Multi-stage collaborative rapid overcurrent protection circuit of frequency converter

Abstract

The invention discloses a frequency converter multi-stage cooperative rapid overcurrent protection circuit, and the overcurrent protection circuit cooperates with an IGBT inverter circuit, an IGBT drive circuit, a current sampling circuit and a master control MCU of a frequency converter to construct a three-stage cooperative protection system composed of hardware-stage extreme rapid protection, hardware-stage rapid protection and software-stage intelligent protection. And independent action and information interaction between the stages can be realized through circuit design. Compared with the prior art, protection blind areas are eliminated, nanosecond-microsecond level extreme protection is achieved, a multiple and high-reliability redundancy protection system is constructed, intelligent grading processing of faults is achieved, and the diagnosability of the system is enhanced.

Description

Technical Field

[0001] This invention relates to the field of inverter protection circuit technology, and in particular to a multi-level coordinated fast overcurrent protection circuit for inverters. Background Technology

[0002] Insulated Gate Bipolar Transistors (IGBTs) are core and expensive power switching devices in frequency converters, and their reliability directly determines the reliability of the entire unit. In industrial frequency converters with power ratings of 18.5kW and similar, the risks of overcurrent and short circuits faced by IGBTs are particularly prominent, such as phase-to-phase short circuits caused by motor cable damage, turn-to-turn short circuits caused by motor insulation aging, and sudden load jamming.

[0003] Existing overcurrent protection solutions typically have the following drawbacks: 1. The contradiction between response speed and protection blind zone: Most protection relies on software sampling and algorithm judgment by a microcontroller (MCU). Since software execution takes time (usually tens to hundreds of microseconds), in the event of a severe short circuit, the current rises extremely quickly (reaching its peak value within a few microseconds). The IGBT may be damaged due to overcurrent before the MCU can react, which forms a "protection blind zone".

[0004] 2. Single Protection Path: Many designs employ a single protection path, such as relying solely on the desaturation (DESAT) protection of the driver chip. While DESAT protection is fast, it may be falsely triggered by interference, or there may be a detection delay under specific fault conditions due to factors such as diode reverse recovery time. Once this path fails, the system will lose protection.

[0005] 3. Lack of fault classification and handling capabilities: Traditional protection circuits often adopt a "one-size-fits-all" approach, immediately shutting down and alarming once triggered. However, for some instantaneous overshoots or minor overloads, the system could try current-limiting operation or adopt a more lenient shutdown strategy to avoid production interruption. Existing circuits lack the ability to distinguish the severity of faults and take different response strategies. Summary of the Invention

[0006] Purpose of the invention: The purpose of this invention is to provide an optimized overcurrent protection circuit for frequency converters, which solves the problems of slow response speed, blind spots, limited reliability, and lack of fault classification and handling capabilities in existing circuits.

[0007] Technical solution: A multi-level collaborative fast overcurrent protection circuit for frequency converters, wherein the overcurrent protection circuit works in conjunction with the frequency converter's IGBT inverter circuit, IGBT drive circuit, current sampling circuit, and main control MCU; An ultrafast recovery diode and a small-value resistor are connected in series between the DESAT detection pin of the IGBT driver chip and the collector of the driven IGBT. When the IGBT is normally turned on, the voltage between the collector and collector is at the saturation voltage drop Vce(sat), and the voltage at the DESAT detection pin is lower than the threshold. When a short circuit occurs, the IGBT exits the saturation region, the voltage drop Vce increases sharply, and the voltage at the DESAT detection pin exceeds the threshold. The driver chip immediately generates a hard turn-off signal to quickly pull the IGBT gate voltage down to a negative voltage, thereby achieving forced turn-off. In the current sampling circuit, the three-phase current sampling points are respectively connected to the negative input of a high-speed voltage comparator, and the positive input is connected to an adjustable reference voltage REF_OC. The outputs of the three comparators are combined into a fault signal OC_LOGIC. Once any phase current exceeds the adjustable reference voltage REF_OC, the comparator flips, and the fault signal OC_LOGIC immediately becomes valid. It is directly sent to the dedicated high-priority fault interrupt pin of the main control MCU, and the output blocking function of its PWM module is directly triggered by the external signal at the hardware level, setting all PWM outputs to an invalid state. The analog-to-digital converter (ADC) inside the main control MCU periodically samples the three-phase output current and DC bus voltage, and classifies faults and handling strategies according to the degree and duration of overcurrent.

[0008] Furthermore, the adjustable reference voltage REF_OC is set to a current value that is higher than the rated current but lower than the DESAT protection threshold.

[0009] Preferably, the adjustable reference voltage REF_OC is set to 150% to 200% of the rated current.

[0010] Furthermore, the graded faults and handling strategies are as follows: Slight overload: Implements automatic current limiting, reduces output frequency and voltage, and limits current to a safe range; Continuous overload: Inverse time-limit protection is activated; the more severe the overcurrent, the faster the shutdown action.

[0011] Beneficial effects: The optimized overcurrent protection circuit of this invention eliminates the protection blind zone for the frequency converter, achieves ultimate protection at the nanosecond to microsecond level, constructs a multi-layered and highly reliable redundant protection system, realizes intelligent hierarchical processing of faults, and enhances the system's diagnosability. Attached Figure Description

[0012] Figure 1 This is the overall logic block diagram of the overcurrent protection circuit of the present invention; Figure 2 This is the schematic diagram of the first-level protection circuit, taking one phase upper bridge arm as an example; Figure 3 This is the schematic diagram of the second-level protection circuit; Figure 4 This is a time-series flowchart for the coordinated operation of the three-level protection system. Detailed Implementation

[0013] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0014] A multi-stage coordinated fast overcurrent protection circuit for frequency converters, as shown in the attached diagram. Figure 1 As shown, the overcurrent protection circuit is integrated with the inverter's IGBT inverter circuit, IGBT drive circuit, current sampling circuit, and main control MCU to construct a three-level collaborative protection system consisting of hardware-level ultra-fast protection, hardware-level fast protection, and software-level intelligent protection. Through circuit design, each level can operate independently and exchange information.

[0015] The first-level protection circuit is designed to achieve ultra-fast hardware-level protection. For extreme short-circuit faults such as shoot-through of IGBTs, the protection target is to achieve a response time of less than 10 microseconds.

[0016] Combined with appendix Figure 2 As shown, the desaturation (DESAT) detection function built into the IGBT driver chip is used. An ultra-fast recovery diode US1M and a small-value resistor R72 are connected in series between the DESAT detection pin of the IGBT driver chip and the collector of the driven IGBT. When the IGBT is normally turned on, the voltage between the collector and collector is very low, at the saturation voltage drop Vce(sat). The voltage at the DESAT detection pin is below the threshold. When a short circuit occurs, the IGBT exits the saturation region, the voltage drop Vce increases sharply, and the voltage at the DESAT detection pin exceeds the threshold. The driver chip immediately (usually within 2-5 microseconds) generates a hard turn-off signal, rapidly pulling the IGBT gate voltage down to a negative voltage, thus achieving forced turn-off.

[0017] The first-level protection circuit is the fastest and most direct protective barrier.

[0018] The design of the second-level protection circuit enables fast hardware-level protection, serving as a redundant backup for the first-level protection circuit and covering a wider range of overcurrent conditions, such as non-shoo-through faults with a sharp increase in current. The protection target is to achieve a response time in the range of 10 to 20 microseconds.

[0019] Combined with appendix Figure 3As shown, a three-phase output current sampling circuit is used. The three-phase current sampling points are each connected to the negative input of a high-speed voltage comparator LM2903DR2G. The positive input is connected to an adjustable reference voltage REF_OC, which is set to a current value higher than the rated current but lower than the DESAT protection threshold, for example, 150% to 200% of the rated current. The outputs of the three comparators are combined into a single fault signal OC_LOGIC. Once any phase current exceeds the adjustable reference voltage REF_OC, the comparator flips, and the fault signal OC_LOGIC immediately becomes active. This fault signal is directly sent to the dedicated high-priority fault interrupt pin of the main control MCU. Upon receiving this interrupt, the main control MCU, without software judgment, directly triggers the output blocking function of its PWM module at the hardware level via an external signal, setting all PWM outputs to an invalid state.

[0020] For example: For an 18.5kW / 380V frequency converter, the rated current is approximately 37A. The circuit uses a 1mΩ shunt amplifier for current signal acquisition, with an output voltage of Iac*1.414*0.001*8.2*3.9. When the current is 37A, the output is approximately 1.67V. The trigger threshold for the second-stage protection is set to 180% of the rated current, approximately 66.6A. The corresponding sensor output voltage is 66.6*1.414*0.001*8.2*3.9≈3.01V. Therefore, the adjustable reference voltage REF_OC at the positive input of the high-speed voltage comparator is set to 3.01V. When the output voltage of any phase sensor exceeds this value, the comparator immediately activates and issues an interrupt signal.

[0021] The design of the third-level protection circuit realizes software-level intelligent protection. For handling non-emergency faults such as continuous overload and imbalance, it is necessary to achieve the protection goals of fault recording, hierarchical processing and system coordination.

[0022] The analog-to-digital converter (ADC) inside the main control MCU periodically samples the three-phase output current and DC bus voltage, and uses software algorithms to make judgments. Based on the degree and duration of overcurrent, the main control MCU can take different levels of fault and handling strategies: Slight overload: Implements automatic current limiting, reduces output frequency and voltage, and limits current to a safe range; Continuous overload: Inverse time-limit protection is activated; the more severe the overcurrent, the faster the shutdown action.

[0023] Combined with appendix Figure 4 As shown, the collaborative working mechanism of the three-level protection system: Independence: The first and second level protections are implemented entirely in hardware, without relying on the main control MCU software, ensuring extreme speed.

[0024] Redundancy: The second-level protection is an effective backup for the first-level protection. Even if the DESAT function of a certain IGBT fails, the second-level protection can still trigger the shutdown by detecting the output current.

[0025] Intelligence: The third-level protection is software-based, handling non-emergency faults and avoiding unnecessary downtime. The main control MCU can also monitor the trigger signals of the first and second-level protections to distinguish fault types: if the fault signal OC_LOGIC is received but the fault signal DESAT is not received, it may be judged as a moderately severe overcurrent; if the fault signal DESAT is received simultaneously, it is judged as an extreme short circuit. Information interaction greatly enhances the maintainability and intelligence level of the system.

[0026] Advantages of the multi-level coordinated fast overcurrent protection circuit of this invention: 1. Eliminates protection blind spots and achieves ultimate protection at the nanosecond to microsecond level, providing blind-spot-free protection against extreme short-circuit faults occurring within microseconds: Through pure hardware DESAT protection and hardware comparator protection, it achieves unparalleled response speed to the most dangerous short-circuit faults, fundamentally eliminating IGBT damage caused by response delay.

[0027] 2. A multi-layered and highly reliable redundant protection system has been constructed: Two independent hardware protection paths form a "double insurance" to ensure that the system can still shut down safely when a single protection path fails, which greatly improves the fault tolerance and overall reliability of the protection circuit and meets the high reliability requirements of industrial applications.

[0028] 3. Intelligent fault classification and handling: Through the combination of hardware and software, the system can distinguish the severity level of faults, such as instantaneous overcurrent, continuous overload, and severe short circuit, and execute different protection strategies. It adopts flexible handling for minor faults and immediate shutdown for severe faults, which not only protects the equipment, but also ensures the continuity of production to the greatest extent and improves the user experience.

[0029] 4. Enhanced system diagnostics: The main control MCU can record the trigger status and sequence of each level of protection, providing clear data support for after-sales service personnel to conduct fault analysis, and can quickly locate whether the problem is caused by external load or internal components.

Claims

1. A multi-stage coordinated fast overcurrent protection circuit for a frequency converter, characterized in that: The overcurrent protection circuit works in conjunction with the inverter's IGBT inverter circuit, IGBT drive circuit, current sampling circuit, and main control MCU. An ultrafast recovery diode and a small-value resistor are connected in series between the DESAT detection pin of the IGBT driver chip and the collector of the driven IGBT. When the IGBT is normally turned on, the voltage between the collector and collector is at the saturation voltage drop Vce(sat), and the voltage at the DESAT detection pin is lower than the threshold. When a short circuit occurs, the IGBT exits the saturation region, the voltage drop Vce increases sharply, and the voltage at the DESAT detection pin exceeds the threshold. The driver chip immediately generates a hard turn-off signal to quickly pull the IGBT gate voltage down to a negative voltage, thereby achieving forced turn-off. In the current sampling circuit, the three-phase current sampling points are respectively connected to the negative input of a high-speed voltage comparator, and the positive input is connected to an adjustable reference voltage REF_OC. The outputs of the three comparators are combined into a fault signal OC_LOGIC. Once any phase current exceeds the adjustable reference voltage REF_OC, the comparator flips, and the fault signal OC_LOGIC immediately becomes valid. It is directly sent to the dedicated high-priority fault interrupt pin of the main control MCU, and the output blocking function of its PWM module is directly triggered by the external signal at the hardware level, setting all PWM outputs to an invalid state. The analog-to-digital converter (ADC) inside the main control MCU periodically samples the three-phase output current and DC bus voltage, and classifies faults and handling strategies according to the degree and duration of overcurrent.

2. The inverter multi-level coordinated fast overcurrent protection circuit according to claim 1, characterized in that: The adjustable reference voltage REF_OC is set to a current value that is higher than the rated current but lower than the DESAT protection threshold.

3. The inverter multi-level coordinated fast overcurrent protection circuit according to claim 2, characterized in that: The adjustable reference voltage REF_OC is set to 150% to 200% of the rated current.

4. The inverter multi-level coordinated fast overcurrent protection circuit according to claim 1, characterized in that: The graded faults and handling strategies are as follows: Slight overload: Implements automatic current limiting, reduces output frequency and voltage, and limits current to a safe range; Continuous overload: Inverse time-limit protection is activated; the more severe the overcurrent, the faster the shutdown action.

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