Input under-voltage protection control method and control circuit

Abstract

The invention provides an input under-voltage protection control method, which is used for a switching power supply and comprises the following steps of: sampling an input voltage signal of the switching power supply; the sampling value of the input voltage signal is compared with an input under-voltage threshold value, and when the sampling value of the input voltage signal is smaller than the input under-voltage threshold value, input under-voltage protection triggering judgment is carried out; and controlling the input under-voltage protection of the switching power supply according to the judgment result. When the input voltage is reduced, the input voltage sampling value touches the input under-voltage protection threshold value, and the input voltage sampling value cannot rise above a certain safety value within a certain time, the switching power supply system is controlled to trigger the input under-voltage protection; the problem that input under-voltage protection cannot be triggered when low input voltage oscillates can be effectively avoided, and meanwhile the system reliability is improved.

Description

Technical Field

[0001] The present invention relates to the field of electronics, and specifically but not limited to, relates to an input undervoltage protection control method and a control circuit. Background Art

[0002] BOP (Brown-Out Protection) is input undervoltage protection, which is a common protection function of ACDC switching power supply.

[0003] The input range of the AC power supply must be considered at the beginning of the design of the ACDC power supply, because input voltage that is too high or too low will cause excessive stress on the power supply system. If the input voltage is too low, the input average current may be too large, causing serious heating of the system and even damage to some components. The BOP can shut down the switching power supply to protect the system when the input voltage fluctuates to a low point.

[0004] Common BOP designs include the following (taking flyback topology as an example):

[0005] (1) Figure 1 As shown, the power supply directly samples the input voltage information and uses this voltage information to design the BOP function.

[0006] (2) Figure 2 As shown, the power supply obtains a sampling voltage proportional to the voltage before or after the input rectifier bridge, thereby realizing input voltage detection and protection.

[0007] (3) Figure 3 As shown in FIG. 1 , when the primary MOS is turned on, the DEM connected to the auxiliary winding can detect the bus voltage. The power supply performs input undervoltage protection based on the bus voltage information sampled by the DEM pin.

[0008] (4) Figure 4 As shown, the protection design is performed using the Drain terminal voltage information when the switch tube is turned off.

[0009] The above-mentioned various ways to realize the BOP function, the commonly used BOP threshold control method, the protection threshold and hysteresis design are basically the same. Generally, by setting a fixed threshold BOP_in for triggering BOP, as long as the system detects that the input voltage is continuously lower than the BOP trigger threshold BOP_in within a certain debounce time, the system triggers BOP. A fixed hysteresis is set above the BOP trigger threshold. The hysteresis value is generally set to 10%-15% of the BOP threshold, that is, BOP_out≈1.1*BOP_in.

[0010] This method logical design is as follows Figure 5As shown, the system input bus voltage sampling value Vbus_sense is compared with the protection threshold Vbop_in. When Vbus_sense is less than Vbop_in and lasts for a debounce time, BOP protection can be triggered, and the comparator threshold is switched to Vbop_out. After entering the protection, Vbus_sense needs to be greater than Vbop_out and last for a debounce time to exit the BOP protection.

[0011] By adopting this BOP hysteresis function, it is possible to avoid the BOP function repeatedly triggering protection near the trigger threshold, causing the output to start and stop repeatedly.

[0012] The aforementioned BOP threshold control method (fixed threshold + fixed hysteresis) commonly used in current power chips has the following problems.

[0013] When the power supply is lightly loaded and fully loaded, the bus voltage Vbus is inconsistent, and there are differences in the peak / valley / average values, but the peak values ​​are similar (see Figure 7 ). Generally, when the power system is fully loaded, the bus voltage ripple can reach 15% (the ripple of the high-voltage system is about 45V).

[0014] from Figure 8 It can be seen that for the same BOP trigger threshold, the heavier the load, the more Vbus is lower than Vbop_in and the longer the time. In addition, the heavier the power load, the closer the input current stress is to the design rated value. In a power system with insufficient design margin, the input current stress may exceed the rated value during the time when the bus voltage is lower than Vbop_in, increasing the risk of failure.

[0015] As the power supply is used for a longer time, the capacitance of the input electrolytic capacitor will decay, the input ripple will gradually increase, and the stress risk of low-voltage heavy load will be further aggravated.

[0016] like Fig. 9 As shown in the figure, when the input voltage of the power system is abnormally low and continues to be low, in extreme cases, Vbus_sense only touches Vbop_in for a short time and does not reach the debounce time to trigger the protection, then the power system will continue to work under low voltage conditions, causing the system temperature to rise and even reliability problems. Especially as mentioned above, under abnormal low voltage conditions and system overload, the current BOP protection triggering logic will face greater risks.

[0017] In view of this, it is necessary to provide a new structure or control method to solve at least part of the above problems. Summary of the invention

[0018] At least in response to one or more problems in the background technology, the present invention proposes an input undervoltage protection control method and control circuit, which can avoid the problem of failure to trigger input undervoltage protection due to low input voltage oscillation, thereby improving the reliability of the switching power supply system.

[0019] According to one aspect of the present invention, an input undervoltage protection control method is used for a switching power supply, comprising:

[0020] Sampling an input voltage signal of the switching power supply;

[0021] Comparing the sampling value of the input voltage signal with the input undervoltage threshold, and when the sampling value of the input voltage signal is less than the input undervoltage threshold, performing input undervoltage protection triggering judgment;

[0022] The input undervoltage protection of the switching power supply is controlled according to the determination result.

[0023] In one embodiment, the input undervoltage protection triggering determination includes:

[0024] The timing starts when the sampling value of the input voltage signal is less than the input under-voltage threshold. If the duration of the sampling value of the input voltage signal being less than the safety value of the input under-voltage threshold reaches the de-jitter time, it is determined that the switching power supply triggers the input under-voltage protection.

[0025] In one embodiment, the input undervoltage protection triggering determination includes:

[0026] Starting from the time when the sampling value of the input voltage signal is less than the input undervoltage threshold, if the duration of the sampling value of the input voltage signal being less than the input undervoltage threshold reaches a first de-jitter time, switching the input undervoltage threshold to a safe value of the input undervoltage threshold;

[0027] In a second de-jitter time after the first de-jitter time, the sampling value of the input voltage signal is compared with the safety value of the input under-voltage threshold, and it is determined whether the switching power supply triggers the input under-voltage protection according to the comparison result.

[0028] In one embodiment, determining whether the switching power supply triggers input undervoltage protection according to the comparison result includes:

[0029] During the second de-bouncing time, if the sampling value of the input voltage signal is greater than the safety value of the input under-voltage threshold for a duration that reaches the preset first de-bouncing time, the second de-bouncing time timing is reset, and the safety value of the input under-voltage threshold is switched to the input under-voltage threshold; otherwise, it is determined that the switching power supply triggers the input under-voltage protection.

[0030] In one embodiment, the safety value of the input under-voltage threshold=the input under-voltage threshold*1.1.

[0031] In one embodiment, the first de-bounce time≤the second de-bounce time.

[0032] According to another aspect of the present invention, an input undervoltage protection control circuit is used for a switching power supply, comprising:

[0033] A sampling circuit, used for sampling an input voltage signal of the switching power supply;

[0034] A comparison circuit, a first end of which is coupled to the output end of the sampling circuit, and a second end of which is connected to the input undervoltage threshold, for comparing the sampling value of the input voltage signal with the input undervoltage threshold and outputting the result;

[0035] The trigger circuit has an input terminal coupled to the output terminal of the comparison circuit and is used to control the input undervoltage protection of the switching power supply according to the output signal of the comparison circuit.

[0036] In one embodiment, the control circuit further includes:

[0037] A de-jitter time timing circuit, whose input end is coupled to the output end of the comparison circuit, and whose output end is coupled to the input end of the trigger circuit, is used to calculate the duration that the sampling value of the input voltage signal is less than the input under-voltage threshold, so that the trigger circuit triggers the input under-voltage protection of the switching power supply based on the output signal of the comparison circuit and the duration.

[0038] In one embodiment, the control circuit further includes:

[0039] A threshold switching circuit, whose input end is coupled to the output end of the de-jitter time timing circuit, and whose output end is coupled to the second end of the comparison circuit, is used to switch the input under-voltage threshold to a safe value of the input under-voltage threshold and output it to the comparison circuit when the duration of the sampling value of the input voltage signal being less than the input under-voltage threshold reaches the de-jitter time.

[0040] In one embodiment, the control circuit further includes:

[0041] A first de-jitter time timing circuit, whose input end is coupled to the output end of the comparison circuit, and whose output end is coupled to the second de-jitter time timing circuit, is used to calculate the duration of the sampling value of the input voltage signal being less than the input undervoltage threshold;

[0042] The second de-jitter time timing circuit has an input end coupled to the output end of the first de-jitter time timing circuit and an output end coupled to the input end of the trigger circuit, and is used to calculate the duration that the sampling value of the input voltage signal is greater than the safety value of the input undervoltage threshold.

[0043] In one embodiment, the control circuit further includes:

[0044] A threshold switching circuit, whose input end is coupled to the output end of the first de-jitter time timing circuit, and whose output end is coupled to the second end of the comparison circuit, is used to switch the input under-voltage threshold to a safe value of the input under-voltage threshold when the duration of the sampling value of the input voltage signal being less than the input under-voltage threshold reaches the first de-jitter time.

[0045] In one embodiment, the control circuit further includes:

[0046] An input undervoltage protection determination circuit, whose input end is coupled to the output end of the second de-bouncing time timing circuit, and whose output end is coupled to the input end of the trigger circuit, is used to reset the second de-bouncing time timing circuit when the duration of the sampling value of the input voltage signal being greater than the safety value of the input undervoltage threshold reaches the first de-bouncing time, otherwise it is determined that the switching power supply input is undervoltage, and an input undervoltage protection trigger signal of the switching power supply is output.

[0047] The input undervoltage protection control method and control circuit proposed in the present invention can effectively avoid the problem of failing to trigger the input undervoltage protection due to low input voltage oscillation, by triggering the input undervoltage protection when the input voltage sampling value fails to rise above a safe value within a certain period of time after the input voltage decreases and the input voltage sampling value touches the input undervoltage protection threshold, thereby improving the reliability of the switching power supply system. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings are used to provide a further understanding of the present invention and are used together with the description to explain the embodiments of the present invention, but do not constitute a limitation of the present invention. In the accompanying drawings:

[0049] Figure 1 An input undervoltage protection circuit for directly sampling an input voltage signal is shown;

[0050] Figure 2 An input undervoltage protection circuit is shown which feeds back the input voltage signal by sampling the voltage before the rectifier bridge;

[0051] Figure 3 An input undervoltage protection circuit is shown which feeds back the input voltage signal through voltage sampling after the rectifier bridge;

[0052] Figure 4 An input undervoltage protection circuit is shown which detects an input voltage signal through an auxiliary winding;

[0053] Figure 5 The input undervoltage protection circuit is shown by detecting the Drain voltage of the switch tube;

[0054] Figure 6 A schematic diagram of an input undervoltage protection circuit in the prior art is shown;

[0055] Figure 7 The waveform diagram of the input undervoltage protection circuit in the prior art is shown;

[0056] Figure 8 A schematic diagram of bus voltage under different power supply load conditions in the prior art is shown;

[0057] Fig. 9 A schematic diagram showing the same input undervoltage protection triggering threshold under different power supply load conditions in the prior art;

[0058] Fig.10 The schematic diagram of input undervoltage protection triggering logic under low voltage oscillation condition is shown;

[0059] Fig.11 A schematic diagram of input undervoltage protection triggering logic under low voltage conditions according to an embodiment of the present invention is shown;

[0060] Fig.12 A schematic diagram of input undervoltage protection triggering logic under low voltage oscillation conditions according to an embodiment of the present invention is shown;

[0061] Fig.13 A flow chart of an input undervoltage protection control method according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0062] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0063] The description in this section is only for several typical embodiments, and the present invention is not limited to the scope of the embodiments. The combination of different embodiments, the mutual replacement of some technical features in different embodiments, and the mutual replacement of the same or similar prior art means with some technical features in the embodiments are also within the scope of the description and protection of the present invention.

[0064] "Coupled" or "connected" in the specification includes both direct connection and indirect connection. Indirect connection is a connection through an intermediate medium, such as a connection through an electrical conductive medium such as a conductor, wherein the electrical conductive medium may contain parasitic inductance or parasitic capacitance, or a connection through an intermediate circuit or component described in the embodiments in the specification; indirect connection may also include a connection through other active or passive devices on the basis of achieving the same or similar functions, such as a connection through circuits or components such as switches, signal amplification circuits, and follower circuits. "Multiple" or "many" means two or more.

[0065] According to an embodiment of the present invention, an input undervoltage protection control method is used for a switching power supply. Fig.13As shown, the control method comprises the following steps:

[0066] S1, sampling the input voltage signal of the switching power supply to obtain a sampled value Vbus_sense of the input voltage signal.

[0067] S2. Compare the sampling value Vbus_sense of the input voltage signal with the input undervoltage threshold Vbop_in_low. When the sampling value Vbus_sense of the input voltage signal is less than the input undervoltage threshold Vbop_in_low, perform input undervoltage protection triggering determination.

[0068] S3. Controlling input undervoltage protection of the switching power supply according to the determination result.

[0069] In a preferred embodiment, the input undervoltage protection triggering determination includes:

[0070] S2-1, start timing from the time when the sampling value Vbus_sense of the input voltage signal is less than the input undervoltage threshold Vbop_in_low, if the duration of the sampling value Vbus_sense of the input voltage signal being less than the safety value Vbop_in_high of the input undervoltage threshold reaches the preset debounce time Tbop_debounce, then determine that the switching power supply triggers the input undervoltage protection. Fig.11 The figure shows the logic diagram of input undervoltage protection triggering judgment. The dotted line indicates that the switching power supply has not triggered the input undervoltage protection, and the solid line indicates that the switching power supply has triggered the input undervoltage protection. When the sampling value Vbus_sense of the input voltage signal decreases, after the sampling value Vbus_sense of the input voltage signal touches the input undervoltage protection threshold Vbop_in_low, if the sampling value Vbus_sense of the input voltage signal cannot rise to the input undervoltage protection safety value Vbop_in_high (Vbop_in_high is approximately Vbop_in*1.1) within a certain time Tbop_debounce, it is determined that the switching power supply system triggers the input undervoltage protection.

[0071] According to the above input undervoltage protection triggering logic, it can effectively avoid the situation where the input voltage signal repeatedly and briefly touches the input undervoltage protection threshold but fails to trigger the input undervoltage protection when the input voltage fluctuates. Fig.12 As shown in the figure, this logic can provide safer input undervoltage protection for the system when the low voltage oscillates.

[0072] Specifically, the safety value of the input undervoltage threshold Vbop_in_high=the input undervoltage threshold Vbop_in_low*1.1, and the first de-bounce time Tbop_debounce_1≤the second de-bounce time Tbop_debounce_2.

[0073] In another preferred embodiment, Fig.13 As shown, the input undervoltage protection triggering determination includes:

[0074] S2-a, starting timing from when the sampling value Vbus_sense of the input voltage signal is less than the input undervoltage threshold Vbop_in_low; if the duration of the sampling value Vbus_sense of the input voltage signal being less than the input undervoltage threshold Vbop_in_low reaches a preset first de-jitter time Tbop_debounce_1, switching the input undervoltage threshold Vbop_in_low to a safe value Vbop_in_high of the input undervoltage threshold.

[0075] S2-b, then within the second de-bounce time Tbop_debounce_2 after the preset first de-bounce time Tbop_debounce_1, compare the sampling value Vbus_sense of the input voltage signal with the safety value Vbop_in_high of the input under-voltage threshold, and determine whether the switching power supply triggers the input under-voltage protection according to the comparison result.

[0076] S2-c. Repeat S2-a and S2-b.

[0077] Specifically, the safety value of the input undervoltage threshold=the input undervoltage threshold*1.1, and the first debounce time Tbop_debounce_1≤the second debounce time Tbop_debounce_2.

[0078] In a specific embodiment, Fig.12 As shown, the step of determining whether the switching power supply triggers input undervoltage protection according to the comparison result includes:

[0079] S2-b-1. Within the preset second de-bouncing time Tbop_debounce_2, if the duration of the sampling value Vbus_sense of the input voltage signal being greater than the safety value Vbop_in_high of the input under-voltage threshold reaches the preset first de-bouncing time Tbop_debounce_1, the second de-bouncing time Tbop_debounce_2 timing is reset, and the safety value Vbop_in_high of the input under-voltage threshold is switched to the input under-voltage threshold Vbop_in_low; otherwise, it is determined that the switching power supply triggers the input under-voltage protection.

[0080] According to another embodiment of the present invention, an input undervoltage protection control circuit is used for a switching power supply. The control circuit includes:

[0081] A sampling circuit, used for sampling an input voltage signal of the switching power supply;

[0082] A comparison circuit, a first end of which is coupled to the output end of the sampling circuit, and a second end of which is connected to the input undervoltage threshold, for comparing the sampling value of the input voltage signal with the input undervoltage threshold;

[0083] The trigger circuit has an input terminal coupled to the output terminal of the comparison circuit and is used to control the input undervoltage protection of the switching power supply according to the output signal of the comparison circuit.

[0084] In a preferred embodiment, the control circuit further includes:

[0085] A de-jitter time timing circuit, whose input end is coupled to the output end of the comparison circuit, and whose output end is coupled to the input end of the trigger circuit, is used to calculate the duration that the sampling value of the input voltage signal is less than the input under-voltage threshold, so that the trigger circuit triggers the input under-voltage protection of the switching power supply according to the output signal of the comparison circuit and the duration.

[0086] Furthermore, in a specific embodiment, the control circuit further includes:

[0087] A threshold switching circuit, whose input end is coupled to the output end of the de-jitter time timing circuit, and whose output end is coupled to the second end of the comparison circuit, is used to switch the input under-voltage threshold to a safe value of the input under-voltage threshold and output it to the comparison circuit when the duration of the sampling value of the input voltage signal being less than the input under-voltage threshold reaches a preset de-jitter time.

[0088] In another preferred embodiment, the control circuit further includes:

[0089] A first de-jitter time timing circuit, whose input end is coupled to the output end of the comparison circuit, and whose output end is coupled to the second de-jitter time timing circuit, is used to calculate the duration of the sampling value of the input voltage signal being less than the input undervoltage threshold;

[0090] The second de-jitter time timing circuit has an input end coupled to the output end of the first de-jitter time timing circuit and an output end coupled to the input end of the trigger circuit, and is used to calculate the duration that the sampling value of the input voltage signal is greater than the safety value of the input undervoltage threshold.

[0091] Furthermore, in a specific embodiment, the control circuit further includes:

[0092] A threshold switching circuit, whose input end is coupled to the output end of the first de-jitter time timing circuit, and whose output end is coupled to the second end of the comparison circuit, is used to switch the input under-voltage threshold to a safe value of the input under-voltage threshold when the duration of the sampling value of the input voltage signal being less than the input under-voltage threshold reaches a preset first de-jitter time.

[0093] Furthermore, in a specific embodiment, the control circuit further includes:

[0094] An input undervoltage protection determination circuit, whose input end is coupled to the output end of the second de-jitter time timing circuit, and whose output end is coupled to the input end of the trigger circuit, is used to reset the second de-jitter time timing circuit when the duration of the sampling value of the input voltage signal being greater than the safety value of the input undervoltage threshold reaches a preset first de-jitter time, otherwise it is determined that the switching power supply input is undervoltage, and an input undervoltage protection trigger signal of the switching power supply is output.

[0095] In another preferred embodiment, the sampling circuit directly samples the input voltage signal (such as Figure 1 As shown), the input voltage signal is fed back through the voltage sampling before the rectifier bridge (as shown Figure 2 As shown), the voltage is sampled and fed back to the input voltage signal after the rectifier bridge (as shown Figure 3 As shown), the input voltage signal is detected by the auxiliary winding (such as Figure 4 as shown), or by detecting the switch tube Drain voltage (as shown Figure 5 As shown) to obtain the sampling value of the input voltage signal of the switching power supply.

[0096] Based on the above input undervoltage protection control circuit, when the input voltage decreases and the input voltage sampling value touches the input undervoltage protection threshold, and the input voltage sampling value cannot rise above a certain safety value within a certain period of time, the switching power supply system is controlled to trigger the input undervoltage protection, which can avoid the problem of being unable to trigger the BOP due to low input voltage oscillation, while improving system reliability.

[0097] Those skilled in the art should know that the logic controls such as "high level" and "low level", "set" and "reset", "AND gate" and "OR gate", "non-inverting input terminal" and "inverting input terminal" in the logic controls involved in the specification or drawings can be interchanged or changed with each other, and the same function or purpose as the above embodiment can be achieved by adjusting the subsequent logic controls.

[0098] The description and application of the present invention here are illustrative, and it is not intended to limit the scope of the present invention to the above-mentioned embodiments. The effects or advantages involved in the specification may not be reflected in the actual experimental examples due to the uncertainty of specific condition parameters or other factors, and the effects or advantages are not used to limit the scope of the invention. The deformation and change of the embodiments disclosed here are possible, and the replacement of the embodiments and the various equivalent parts are well known to those of ordinary skill in the art. It should be clear to those skilled in the art that the present invention can be implemented in other forms, structures, arrangements, proportions, and with other components, materials and parts without departing from the spirit or essential features of the present invention. Other deformations and changes can be made to the embodiments disclosed here without departing from the scope and spirit of the present invention.

Claims

1. An input undervoltage protection control method for a switching power supply, characterized in that: include: Sampling an input voltage signal of the switching power supply; Comparing the sampling value of the input voltage signal with the input undervoltage threshold, and when the sampling value of the input voltage signal is less than the input undervoltage threshold, performing input undervoltage protection triggering judgment; The input undervoltage protection of the switching power supply is controlled according to the determination result.

2. The input undervoltage protection control method according to claim 1, characterized in that: The input undervoltage protection triggering determination includes: The timing starts when the sampling value of the input voltage signal is less than the input under-voltage threshold. If the duration of the sampling value of the input voltage signal being less than the safety value of the input under-voltage threshold reaches the de-jitter time, it is determined that the switching power supply triggers the input under-voltage protection.

3. The input undervoltage protection control method according to claim 1 or 2, characterized in that: The input undervoltage protection triggering determination includes: Starting from the time when the sampling value of the input voltage signal is less than the input undervoltage threshold, if the duration of the sampling value of the input voltage signal being less than the input undervoltage threshold reaches a first de-jitter time, switching the input undervoltage threshold to a safe value of the input undervoltage threshold; In a second de-jitter time after the first de-jitter time, the sampling value of the input voltage signal is compared with the safety value of the input under-voltage threshold, and it is determined whether the switching power supply triggers the input under-voltage protection according to the comparison result.

4. The input undervoltage protection control method according to claim 3, characterized in that: Determining whether the switching power supply triggers input undervoltage protection according to the comparison result includes: During the second de-bouncing time, if the sampling value of the input voltage signal is greater than the safety value of the input under-voltage threshold for a duration that reaches the preset first de-bouncing time, the second de-bouncing time timing is reset, and the safety value of the input under-voltage threshold is switched to the input under-voltage threshold; otherwise, it is determined that the switching power supply triggers the input under-voltage protection.

5. The input undervoltage protection control method according to any one of claims 2 to 4, characterized in that: The safety value of the input undervoltage threshold=the input undervoltage threshold*1.

1.

6. The input undervoltage protection control method according to claim 3 or 4, characterized in that: The first de-jitter time≤the second de-jitter time.

7. An input undervoltage protection control circuit for a switching power supply, characterized in that: include: A sampling circuit, used for sampling an input voltage signal of the switching power supply; A comparison circuit, a first end of which is coupled to the output end of the sampling circuit, and a second end of which is connected to the input undervoltage threshold, for comparing the sampling value of the input voltage signal with the input undervoltage threshold and outputting the result; The trigger circuit has an input terminal coupled to the output terminal of the comparison circuit and is used to control the input undervoltage protection of the switching power supply according to the output signal of the comparison circuit.

8. The input undervoltage protection control circuit according to claim 7, characterized in that: The control circuit further comprises: A de-jitter time timing circuit, whose input end is coupled to the output end of the comparison circuit, and whose output end is coupled to the input end of the trigger circuit, is used to calculate the duration that the sampling value of the input voltage signal is less than the input under-voltage threshold, so that the trigger circuit triggers the input under-voltage protection of the switching power supply based on the output signal of the comparison circuit and the duration.

9. The input undervoltage protection control circuit according to claim 8, characterized in that: The control circuit further comprises: A threshold switching circuit, whose input end is coupled to the output end of the de-jitter time timing circuit, and whose output end is coupled to the second end of the comparison circuit, is used to switch the input under-voltage threshold to a safe value of the input under-voltage threshold and output it to the comparison circuit when the duration of the sampling value of the input voltage signal being less than the input under-voltage threshold reaches the de-jitter time.

10. The input undervoltage protection control circuit according to claim 7, characterized in that: The control circuit further comprises: A first de-jitter time timing circuit, whose input end is coupled to the output end of the comparison circuit, and whose output end is coupled to the second de-jitter time timing circuit, is used to calculate the duration of the sampling value of the input voltage signal being less than the input undervoltage threshold; The second de-jitter time timing circuit has an input end coupled to the output end of the first de-jitter time timing circuit and an output end coupled to the input end of the trigger circuit, and is used to calculate the duration that the sampling value of the input voltage signal is greater than the safety value of the input undervoltage threshold.

11. The input undervoltage protection control circuit according to claim 10, characterized in that: The control circuit further comprises: A threshold switching circuit, whose input end is coupled to the output end of the first de-jitter time timing circuit, and whose output end is coupled to the second end of the comparison circuit, is used to switch the input under-voltage threshold to a safe value of the input under-voltage threshold when the duration of the sampling value of the input voltage signal being less than the input under-voltage threshold reaches the first de-jitter time.

12. The input undervoltage protection control circuit according to claim 10 or 11, characterized in that: The control circuit further comprises: An input undervoltage protection determination circuit, whose input end is coupled to the output end of the second de-bouncing time timing circuit, and whose output end is coupled to the input end of the trigger circuit, is used to reset the second de-bouncing time timing circuit when the duration of the sampling value of the input voltage signal being greater than the safety value of the input undervoltage threshold reaches the first de-bouncing time, otherwise it is determined that the switching power supply input is undervoltage, and an input undervoltage protection trigger signal of the switching power supply is output.

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