A low-cost switching power supply for high-precision electric energy metering and a control method thereof

Abstract

The application provides a low-cost switching power supply for high-precision electric energy metering and a control method thereof, comprising that the voltage output pin of a switching power supply chip is grounded through a first capacitor; an external low-voltage power supply is connected to and charges the first capacitor through a first resistor and a first diode pair, and the maximum soft start current of a boost power supply circuit is limited to less than 100mA by setting the resistance value of the first resistor; the external low-voltage power supply is connected to the power input pin of the switching power supply chip through a sampling resistor, the sampling resistor is connected to the non-inverting input terminal and the inverting input terminal of a voltage comparator, and whether overcurrent occurs is judged according to the high or low level signal output by the voltage comparator; and the overcurrent protection threshold of a USB power supply circuit is limited by setting the resistance value tolerance level and the input offset voltage of the voltage comparator. The application provides the USB power supply with the functions of surge current prevention and accurate overcurrent protection by designing the soft start and overcurrent protection circuits, so that the cost is greatly reduced, the circuit design is simplified, and the circuit reliability is improved.

Description

TECHNICAL FIELD

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

[0002] With the rapid development of smart grid, the traditional electronic electric energy meter adopts infrared communication or RS485 communication interface to carry out precision adjustment, production test, daily record, data processing, technical maintenance, on-site service and other communication means, which cannot meet the needs of the development of intelligent electric meter. Now high-end intelligent electric energy meter needs more precise measurement and more complex functions, including but not limited to complex data processing such as event log, measurement parameter, harmonic analysis, which makes the traditional infrared or RS485 serial communication mode with low data transmission rate and poor stability unable to be applied to high-end intelligent electric energy meter. Therefore, the USB serial communication with high communication efficiency, stability and reliability, expandable function and good data security and integrity becomes the preferred scheme.

[0003] However, due to the complexity of the functions of the smart meter and the high requirements of the power supply system, the power supply system of the smart meter is usually composed of an AC to DC low-voltage main power supply and a multi-channel low-voltage to low-voltage power supply system. In addition, it also needs to deal with the harsh industrial and mining environment of power grid voltage fluctuation and flashover. The power supply system generally deploys a large number of energy storage capacitors. In this case, when the external low-voltage USB power supply is directly connected to the meter system, the inrush current caused by several thousand uF capacitors or even several F capacitors in the power supply system will be very large, which not only easily damages the external low-voltage USB power supply, but also affects the start of the entire external device and the entire meter power supply system. Therefore, a low-voltage power supply with soft start and overcurrent protection function is needed as the basis for implementing the USB function of the electric energy meter.

[0004] However, the soft start and overcurrent protection modules built-in in ordinary low-voltage power supply integrated chips generally have the shortcomings of short soft start time, large overcurrent protection error, easy mis-triggering and overcurrent protection lock after overcurrent protection.

[0005] Although the programmable high-end power supply chip can realize precise power supply control by dynamically adjusting voltage, current and other parameters, it usually uses complex process and precise components, resulting in high price, high direct development or maintenance threshold, and high power consumption due to high integration and strong processing capacity, which may cause additional burden on the meter battery or heat dissipation system.

[0006] Therefore, there is an urgent need for a low-cost USB power supply circuit with adaptive soft start and precise overcurrent protection function for high-precision electric energy metering. SUMMARY

[0007] In order to solve the problems existing in the prior art, the present application aims to provide a low-cost switching power supply for high-precision electric energy metering and a control method thereof, which combines the pin functions of a switching power supply chip with a low-cost triode, a resistor and a comparator to design a soft start and overcurrent protection circuit, thereby providing surge current protection and accurate overcurrent protection under the premise of meeting the power requirements of the USB function of a high-end smart meter, and achieving the effects of greatly reducing cost, simplifying circuit design and improving circuit reliability.

[0008] The present application achieves the above-mentioned purposes through the following technical solutions:

[0009] A low-cost switching power supply for high-precision electric energy metering comprises a boost power supply circuit, a soft start circuit and an overcurrent protection circuit, the boost power supply circuit comprises 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 a working voltage for supplying a power supply system of a meter; the soft start circuit comprises a first resistor, a first diode, a first switch tube and a second switch tube, 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 the first capacitor through the first resistor and the first diode, and is connected to the base of the second switch tube through the first switch tube, the emitter and the base of the first switch tube are connected in parallel across the first resistor, and the collector of the second switch tube is connected to the switch enable pin of the switching power supply chip, and the emitter is grounded.

[0010] The resistance of the first resistor is used to limit the maximum soft start current of the boost power supply circuit.

[0011] The overcurrent protection circuit comprises 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 supply input pin of the switching power supply chip through the sampling resistor, the power supply 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 voltages at the non-inverting input end and the inverting input end, and outputs a high or low level signal to the feedback pin of the switching power supply chip according to the comparison result, and the switching power supply chip determines whether to be turned off according to the voltage at the feedback pin.

[0012] The resistance tolerance level of the sampling resistor and the first input resistor and the input offset voltage of the voltage comparator are used to limit the overcurrent protection threshold of the USB power supply circuit.

[0013] The low-cost switching power supply for high-precision electric energy metering provided by the application further comprises a USB-SUP power input circuit for inputting a second USB low-voltage power supply supporting a multi-grade voltage output of a USB PD 3.0 and above standard interface, comprising a second capacitor, a first voltage dividing resistor and a second voltage dividing resistor constituting an RC delay network, and the second USB low-voltage power supply is connected to the power input end of a soft start circuit and an overcurrent protection circuit through the RC delay network respectively; wherein the first USB low-voltage power supply is 5V low-voltage direct current, and the second USB low-voltage power supply comprises 5V, 9V / 12V and 20V low-voltage direct current.

[0014] The low-cost switching power supply for high-precision electric energy metering provided by the application is characterized in that one end of the second capacitor is connected in parallel with the first voltage dividing resistor and grounded, the other end is connected to the second USB low-voltage power supply, and the power input end of the first resistor and a sampling resistor is connected through the second voltage dividing resistor respectively; wherein the rising rate of the base voltage of the second switch tube in the soft start stage is limited by adjusting the charging time constant of the second capacitor and the saturation voltage drop of the second switch tube, so as to limit the maximum soft start current of the boost power supply circuit.

[0015] The low-cost switching power supply for high-precision electric energy metering provided by the application is characterized in that in the soft start stage of connecting to the first USB low-voltage power supply, the voltage difference between the two ends of the first resistor is equal to the difference between the voltage output pin voltage of the switching power supply chip and the first diode voltage drop and the first USB low-voltage power supply; wherein the first resistor adopts a resistor with a resistance value of 50 ohm or less, so that the maximum soft start current of the boost power supply circuit is less than 100 mA.

[0016] The low-cost switching power supply for high-precision electric energy metering provided by the application is characterized in that the overcurrent protection circuit further comprises a third voltage dividing resistor and a third capacitor, the first input resistor is connected in series with the third voltage dividing resistor and grounded, and the third capacitor is connected in parallel across the third voltage dividing resistor; the fixed value voltage is obtained by the non-inverting input end of the voltage comparator through the first input resistor and the third voltage dividing resistor voltage dividing.

[0017] When the voltage value of the inverting input end of the voltage comparator is greater than the fixed value voltage, the output end of the voltage comparator outputs a low level signal to the feedback pin of the switching power supply chip, otherwise a high level signal is outputted.

[0018] The application provides a low-cost switching power supply for high-precision electric energy metering.

[0019] The voltage output pin of the switching power supply chip is connected with the feedback pin thereof through the power output feedback loop, and the switching power supply chip is provided with an output threshold voltage; when the voltage of the feedback pin of the switching power supply chip is greater than the output threshold voltage, the switching power supply chip controls the internal switch tube to be closed; otherwise, the switching power supply chip works normally.

[0020] The application provides a low-cost switching power supply for high-precision electric energy metering.

[0021] A control method of a low-cost switching power supply for high-precision electric energy metering is applied to the low-cost switching power supply for high-precision electric energy metering, and includes the following steps.

[0022] S1: the switching power supply is connected to the first USB low-voltage power supply or the second USB low-voltage power supply, and a first capacitor of a boost power supply circuit output node is slowly charged through a first resistor and a first diode.

[0023] S2: the voltage difference between the two ends of the first resistor gradually decreases with the charging of the first capacitor, when the voltage difference between the two ends of the first resistor is greater than 0.55V, a first switch tube is turned on and drives a second switch tube to be turned 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 unopened state.

[0024] S3: when the voltage difference between the two ends of the first resistor is less than or equal to 0.55V, the first switch tube and the second switch tube are turned off, at this time, the pull-down state of the switch enable pin of the switching power supply chip is released to a high level, and the switching power supply chip is started.

[0025] S4: Real-time monitoring of the voltage drop across the sampling resistor due to the load current. 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 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.

[0026] S5: The power supply chip remains in the off state until the load current drops below the overcurrent protection threshold, at which point the soft-start steps S1~S3 are automatically re-executed.

[0027] S6: While executing steps S4~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 switching transistor to turn off; otherwise, it works normally.

[0028] According to the present invention, a control method for a low-cost switching power supply for high-precision power metering is provided, wherein in step S2:

[0029] The conduction threshold of the first switching transistor is set to 0.55V±0.02V. The resistance value of the first resistor is adjusted to match the difference in the conduction threshold of the first switching transistor.

[0030] During charging, 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.

[0031] According to the control method of a low-cost switching power supply for high-precision power metering provided by the present invention, steps S4 to S5 further include:

[0032] Error compensation is performed based on the total error of the overcurrent point protection, where the total error of the overcurrent point protection is:

[0033]

[0034] in, The target overcurrent threshold; This is the error value of the sampling resistor; This is the combined error of the first input resistor and the third voltage divider resistor of the voltage comparator. This is the normalized error of the input offset voltage of the voltage comparator.

[0035] Therefore, compared with the prior art, the present invention has the following beneficial effects:

[0036] 1. The switching power supply of this invention, through circuit design and the use of simple discrete components, builds a soft-start circuit and a precise overcurrent protection circuit. It can achieve the soft-start function to charge the output voltage from 0V to 5V, while the overcurrent protection threshold is 600mA±5%, thus fully meeting the minimum 500mA current limiting standard of USB. Under the premise of meeting the power supply requirements of high-end smart meters implementing USB functions, it can provide surge current protection and precise overcurrent protection.

[0037] 2. This invention utilizes low-cost discrete components to design the circuit. During the soft-start process, the first switching transistor V202 is turned on by the voltage difference control of resistor R226, ensuring that the rising slope of the output voltage VCS from 0V to 5V is stable. The circuit structure is simple and has good stability, which greatly reduces the cost of traditional dedicated switching power supply chips with soft-start function. At the same time, the discrete component setting can reduce the maintenance cost of circuit failure.

[0038] 3. The overcurrent protection circuit of this 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 overcurrent protection threshold of traditional chips with overcurrent protection function of 2.0~3.2A, the overcurrent protection circuit of this invention can achieve more accurate overcurrent protection and can respond quickly when the circuit is overcurrent, effectively avoiding malfunction or damage to the meter due to excessively low or high current.

[0039] 4. The switching power supply of this invention is compatible with various USB peripherals, meets the minimum USB 2.0 standard power supply and the power supply capacity of as low as 500mA / 5V. The boost power supply circuit that supports USB power supply can provide an adaptive large capacitive soft start function through the first capacitor, overcoming the problem that traditional communication methods cannot be applied to high-end smart meters.

[0040] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0041] Figure 1 This is a circuit diagram of an embodiment of a low-cost switching power supply for high-precision power metering according to the present invention.

[0042] Figure 2 This is a flowchart of an embodiment of a control method for a low-cost switching power supply for high-precision power metering according to the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0044] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0045] See Figure 1 This invention provides a low-cost switching power supply for high-precision electricity 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 includes a switching power supply chip IC202 and a first capacitor C224. The voltage output pin of the switching power supply chip IC202 is grounded through the first capacitor C224, used to boost an externally input first USB low-voltage power supply VUSB to the operating voltage supplied to the electricity meter power system. The soft-start circuit 20 includes a first resistor R226, a first diode D223, and a second... A first switching transistor V202 and a second switching transistor V205 are connected in series with a first resistor R226 and a first diode D223 to the voltage output pin of the switching power supply chip IC202. A first USB low-voltage power supply VUSB is connected and charges a first capacitor C224 through the first resistor R226 and the first diode D223, and is connected to the base of the second switching transistor V205 through the first switching transistor V202. The emitter and base of the first switching transistor V202 are connected in parallel with the two ends of the first resistor R226. The collector of the second switching transistor V205 is connected to the switch enable pin of the switching power supply chip IC202, and its emitter is grounded.

[0046] The maximum soft-start current of the boost power supply circuit 10 is limited by setting the resistance value of the first resistor R226.

[0047] 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. The first USB low-voltage power supply VUSB is connected to the power input pin of the switching power supply chip IC202 through the sampling resistor R530. The power input terminal of the sampling resistor R530 is connected to the non-inverting input terminal of the voltage comparator IC206 through the first input resistor R531, and the other end is connected to the inverting input terminal of the voltage comparator IC206 through the second input resistor R532. The voltage comparator IC206 compares the voltage at its non-inverting input terminal with the voltage at its inverting input terminal, and outputs a high or low level signal to the feedback pin of the switching power supply chip IC202 according to the comparison result. The switching power supply chip IC202 determines whether to turn off based on the voltage magnitude at its feedback pin.

[0048] Specifically, the overcurrent protection threshold of the USB power supply circuit is limited by setting the resistance tolerance levels of the sampling resistor R530 and the first input resistor R531, as well as the input offset voltage of the voltage comparator IC206.

[0049] Specifically, in this embodiment, the switching power supply chip IC202 uses a boost power supply integrated chip of model TLV61046ADB, with a switching frequency of 1050KHz and an integrated 30V switching MOSFET. The switching power supply chip IC202 includes six pins P1~P6, which are: P1 pin SW, which is connected to the drain of the internal switching MOSFET of the switching power supply chip IC202; P2 pin GND, which is the chip reference ground; P3 pin FB, which is the feedback pin of the switching power supply chip IC202; P4 pin EN, which is the switch enable pin of the switching power supply chip IC202, and its control logic is set as follows: logic low shuts down the chip, logic high starts the power supply; P5 pin VOUT, which is the voltage output pin of the switching power supply chip IC202; and P6 pin VIN, which is the power input pin of the switching power supply chip IC202.

[0050] Specifically, in this embodiment, a switching inductor is connected between the drain pin of the internal switching transistor of the switching power supply chip IC202 and the power input pin. In the boost power supply circuit 10, the switching inductor is used for energy storage and conversion. During the charging phase of the switching inductor, the internal MOSFET of the chip is turned on, and the voltage at the SW pin is approximately 0V. The current path is that the power supply flows from the VIN pin through the switching inductor and the MOSFET to ground. At this time, the switching inductor stores energy, and the current increases linearly. During the discharging phase 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 that the switching inductor flows through the first capacitor C224 to the load meter and then to ground. The energy of the switching inductor is released to the output terminal, realizing the boost.

[0051] 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 that supports multi-level voltage output of USB PD 3.0 and above standard interfaces. It includes a second capacitor C231, a first voltage divider resistor R268, and a second voltage divider resistor R267 forming 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 through the RC delay network. The first USB low-voltage power supply VUSB is 5V low-voltage DC, and the second USB low-voltage power supply USBSUP includes 5V, 9V / 12V, and 20V low-voltage DC.

[0052] Specifically, in order to be compatible with various USB peripherals and meet the minimum USB 2.0 standard power supply and the minimum power supply capacity of 500mA / 5V, the USB power supply boost circuit 10 can provide an adaptive large capacitive soft-start function through the first capacitor C224.

[0053] In this embodiment, one end of the second capacitor C231 is connected in parallel with the first voltage divider resistor R268 to ground, and the other end is connected to the second USB low-voltage power supply USBSUP. It is also connected to the power input terminals of the first resistor R226 and the sampling resistor R530 through the second voltage divider resistor R267. The maximum soft-start current of the boost power supply circuit 10 is limited by adjusting the charging time constant of the second capacitor C231 and the saturation voltage drop of the second switch V205 to limit the rise rate of the base voltage of the second switch V205 during the soft-start phase.

[0054] Specifically, in this embodiment, the charging time constant τ of the second capacitor C231 is approximately 2.2m. During the startup phase, the voltage rise rate of the first switching transistor V202 is limited to within 1.2V / ms, thereby reducing the peak surge current to less than 15% of the steady-state value.

[0055] In this embodiment, during the soft-start phase of connecting the first USB low-voltage power supply VUSB, the voltage difference across the first resistor R226 is equal to the voltage output pin voltage of the switching power supply chip IC202 and the difference between the voltage drop of the first diode D223 and the first USB low-voltage power supply VUSB. The first resistor R226 is a resistor with a resistance value of less than 50 ohms, which can limit the maximum soft-start current of the boost power supply circuit 10 to less than 100mA.

[0056] In this embodiment, the overcurrent protection circuit 30 further includes a third voltage divider resistor R537 and a third capacitor C228. The first input resistor R531 is grounded through series connection with the third voltage divider resistor R537, and the third capacitor C228 is connected in parallel across the third voltage divider resistor R537. The non-inverting input terminal of the voltage comparator IC206 obtains a fixed voltage through voltage division between the first input resistor R531 and the third voltage divider resistor R537.

[0057] When the voltage value at the inverting input terminal of voltage comparator IC206 is greater than the fixed voltage value, the output terminal of voltage comparator IC206 outputs a low-level signal to the feedback pin of switching power supply chip IC202; otherwise, it outputs a high-level signal.

[0058] Specifically, in this embodiment, the first input resistor R531 and the third voltage divider resistor R537 are resistors with resistance values ​​of 12 kΩ and 220 kΩ, respectively. The specific calculation of the fixed voltage is as follows:

[0059]

[0060] in, The first USB low-voltage power supply is VUSB5V. The value of the first input resistor R531 is given. The resistance value of the third voltage divider resistor R537 is given. Therefore, the fixed voltage is 4.74V.

[0061] In this embodiment, a second diode and a first current-limiting resistor are connected in series on the feedback path from the output 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 switching transistor to turn off; otherwise, it operates normally.

[0062] Specifically, in this embodiment, the first current-limiting resistor is a resistor with a resistance of 47 kΩ; the feedback threshold voltage is 0.795 V.

[0063] The boost power supply circuit 10 also includes a power output feedback loop composed of several 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 voltage of the feedback pin of the switching power supply chip IC202 is greater than the output threshold voltage, the switching power supply chip IC202 controls the internal switching transistor to turn off; otherwise, it works normally.

[0064] In this embodiment, the sampling resistor R530 is a 0.51 ohm resistor, and by setting the resistance tolerance level of the sampling resistor R530 and the first input resistor R531 to ±1%, and the input offset voltage of the voltage comparator IC206 to 110uV, the overcurrent protection threshold of the switching power supply is limited to 510mA±2%.

[0065] Specifically, in this embodiment, the aforementioned switching power supply, through circuit design and the use of simple discrete components, constructs a low-cost soft-start circuit and a precise overcurrent protection circuit 30. It is comparable to the TI TPS61085, a typical switching chip integrating soft-start and overcurrent protection. Its typical application input voltage is 6V, boost output is 12V / 600mA, and its overcurrent protection threshold is 2.0~3.2A. However, its soft-start function charges the VSS from 0.3V to 0.8V. Clearly, these typical functional characteristics do not meet the requirements of this design application. This invention achieves a soft-start function that charges the output voltage VCS from 0V to 5V, while the overcurrent protection threshold is 600mA ± 5%, thus fully meeting the USB minimum 500mA current limiting standard.

[0066] See Figure 2 This invention provides a control method for a low-cost switching power supply used for high-precision energy metering, applied to the low-cost switching power supply used for high-precision energy metering, comprising:

[0067] 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 of the output node VCS of the boost power supply circuit 10 through the first resistor R226 and the first diode D223.

[0068] S2: The voltage difference across the first resistor R226 gradually decreases as the first capacitor C224 is charged. When the voltage difference across it is greater than 0.55V, the first switch V202 is turned on and drives the second switch V205 to turn on as well. 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 an off state.

[0069] S3: When the voltage difference across the first resistor R226 is ≤0.55V, the first switch V202 and the second switch V205 are turned off. At this time, the pull-down 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 up automatically.

[0070] S4: Real-time monitoring of the voltage drop across the sampling resistor R530 due to the load current. When this 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 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 is turned off; otherwise, it operates normally.

[0071] S5: The power supply chip IC202 remains in the off state until the load current drops below the overcurrent protection threshold, at which point the soft-start steps S1~S3 are automatically re-executed.

[0072] S6: While executing steps S4~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 switching transistor to turn off; otherwise, it works normally.

[0073] In this embodiment, in step S2, the conduction threshold of the first switch V202 is set to 0.55V±0.02V, and the resistance value of the first resistor R226 is adjusted to match the difference in the conduction threshold of the different first switch V202.

[0074] During charging, the maximum soft-start current is limited to less than 100mA until the VCS voltage at the output node of the boost power supply circuit 10 rises to the input voltage value of the switching power supply.

[0075] In this embodiment, steps S4-S5 further include:

[0076] Error compensation is performed based on the total error of the overcurrent point protection, where the total error of the overcurrent point protection is:

[0077]

[0078] in, The target overcurrent threshold; This is the error value of the sampling resistor R530. The combined error of the first input resistor R531 and the third voltage divider resistor R537 of the voltage comparator IC206 is... This is the normalized error of the input offset voltage of voltage comparator IC206.

[0079] Specifically, in this embodiment, the target overcurrent threshold is 510mA, and the error values ​​of the sampling resistor R530, the first input resistor R531, and the third voltage divider resistor R537 are all 1%. The normalized error of the input offset voltage is calculated from the input offset voltage of 110uV, and the total error of the overcurrent point protection is less than ±2%.

[0080] The technical features of the above embodiments can be combined in any way. For the sake of brevity, 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.

[0081] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A low-cost switching power supply for high-precision power metering, characterized in that, include: The system includes a boost power supply circuit, a soft-start circuit, and an overcurrent protection circuit. The boost power supply circuit comprises 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, used to boost an externally input first USB low-voltage power supply to the operating voltage supplied to the power meter system. The soft-start circuit comprises 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 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 with the two 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. 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 terminal of the sampling resistor is connected to the non-inverting input terminal of the voltage comparator through the first input resistor, and the other end is connected to the inverting input terminal of the voltage comparator through the second input resistor. The voltage comparator compares the voltage at its non-inverting input terminal with the voltage at its inverting input terminal, and outputs a high or low level signal to the feedback pin of the switching power supply chip according to the comparison result. The switching power supply chip determines whether to shut down based on the voltage magnitude at its feedback pin. Specifically, the overcurrent protection threshold of the USB power supply circuit is limited by setting the resistance tolerance levels of the sampling resistor and the first input resistor, as well as the input offset voltage of the voltage comparator.

2. The low-cost switching power supply for high-precision power 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 that supports multi-level voltage output of USB PD 3.0 and above standard interfaces. The circuit includes a second capacitor, a first voltage divider resistor, and a second voltage divider resistor forming 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 through the RC delay network. The first USB low-voltage power supply is a 5V low-voltage DC power supply, and the second USB low-voltage power supply includes 5V, 9V / 12V, and 20V low-voltage DC power supplies.

3. The low-cost switching power supply for high-precision power metering according to claim 2, characterized in that: One end of the second capacitor is connected in parallel with the first voltage divider resistor to ground, and the other end is connected to the second USB low-voltage power supply. It is also connected to the power input terminals of the first resistor and the sampling resistor through the second voltage divider resistor. The maximum soft-start current of the boost power supply circuit is limited by adjusting the charging time constant of the second capacitor and the saturation voltage drop of the second switch to limit the rise rate of the base voltage of the second switch during the soft-start phase.

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

5. The low-cost switching power supply for high-precision power metering according to claim 1, characterized in that: The overcurrent protection circuit further includes a third voltage divider resistor and a third capacitor. The first input resistor is grounded in series through the third voltage divider resistor, and the third capacitor is connected in parallel across the third voltage divider resistor. The non-inverting input terminal of the voltage comparator obtains a fixed voltage value through voltage division between the first input resistor and the third voltage divider resistor. When the voltage value at the inverting input terminal of the voltage comparator is greater than the fixed voltage value, 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 power metering according to claim 5, characterized in that: A second diode and a first current-limiting resistor are connected in series on the feedback path from the output 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 switching transistor to turn off; otherwise, it operates normally. The boost power supply circuit further includes a power output feedback loop composed of several 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 set with an output threshold voltage. When the voltage at the feedback pin of the switching power supply chip is greater than the output threshold voltage, the switching power supply chip controls the internal switching transistor to turn off; otherwise, it operates normally.

7. The low-cost switching power supply for high-precision power 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 110uV, the overcurrent protection threshold of the switching power supply is limited to 510mA±2%.

8. A control method for a low-cost switching power supply for high-precision power metering, characterized in that, The low-cost switching power supply for high-precision power metering as described in any one of claims 1-7 comprises: 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 of 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 is charged. When the voltage difference across the first resistor is greater than 0.55V, the first switch is turned on and drives the second switch to turn on as well. At this time, the switch enable pin of the power supply chip is pulled to a low level, and the power supply chip is in an off state. S3: When the voltage difference across the first resistor is ≤0.55V, the first and second switching transistors are turned off. At this time, the pull-down 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 up automatically. S4: Real-time monitoring of the voltage drop across the sampling resistor due to the load current. 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 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 power supply chip remains off until the load current drops below the overcurrent protection threshold, at which point the soft-start steps S1~S3 are automatically re-executed; S6: While executing steps S4~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 switching transistor to turn off; otherwise, it works normally.

9. The control method for a low-cost switching power supply for high-precision power metering according to claim 8, characterized in that... In step S2: The conduction threshold of the first switching transistor is set to 0.55V±0.02V. The difference in conduction threshold of different first switching transistors is matched by adjusting the resistance value of the first resistor. During charging, 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 for a low-cost switching power supply for high-precision power 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, where the total error of the overcurrent point protection is: in, The target overcurrent threshold; This is the error value of the sampling resistor; This is the combined error of the first input resistor and the third voltage divider resistor of the voltage comparator. This is the normalized error of the input offset voltage of the voltage comparator.

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