CN112532034A - Quasi-resonant switching power supply variable-frequency power supply control system and control method thereof - Google Patents

Abstract

The invention discloses a quasi-resonance switch power supply variable frequency power supply control system and a control method thereof. According to the invention, the single-phase alternating voltage or the three-phase alternating voltage is introduced into the rectification module, and the direct-current voltage output by the rectification module is processed by the transformation module, so that power is supplied to the inversion module, the voltage range input by the rectification module is widened, the application range is wide, and the universality is strong. The invention can be widely applied to the technical field of electronic circuits.

Description

Quasi-resonant switching power supply variable-frequency power supply control system and control method thereof

Technical Field

The invention relates to the technical field of electronic circuits, in particular to a frequency conversion power supply control system of a quasi-resonant switching power supply and a control method thereof.

Background

The quasi-resonance switch power supply uses a direct current power supply obtained by rectifying and filtering (realized by a rectifying circuit and a filtering circuit) single-phase alternating current or three-phase alternating current as an input power supply, and the input power supply is used for supplying power to the inversion module. However, the quasi-resonant switching power supply in the prior art is applied to either a single-phase low-voltage power supply circuit or a three-phase high-voltage power supply circuit, and the input voltage range is narrow and the universality is not strong due to the fact that the quasi-resonant switching power supply cannot be compatible with the single-phase low-voltage switching power supply and the three-phase high-voltage switching power supply.

Disclosure of Invention

To solve the above technical problems, the present invention aims to: a quasi-resonance switch power supply variable frequency power supply control system and a control method thereof are provided.

The technical scheme adopted by the first aspect of the invention is as follows:

a frequency conversion power supply control system of a quasi-resonance switch power supply comprises a rectification module, a filtering module, an inversion module and a quasi-resonance switch power supply, wherein the quasi-resonance switch power supply comprises a voltage transformation module, and the rectification module is a single-phase rectification circuit or a three-phase rectification circuit;

a first output end of the rectifying module is connected with a first end of the filtering module, and a second output end of the rectifying module is connected with a second end of the filtering module;

the first end of the filtering module is connected with the first input end of the inversion module, and the second end of the filtering module is connected with the second input end of the inversion module;

the input end of the voltage transformation module is connected with the first end of the filtering module, and the output end of the voltage transformation module is connected with the third input end of the inversion module.

Further, the quasi-resonance switch power supply variable frequency power supply control system also comprises a controller, a change-over switch and a protective resistor;

one end of the protection resistor is connected with the second output end of the rectification module, and the other end of the protection resistor is connected with the second end of the filtering module;

the switch is connected with the protection resistor in parallel;

the input end of the controller is connected with the output end of the quasi-resonant switching power supply, and the output end of the controller is also connected with the change-over switch.

Further, the voltage transformation module includes:

the primary winding of the transformer is connected with the first end of the filtering module;

the input end of the rectification filtering unit is connected with the secondary winding of the transformer;

and the input end of the voltage stabilizing unit is connected with the output end of the rectifying and filtering unit, and the output end of the voltage stabilizing unit is connected with the third input end of the inversion module. The voltage is used for reducing the rectified and filtered voltage so as to provide the grid voltage of the inversion module.

Further, the quasi-resonance switching power supply comprises a quasi-resonance control chip, a sampling voltage module and a sampling current module.

Further, the sampling current module comprises a first field effect transistor, an eighth resistor and a ninth resistor.

Further, the sampling current module further comprises a filtering unit.

Further, the sampling voltage module comprises a feedback unit, one end of the feedback unit is connected with the feedback winding of the transformer, and the other end of the feedback unit is connected with the quasi-resonance control chip.

Further, the quasi-resonant switching power supply further comprises a spike voltage absorption module.

The second aspect of the invention adopts the technical scheme that:

a control method applied to the variable-frequency power supply control system of the quasi-resonant switching power supply according to the first aspect includes the following steps:

acquiring the bus voltage of the filtering module;

and controlling the action of the change-over switch according to the bus voltage to enable the quasi-resonant switching power supply variable-frequency power supply control system to enter a low-power consumption mode or a fault detection mode.

Further, the step of controlling the action of the change-over switch according to the bus voltage to enable the quasi-resonant switching power supply variable frequency power supply control system to enter a low power consumption mode or a fault detection mode comprises the following steps:

determining that the bus voltage is greater than a first threshold voltage and reaches preset time, and controlling the switch to be closed so that the quasi-resonant switch power supply variable-frequency power supply control system is switched to a low-power-consumption mode;

and determining that the bus voltage is smaller than a second threshold voltage, and controlling the change-over switch to be switched off, so that the quasi-resonant switch power supply variable-frequency power supply control system is switched to a fault detection mode.

The invention has the beneficial effects that: through introducing single-phase alternating voltage or three-phase alternating voltage into the rectifier module, utilize the transform module to handle the direct current voltage of rectifier module output to for the power supply of contravariant module, widened the voltage range of rectifier module input, application scope is wide, and the commonality is strong.

Drawings

Fig. 1 is a schematic circuit diagram of a frequency conversion power supply control system of a quasi-resonant switching power supply according to an embodiment of the present application;

FIG. 2 is a schematic circuit diagram of a variable frequency power supply control system of a quasi-resonant switching power supply with a single-phase AC voltage as an input according to an embodiment of the present application;

FIG. 3 is a schematic circuit diagram of a variable frequency power supply control system of a quasi-resonant switching power supply with an input of three-phase AC voltage according to an embodiment of the present application;

FIG. 4 is a schematic circuit diagram of a quasi-resonant switching power supply according to an embodiment of the present application;

fig. 5 is a flowchart of the steps of a control method provided in the embodiment of the present application.

Detailed Description

Reference will now be made in detail to the present embodiments of the present application, preferred embodiments of which are illustrated in the accompanying drawings, which are for the purpose of visually supplementing the description with figures and detailed description, so as to enable a person skilled in the art to visually and visually understand each and every feature and technical solution of the present application, but not to limit the scope of the present application.

In the present application, if directions (up, down, left, right, front, and rear) are described, it is only for convenience of describing the technical aspects of the present application, and it is not intended or implied that the technical features referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus, should not be construed as limiting the present application.

In the present application, "a plurality" means one or more, "a plurality" means two or more, "more than", "less than", "more than" and the like are understood as excluding the number; the terms "above", "below", "within" and the like are understood to include the instant numbers. In the description of the present application, the descriptions of "first" and "second" are only used for distinguishing technical features, but are not understood to indicate or imply relative importance or implicitly indicate the number of the indicated technical features or implicitly indicate the precedence of the indicated technical features.

In this application, unless explicitly defined otherwise, the terms "disposed," "mounted," "connected," and the like are to be construed broadly, e.g., directly connected or indirectly connected through intervening media; can be fixedly connected, can also be detachably connected and can also be integrally formed; may be mechanically coupled, may be electrically coupled or may be capable of communicating with each other; either as communication within the two elements or as an interactive relationship of the two elements. The specific meaning of the above-mentioned words in this application can be reasonably determined by those skilled in the art in combination with the details of the technical solution.

The invention will be further explained and explained with reference to the drawings and the embodiments in the description.

In order to at least partially solve one of the above problems, referring to fig. 1, the present invention provides a quasi-resonant switching power supply variable frequency power supply control system, which includes a rectification module 10, a filtering module 20, an inversion module 30 and a quasi-resonant switching power supply 40, where the quasi-resonant switching power supply 40 includes a transformation module, and referring to fig. 2 and 3, the rectification module 10 is a single-phase rectification circuit or a three-phase rectification circuit;

a first output end of the rectifying module 10 is connected with a first end of the filtering module 20, and a second output end of the rectifying module 10 is connected with a second end of the filtering module 20;

a first end of the filter module 20 is connected with a first input end of the inverter module 30, and a second end of the filter module 20 is connected with a second input end of the inverter module 30;

the input end of the voltage transformation module is connected with the first end of the filter module 20, and the output end of the voltage transformation module is connected with the third end of the inverter module 30.

Specifically, the rectification module 10 includes a single-phase rectification module 10 or a three-phase rectification module 10 for rectifying an input single-phase alternating voltage or three-phase alternating voltage into a direct-current voltage, and referring to fig. 2 and 3, the single-phase rectification circuit may employ a single-phase full-bridge rectification circuit, and the three-phase rectification circuit may employ a three-phase full-bridge rectification circuit.

And a filtering module 20, configured to filter the dc voltage output by the rectifying module 10, so as to obtain a smoother dc voltage.

And an inverting module 30 for inverting the dc voltage at both ends of the filtering module 20 into a three-phase ac voltage, thereby supplying power to the load using the three-phase ac voltage. The inverter module 30 includes a plurality of Insulated Gate Bipolar Transistors (IGBTs).

The quasi-resonant switching power supply 40 is configured to supply power to the inverter module 30, and in more detail, to supply a gate voltage to the IGBTs in the inverter module 30, so that the IGBTs operate. Therefore, referring to fig. 1 and 4, the quasi-resonant switching power supply 40 is connected across the two ends (between P, N) of the filter module 20, so as to supply power to each circuit module in the quasi-resonant switching power supply 40 by using the dc voltage output by the filter module 20. The transformation module processes the input dc voltage, so as to obtain a gate voltage suitable for the inverter module 30 at the output terminal of the transformation module.

As can be seen from the above embodiments, compared with the method that only one of a 220V single-phase ac voltage and a 330V three-phase ac voltage can be introduced into the rectification module 10, in the present application, the single-phase ac voltage or the three-phase ac voltage is introduced into the rectification module 10, and the dc voltage output by the rectification module 10 is processed by using the transformation module, so as to supply power to the inversion module 30.

As a further optional implementation, the quasi-resonant switching power supply 40 variable-frequency power supply control system further includes a controller 50, a switch K, and a protection resistor Rs;

one end of the protection resistor Rs is connected with the second output end of the rectifying module 10, and the other end of the protection resistor Rs is connected with the second end of the filtering module 20;

the switch K is connected with the protective resistor Rs in parallel;

the input end of the controller 50 is connected with the output end of the quasi-resonant switching power supply 40, and the output end of the controller 50 is also connected with the change-over switch K.

Specifically, referring to fig. 1, in the present application, a protection resistor Rs, a change-over switch K and a controller 50 are further added in the frequency conversion power supply control system of the quasi-resonant switching power supply 40, at the power-on time of the frequency conversion power supply control system of the quasi-resonant switching power supply 40, the change-over switch K is kept off, and the protection resistor Rs is connected in series in a circuit loop, so as to prevent the circuit module such as the rectification module 10 and the inversion module 30 from being burnt out due to a large impact current generated in the circuit loop at the power-on time of the system. When detecting that the bus voltage at the two ends of the filtering module 20 reaches a stable state, the controller 50 controls the switch K to be closed, so that the protection resistor Rs is short-circuited by the switch K, power consumption caused by the fact that current continuously flows through the protection resistor Rs is reduced, and system power efficiency is improved. And when the bus voltage is detected to be abnormal, the change-over switch K is controlled to be switched off, and the fault detection is carried out on the system.

In addition, in order to measure the bus voltage output by the filtering module 20, a bus voltage detection module is further provided in the variable-frequency power supply control system of the quasi-resonant switching power supply 40, and referring to fig. 2 and 3, the bus voltage detection module includes a first resistor R1 and a second resistor R2, and the bus voltage at both ends of the bus voltage detection module is detected by the controller 50.

As a further optional embodiment, the voltage transformation module comprises:

a primary winding of the transformer TR1, the primary winding of the transformer TR1 is connected with the first end of the filter module 20;

the input end of the rectifying and filtering unit is connected with a secondary winding of the transformer TR 1;

and the input end of the voltage stabilizing unit is connected with the output end of the rectifying and filtering unit, and the output end of the voltage stabilizing unit is connected with the third input end of the inversion module 30. For stepping down the rectified and filtered voltage to provide the gate voltage of the inverter module 30.

Specifically, referring to fig. 4, this embodiment provides a specific implementation manner of a transformer module, an input dc voltage is input to a primary winding of a transformer TR1 by using a transformer TR1, a transformed dc voltage is output from a secondary winding, the transformed dc voltage further needs to be rectified and filtered by a rectifying and filtering unit, the rectifying and filtering unit includes a second inductor L2, a seventh diode D7, an eleventh capacitor C11, a thirteenth capacitor C13, and a fifteenth capacitor C15, where the seventh diode D7 is used for rectification, the eleventh capacitor C11 is used for first-stage capacitor filtering, the second inductor L2, the thirteenth capacitor C13, and the fifteenth capacitor C15 are used for second-stage inductor filtering, and the fifteenth capacitor C15 outputs 15V dc voltage. The electric voltage value of the direct current voltage after rectification and filtering is higher, therefore, the voltage reduction is carried out through the voltage stabilizing unit, wherein the voltage stabilizing unit mainly comprises a first voltage reduction chip and is used for converting the 15V direct current voltage output by the transformer TR1 into 12V direct current voltage, a second voltage reduction chip and a third voltage reduction chip are used for reducing the 12V direct current voltage output by the first voltage reduction chip into 5V direct current voltage, and the third voltage reduction chip is used for reducing the 5V direct current voltage into 3.3V direct current voltage, so that a proper grid voltage is provided for the IGBT of the inversion module 30.

Further as an alternative embodiment, the quasi-resonant switching power supply 40 includes a quasi-resonant control chip U1, a sampled voltage module, and a sampled current module.

Specifically, the quasi-resonant switching power supply 40 of the present application further includes a quasi-resonant control chip U1 having a quasi-resonant control function, and a sampling voltage module and a sampling current module based on the quasi-resonant control chip U1, where the quasi-resonant control chip U1 may employ a typical quasi-resonant control chip of the company on america, such as NCP1377, NCP1380, and the like, and may also employ a quasi-resonant flyback control chip UCC28600 of the company TI, and the like.

The sampling voltage module is used for collecting sampling current output by the first field effect transistor Q1, after the sampling current is collected by the quasi-resonance control chip U1, the quasi-resonance control chip U1 judges whether the quasi-resonance control chip U1 is in an overcurrent working state or not according to the magnitude of the sampling current, and if the quasi-resonance control chip U1 is judged to be in the overcurrent working state, the quasi-resonance control chip U1 stops working.

The sampling voltage module is used for acquiring sampling voltage after the direct-current voltage output by the filtering module 20 is converted, after the sampling voltage is acquired by the quasi-resonance control chip U1, the quasi-resonance control chip U1 judges whether the sampling voltage is in an overvoltage or undervoltage working state according to the magnitude of the sampling voltage, and if the sampling voltage is judged to be in the overvoltage or undervoltage working state, the quasi-resonance control chip U1 stops outputting DRV pulses.

Further as an optional implementation, the sampling current module includes a first field effect transistor Q1, an eighth resistor R8, and a ninth resistor R9.

Specifically, referring to fig. 4, the first fet Q1 of the present application is a high voltage fet, the eighth resistor R8 and the ninth resistor R9 are sampling resistors, the eighth resistor R8 and the ninth resistor R9 are used to sample a current flowing through the first fet Q1, i.e., a sampling current, the sampling current is sent to a comparator inside the quasi-resonant power chip, and the comparator determines whether the quasi-resonant power chip is in an overcurrent state according to the sampling current. And when the quasi-resonant power control chip judges that the quasi-resonant power control chip is in an overcurrent state according to the sampling current, the DRV pulse of the fifth pin is cut off. And when the overcurrent state disappears, controlling the fifth pin to output the DRV pulse again.

In addition, in fig. 4, the thirteenth resistor R13 is a gate resistor of the first fet Q1, and provides a gate charging resistor loop for the first fet Q1, the sixth diode D2 provides a gate discharging loop for the first fet Q1, the third resistor R3 is a miller resistor, so as to prevent the first fet Q1 from being damaged due to the fact that energy accumulated at the collector of the first fet Q1 cannot be discharged when the first fet Q1 is turned off, and the third capacitor C3 is a quasi-resonant capacitor of the first fet Q1, and provides a resonant loop for the valley voltage to be turned on.

Further as an optional implementation manner, the sampling current module further includes a filtering unit, and the filtering unit includes a second resistor R2 and a seventh capacitor C7.

Specifically, the second resistor R2 and the seventh capacitor C7 form a filtering unit of the sampling current, and impurity signals, particularly high-frequency signals, in the sampling current are filtered by the filtering unit, so that the accuracy of the sampling current is guaranteed.

Further as an optional implementation manner, the sampling voltage module includes a feedback unit, one end of the feedback unit is connected to the feedback winding of the transformer TR1, and the other end of the feedback unit is connected to the quasi-resonant control chip U1.

Specifically, the feedback winding is connected to a feedback loop formed by a twelfth resistor R12, a fourth diode D2 and a fourth capacitor C4, so as to supply power to a sixth pin of the quasi-resonant control chip U1, and the fifth voltage regulator tube D5 is used for preventing the feedback winding of the transformer TR1 from supplying too high voltage, so that the function of voltage stabilization is achieved.

The tenth resistor R10, the eleventh resistor R11 and the twelfth resistor R12 form a feedback unit, so as to provide sampling voltage for the quasi-resonant control chip U1, and when the quasi-resonant control chip U1 determines that the gas outlet is in an overvoltage or undervoltage state according to the sampling voltage, the DRV pulse of the fifth pin is turned off. And when the overvoltage or undervoltage state disappears, controlling the fifth pin to output the DRV pulse again.

As a further alternative embodiment, the quasi-resonant switching power supply 40 further includes a spike voltage absorption module.

Specifically, the quasi-resonant power supply further comprises a spike voltage absorption module, wherein the spike voltage absorption module comprises a fourth resistor R4, a fifth resistor R5, a sixth resistor R6, a seventh resistor R7, a second capacitor C2 and a third diode D3, and the spike voltage absorption module is used for absorbing energy generated by a leakage inductance spike of the transformer TR 1.

Referring to fig. 5, the present invention further provides a control method, which is applied to the above-mentioned frequency-conversion power supply control system of the quasi-resonant switching power supply 40, and includes the following steps S101 to S102:

s101, acquiring bus voltage of the filtering module 20;

and S102, controlling the action of the change-over switch K according to the bus voltage, so that the frequency conversion power supply control system of the quasi-resonant switching power supply 40 enters a low power consumption mode or a fault detection mode.

In the embodiment of the present application, a control method is provided based on the frequency conversion power supply control system of the quasi-resonant switching power supply 40 in the above embodiments, and the bus voltage at two ends of the filter module 20 can be detected to control the action of the switch K, so as to control the frequency conversion power supply control system of the quasi-resonant switching power supply 40 to enter a low power consumption mode or a fault detection mode, where the low power consumption mode refers to that the frequency conversion power supply control system of the quasi-resonant switching power supply 40 operates in an operation mode that reduces the power consumption of the system, and the fault detection mode refers to that the current operation state of the frequency conversion power supply control system of the quasi-resonant switching power supply 40 is a fault state, and circuit faults need to be.

Further as an alternative embodiment, step S102 includes the following steps S1021-S1022:

s1021, determining that the bus voltage is greater than the first threshold voltage and reaches preset time, and controlling a switch K to be closed, so that the variable-frequency power supply control system of the quasi-resonant switching power supply 40 is switched to a low-power-consumption mode;

and S1022, determining that the bus voltage is smaller than the second threshold voltage, and controlling the switch K to be switched off, so that the frequency conversion power supply control system of the quasi-resonant switching power supply 40 is switched to a fault detection mode.

Specifically, the first threshold voltage and the second threshold voltage may be set according to actual conditions, for example, when the input voltage is a single-phase alternating-current voltage, the first threshold voltage is set to 130V and the second threshold voltage is set to 80V, and when the input voltage is a three-phase alternating-current voltage, the first threshold voltage is set to 380V and the second threshold voltage is set to 320V. The preset time is set to 1 s.

When the measured bus voltage is greater than the first threshold voltage and reaches the preset time, it is indicated that the circuit modules such as the rectifying module 10 and the filtering module 20 work normally, at this time, the switch K is controlled to be closed, the protection resistor Rs is short-circuited by the switch K, and no current flows through the protection resistor Rs, so that the protection resistor Rs does not consume the power of the system any more, and thus, the system enters a low power consumption mode;

when the measured bus voltage is lower than the second threshold voltage, it indicates that the rectifying module 10, the filtering module 20, etc. are working abnormally, and therefore, the switch K is controlled to be turned off, so that the system enters a fault detection mode, and in the fault detection mode, the fault cause of the system circuit is checked.

It will be understood that all or some of the steps, systems of methods disclosed above may be implemented as software, firmware, hardware, and suitable combinations thereof. Some or all of the physical components may be implemented as software executed by a processor, such as a central processing unit, digital signal processor, or microprocessor, or as hardware, or as an integrated circuit, such as an application specific integrated circuit. Such software may be distributed on computer readable media, which may include computer storage media (or non-transitory media) and communication media (or transitory media). The term computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data, as is well known to those of ordinary skill in the art. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, Digital Versatile Disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can accessed by a computer. In addition, communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media as known to those skilled in the art.

The embodiments of the present application have been described in detail with reference to the drawings, but the present application is not limited to the embodiments, and various changes can be made without departing from the spirit of the present application within the knowledge of those skilled in the art.

Claims (10)

1. A frequency conversion power supply control system of a quasi-resonance switch power supply is characterized by comprising a rectification module, a filtering module, an inversion module and a quasi-resonance switch power supply, wherein the quasi-resonance switch power supply comprises a voltage transformation module, and the rectification module is a single-phase rectification circuit or a three-phase rectification circuit;

a first output end of the rectifying module is connected with a first end of the filtering module, and a second output end of the rectifying module is connected with a second end of the filtering module;

the first end of the filtering module is also connected with the first input end of the inversion module, and the second end of the filtering module is also connected with the second input end of the inversion module;

the input end of the voltage transformation module is connected with the first end of the filtering module, and the output end of the voltage transformation module is connected with the third input end of the inversion module.

2. The quasi-resonant switching power supply variable frequency power supply control system according to claim 1, further comprising a controller, a switch and a protection resistor;

one end of the protection resistor is connected with the second output end of the rectification module, and the other end of the protection resistor is connected with the second end of the filtering module;

the switch is connected with the protection resistor in parallel;

the input end of the controller is connected with the output end of the quasi-resonant switching power supply, and the output end of the controller is also connected with the change-over switch.

3. The system of claim 2, wherein the voltage transformation module comprises:

the primary winding of the transformer is connected with the first end of the filtering module;

the input end of the rectification filtering unit is connected with the secondary winding of the transformer;

and the input end of the voltage stabilizing unit is connected with the output end of the rectifying and filtering unit, and the output end of the voltage stabilizing unit is connected with the third input end of the inversion module.

4. The system according to claim 1, wherein the quasi-resonant switching power supply comprises a quasi-resonant control chip, a sampling voltage module, and a sampling current module.

5. The variable frequency power supply control system of claim 4, wherein the sampling current module comprises a first field effect transistor, an eighth resistor and a ninth resistor.

6. The system according to claim 5, wherein the sampling current module further comprises a filtering unit.

7. The variable-frequency power supply control system of the quasi-resonant switching power supply according to claim 4, wherein the sampling voltage module comprises a feedback unit, one end of the feedback unit is connected with a feedback winding of the transformer, and the other end of the feedback unit is connected with the quasi-resonant control chip.

8. The system according to claim 3, wherein the quasi-resonant switching power supply further comprises a spike voltage absorption module.

9. A control method, applied to the variable frequency power supply control system of the quasi-resonant switching power supply of claim 2 or 3, comprising the following steps:

acquiring the bus voltage of the filtering module;

and controlling the action of the change-over switch according to the bus voltage to enable the quasi-resonant switching power supply variable-frequency power supply control system to enter a low-power consumption mode or a fault detection mode.

10. The control method according to claim 9, wherein the step of controlling the action of the change-over switch according to the bus voltage to make the quasi-resonant switching power supply variable frequency power supply control system enter a low power consumption mode or a fault detection mode comprises the following steps:

determining that the bus voltage is greater than a first threshold voltage and reaches preset time, and controlling the switch to be closed so that the quasi-resonant switch power supply variable-frequency power supply control system is switched to a low-power-consumption mode;

and determining that the bus voltage is smaller than a second threshold voltage, and controlling the change-over switch to be switched off, so that the quasi-resonant switch power supply variable-frequency power supply control system is switched to a fault detection mode.

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