Circuit for realizing inductive current sampling of inverter by using current transformer

Abstract

The invention belongs to the technical field of inverters, and particularly relates to a circuit for realizing inductive current sampling of an inverter by using a current transformer, which comprises a T1, a controller and a Hall sensor, the driver and the PWM inverter are connected with the controller, the driver uses a Hall sensor as a current sensor of an inversion inductor Linv, acquires an inductive current and feeds back the inductive current to the controller to realize current closed-loop control, and by using a current transformer and a specially designed secondary circuit and combining with switching time sequence control of the inverter, the current closed-loop control of the inverter is realized. The reset of the exciting current of the current transformer is realized, the inductive current of the inverter can be sampled by using the current transformer, and as the current transformer has no temperature drift problem, fixed transformation ratio, low cost and relatively high bandwidth, the inversion inductive current sampling is realized by using the mode.

Description

Technical Field

[0001] The invention belongs to the technical field of inverters, and in particular relates to a circuit for implementing inductor current sampling of an inverter by using a current transformer. Background Art

[0002] In the inverter circuit, it is usually necessary to sample the current of the inverter inductor as the control object, and combine it with some other signals (such as bus voltage, output voltage or grid voltage, etc.) to realize the closed-loop control of the current loop. The inductor current of the inverter usually superimposes a certain switching ripple. In order to ensure a small delay, the required sensor bandwidth is relatively high, generally 3-10 times the switching frequency. High-power inverters usually use Hall sensors (current type or voltage type) to realize inductor current sampling, which is converted into a suitable voltage through a matching circuit and given to the controller's analog-to-digital converter ADC for signal sampling as feedback to realize closed-loop control. When the controller is required to be isolated from the main circuit, or when the three-phase controller is shared, the use of Hall sensors is a better choice, and it is almost the only choice. For some low-power or single-phase circuits, there is no need for controller isolation. Resistors and signal amplifiers can be used to realize current sampling, which can meet the requirements of high bandwidth while reducing costs. Low-power inverters also use sampling resistors and isolation amplifiers. However, they are affected by the bandwidth and delay of the isolation amplifier, as well as the noise limitation of the sampling resistor (to reduce the heating of the sampling resistor, the sampling resistor value is generally very small, in the milliohm range, and the voltage division is generally only tens of millivolts, so an amplifier with a higher amplification factor must be used. In inverters based on power electronics, the switching voltage of the switch tube generally reaches 400V to 800V, which can easily interfere with the sensitive sampling amplifier circuit, thereby limiting the minimum resistance value of the sampling resistor). They can only be used in some occasions with low bandwidth and low power.

[0003] Current transformers are commonly used to measure AC currents. They convert the secondary current, which is proportional to the primary current, into a corresponding voltage signal output through a sampling resistor to match the measuring instrument or related circuit to achieve current measurement. The measurement principle of the current transformer is simple, the current transformer itself is simple and reliable, can achieve electrical isolation, and has low cost, so it is widely used in various industries. In order to improve the current measurement accuracy, the current transformer generally uses a high magnetic permeability core and does not open an air gap. The purpose is to try to increase the secondary excitation reactance value, thereby reducing the excitation current error caused by the load voltage, so as to achieve a small size or high measurement accuracy. In the inverter circuit, since the inverter output current contains a certain amount of DC component (usually due to factors such as asymmetric actual duty cycle, half-wave rectifier type load, amplifier circuit and controller ADC containing a certain offset voltage), the DC component will cause a certain magnetic field bias in the transformer, so that the normal working point of the transformer is no longer 0, thereby reducing the working range of the magnetic core. At the same time, due to the existence of bias magnetism, the magnetic permeability of the magnetic core will also decrease (the magnetic permeability of the magnetic core decreases with the increase of magnetic field strength until saturation), which is equivalent to reducing the secondary reactance of the transformer, which will greatly reduce the measurement accuracy of the transformer, and at the same time produce a large output signal phase shift, or even make it unusable.

[0004] In addition, the DC component of the inverter's inductor current cannot be sensed by the current transformer and output to the secondary, resulting in the signal given to the controller not being able to truly reflect the inductor current, thus causing control problems. In view of this, we propose a circuit that uses a current transformer to implement inverter inductor current sampling. Summary of the invention

[0005] The purpose of the present invention is to provide a circuit for sampling the inductor current of an inverter by using a current transformer in view of the above-mentioned technical problems, so as to solve the problems raised in the above-mentioned background technology.

[0006] In view of this, the present invention provides a circuit for implementing inductor current sampling of an inverter using a current transformer, comprising: T1, controller, Hall sensor; Driver, PWM inverter, the driver is connected to the controller. The driver uses a Hall sensor as the current sensor of the inverter inductor Linv to collect the inductor current and feed it back to the controller to achieve current closed-loop control; The analog switch SW1, hysteresis comparison module, timing control module, analog switch SW1, D1-D3, hysteresis comparison module and timing control module constitute a circuit that can magnetically reset the current transformer T1 under specific conditions. For the convenience of analysis, L and T1 are equivalent circuits of real transformers, where L is the equivalent excitation inductance of the transformer and T1 is an ideal transformer.

[0007] In the above technical solution, further, the inverter mainly outputs AC current, and IL must be close to 0 at a certain moment. This situation is detected by the "hysteresis comparison" module to obtain the corresponding Rang output. After being processed by the timing control module, Goff outputs a low-level pulse, and CA outputs a delayed low-level pulse; The low-level Goff executes the AND logic with PWMH and PWML, making the gate level of the switch tube low, and both the upper and lower switch tubes are turned off. Since the absolute value of the bus voltage is higher than the peak value of Ug, the inductor Linv will continue to flow, and the initial value of the inductor current is very close to 0, so the continuous flow will be completed quickly, so that the inductor current truly reaches 0. At this time, CA also outputs a low level, so that the analog switch SW1 is disconnected, and the secondary current of T1 passes through D1 to D2 or D3 for continuous flow. The primary of T1 is in an open circuit state. Under the action of the excitation current, the secondary coil of T1 can output a higher voltage, so that the excitation current of T1 decreases rapidly and linearly, and T1 enters the reset state. After the set time, CA and Goff return to the default high level, and the next reset can be started.

[0008] In the above technical solution, further, the hysteresis comparison module, when the input IL is in a specific range, the output Range is flipped to a high level, and after the Range is processed by the timing control module, the Goff signal and the CA signal are output. Under normal circumstances, Goff and CA are both high levels. At this time, the analog switch is turned on, and the secondary current of the current transformer flows through Rs to obtain a signal IL proportional to the inductor current. Since Goff is a high level, PWMH and PWML can also be transmitted normally to the switch tube, and the inverter circuit works normally.

[0009] In the above technical solution, further, the timing control includes a rising edge extraction circuit and a timeout timing circuit; The rising edge extraction circuit outputs a high level pulse at point B at the rising edge of the input Range signal, and the width is determined by the delay unit; The "timeout timing" module implements the timing function and has a level reset signal input. When the reset is low, the output remains high after the timer exceeds a certain time. When the input reset is high, the internal timer is reset and the output is low. Point A can be high only when the output of C is high, thus preventing the frequent operation of the reset circuit. Ideally, it should be operated once in each inverter power frequency cycle. In special cases, such as when the inverter output current is 0, the Rang signal may have multiple edges within one power frequency cycle, and the reset cycle of the current transformer is controlled by the timeout circuit to ensure that the reset cycle is not less than the power frequency cycle; When C is at a high level, the high-level narrow pulse at point B can set A to high, and point C will be reset to low at the same time, but due to the effect of delay 1, point A will remain high until the delay ends; After a delay of 2, point A is ANDed with the original signal to achieve a falling edge of CA delayed from A. Delay 2 is the shortest time required for the inductor current to return to zero after the main switch is disconnected, and delay 1 should be greater than the sum of the time required for the CT excitation current to reset to 0 and delay 2 in the worst case.

[0010] In the above technical solution, further, the function of the hysteresis comparison module is that when the transformer output current IL is near zero crossing, for example, when the negative current crosses zero and increases to +2% of the rated value, the hysteresis comparison module outputs 0; The output is high when the positive current crosses zero and increases to -2% of the rated value; Using the hysteresis comparator module to capture the zero crossing point of the current signal has two advantages: 1. A relatively clean output waveform can be obtained; 2. The inverter is a triangular wave with a high-frequency component superimposed on the inductor current. When the current passes through zero for the first time, its average value is not 0. If the CT reset is started at this moment, the inductor current will continue to flow and become 0, which will have a certain adverse effect on the output. When the current changes from negative to positive or from positive to negative, the average value of the inductor current can be very close to 0, which can achieve the best effect and minimize the impact of CT reset on the inverter current.

[0011] In the above technical solution, further, the actual working waveform of the inverter sampled by the current transformer contains a large DC component, but still meets the zero-crossing condition, so the circuit can work normally, proving that there is no problem in actual use, wherein CH3 is the inductor current, CH2 is the voltage signal converted by the secondary current of the transformer, and CH1 is the voltage output by the inverter. It can be seen from the waveform that CH2 can completely overlap with CH3, proving that under this load condition, the current transformer is not saturated and can work normally. In the above technical solution, further, the PWM inverter circuit part can use any other inverter topology as long as the condition that the sampled inductor current can drop to zero when the driving pulse is turned off is met.

[0012] The beneficial effects of the present invention are: The circuit for sampling the inductance current of the inverter using a current transformer can solve the problems of measurement accuracy, temperature drift, bandwidth, and delay of the inverter current, effectively solve the problems of measurement accuracy, temperature drift, bandwidth, and delay of the inverter current, and can effectively reduce the cost of the current sampling circuit. It is particularly suitable for applications with medium and low current levels that require isolation, have high performance requirements, and are low in cost, and can easily realize electrical isolation functions. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic diagram of an existing two-level inverter circuit; Figure 2 It is a schematic diagram of an inverter circuit for current transformer sampling of the present invention; Figure 3 It is a waveform schematic diagram of the present invention; Figure 4 is a schematic diagram of a timing control module of the present invention; Figure 5 is a schematic diagram of a hysteresis comparator of the present invention; Figure 6 is a schematic diagram of a reference waveform of a hysteresis comparator of the present invention; Figure 7 is a schematic diagram of the actual inductor current of the present invention; Figure 8 It is a schematic diagram of the working waveform of the present invention with a half-wave rectifier load; Fig. 9 It is a schematic diagram of the local waveform expansion of the present invention; Fig.10 It is a waveform diagram of multiple power frequency cycles of the present invention; Fig.11 It is a schematic diagram of an inverter circuit for current transformer sampling of a three-level inverter of the present invention; Fig.12 It is a schematic diagram of a PWM rectifier circuit for current transformer sampling of the present invention. DETAILED DESCRIPTION

[0014] The following will be combined with the drawings in the embodiments of the present application to clearly describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments in the present application belong to the scope of protection of this application.

[0015] In the description of the present application, it should be noted that the terms used herein are only for describing specific embodiments, and are not intended to limit the exemplary embodiments according to the present application. For ease of description, the sizes of the various parts shown in the drawings are not drawn according to the actual proportional relationship. The technology, methods and equipment known to ordinary technicians in the relevant field may not be discussed in detail, but in appropriate cases, the technology, methods and equipment should be regarded as part of the authorization specification. In all examples shown and discussed here, any specific value should be interpreted as merely exemplary, rather than as a limitation. Therefore, other examples of the exemplary embodiments may have different values. It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further discussed in subsequent drawings.

[0016] It should be noted that, in the present application, the terms "comprise", "include" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises one..." does not exclude the presence of other identical elements in the process, method, article or device including the element. In addition, it should be noted that the scope of the method and device in the embodiment of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in reverse order according to the functions involved, for example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0017] Embodiment 1: See also Figure 1-Figure 12 As shown, this embodiment provides a circuit for sampling the inductor current of an inverter by using a current transformer.

[0018] include: T1, controller, Hall sensor; Driver, PWM inverter, the driver is connected to the controller. The driver uses a Hall sensor as the current sensor of the inverter inductor Linv to collect the inductor current and feed it back to the controller to achieve current closed-loop control; The analog switch SW1, hysteresis comparison module, timing control module, analog switch SW1, D1-D3, hysteresis comparison module and timing control module constitute a circuit that can magnetically reset the current transformer T1 under specific conditions. For the convenience of analysis, L and T1 are equivalent circuits of real transformers, where L is the equivalent excitation inductance of the transformer and T1 is an ideal transformer.

[0019] Embodiment 2: This embodiment provides a circuit for implementing inductor current sampling of an inverter using a current transformer. In addition to the technical solutions of the above embodiments, it also has the following technical features.

[0020] refer to Figure 2 and Figure 3 , the inverter mainly outputs AC current, and IL must be close to 0. This situation is detected by the "hysteresis comparison" module to obtain the corresponding Rang output. After being processed by the timing control module, Goff outputs a low-level pulse, and CA outputs a delayed low-level pulse (the delay can ensure that the switch tube is completely turned off, and it is best that the inductor current can drop to 0); The low-level Goff executes the AND logic with PWMH and PWML, making the gate level of the switch tube low, and both the upper and lower switch tubes are turned off. Since the absolute value of the bus voltage (+BUS, -BUS) is higher than the peak value of Ug, the inductor Linv will continue to flow, and the initial value of the inductor current is very close to 0, so the continuous flow will be completed quickly, so that the inductor current truly reaches 0. At this time, CA also outputs a low level, so that the analog switch SW1 is disconnected, and the secondary current of T1 passes through D1 to reach D2 or D3 for continuous flow. The primary of T1 is in an open circuit state. Under the action of the excitation current, the secondary coil of T1 can output a higher voltage (determined by the voltage regulator), so that the excitation current of T1 decreases rapidly and linearly, and T1 enters the reset state. After the set time (it should be able to ensure that the current of T1's excitation inductor L drops to zero to complete the magnetic reset), CA and Goff return to the default high level, and the next reset can be started.

[0021] Embodiment 3: This embodiment provides a circuit for implementing inductor current sampling of an inverter using a current transformer. In addition to the technical solutions of the above embodiments, it also has the following technical features.

[0022] refer to Figure 2, hysteresis comparison module, when the input IL is in a specific range, the output Range flips to a high level. After being processed by the timing control module, Range outputs the Goff signal and the CA signal. Under normal circumstances, Goff and CA are both high levels. At this time, the analog switch is turned on, and the secondary current of the current transformer flows through Rs, and a signal IL proportional to the inductor current is obtained. Since Goff is a high level, PWMH and PWML can also be transmitted to the switch tube normally, and the inverter circuit works normally.

[0023] Embodiment 4: This embodiment provides a circuit for implementing inductor current sampling of an inverter using a current transformer. In addition to the technical solutions of the above embodiments, it also has the following technical features.

[0024] refer to Figure 4 ,The timing control includes a rising edge extraction circuit and a timeout timing circuit; The rising edge extraction circuit outputs a high level pulse at point B at the rising edge of the input Range signal, and the width is determined by the delay unit; The "timeout timing" module implements the timing function and has a level reset signal input. When the reset is low, the output remains high after the timer has counted for a certain period of time (usually more than half a power frequency cycle, for 50Hz, it can be set to about 10ms). When the input reset is high, the internal timer is reset and the output is low. Point A can be high only when the output of C is high, thus preventing the frequent operation of the reset circuit. Ideally, it should be operated once in each inverter power frequency cycle (because the power frequency current transformer can withstand at least half of the power frequency cycle without saturation); In special cases, such as when the inverter output current is 0 (the inductor current is a triangle wave with an average value of zero), the Rang signal may have multiple edges in one power frequency cycle, and the reset period of the current transformer is controlled by the timeout circuit to ensure that the reset period is not less than the power frequency period; When C is at a high level, the high-level narrow pulse at point B can set A to high, and point C will be reset to low at the same time, but due to the effect of delay 1, point A will remain high until the delay ends; After a delay of 2, point A is ANDed with the original signal to achieve a falling edge of CA delayed from A. Delay 2 is the shortest time required for the inductor current to return to zero after the main switch is disconnected, and delay 1 should be greater than the sum of the time required for the CT excitation current to reset to 0 and delay 2 in the worst case.

[0025] Embodiment 5: This embodiment provides a circuit for implementing inductor current sampling of an inverter using a current transformer. In addition to the technical solutions of the above embodiments, it also has the following technical features.

[0026] refer to Figure 5 and Figure 6 ,The function of the hysteresis comparison module is to output 0 when the transformer output current IL (close to the actual inductor current) is near zero, such as when the negative current crosses zero and increases to +2% of the rated value (adjust as needed, the ideal value is half of the ripple current); The output is high when the positive current crosses zero and increases to -2% of the rated value; Using the hysteresis comparator module to capture the zero crossing point of the current signal has two advantages: 1. A relatively clean output waveform can be obtained; Second, the inverter is a triangular wave with a high-frequency component superimposed on the inductor current. When the current first passes through zero, its average value is not 0. If the CT reset is started at this moment, the inductor current will continue to flow and become 0, which will have a certain adverse effect on the output, such as Figure 7 Position 1. When the current changes from negative to positive or from positive to negative, the average value of the inductor current can be very close to 0, which can achieve the best effect and minimize the impact of CT reset on the inverter current.

[0027] Embodiment 6: This embodiment provides a circuit for implementing inductor current sampling of an inverter using a current transformer. In addition to the technical solutions of the above embodiments, it also has the following technical features.

[0028] refer to Figure 8 and Fig.10 , the actual working waveform of the inverter sampled by the current transformer, the output contains a large DC component, but still meets the zero-crossing condition, so the circuit can work normally, proving that there is no problem in actual use (Note: the hardware of the waveform in the verification diagram uses zero-crossing comparison, because the transformer current is not 0 when reset, there is a certain oscillation). Among them, CH3 is the inductor current, CH2 is the voltage signal converted by the transformer secondary current, and CH1 is the inverter output voltage. It can be seen from the waveform that CH2 can completely overlap with CH3, proving that under this load condition, the current transformer is not saturated and can work normally.

[0029] Embodiment 7: This embodiment provides a circuit for implementing inductor current sampling of an inverter using a current transformer. In addition to the technical solutions of the above embodiments, it also has the following technical features.

[0030] refer to Fig.11 and Fig.12The PWM inverter circuit part can use any other inverter topology as long as the condition that the sampled inductor current can drop to zero when the drive pulse is turned off is met.

[0031] The embodiments of the present application are described above in conjunction with the accompanying drawings. In the absence of conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present application, ordinary technicians in this field can also make many forms without departing from the purpose of the present application and the scope of protection of the claims, all of which are within the protection of the present application.

Claims

1. A circuit for sampling the inductor current of an inverter using a current transformer, characterized in that: include: T1, controller, Hall sensor; Driver, PWM inverter, the driver is connected to the controller. The driver uses a Hall sensor as the current sensor of the inverter inductor Linv to collect the inductor current and feed it back to the controller to achieve current closed-loop control; The analog switch SW1, hysteresis comparison module, timing control module, analog switch SW1, D1-D3, hysteresis comparison module and timing control module constitute a circuit that can magnetically reset the current transformer T1 under specific conditions. For the convenience of analysis, L and T1 are equivalent circuits of real transformers, where L is the equivalent excitation inductance of the transformer and T1 is an ideal transformer.

2. The circuit for implementing inductor current sampling of an inverter using a current transformer according to claim 1, characterized in that: The inverter mainly outputs AC current, and IL must be close to 0 at some point. This situation is detected by the "hysteresis comparison" module to obtain the corresponding Rang output. After being processed by the timing control module, Goff outputs a low-level pulse, and CA outputs a delayed low-level pulse. The low-level Goff executes the AND logic with PWMH and PWML, making the gate level of the switch tube low, and both the upper and lower switch tubes are turned off. Since the absolute value of the bus voltage is higher than the peak value of Ug, the inductor Linv will continue to flow, and the initial value of the inductor current is very close to 0, so the continuous flow will be completed quickly, so that the inductor current truly reaches 0. At this time, CA also outputs a low level, so that the analog switch SW1 is disconnected, and the secondary current of T1 passes through D1 to D2 or D3 for continuous flow. The primary of T1 is in an open circuit state. Under the action of the excitation current, the secondary coil of T1 can output a higher voltage, so that the excitation current of T1 decreases rapidly and linearly, and T1 enters the reset state. After the set time, CA and Goff return to the default high level, and the next reset can be started.

3. The circuit for implementing inductor current sampling of an inverter using a current transformer according to claim 1, characterized in that: The hysteresis comparison module outputs Range to a high level when the input IL is within a specific range. After being processed by the timing control module, Range outputs Goff signal and CA signal. Under normal circumstances, Goff and CA are both high levels. At this time, the analog switch is turned on, and the secondary current of the current transformer flows through Rs to obtain a signal IL proportional to the inductor current. Since Goff is a high level, PWMH and PWML can also be transmitted to the switch tube normally, and the inverter circuit works normally.

4. The circuit for implementing inductor current sampling of an inverter using a current transformer according to claim 1, characterized in that: The timing control includes a rising edge extraction circuit and a timeout timing circuit; The rising edge extraction circuit outputs a high level pulse at point B at the rising edge of the input Range signal, and the width is determined by the delay unit; The "timeout timing" module implements the timing function and has a level reset signal input. When the reset is low, the output remains high after the timer exceeds a certain time. When the input reset is high, the internal timer is reset and the output is low. Point A can be high only when the output of C is high, thus preventing the frequent operation of the reset circuit. Ideally, it should be operated once in each inverter power frequency cycle. In special cases, such as when the inverter output current is 0, the Rang signal may have multiple edges within one power frequency cycle, and the reset cycle of the current transformer is controlled by the timeout circuit to ensure that the reset cycle is not less than the power frequency cycle; When C is at a high level, the high-level narrow pulse at point B can set A to high, and point C will be reset to low at the same time, but due to the effect of delay 1, point A will remain high until the delay ends; After a delay of 2, point A is ANDed with the original signal to achieve a falling edge of CA delayed from A. Delay 2 is the shortest time required for the inductor current to return to zero after the main switch is disconnected, and delay 1 should be greater than the sum of the time required for the CT excitation current to reset to 0 and delay 2 in the worst case.

5. The circuit for implementing inductor current sampling of an inverter using a current transformer according to claim 1, characterized in that: The function of the hysteresis comparison module is to output 0 when the transformer output current IL is near zero, for example, when the negative current crosses zero and increases to +2% of the rated value; The output is high when the positive current crosses zero and increases to -2% of the rated value; Using the hysteresis comparator module to capture the zero crossing point of the current signal has two advantages:

1. A relatively clean output waveform can be obtained; 2. The inverter is a triangular wave with a high-frequency component superimposed on the inductor current. When the current passes through zero for the first time, its average value is not 0. If the CT reset is started at this moment, the inductor current will continue to flow and become 0, which will have a certain adverse effect on the output. When the current changes from negative to positive or from positive to negative, the average value of the inductor current can be very close to 0, which can achieve the best effect and minimize the impact of CT reset on the inverter current.

6. The circuit for implementing inductor current sampling of an inverter using a current transformer according to claim 1, characterized in that: The actual working waveform of the inverter sampled by the current transformer has a large DC component in the output, but still meets the zero-crossing condition, so the circuit can work normally, proving that there is no problem in actual use, wherein CH3 is the inductor current, CH2 is the voltage signal converted from the secondary current of the transformer, and CH1 is the voltage output by the inverter. It can be seen from the waveform that CH2 can completely overlap with CH3, proving that under this load condition, the current transformer is not saturated and can therefore work normally.

7. The circuit for implementing inductor current sampling of an inverter using a current transformer according to claim 1, characterized in that: The PWM inverter circuit part may use any other inverter topology as long as the condition that the sampled inductor current can drop to zero when the driving pulse is turned off is met.

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