A PWM control-based BMS pre-charging device and method

Abstract

This invention relates to the field of lithium battery application technology, and in particular to a BMS pre-charging device and method based on PWM control. The invention includes: a lithium battery, a control system, a chopper drive module, a discharge MOSFET, a positive terminal PACK+ of the battery pack, and a negative terminal PACK- of the battery pack. The positive terminal of the lithium battery is connected to the positive terminal PACK+, and the negative terminal of the lithium battery is connected to the negative terminal PACK- through the discharge MOSFET. A load capacitor is connected in series between the positive and negative terminals PACK+ and PACK-. The chopper drive module is connected to both the control system and the discharge MOSFET. During the pre-charging phase of the load capacitor, the control system outputs a PWM pulse signal, which controls the discharge MOSFET to turn on or off through the chopper drive module, achieving intermittent charging of the load capacitor. This invention overcomes the problems of high hardware losses and poor adaptability in the pre-charging hardware of existing technologies. By controlling the charging of the load capacitor through a pulse signal, the hardware circuit is simplified, and heat loss is reduced.

Description

Technical Field

[0001] This invention relates to the field of lithium battery application technology, and in particular to a BMS pre-charge device and method based on PWM control. Background Technology

[0002] With the rapid development of new energy vehicles, electric motorcycles and other medium- and high-voltage platform vehicles are becoming increasingly popular. Their motor drive controllers and other load terminals typically have large-capacity bus filter capacitors connected in parallel. During initial power-on or wake-up from sleep mode, if the main circuit switch between the battery pack and the load is closed directly, a huge surge current will be generated instantaneously because the capacitor voltage is initially zero. This could damage switching devices, lower the system voltage, or even pose a safety risk. Therefore, charging the load capacitors using a pre-charging device before closing the main circuit has become a critical protection feature in the BMS (Battery Management System).

[0003] The most common technical solution currently is to use an independent resistor-limited pre-charge circuit, which is a branch connected in parallel next to the main circuit switch, consisting of a dedicated pre-charge MOSFET and a high-power current-limiting resistor in series. However, the continuous conduction of the pre-charge resistor leads to a large amount of Joule heat loss, resulting in low pre-charge efficiency. In addition, the high-power resistor is bulky, expensive, and occupies too much PCB space. In addition, existing technologies also include high-voltage pre-charging circuits based on pure hardware logic, such as the one proposed in publication number CN121105902A. These circuits utilize an RC timing module to set a fixed pre-charging time and achieve automatic control through a self-locking and bypass module. However, the pre-charging strategy is rigid, relying on a fixed RC constant to determine the pre-charging time. This makes it unable to adapt to changes in operating conditions such as battery voltage fluctuations and capacitor value deviations, and it is prone to risks such as "bypassing before full charge" or "excessive waiting after full charge". Summary of the Invention

[0004] To overcome the shortcomings of high hardware loss and poor adaptability of the pre-charging hardware in the existing technology, this invention proposes a BMS pre-charging device based on PWM control, which controls the charging of the load capacitor through pulse signals, simplifies the hardware circuit and reduces heat loss.

[0005] The present invention proposes a BMS pre-charge device based on PWM control, comprising: a lithium battery, a control system, a chopper drive module, a discharge MOSFET, a battery pack positive terminal PACK+, and a battery pack negative terminal PACK-; The positive terminal of the lithium battery is connected to the positive terminal PACK+ of the battery pack, and the negative terminal of the lithium battery is connected to the negative terminal PACK- of the battery pack through a discharge MOSFET; the load capacitor is connected in series between the positive terminal PACK+ and the negative terminal PACK- of the battery pack; the chopper drive module is connected to the control system and the discharge MOSFET respectively; During the pre-charging phase of the load capacitor, the control system outputs a PWM pulse signal, which controls the discharge MOSFET to turn on or off through the chopper drive module, thereby achieving intermittent charging of the load capacitor.

[0006] Preferably, it also includes a voltage acquisition module, which is used to monitor the load voltage of the load capacitor and feed it back to the control system; the control system stores the mapping relationship between the load voltage and the PWM pulse signal parameters, and adjusts the PWM pulse signal according to the load voltage; the PWM pulse signal parameters include duty cycle and frequency.

[0007] Preferably, the discharge MOSFET is an N-channel MOSFET; the chopper drive module includes a chopper control circuit and a discharge circuit; The chopper control circuit includes: a third MOSFET and a fifth transistor; the third MOSFET is a P-channel MOSFET, and its source is used as the constant voltage input terminal of the chopper control circuit; the fifth transistor is an N-type transistor, and its base is used as the chopper signal input terminal; the gate of the third MOSFET is grounded through the fifth transistor; the drain of the third MOSFET is used as the output terminal of the chopper control circuit. The discharge circuit includes: a first diode and a sixth transistor; the positive terminal of the first diode serves as the input terminal of the discharge circuit, and its negative terminal serves as the output terminal of the discharge circuit, which is used to connect to the gate of the discharge MOSFET; the sixth transistor is a P-type transistor, the negative terminal of the first diode is also grounded through the sixth transistor, and the base of the sixth transistor is connected to the input terminal of the discharge circuit. The control system is connected to the constant voltage input terminal and the chopper signal input terminal of the chopper control circuit, respectively.

[0008] Preferably, it also includes a voltage comparison module, which is used to monitor the voltage difference between the negative terminal of the lithium battery and the negative terminal PACK- of the battery pack and feed it back to the control system. When the positive terminal PACK+ and the negative terminal PACK- of the battery pack are connected to the positive and negative terminals of the bus respectively, the lithium battery enters the grid-connected power supply state. The control system controls the discharge MOSFET to turn on or off through the chopper drive module according to the feedback from the voltage comparison module, so as to realize the grid connection and disconnection of the lithium battery.

[0009] Preferably, the drain of the discharge MOSFET is connected to the negative terminal of the bus via the charging MOSFET; the charging MOSFET is an N-channel MOSFET and is connected to a charging control circuit.

[0010] Preferably, the charging control circuit includes a second diode and a fourth transistor; the fourth transistor is a P-type transistor. The negative terminal of the second diode is connected to the gate of the charging MOSFET, and its positive terminal is connected to the charging signal input terminal through the fourth transistor; the emitter of the fourth transistor is connected to its base through the eighth resistor, and the base is grounded through the tenth resistor; the control system is also connected to the charging signal input terminal of the charging control circuit.

[0011] Preferably, the negative terminal of the second diode is connected to the gate of the second MOS transistor through a third resistor, and the fourth transistor is connected to the charging signal input terminal through a fourth resistor.

[0012] Preferably, the base of the fifth transistor is connected to the chopper signal input terminal through the ninth resistor, the base of the fifth transistor is also grounded through the eleventh resistor, and the emitter of the fifth transistor is grounded.

[0013] The present invention proposes a control method for a BMS pre-charge device based on PWM control, comprising the following steps: St1, when the load capacitor is connected between the positive terminal PACK+ and the negative terminal PACK- of the battery pack, the control system starts and outputs a PWM pulse signal with initial parameters; the parameters of the PWM pulse signal include duty cycle and frequency; St2, the voltage acquisition module detects the voltage of the load capacitor in real time, and the control system adjusts the PWM chopping parameters according to the detected voltage; St3. When the voltage of the load capacitor reaches the set condition, the control system outputs a continuous high-level chopper signal, and the charging of the load capacitor switches from pulse charging to constant current charging mode.

[0014] Preferably, the setting condition in step St3 is: the voltage of the load capacitor reaches the set value.

[0015] The advantages of this invention are: (1) The present invention adopts PWM pulse control and uses the milliohm-level on-resistance of the discharge MOS transistor to replace the ohm-level current limiting resistor. The current flows through the channel instead of the resistive body to generate heat, and the electrical energy is mainly converted into capacitor energy storage. This significantly reduces the Joule heat loss during the pre-charging process and improves the energy conversion efficiency.

[0016] (2) This invention eliminates the need for independent high-power pre-charge resistors and dedicated pre-charge MOSFETs and their driving circuits, and directly reuses the existing discharge MOSFETs and driving links in the main discharge circuit, which significantly reduces the number of power devices, PCB layout area and bill of materials (BOM) items, and realizes a high degree of integration of hardware resources and miniaturized and economical design.

[0017] (3) The present invention samples the load voltage in real time and performs closed-loop feedback control through the control system, so that the pre-charge result is accurate and reliable, and completely avoids the risk of "not fully charged" or "overcharge waiting" in the fixed timing scheme; at the same time, the duty cycle and frequency of PWM can be dynamically adjusted by software, and the pre-charge current curve can be adaptively optimized according to the battery status to achieve a smoother and faster charging process.

[0018] (4) The discharge circuit can ensure that the discharge MOS transistor is turned off quickly during the PWM low level, effectively overcome the turn-off delay problem caused by parasitic capacitance, improve the timing accuracy under high frequency switching, eliminate potential short circuit risk, and enhance the overall system reliability and safety. Attached Figure Description

[0019] Figure 1 This is the circuit diagram of the chopper driver module; Figure 2 For chopper drive module and charging control circuit; Figure 3 This is a BMS pre-charge device based on PWM control; Figure 4 For BMS pre-charging device under grid-connected power supply conditions; Figure 5 A BMS pre-charge device for charging the load capacitor; Figure 6 The waveform is shown for an example. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] like Figure 1 As shown, the BMS pre-charging device based on PWM control proposed in this embodiment includes: a lithium battery, a control system, a chopper drive module, a discharge MOSFET, a voltage comparison module, a voltage acquisition module, the positive terminal PACK+ of the battery pack, and the negative terminal PACK- of the battery pack. The chopper drive module includes a chopper control circuit and a discharge circuit, as detailed in [link to documentation]. Figure 1 , Figure 3 .

[0022] The control system, chopper drive module, voltage comparison module, and discharge MOSFET form a parallel device used to connect the lithium battery and the bus to achieve grid-connected power supply for the lithium battery.

[0023] The discharge MOSFET Q1 is an N-channel MOSFET, which is connected in series between the negative terminal of the lithium battery and the negative terminal of the bus. The chopper control circuit includes: a third MOSFET Q3 and a fifth transistor Q5; the third MOSFET Q3 is a P-channel MOSFET, and its source serves as the constant voltage input terminal of the chopper control circuit, used to connect the constant voltage signal DSG_EN; the fifth transistor Q5 is an N-type transistor, and its base serves as the chopper signal input terminal, used to connect the chopper signal PWM_EN; the gate of the third MOSFET Q3 is grounded through the fifth transistor Q5; the drain of the third MOSFET Q3 serves as the output terminal of the chopper control circuit.

[0024] Specifically, the fifth resistor R5 is connected in series between the source and gate of the third MOSFET Q3; the base of the fifth MOSFET Q5 is connected to the chopper signal input terminal through the ninth resistor R9, the base of the fifth MOSFET Q5 is also grounded through the eleventh resistor R11, and the emitter of the fifth MOSFET Q5 is grounded.

[0025] The bleeder circuit includes: a first diode D1 and a sixth transistor Q6; the first diode D1 is a switching diode, and its anode serves as the input terminal of the bleeder circuit, used to connect to the output terminal of the chopper control circuit. In a specific implementation, a second resistor R2 can be connected in series between the input terminal of the bleeder circuit and the output terminal of the chopper control circuit. The cathode of the first diode D1 serves as the output terminal of the bleeder circuit, used to connect to the gate of the discharge MOSFET.

[0026] The gate of a power MOSFET has parasitic capacitance. After the driving voltage is removed, the gate charge cannot disappear instantly, causing a delay in the MOSFET's turn-off and affecting control accuracy. This embodiment includes a bleeder circuit. When the chopper signal PWM_EN goes low, the gate of the discharge MOSFET Q1 remains high due to the parasitic capacitance. At this time, this high level, through the first diode D1 (currently forward biased), causes the base-emitter voltage of the sixth transistor Q6 to meet the conduction condition, and Q6 quickly saturates and turns on. The conducting sixth transistor Q6 forms a low-impedance path between the gate and source (ground) of the discharge MOSFET Q1, forcibly removing the residual charge on the gate quickly, causing the gate voltage to drop rapidly to zero. This ensures that the discharge MOSFET Q1 can be quickly and hard turned off, guaranteeing the effectiveness of the chopper signal control at high frequencies and system safety. When the gate charge of the discharge MOSFET Q1 disappears, the potential difference between the emitter and base of the sixth transistor Q6 disappears, thus turning off the sixth transistor Q6.

[0027] The sixth transistor Q6 is a P-type transistor. The cathode of the first diode D1 is also grounded through the sixth transistor Q6. The base of the sixth transistor Q6 is connected to the input terminal of the bleeder circuit through the sixth resistor R6.

[0028] like Figure 3 , Figure 4As shown, the input terminals of the voltage comparator module are connected to the negative terminal BATT- of the lithium battery and the negative terminal PACK- of the battery pack, respectively, while the positive terminal BATT+ of the lithium battery is directly connected to the positive terminal PACK+ of the battery pack. The output terminal of the voltage comparator module is connected to the control system, which is also connected to the constant voltage input terminal and the chopper signal input terminal of the chopper control circuit, respectively, for outputting the constant voltage signal DSG_EN and the chopper signal PWM_EN. Specifically, in the grid-connected state of the lithium battery, the positive terminal PACK+ and the negative terminal PACK- of the battery pack are connected to the positive and negative terminals of the bus, respectively.

[0029] When the lithium battery is in grid-connected discharge mode, the constant voltage signal DSG_EN remains high during control system operation. When the chopper signal PWM_EN is low, the fifth transistor Q5 is cut off, causing the third MOSFET Q3 to reach a high level and be cut off, thus the first diode D1 is cut off. The sixth transistor Q6 is briefly turned on to help the gate of the discharge MOSFET Q1 discharge quickly. Then, the discharge MOSFET Q1 is turned off due to the gate being de-energized, and the lithium battery is disconnected from the bus. When the chopper signal PWM_EN is high, the fifth transistor Q5 is turned on, causing the third MOSFET Q3 to reach a low level and be turned on. The first diode D1 is turned on, and the sixth transistor Q6 is cut off. The discharge MOSFET Q1 receives power from the constant voltage signal DSG_EN through the turned-on third MOSFET Q3 and the first diode D1, and the discharge MOSFET Q1 is turned on, connecting the lithium battery to the bus.

[0030] like Figure 5 As shown, the voltage acquisition module is connected to the positive terminal PACK+, the negative terminal PACK- of the battery pack, and the control system. When a load capacitor is connected between the positive terminal PACK+ and the negative terminal PACK- of the battery pack, the voltage acquisition module is used to detect the load voltage (i.e., the voltage of the load capacitor) and feed it back to the control system.

[0031] In practice, the source of the discharge MOSFET Q1 is connected to the negative terminal of the lithium battery through the first resistor R1; the drain of the discharge MOSFET Q1 is connected to the negative terminal of the bus through the charging MOSFET Q2, which is an N-channel MOSFET.

[0032] The gate of the charging MOSFET Q2 is connected to a charging control circuit. The charging control circuit includes a second diode D2 and a fourth transistor Q4; The second diode D2 is a switching diode, and the fourth transistor Q4 is a P-type transistor. The cathode of the second diode D2 is connected to the gate of the charging MOSFET Q2, and its cathode is connected to the charging signal input terminal through the fourth transistor Q4 to receive the charging signal CHG. The emitter of the fourth transistor Q4 is connected to its base through the eighth resistor R8, and the base is grounded through the tenth resistor R10. The tenth resistor R10 and the eighth resistor R8 form a voltage divider combination, so that when CHG is high, a potential difference is formed between the emitter and base of the fourth transistor Q4, thus turning it on. CHG supplies power to the gate of the charging MOSFET Q2 through the fourth transistor Q4 and the second diode D2, turning on the charging MOSFET Q2. When CHG is low, the charging MOSFET Q2 is off.

[0033] In practice, the negative terminal of the second diode D2 is connected to the gate of the second MOS transistor through the third resistor R3, and the fourth transistor Q4 is connected to the charging signal input terminal through the fourth resistor.

[0034] The control system is also connected to the charging signal input terminal of the charging control circuit, which is used to output the charging signal CHG.

[0035] During lithium battery charging, the constant voltage signal DSG_EN and the chopper signal PWM_EN remain at a high level. When the charging signal CHG is at a high level, the charging MOSFET Q2 is turned on, and the lithium battery is charged in parallel with the grid. When the charging signal CHG is at a low level, the charging MOSFET Q2 is turned off because its gate is de-energized, and the lithium battery stops charging offline.

[0036] The lithium battery grid-connected charging control method is as follows: When the lithium battery enters the charging state, the control system is turned on and outputs a high-level constant voltage signal DSG_EN, a chopper signal PWM_EN, and a charging signal CHG, so that both the discharge MOSFET Q1 and the charging MOSFET Q2 are turned on, and the lithium battery enters the grid-connected state for charging until the negative terminal BATT- of the lithium battery and the negative terminal voltage of the bus are equalized.

[0037] The charging status of the lithium battery is controlled by an external charging controller.

[0038] When the lithium battery is discharged in grid-connected mode, the charging signal CHG remains high, causing the charging MOSFET Q2 to remain on. The control system controls the chopper signal PWM_EN to switch between high and low states, thereby controlling the lithium battery to discharge in grid-connected mode. When the lithium battery is charged in grid-connected mode, the chopper signal PWM_EN remains high, causing the discharging MOSFET Q1 to remain on. The control system controls the charging signal CHG to switch between high and low states, thereby controlling the lithium battery to charge in grid-connected mode.

[0039] Specifically, the lithium battery grid-connected discharge control method includes the following steps: S1. When the lithium battery enters the discharge state, the control system is turned on, and the stable output of the normal voltage signal DSG_EN and the high-level chopper signal PWM_EN are output to drive the fifth transistor Q5 to conduct, so that Q1 is turned on, the negative terminal of the lithium battery is connected to the negative terminal of the bus, and the lithium battery enters the grid-connected state as a power supply. S2. In grid-connected mode, the voltage comparison module detects the voltage difference between the negative terminal of the lithium battery and the negative terminal of the bus in real time and feeds it back to the control system. When the power imbalance condition is reached, the control system switches to the low-level chopping signal PWM_EN, the fifth transistor Q5 is turned on and off, the discharge MOSFET Q1 is turned off, so that the lithium battery is disconnected from the grid; the sixth transistor Q6 is briefly turned on, guiding the gate of the discharge MOSFET Q1 to discharge quickly.

[0040] S3. After the control system maintains the low-level chopper signal PWM_EN for a set time (e.g., 10ms), it switches back to the high-level chopper signal PWM_EN and then returns to step S2.

[0041] Thus, during lithium battery discharge, the control system performs chopper control via DSG_EN and PWM_EN; the voltage comparison module compares the BATT-voltage with the negative bus voltage, and if a voltage difference exists, it feeds back to the control system; the control system detects the lithium battery discharge current; and the control system controls the switching of the discharge MOSFET Q1 via the chopper drive circuit. When a voltage difference exists and the current exceeds a threshold, the discharge MOSFET Q1 is turned off for a set time (e.g., 10ms) and then closed again.

[0042] When the lithium battery is in discharge mode, both DSG_EN and CHG are constant voltages.

[0043] During lithium battery charging, both DSG_EN and PWM_EN are constant voltages; the control system controls the charging process through the CHG signal to prevent overcharging.

[0044] like Figure 5 As shown, the voltage acquisition module is connected between the positive terminal PACK+ and the negative terminal PACK- of the battery pack. It is used to acquire the voltage between these terminals and feed it back to the control system. When a load capacitor is connected between the positive and negative terminals, the voltage acquisition module monitors the voltage of the load capacitor in real time, thereby executing the following BMS pre-charge method based on PWM control to charge the load capacitor.

[0045] St1, the load capacitor is connected between the positive terminal PACK+ and the negative terminal PACK- of the battery pack. The control system starts and outputs a PWM chopper with initial parameters. The parameters of the PWM chopper include duty cycle and frequency.

[0046] use Figure 1 In the circuit shown, the constant voltage signal DSG_EN remains high, and the chopping signal PWM_EN uses PWM chopping; Figure 2 In the circuit shown, the charging signal CHG also remains high. Thus, when the chopper signal PWM_EN is high, the discharge MOSFET Q1 closes, and the lithium battery supplies power to the load capacitor; when the chopper signal PWM_EN is low, the discharge MOSFET Q1 is off, and the lithium battery stops supplying power. Therefore, by controlling the duty cycle of the chopper signal PWM_EN, the amount of charge on the load capacitor in a single charging cycle can be controlled, and by controlling the frequency of the chopper signal PWM_EN, the charging frequency of the load capacitor can be controlled.

[0047] St2, the voltage acquisition module detects the voltage of the load capacitor in real time, and the control system adjusts the PWM chopping parameters according to the detected voltage.

[0048] In practice, a mapping relationship between the load voltage range and the PWM chopper parameters can be set, and the PWM chopper parameters such as duty cycle and frequency can be adjusted according to the mapping relationship.

[0049] St3. When the voltage of the load capacitor reaches the set condition (e.g., the load capacitor is 95% charged), the control system outputs a continuously high-level chopper signal PWM_EN, and the charging of the load capacitor switches from pulse charging to constant current charging mode.

[0050] In a specific embodiment, PWN chopper ( Figure 6 The parameters for the orange waveform are set as follows: duty cycle 10%, frequency 1kHz; pre-charge setting condition is that the load capacitor is 95% charged; the result is as follows. Figure 6 As shown, the blue waveform is the voltage waveform of PACK+ / - (i.e., the load voltage). It can be seen that as the PWM chopper continuously pulses to close and discharge the MOS, the precharge voltage increases until precharge is complete.

[0051] Of course, those skilled in the art will recognize that the present invention is not limited to the details of the exemplary embodiments described above, but also includes the same or similar structures that can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0052] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0053] The technologies, shapes, and structures not described in detail in this invention are all known technologies.

Claims

1. A BMS pre-charge device based on PWM control, characterized in that, include: Lithium battery, control system, chopper drive module, discharge MOSFET, battery pack positive terminal PACK+ and battery pack negative terminal PACK-; The positive terminal of the lithium battery is connected to the positive terminal PACK+ of the battery pack, and the negative terminal of the lithium battery is connected to the negative terminal PACK- of the battery pack through a discharge MOSFET; the load capacitor is connected in series between the positive terminal PACK+ and the negative terminal PACK- of the battery pack; the chopper drive module is connected to the control system and the discharge MOSFET respectively; During the pre-charging phase of the load capacitor, the control system outputs a PWM pulse signal, which controls the discharge MOSFET to turn on or off through the chopper drive module, thereby achieving intermittent charging of the load capacitor.

2. The BMS pre-charge device based on PWM control as described in claim 1, characterized in that, It also includes a voltage acquisition module, which monitors the load voltage of the load capacitor and feeds it back to the control system. The control system stores the mapping relationship between the load voltage and PWM pulse signal parameters and adjusts the PWM pulse signal according to the load voltage. The PWM pulse signal parameters include duty cycle and frequency.

3. The BMS pre-charge device based on PWM control as described in claim 2, characterized in that, The discharge MOSFET (Q1) is an N-channel MOSFET; the chopper drive module includes a chopper control circuit and a discharge circuit; The chopper control circuit includes a third MOSFET (Q3) and a fifth transistor (Q5). The third MOSFET (Q3) is a P-channel MOSFET, and its source serves as the constant voltage input terminal of the chopper control circuit. The fifth transistor (Q5) is an N-type transistor, and its base serves as the chopper signal input terminal. The gate of the third MOSFET (Q3) is grounded through the fifth transistor (Q5). The drain of the third MOSFET (Q3) serves as the output terminal of the chopper control circuit. The discharge circuit includes: a first diode (D1) and a sixth transistor (Q6); the anode of the first diode (D1) serves as the input terminal of the discharge circuit, and its cathode serves as the output terminal of the discharge circuit, used to connect to the gate of the discharge MOSFET; the sixth transistor (Q6) is a P-type transistor, the cathode of the first diode (D1) is also grounded through the sixth transistor (Q6), and the base of the sixth transistor (Q6) is connected to the input terminal of the discharge circuit; The control system is connected to the constant voltage input terminal and the chopper signal input terminal of the chopper control circuit, respectively.

4. The BMS pre-charge device based on PWM control as described in claim 3, characterized in that, It also includes a voltage comparison module, which monitors the voltage difference between the negative terminal of the lithium battery and the negative terminal PACK- of the battery pack and feeds it back to the control system. When the positive terminal PACK+ and the negative terminal PACK- of the battery pack are connected to the positive and negative terminals of the bus respectively, the lithium battery enters the grid-connected power supply state. The control system controls the discharge MOSFET to turn on or off through the chopper drive module according to the feedback from the voltage comparison module, so as to realize the grid connection and disconnection of the lithium battery.

5. The BMS pre-charge device based on PWM control as described in claim 3, characterized in that, The drain of the discharge MOSFET (Q1) is connected to the negative terminal of the bus through the charging MOSFET (Q2); the charging MOSFET (Q2) is an N-channel MOSFET and is connected to a charging control circuit.

6. The BMS pre-charge device based on PWM control as described in claim 5, characterized in that, The charging control circuit includes a second diode (D2) and a fourth transistor (Q4); the fourth transistor (Q4) is a P-type transistor. The negative terminal of the second diode (D2) is connected to the gate of the charging MOSFET (Q2), and its positive terminal is connected to the charging signal input terminal through the fourth transistor (Q4). The emitter of the fourth transistor (Q4) is connected to its base through the eighth resistor (R8), and the base is grounded through the tenth resistor (R10). The control system is also connected to the charging signal input terminal of the charging control circuit.

7. The BMS pre-charge device based on PWM control as described in claim 6, characterized in that, The negative terminal of the second diode (D2) is connected to the gate of the second MOS transistor through the third resistor (R3), and the fourth transistor (Q4) is connected to the charging signal input terminal through the fourth resistor.

8. The BMS pre-charge device based on PWM control as described in claim 3, characterized in that, The base of the fifth transistor (Q5) is connected to the chopper signal input terminal through the ninth resistor (R9). The base of the fifth transistor (Q5) is also grounded through the eleventh resistor (R11). The emitter of the fifth transistor (Q5) is grounded.

9. A control method for a BMS pre-charge device based on PWM control as described in any one of claims 2-8, characterized in that, Includes the following steps: St1, when the load capacitor is connected between the positive terminal PACK+ and the negative terminal PACK- of the battery pack, the control system starts and outputs a PWM pulse signal with initial parameters; the parameters of the PWM pulse signal include duty cycle and frequency; St2, the voltage acquisition module detects the voltage of the load capacitor in real time, and the control system adjusts the PWM chopping parameters according to the detected voltage; St3. When the voltage of the load capacitor reaches the set condition, the control system outputs a continuously high-level chopping signal, and the charging of the load capacitor switches from pulse charging to constant current charging mode.

10. The control method as described in claim 9, characterized in that, The setting condition in step St3 is: the voltage of the load capacitor reaches the set value.

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