Low-cost switching power supply for high-precision electric energy metering and control method thereof

Abstract

The invention provides a low-cost switching power supply for high-precision electric energy metering and a control method thereof. A voltage output pin of a switching power supply chip is grounded through a first capacitor; an external low-voltage power supply is connected and charges the first capacitor through the first resistor and the first diode, and the maximum soft start current of the boost power supply circuit is limited to be less than 100mA by setting the resistance value of the first resistor; an external low-voltage power supply is connected to a power supply input pin of the switching power supply chip through a sampling resistor, the sampling resistor is connected with an in-phase input end and an inverted input end of a voltage comparator, and whether overcurrent occurs or not is judged according to high or low level signals output by the voltage comparator; the over-current protection threshold value of the USB power supply circuit is limited by setting the resistance tolerance level and the input offset voltage of the voltage comparator. By designing the soft start and overcurrent protection circuit, the USB power supply with surge current prevention and precise overcurrent protection functions is provided, and the effects of greatly reducing the cost, simplifying the circuit design and improving the circuit reliability are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of low-voltage switching power supplies, and in particular to a low-cost switching power supply for high-precision electric energy metering and a control method thereof. Background Art

[0002] With the rapid development of smart grids, traditional electronic electricity meters that use infrared communication or RS485 communication interfaces for precision adjustment, production testing, diary recording, data processing, technical maintenance, on-site service and other task processing methods can no longer meet the development needs of intelligent electricity meters. Now high-end intelligent electricity meters require more precise measurement and more complex functions, including but not limited to configured event logs, metering parameters, harmonic analysis and other complex data processing. This makes traditional infrared or RS485 serial communication methods with low data transmission rate and poor stability unsuitable for high-end smart electricity meters. Therefore, USB serial communication with high communication efficiency, stability and reliability, scalable functions and good data security and integrity has become the preferred solution.

[0003] Due to the functional complexity of smart meters and their high power system requirements, their power systems typically consist of a low-voltage AC-to-DC main power supply supplemented by multiple low-voltage-to-low-voltage power systems. Furthermore, these systems must also withstand harsh industrial and mining environments, such as grid voltage fluctuations and flashovers. Consequently, these power systems typically employ a large number of energy storage capacitors. In this scenario, when an external low-voltage USB power supply is directly connected to the meter system, the thousands of uF or even several F-class capacitors in the power system can cause significant inrush currents. This can easily damage the external low-voltage USB power supply and even affect the startup of external devices and the entire meter power system. Therefore, a low-voltage power supply with soft-start and overcurrent protection is essential for implementing USB functionality in energy meters.

[0004] However, the built-in soft start and overcurrent protection modules of ordinary low-voltage power supply integrated chips generally have shortcomings such as short soft start time, large overcurrent protection error, easy false triggering and lockout after overcurrent protection. Although programmable high-end power chips can achieve precise power supply control by dynamically adjusting parameters such as voltage and current, they usually use complex processes and precision components, resulting in high prices and high direct development or maintenance barriers. In addition, their high integration and strong processing capabilities lead to increased power consumption, which may place an additional burden on the meter battery or cooling system.

[0005] Therefore, there is an urgent need for a low-cost USB power supply circuit with adaptive soft start and accurate overcurrent protection functions for high-precision energy measurement. Summary of the Invention

[0006] In order to solve the common problems in the prior art, the purpose of the present invention is to provide a low-cost switching power supply and a control method for high-precision electric energy metering. The invention utilizes the pin function of the switching power supply chip in combination with inexpensive transistors, resistors and comparators to design soft start and overcurrent protection circuits. While meeting the power supply requirements of high-end smart meters implementing USB functions, it can provide surge current protection and precise overcurrent protection functions, and can achieve the effects of significantly reducing costs, simplifying circuit design and improving circuit reliability.

[0007] The present invention achieves the above-mentioned purpose through the following technical solutions: A low-cost switching power supply for high-precision electric energy metering includes: a boost power supply circuit, a soft-start circuit, and an overcurrent protection circuit. The boost power supply circuit includes a switching power supply chip and a first capacitor. A voltage output pin of the switching power supply chip is grounded through the first capacitor, and is used to boost an externally input first USB low-voltage power supply to an operating voltage supplied to the power supply system of the electric meter. The soft-start circuit includes a first resistor, a first diode, a first switching transistor, and a second switching transistor. The first resistor and the first diode are connected in series to the voltage output pin of the switching power supply chip. The first USB low-voltage power supply is connected to and charges the first capacitor through the first resistor and the first diode, and is connected to the base of the second switching transistor through the first switching transistor. The emitter and base of the first switching transistor are connected in parallel to both ends of the first resistor. The collector of the second switching transistor is connected to the switch enable pin of the switching power supply chip, and its emitter is grounded.

[0008] The maximum soft-start current of the boost power supply circuit is limited by setting the resistance value of the first resistor.

[0009] The overcurrent protection circuit includes a sampling resistor, a voltage comparator, a first input resistor, and a second input resistor. The first USB low-voltage power supply is connected to the power input pin of the switching power supply chip through the sampling resistor. The power input end of the sampling resistor is connected to the non-inverting input end of the voltage comparator through the first input resistor, and the other end is connected to the inverting input end of the voltage comparator through the second input resistor. The voltage comparator compares the voltage at its non-inverting input end with the voltage at its inverting input end, and outputs a high or low level signal to the feedback pin of the switching power supply chip based on the comparison result. The switching power supply chip determines whether to shut down based on the voltage level of its feedback pin.

[0010] The overcurrent protection threshold of the USB power supply circuit is limited by setting the resistance tolerance level of the sampling resistor and the first input resistor and the input offset voltage of the voltage comparator.

[0011] According to the present invention, a low-cost switching power supply for high-precision electric energy metering is provided, which also includes a USB-SUP power input circuit. The USB-SUP power input circuit is used to input a second USB low-voltage power supply with multi-speed voltage output that supports USB PD 3.0 and above standard interfaces, including a second capacitor, a first voltage divider resistor, and a second voltage divider resistor that constitute an RC delay network. The second USB low-voltage power supply is respectively connected to the power input terminals of the soft start circuit and the overcurrent protection circuit via the RC delay network; wherein the first USB low-voltage power supply is a 5V low-voltage direct current, and the second USB low-voltage power supply includes 5V, 9V / 12V, and 20V low-voltage direct current.

[0012] According to a low-cost switching power supply for high-precision electric energy metering provided by the present invention, one end of the second capacitor is connected to ground in parallel with the first voltage-divider resistor, and the other end is connected to the second USB low-voltage power supply and is respectively connected to the power input terminals of the first resistor and the sampling resistor through the second voltage-divider resistor. The charging time constant of the second capacitor and the saturation voltage drop of the second switching tube are adjusted to limit the rise rate of the base voltage of the second switching tube during the soft-start phase, thereby limiting the maximum soft-start current of the boost power supply circuit.

[0013] According to the present invention, a low-cost switching power supply for high-precision electric energy metering is provided. During a soft-start phase when connected to a first USB low-voltage power supply, the voltage difference across the first resistor is equal to the difference between the voltage at the voltage output pin of the switching power supply chip and the voltage drop of the first diode and the first USB low-voltage power supply. Furthermore, by using a first resistor with a resistance of 50 ohms or less, the maximum soft-start current of the boost power supply circuit can be limited to less than 100 mA.

[0014] According to a low-cost switching power supply for high-precision electric energy metering provided by the present invention, the overcurrent protection circuit also includes a third voltage-dividing resistor and a third capacitor. The first input resistor is connected to ground in series with the third voltage-dividing resistor, and the third capacitor is connected in parallel across the third voltage-dividing resistor. The non-inverting input terminal of the voltage comparator obtains a fixed-value voltage by dividing the voltage by the first input resistor and the third voltage-dividing resistor.

[0015] When the voltage value of the inverting input terminal of the voltage comparator is greater than the fixed value voltage, the output terminal of the voltage comparator outputs a low level signal to the feedback pin of the switching power supply chip, otherwise it outputs a high level signal.

[0016] According to a low-cost switching power supply for high-precision electric energy metering provided by the present invention, a second diode and a first current-limiting resistor are further connected in series on a feedback path from the output end of the voltage comparator to the feedback pin of the switching power supply chip. The feedback threshold voltage is determined by setting the voltage drop of the second diode and the resistance value of the first current-limiting resistor. When the voltage at the feedback pin of the switching power supply chip is greater than the feedback threshold voltage, the switching power supply chip controls the internal switch tube to turn off; otherwise, it operates normally.

[0017] In which, the boost power supply circuit also includes a power output feedback loop composed of a plurality of feedback voltage divider resistors, the voltage output pin of the switching power supply chip is connected to its feedback pin through the source output feedback loop, and the switching power supply chip is provided with an output threshold voltage; when the feedback pin voltage of the switching power supply chip is greater than the output threshold voltage, the switching power supply chip controls the internal switch tube to turn off; otherwise, it operates normally.

[0018] According to a low-cost switching power supply for high-precision electric energy metering provided by the present invention, the sampling resistor adopts a resistor with a resistance value of 0.51 ohm, and by setting the resistance tolerance level of the sampling resistor and the first input resistor to ±1%, and the input offset voltage of the voltage comparator to 110 uV, the overcurrent protection threshold of the switching power supply is limited to 510 mA ±2%.

[0019] A control method for a low-cost switching power supply for high-precision electric energy metering, applied to the low-cost switching power supply for high-precision electric energy metering, comprising: S1: The switching power supply is connected to the first USB low-voltage power supply or the second USB low-voltage power supply, and slowly charges the first capacitor at the output node of the boost power supply circuit through the first resistor and the first diode. S2: The voltage difference across the first resistor gradually decreases as the first capacitor charges. When the voltage difference across the first resistor is greater than 0.55V, the first switch tube is turned on and drives the second switch tube to turn on. At this time, the switch enable pin of the switching power supply chip is pulled to a low level, and the switching power supply chip is in an off state.

[0020] S3: When the voltage difference across the first resistor is ≤0.55V, the first switch tube and the second switch tube are turned off. At this time, the low state of the switch enable pin of the switching power supply chip is released to a high level, and the switching power supply chip starts automatically.

[0021] S4: Real-time monitoring of the voltage drop of the load current on the sampling resistor. When the voltage drop is less than the fixed voltage at the non-inverting input of the voltage comparator, the voltage comparator outputs a high-level signal and applies it to the feedback pin of the switching power supply chip. When the feedback pin voltage of the switching power supply chip is greater than the feedback threshold voltage, the switching power supply chip is turned off; otherwise, it operates normally.

[0022] S5: The off state of the switching power supply chip continues until the load current drops below the overcurrent protection threshold, and the soft start steps S1 to S3 are automatically re-executed.

[0023] S6: While executing steps S4 to S5, the output voltage of the switching power supply chip is sampled in real time. When the feedback pin voltage of the switching power supply chip is greater than the set output threshold voltage, the switching power supply chip controls the internal switch tube to turn off; otherwise, it operates normally.

[0024] According to a control method for a low-cost switching power supply for high-precision electric energy metering provided by the present invention, in step S2: The conduction threshold of the first switch tube is set to 0.55V±0.02V, and the resistance value of the first resistor is adjusted to match the conduction threshold difference of different first switch tubes.

[0025] During the charging process, the maximum soft-start current is limited to less than 100mA until the output node voltage of the boost power supply circuit rises to the input voltage value of the switching power supply.

[0026] According to the present invention, a control method for a low-cost switching power supply for high-precision electric energy metering further includes, in steps S4-S5: Error compensation is performed based on the total error of the overcurrent point protection, which is:

[0027] in, is the target overcurrent threshold; is the error value of the sampling resistor; is the combined error of the first input resistor and the third voltage divider resistor of the voltage comparator, is the normalized error of the input offset voltage of the voltage comparator.

[0028] It can be seen that compared with the prior art, the present invention has the following beneficial effects: 1. The switching power supply of the present invention uses simple discrete components through circuit design to build a soft start circuit and a precise overcurrent protection circuit. The soft start function can charge the output voltage from 0V to 5V, and the overcurrent protection threshold is 600mA±5%, thereby fully meeting the USB minimum current limit standard of 500mA. Under the premise of meeting the power supply requirements for implementing the USB function of high-end smart meters, it can provide anti-surge current and precise overcurrent protection functions.

[0029] 2. The present invention uses low-cost discrete components to design the circuit. During the soft start process, the voltage difference of the resistor R226 is used to control the conduction of the first switch tube V202, ensuring that the rising slope of the output voltage VCS from 0V to 5V is stable. The circuit structure is simple and the stability is good, which greatly reduces the cost of the traditional dedicated switching power supply chip with a soft start function. At the same time, the discrete component setting can reduce the maintenance cost of circuit failure.

[0030] 3. The overcurrent protection circuit of the present invention limits the overcurrent protection threshold of the USB power supply circuit to 510mA±2% by setting the resistance tolerance level of the sampling resistor and the first input resistor, as well as the input offset voltage of the voltage comparator. This accurately covers the minimum standard of 500mA while reserving 2% redundancy to prevent false triggering. Compared with the current protection threshold of 2.0~3.2A of traditional chips with overcurrent protection function, the overcurrent protection circuit of the present invention can achieve more accurate overcurrent protection and can respond quickly when the circuit is overcurrent, effectively avoiding malfunction or damage of the meter due to excessively low or high current.

[0031] 4. The switching power supply of the present invention is compatible with various USB peripherals, meets the minimum USB2.0 standard power supply and the power supply capacity limit as low as 500mA / 5V, and supports the USB-powered boost power supply circuit to provide an adaptive large-capacitive reactance soft start function through the first capacitor, overcoming the problem that traditional communication methods are not applicable to high-end smart meters.

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The diagram is a circuit diagram of a low-cost switching power supply embodiment for high-precision electric energy metering according to the present invention.

[0034] Figure 2 The present invention is a flowchart of a control method embodiment of a low-cost switching power supply for high-precision electric energy metering. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0036] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0037] See also Figure 1 The present invention provides a low-cost switching power supply for high-precision electric energy metering, comprising: a boost power supply circuit 10, a soft start circuit 20, and an overcurrent protection circuit 30. The boost power supply circuit 10, the soft start circuit 20, and the overcurrent protection circuit 30, the boost power supply circuit 10 comprises a switching power supply chip IC202, a first capacitor C224, the voltage output pin of the switching power supply chip IC202 is grounded through the first capacitor C224, and is used to boost an externally input first USB low-voltage power supply VUSB to a working voltage supplied to the power supply system of the electric meter; the soft start circuit 20 comprises a first resistor R226, a first diode D223, a first resistor R227, a first resistor R228, a first resistor R229, a first resistor R230, a first resistor R231, a first resistor R232, a first resistor R233, a first resistor R234, a first resistor R235, a first resistor R236, a first resistor R237, a first resistor R238, a first resistor R239, a first resistor R240, a first resistor R241, a first resistor R242, a first resistor R243, a first resistor R244, a first resistor R245, a first resistor R246, a first resistor R247, a first resistor R248, a first resistor R249, a first resistor R250, a first resistor R251, a first resistor R252, a first resistor R253, a first resistor R254, a first resistor R255, a first resistor R256, a first resistor R257, a first resistor R258, a first resistor R259, a first resistor R260, a first resistor R261, a first resistor R262, a first resistor R263, a first resistor R264, a first resistor R265 A switching tube V202 and a second switching tube V205, a first resistor R226 and a first diode D223 are connected in series to the voltage output pin of the switching power supply chip IC202. The first USB low-voltage power supply VUSB is connected and charges the first capacitor C224 through the first resistor R226 and the first diode D223, and is connected to the base of the second switching tube V205 through the first switching tube V202. The emitter and base of the first switching tube V202 are connected in parallel with the two ends of the first resistor R226. The collector of the second switching tube V205 is connected to the switch enable pin of the switching power supply chip IC202, and its emitter is grounded.

[0038] The maximum soft-start current of the boost power supply circuit 10 is limited by setting the resistance of the first resistor R226 .

[0039] The overcurrent protection circuit 30 includes a sampling resistor R530, a voltage comparator IC206, a first input resistor R531, and a second input resistor R532. A first USB low-voltage power source VUSB is connected to a power input pin of the switching power supply chip IC202 via the sampling resistor R530. The power input end of the sampling resistor R530 is connected to the non-inverting input end of the voltage comparator IC206 via the first input resistor R531, and the other end is connected to the inverting input end of the voltage comparator IC206 via the second input resistor R532. The voltage comparator IC206 compares the voltage at its non-inverting input end with the voltage at its inverting input end and outputs a high or low level signal to the feedback pin of the switching power supply chip IC202 based on the comparison result. The switching power supply chip IC202 determines whether to shut down based on the voltage at its feedback pin.

[0040] The overcurrent protection threshold of the USB power supply circuit is limited by setting the resistance tolerance level of the sampling resistor R530 and the first input resistor R531 and the input offset voltage of the voltage comparator IC206.

[0041] Specifically, the switching power supply chip IC202 of this embodiment adopts a boost power supply integrated chip model TLV61046ADB, whose switching frequency is 1050KHz and integrates a 30V switching MOSFET. The switching power supply chip IC202 includes six pins P1 to P6, which have the following functions: P1 pin SW, the SW pin is connected to the drain of the internal switching tube of the switching power supply chip IC202; P2 pin GND, the GND pin is the chip reference ground; P3 pin FB, the FB pin is the feedback pin of the switching power supply chip IC202; P4 pin EN, the EN pin is the switch enable pin of the switching power supply chip IC202, and its control logic is set as: logic low shuts down the chip, and logic high starts the power supply; P5 pin VOUT, the VOUT pin is the voltage output pin of the switching power supply chip IC202; P6 pin VIN, the VIN pin is the power input pin of the switching power supply chip IC202.

[0042] Specifically, a switching inductor is further connected between the drain lead of the internal switching tube of the switching power supply chip IC202 of this embodiment and the power input pin. In the boost power supply circuit 10, the switching inductor is used for temporary energy storage and conversion. In the charging stage of the switching inductor, the internal MOSFET of the chip is turned on. At this time, the SW pin voltage is approximately 0V, and the current path is that the power supply flows from the VIN pin through the switching inductor and the MOSFET to the ground. At this time, the switching inductor stores energy and the current rises linearly. In the discharging stage of the switching inductor, the internal MOSFET of the chip is turned off, and the switching inductor generates a reverse electromotive force. The current path is from the switching inductor through the first capacitor C224 to the load meter to the ground. The energy of the switching inductor is released to the output end to achieve boosting. In this embodiment, a USB-SUP power input circuit 40 is also included. The USB-SUP power input circuit 40 is used to input a second USB low-voltage power supply USBSUP with multi-level voltage output that supports USB PD 3.0 and above standard interfaces, including a second capacitor C231, a first voltage-dividing resistor R268, and a second voltage-dividing resistor R267 that constitute an RC delay network. The second USB low-voltage power supply USBSUP is connected to the power input terminals of the soft start circuit 20 and the overcurrent protection circuit 30 respectively through the RC delay network; wherein the first USB low-voltage power supply VUSB is a 5V low-voltage direct current, and the second USB low-voltage power supply USBSUP includes 5V, 9V / 12V, and 20V low-voltage direct current.

[0043] Specifically, in order to be compatible with various USB peripherals and meet the minimum USB2.0 standard power supply and the power supply capacity limit as low as 500mA / 5V, the switching power supply of this embodiment supports the USB-powered boost power supply circuit 10 and can provide an adaptive large-capacitive soft-start function through the first capacitor C224.

[0044] In this embodiment, one end of the second capacitor C231 is connected to the ground in parallel with the first voltage-dividing resistor R268, and the other end is connected to the second USB low-voltage power supply USBSUP, and is respectively connected to the power input ends of the first resistor R226 and the sampling resistor R530 through the second voltage-dividing resistor R267; wherein, by adjusting the charging time constant of the second capacitor C231 and the saturation voltage drop of the second switch tube V205, the rising rate of the base voltage of the second switch tube V205 in the soft start phase is limited, thereby limiting the maximum soft start current of the boost power supply circuit 10.

[0045] Specifically, in this embodiment, the charging time constant τ of the second capacitor C231 is 2.2 msec, which limits the voltage rise rate of the first switch tube V202 to within 1.2 V / ms during the startup phase, thereby reducing the peak value of the inrush current to less than 15% of the steady-state value.

[0046] In this embodiment, during the soft-start phase when the first USB low-voltage power source VUSB is connected, the voltage difference across the first resistor R226 is equal to the difference between the voltage at the voltage output pin of the switching power supply chip IC202 and the voltage drop of the first diode D223, and the first USB low-voltage power source VUSB. The first resistor R226 is a resistor with a resistance of 50 ohms or less, which can limit the maximum soft-start current of the boost power supply circuit 10 to less than 100 mA.

[0047] In this embodiment, the overcurrent protection circuit 30 also includes a third voltage-dividing resistor R537 and a third capacitor C228. The first input resistor R531 is connected to ground in series with the third voltage-dividing resistor R537, and the third capacitor C228 is connected in parallel at both ends of the third voltage-dividing resistor R537. The non-inverting input end of the voltage comparator IC206 obtains a fixed voltage by dividing the voltage through the first input resistor R531 and the third voltage-dividing resistor R537.

[0048] When the voltage value of the inverting input terminal of the voltage comparator IC206 is greater than the fixed voltage, the output terminal of the voltage comparator IC206 outputs a low level signal to the feedback pin of the switching power chip IC202, otherwise it outputs a high level signal.

[0049] Specifically, in this embodiment, the first input resistor R531 and the third voltage-dividing resistor R537 are resistors with resistance values ​​of 12kohm and 220kohm, respectively. The specific calculation of the fixed-value voltage is as follows:

[0050] in, The first USB low voltage power supply VUSB5V, is the resistance of the first input resistor R531, is the resistance of the third voltage-dividing resistor R537. The fixed voltage is 4.74V.

[0051] In this embodiment, a second diode and a first current-limiting resistor are further connected in series on the feedback path from the output end of the voltage comparator IC206 to the feedback pin of the switching power supply chip IC202. The feedback threshold voltage is determined by setting the voltage drop of the second diode and the resistance value of the first current-limiting resistor. When the voltage at the feedback pin of the switching power supply chip IC202 is greater than the feedback threshold voltage, the switching power supply chip IC202 controls the internal switch to be turned off; otherwise, the operation is normal.

[0052] Specifically, in this embodiment, the first current limiting resistor is a resistor with a resistance of 47 kohm; and the feedback threshold voltage is 0.795V.

[0053] Among them, the boost power supply circuit 10 also includes a power output feedback loop composed of a plurality of feedback voltage divider resistors. The voltage output pin of the switching power supply chip IC202 is connected to its feedback pin through the source output feedback loop, and the switching power supply chip IC202 is set with an output threshold voltage; when the feedback pin voltage of the switching power supply chip IC202 is greater than the output threshold voltage, the switching power supply chip IC202 controls the internal switch tube to turn off; otherwise, it works normally.

[0054] In this embodiment, the sampling resistor R530 is a 0.51 ohm resistor. By setting the resistance tolerance of the sampling resistor R530 and the first input resistor R531 to ±1%, and the input offset voltage of the voltage comparator IC206 to 110 uV, the overcurrent protection threshold of the switching power supply is limited to 510 mA ±2%.

[0055] Specifically, the switching power supply of this embodiment uses circuit design and simple discrete components to build a low-cost soft-start circuit and a precise overcurrent protection circuit 30. This is compared with TI's typical switching chip TPS61085, which integrates soft-start and overcurrent protection. Its typical application input voltage is 6V, the boost output is 12V / 600mA, its overcurrent protection threshold is 2.0~3.2A, and its soft start is charging VSS from 0.3V to 0.8V. Obviously, these typical functional characteristics do not meet the requirements of this design application. The present invention can achieve the soft-start function of charging the output voltage VCS from 0V to 5V, and the overcurrent protection threshold is 600mA±5%, thereby fully meeting the USB minimum current limit standard of 500mA.

[0056] See also Figure 2 The present invention provides a control method for a low-cost switching power supply for high-precision electric energy metering, which is applied to the low-cost switching power supply for high-precision electric energy metering, comprising: S1: The switching power supply is connected to the first USB low-voltage power supply VUSB or the second USB low-voltage power supply USBSUP, and slowly charges the first capacitor C224 at the output node VCS of the boost power supply circuit 10 through the first resistor R226 and the first diode D223. S2: The voltage difference across the first resistor R226 gradually decreases as the first capacitor C224 charges. When the voltage difference across the first resistor R226 exceeds 0.55V, the first switch tube V202 is turned on and drives the second switch tube V205 to turn on. At this time, the switch enable pin of the switching power supply chip IC202 is pulled to a low level, and the switching power supply chip IC202 is in the off state.

[0057] S3: When the voltage difference across the first resistor R226 is ≤0.55V, the first switch tube V202 and the second switch tube V205 are turned off. At this time, the low state of the switch enable pin of the switching power supply chip IC202 is released to a high level, and the switching power supply chip IC202 starts automatically.

[0058] S4: Real-time monitoring of the voltage drop of the load current on the sampling resistor R530. When the voltage drop is less than the fixed voltage at the non-inverting input of the voltage comparator IC206, the voltage comparator IC206 outputs a high-level signal and applies it to the feedback pin of the switching power supply chip IC202. When the feedback pin voltage of the switching power supply chip IC202 is greater than the feedback threshold voltage, the switching power supply chip IC202 is turned off; otherwise, it works normally.

[0059] S5: The off state of the switching power supply chip IC202 continues until the load current drops below the overcurrent protection threshold, and the soft start steps S1 to S3 are automatically re-executed.

[0060] S6: While executing steps S4 to S5, the output voltage of the switching power supply chip IC202 is sampled in real time. When the feedback pin voltage of the switching power supply chip IC202 is greater than the set output threshold voltage, the switching power supply chip IC202 controls the internal switch tube to turn off; otherwise, it operates normally.

[0061] In this embodiment, in step S2 , the conduction threshold of the first switch V202 is set to 0.55V±0.02V, and the resistance of the first resistor R226 is adjusted to match the conduction thresholds of different first switches V202 .

[0062] During the charging process, the maximum soft-start current is limited to less than 100 mA until the voltage at the output node VCS of the boost power supply circuit 10 rises to the input voltage value of the switching power supply.

[0063] In this embodiment, steps S4-S5 also include: Error compensation is performed based on the total error of the overcurrent point protection, which is:

[0064] in, is the target overcurrent threshold; is the error value of the sampling resistor R530, is the combined error of the first input resistor R531 and the third voltage divider resistor R537 of the voltage comparator IC206, is the normalized error of the input offset voltage of the voltage comparator IC206.

[0065] Specifically, the target overcurrent threshold in this embodiment is 510mA, the error values ​​of the sampling resistor R530, the first input resistor R531, and the third voltage divider resistor R537 are all 1%, then the normalized error of the input offset voltage is calculated by the input offset voltage 110uV, and the total error of the overcurrent point protection is less than ±2%.

[0066] The technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0067] The above embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and replacements made by technicians in this field on the basis of the present invention fall within the scope of protection required by the present invention.

Claims

1. A low-cost switching power supply for high-precision electric energy metering, characterized in that: include: A boost power supply circuit, a soft-start circuit, and an overcurrent protection circuit. The boost power supply circuit includes a switching power supply chip and a first capacitor. The voltage output pin of the switching power supply chip is grounded through the first capacitor, and is used to boost an externally input first USB low-voltage power supply to an operating voltage supplied to the power supply system of the electric meter. The soft-start circuit includes a first resistor, a first diode, a first switching transistor, and a second switching transistor. The first resistor and the first diode are connected in series to the voltage output pin of the switching power supply chip. The first USB low-voltage power supply is connected to and charges the first capacitor through the first resistor and the first diode, and is connected to the base of the second switching transistor through the first switching transistor. The emitter and base of the first switching transistor are connected in parallel to both ends of the first resistor. The collector of the second switching transistor is connected to the switch enable pin of the switching power supply chip, and its emitter is grounded. Wherein, the maximum soft-start current of the boost power supply circuit is limited by setting the resistance value of the first resistor; The overcurrent protection circuit includes a sampling resistor, a voltage comparator, a first input resistor, and a second input resistor. The first USB low-voltage power supply is connected to the power input pin of the switching power supply chip through the sampling resistor. The power input end of the sampling resistor is connected to the non-inverting input end of the voltage comparator through the first input resistor, and the other end is connected to the inverting input end of the voltage comparator through the second input resistor. The voltage comparator compares the voltage of its non-inverting input end with the voltage of its inverting input end, and outputs a high or low level signal to the feedback pin of the switching power supply chip based on the comparison result. The switching power supply chip determines whether to shut down based on the voltage of its feedback pin. The overcurrent protection threshold of the USB power supply circuit is limited by setting the resistance tolerance level of the sampling resistor and the first input resistor and the input offset voltage of the voltage comparator.

2. The low-cost switching power supply for high-precision electric energy metering according to claim 1, characterized in that: It also includes a USB-SUP power input circuit, which is used to input a second USB low-voltage power supply with multi-level voltage output that supports USB PD 3.0 and above standard interfaces, including a second capacitor, a first voltage divider resistor, and a second voltage divider resistor that constitute an RC delay network. The second USB low-voltage power supply is connected to the power input terminals of the soft start circuit and the overcurrent protection circuit respectively through the RC delay network; wherein, the first USB low-voltage power supply is 5V low-voltage DC power, and the second USB low-voltage power supply includes 5V, 9V / 12V, and 20V low-voltage DC power.

3. The low-cost switching power supply for high-precision electric energy metering according to claim 2, characterized in that: One end of the second capacitor is connected to the ground in parallel with the first voltage-dividing resistor, and the other end is connected to the second USB low-voltage power supply and is respectively connected to the power input ends of the first resistor and the sampling resistor through the second voltage-dividing resistor. The charging time constant of the second capacitor and the saturation voltage drop of the second switching tube are adjusted to limit the rising rate of the base voltage of the second switching tube during the soft-start phase, thereby limiting the maximum soft-start current of the boost power supply circuit.

4. The low-cost switching power supply for high-precision electric energy metering according to claim 2, characterized in that: During a soft-start phase when the first USB low-voltage power source is connected, a voltage difference across the first resistor is equal to a difference between a voltage at a voltage output pin of the switching power supply chip and a voltage drop of the first diode and the first USB low-voltage power source. The first resistor having a resistance of 50 ohms or less can limit a maximum soft-start current of the boost power supply circuit to less than 100 mA.

5. The low-cost switching power supply for high-precision electric energy metering according to claim 1, characterized in that: The overcurrent protection circuit further includes a third voltage-dividing resistor and a third capacitor, the first input resistor is connected in series with the third voltage-dividing resistor to be grounded, and the third capacitor is connected in parallel across the third voltage-dividing resistor; the non-inverting input terminal of the voltage comparator obtains a fixed voltage by dividing the voltage between the first input resistor and the third voltage-dividing resistor; When the voltage value of the inverting input terminal of the voltage comparator is greater than the fixed value voltage, the output terminal of the voltage comparator outputs a low level signal to the feedback pin of the switching power supply chip, otherwise it outputs a high level signal.

6. The low-cost switching power supply for high-precision electric energy metering according to claim 5, characterized in that: A second diode and a first current-limiting resistor are further connected in series on a feedback path from the output end of the voltage comparator to the feedback pin of the switching power supply chip. A feedback threshold voltage is determined by setting a voltage drop of the second diode and a resistance value of the first current-limiting resistor. When the voltage at the feedback pin of the switching power supply chip is greater than the feedback threshold voltage, the switching power supply chip controls the internal switch to be turned off; otherwise, the chip operates normally. In which, the boost power supply circuit also includes a power output feedback loop composed of a plurality of feedback voltage divider resistors, the voltage output pin of the switching power supply chip is connected to its feedback pin through the source output feedback loop, and the switching power supply chip is provided with an output threshold voltage; when the feedback pin voltage of the switching power supply chip is greater than the output threshold voltage, the switching power supply chip controls the internal switch tube to turn off; otherwise, it operates normally.

7. The low-cost switching power supply for high-precision electric energy metering according to any one of claims 5 or 6, characterized in that: The sampling resistor is a 0.51 ohm resistor, and by setting the resistance tolerance of the sampling resistor and the first input resistor to ±1%, and the input offset voltage of the voltage comparator to 110 uV, the overcurrent protection threshold of the switching power supply is limited to 510 mA ± 2%.

8. A control method for a low-cost switching power supply for high-precision electric energy metering, characterized in that: A low-cost switching power supply for high-precision electric energy metering as claimed in any one of claims 1 to 7, comprising: S1: The switching power supply is connected to the first USB low-voltage power supply or the second USB low-voltage power supply, and slowly charges the first capacitor at the output node of the boost power supply circuit through the first resistor and the first diode; S2: The voltage difference between the two ends of the first resistor gradually decreases as the first capacitor charges. When the voltage difference between the two ends is greater than 0.55V, the first switch tube is turned on and drives the second switch tube to turn on. At this time, the switch enable pin of the switching power supply chip is pulled to a low level, and the switching power supply chip is in an off state. S3: When the voltage difference across the first resistor is ≤0.55V, the first switch tube and the second switch tube are turned off. At this time, the low state of the switch enable pin of the switching power supply chip is released to a high level, and the switching power supply chip starts automatically. S4: Real-time monitoring of the voltage drop of the load current on the sampling resistor. When the voltage drop is less than the fixed voltage at the non-inverting input terminal of the voltage comparator, the voltage comparator outputs a high-level signal and applies it to the feedback pin of the switching power supply chip. When the voltage at the feedback pin of the switching power supply chip is greater than the feedback threshold voltage, the switching power supply chip is turned off; otherwise, it operates normally. S5: The off state of the switching power supply chip continues until the load current drops below the overcurrent protection threshold, and the soft start steps S1 to S3 are automatically re-executed; S6: While executing steps S4 to S5, the output voltage of the switching power supply chip is sampled in real time. When the feedback pin voltage of the switching power supply chip is greater than the set output threshold voltage, the switching power supply chip controls the internal switch tube to turn off; otherwise, it operates normally.

9. The control method of a low-cost switching power supply for high-precision electric energy metering according to claim 8, characterized in that , in step S2: The conduction threshold of the first switch is set to 0.55V±0.02V, and the resistance value of the first resistor is adjusted to match the conduction threshold difference of different first switch tubes; During the charging process, the maximum soft-start current is limited to less than 100mA until the output node voltage of the boost power supply circuit rises to the input voltage value of the switching power supply.

10. The control method of a low-cost switching power supply for high-precision electric energy metering according to claim 8, characterized in that: Steps S4-S5 also include: Error compensation is performed based on the total error of the overcurrent point protection, which is: in, is the target overcurrent threshold; is the error value of the sampling resistor; is the combined error of the first input resistor and the third voltage divider resistor of the voltage comparator, is the normalized error of the input offset voltage of the voltage comparator.

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