to a microcontroller (MCU) via an isolation drive circuit. The isolation drive circuit uses a magnetic isolation or capacitive isolation chip to achieve electrical isolation between the high-voltage battery pack and the low-voltage control circuit.

4. The multi-string lithium battery voltage detection circuit based on mirror current source according to claim 1, characterized in that: The voltage divider circuit is connected in parallel with a filter capacitor (C1) with a capacitance of 1-10 μF, and the accuracy of the voltage divider resistors (R1, R2) is ±0.1%, and the temperature drift coefficient is ≤25ppm / °C.

5. The multi-string lithium battery voltage detection circuit based on mirror current source according to claim 1, characterized in that: The signal processing unit also includes a voltage calibration module, which pre-stores the measured resistance values ​​and temperature compensation curves of the voltage divider resistors (R1, R2) and performs nonlinear correction on the output value of the analog-to-digital converter (ADC) through a microcontroller (MCU).

6. The multi-string lithium battery voltage detection circuit based on mirror current source according to claim 5, characterized in that: The microcontroller (MCU) is configured to perform the following operations: control the multi-way switch matrix to sequentially select each lithium battery (B1-Bn) at a preset time interval; calculate the real-time battery voltage according to the product of the output current of the mirror current source and the total resistance of the voltage divider circuit; and trigger the protection circuit to disconnect the charge and discharge circuit when it is detected that the voltage of any lithium battery exceeds a preset threshold.

7. The multi-string lithium battery voltage detection circuit based on mirror current source according to claim 1, characterized in that: A voltage follower (U2) is provided between the output end of the differential amplifier (U1) and the analog-to-digital converter (ADC) for reducing output impedance and enhancing signal driving capability.

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