A flyback circuit and control method thereof

Abstract

The present invention discloses a flyback circuit and control method thereof, comprising a main power tube connected to a primary winding. The flyback circuit receives an input power supply to generate a supply voltage to power a switch control circuit of the flyback circuit. After a period of time after detecting a fault trigger signal, the supply voltage is pulled down to a shutdown threshold. When the fault trigger signal is detected, the main power tube is controlled to stop switching. When the supply voltage rises again to the switch-on threshold, the main power tube is controlled to switch normally. The present invention simplifies control and can prevent the main power tube from switching erroneously after an input power outage.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a flyback circuit and a control method thereof. Background Art

[0002] In the prior art, when a flyback circuit has an overvoltage or overcurrent fault, the flyback circuit needs to be protected. The general protection response mechanism is as follows: Figure 1 When a fault signal Fault occurs, the power supply circuit of the flyback circuit is charged with high voltage startup to generate a power supply voltage VCC greater than the shutdown threshold VCC_OFF, and the power tube is controlled to stop switching; after a period of time t FLT After that, the flyback circuit generates a pulse signal DRV to drive the power tube to resume normal operation.

[0003] However, this protective response mechanism exists Figure 2 The problem shown is that for wide-range output applications, the high-voltage startup charger is connected to the AC input power supply side. When the high-voltage output is in progress, the voltage VCC at the power supply point is very high. After the input power AC is powered off, the bus voltage Vbulk will drop. The system detects the fault and starts timing. Since the input power AC has been removed and there is no switch, there is no power source. FLT During this period, the supply voltage VCC will drop out. However, if the supply voltage does not drop to the shutdown threshold VCC_OFF after the timer expires, the controller will restart and generate a normal pulse drive signal. At this time, the secondary-side controller will have secondary-side stress problems. If used with a PFC circuit, the bus voltage Vbulk has not dropped to zero yet, and the PFC circuit can still operate normally for a period of time. This is unacceptable in practice (as shown by the dotted line in the figure). Summary of the Invention

[0004] The object of the present invention is to provide a flyback circuit with simple protection and a control method thereof, so as to solve the problem in the prior art that a switch drive signal is still generated after the input power is lost.

[0005] To achieve the above object, the present invention provides a flyback circuit, which includes a main power tube connected to the primary winding, and further includes:

[0006] a power supply circuit, receiving an input power supply to generate a power supply voltage to supply power to the switch control circuit of the flyback circuit;

[0007] A protection circuit, after detecting a fault trigger signal for a period of time, pulls the supply voltage down to a shutdown threshold;

[0008] The switch control circuit controls the main power tube to stop switching when the fault trigger signal is detected; and controls the main power tube to switch normally when the supply voltage rises to the turn-on threshold again.

[0009] Optionally, the fault trigger signal is a trigger signal generated when overvoltage, undervoltage, overcurrent, overpower or overtemperature occurs in the flyback circuit.

[0010] Optionally, the protection circuit includes:

[0011] a timing circuit, configured to start timing when a fault trigger signal is detected, and end timing when the timing time reaches a threshold time, during which the main power tube stops switching;

[0012] The pull-down circuit pulls down the supply voltage to the shutdown threshold when the timing ends.

[0013] Optionally, the pull-down circuit includes:

[0014] a first switch tube, having a first end connected to an output end of a power supply circuit, the power supply circuit outputting the power supply voltage, a second end connected to a ground, and a control end connected to an output end of the timing circuit; when the timing circuit ends, the timing circuit outputs a conduction trigger signal to control the first switch tube to conduct;

[0015] A first operational amplifier has a first input terminal connected to the output terminal of the power supply circuit, a second input terminal receiving the shutdown threshold, and an output signal used to reset the timing circuit.

[0016] Optionally, if at the end of the timing, the input end of the flyback circuit normally receives the input power, the input power charges the power supply circuit so that the supply voltage rises to reach the start-up threshold.

[0017] The present invention also provides a control method for a flyback circuit, wherein the flyback circuit includes a main power tube connected to a primary winding, receiving an input power supply to generate a power supply voltage to power a switch control circuit of the flyback circuit;

[0018] After a period of time after detecting the fault trigger signal, the supply voltage is pulled down to the shutdown threshold;

[0019] When a fault trigger signal is detected, the main power tube is controlled to stop switching, and when the supply voltage rises to the turn-on threshold again, the main power tube is controlled to switch normally.

[0020] Optionally, the fault trigger signal is a trigger signal generated when input power failure, overvoltage, undervoltage, overcurrent, overpower or overtemperature occurs in the flyback circuit.

[0021] Optionally, timing starts when a fault trigger signal is detected, and ends when the timing time reaches a threshold time. During the timing period, the main power tube stops switching, and the supply voltage is greater than a shutdown threshold.

[0022] When the timing ends, the supply voltage is pulled down to the shutdown threshold.

[0023] Optionally, if at the end of the timing, the input end of the flyback circuit normally receives the input power, the supply voltage rises until it reaches the turn-on threshold.

[0024] Optionally, if the input end of the flyback circuit is not connected to the input power supply at the end of the timing, the supply voltage drops from the shutdown threshold.

[0025] Compared with existing technologies, the technical solution of the present invention has the following advantages: it receives input power to generate a supply voltage to power the flyback circuit; after a period of time after detecting a fault trigger signal, it pulls the supply voltage down to a shutdown threshold; when the fault trigger signal is detected, it controls the main power tube to stop switching, and when the supply voltage rises back to the start threshold, it controls the main power tube to switch normally. The present invention has simple control and a safe protection circuit, and it can prevent the main power tube from switching erroneously after an input power outage. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a protection response waveform diagram of the prior art flyback circuit when the input is normal;

[0027] Figure 2 This is a protection response waveform diagram of the prior art flyback circuit when the input is powered off;

[0028] Figure 3 This is a schematic diagram of the flyback circuit of the present invention;

[0029] Figure 4 This is a protection response waveform diagram of the flyback circuit of the present invention when the input is normal;

[0030] Figure 5 This is a protection response waveform diagram of the flyback circuit of the present invention when the input is powered off;

[0031] Figure 6 It is a schematic diagram of the protection circuit of the present invention. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the present invention is not limited to these embodiments and covers any substitution, modification, equivalent method and solution made within the spirit and scope of the present invention.

[0033] In order to make the public have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, but those skilled in the art can also fully understand the present invention without description of these details.

[0034] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the drawings are simplified and not to exact proportions, and are only used for the purpose of conveniently and clearly illustrating the embodiments of the present invention.

[0035] like Figure 3 As shown, the schematic diagram of the flyback circuit of the present invention is shown, which includes a power supply circuit, a protection circuit and a switch control circuit. The AC input power supply charges the power supply circuit to generate a power supply voltage VCC for powering the controller of the flyback circuit. The controller includes a switch control circuit and a protection circuit. After receiving the fault trigger signal Fault, the power tube stops switching for a period of time t FLT The power supply voltage VCC is then pulled down to the shutdown threshold VCC_OFF. The switch control circuit receives the power supply voltage VCC. When the power supply voltage VCC rises again to the turn-on threshold VCC_ON, the main power transistor is controlled to switch normally. The protection circuit of the present invention only needs to set two thresholds: the turn-on threshold VCC_ON and the turn-off threshold VCC_OFF to protect and restart the controller. The fault trigger signal Fault is a trigger signal generated when the flyback circuit experiences input power failure, undervoltage, overvoltage, overcurrent, short circuit, overpower, or overtemperature.

[0036] like Figure 4 As shown in the figure, the protection response waveform of the present invention when the input is normal is shown. After receiving the fault trigger signal Fault, no pulse drive signal DRV is generated, the flyback circuit stops switching, and the power supply voltage VCC is charged to a value greater than the turn-on threshold VCC_ON. FLT Then the supply voltage VCC is pulled down to the turn-off threshold VCC_OFF. When the supply voltage VCC is charged to the turn-on threshold VCC_ON again, a normal pulse drive signal DRV is generated, and the main power tube switches normally.

[0037] like Figure 5 As shown in the figure, the protection response waveform of the present invention when the input power is lost is shown. After the input end of the flyback circuit loses power, the bus voltage continues to drop. After receiving the fault trigger signal Fault, because there is no power source, the power supply voltage VCC continues to drop. FLT After that, the power supply voltage VCC is further pulled down to the shutdown threshold VCC_OFF, and then continues to drop, and the power tube will not switch normally again. Figure 4 and Figure 5 As shown, the present invention can avoid the problem of the power tube working again after the input power is cut off, thereby protecting the safety of the system.

[0038] like Figure 6As shown, the protection circuit schematic diagram of the present invention is shown, which includes a trigger U01, a timing circuit U02, a trigger U03, a switch tube M1 and an operational amplifier U04. When a fault trigger signal FAULT is detected, the trigger U01 outputs a high level to control the timing circuit U02 to be enabled, and the timing circuit U02 starts timing. When the timing time reaches the threshold time t FLT When the switch tube M1 is turned on, a high-level signal is output to the set terminal of the trigger U03, and the trigger U03 outputs a high-level signal to control the switch tube M1 to turn on. The drain of the switch tube M1 is connected to the output terminal of the power supply circuit, and its source is grounded. The inverting input terminal of the operational amplifier U04 is connected to the switch drain, its non-inverting input terminal receives the shutdown threshold VCC_OFF, and its output terminal is connected to the reset terminal of the trigger U01 and U03. When the switch tube M1 is turned on, the power supply voltage output by the power supply circuit is pulled down to the shutdown threshold VCC_OFF. The present invention is turned on for a period of time t FLT The main power tube stops switching, and the timing circuit is used to ensure that the time t FLT Controllable.

[0039] In addition, although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that they can be replaced and integrated between the embodiments. For content that is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0040] The above-described embodiments do not constitute a limitation on the scope of protection of this technical solution. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the above-described embodiments shall be included in the scope of protection of this technical solution.

Claims

1. A flyback circuit, comprising a main power transistor connected to a primary winding, characterized in that: Also includes, a power supply circuit, receiving an input power supply to generate a power supply voltage to supply power to the switch control circuit of the flyback circuit; A protection circuit, after detecting a fault trigger signal for a period of time, pulls the supply voltage down to a shutdown threshold; The switch control circuit controls the main power tube to stop switching when the fault trigger signal is detected; and controls the main power tube to switch normally when the supply voltage rises to the turn-on threshold again.

2. The flyback circuit according to claim 1, wherein: The fault trigger signal is a trigger signal generated when input power failure, overvoltage, undervoltage, overcurrent, overpower or overtemperature occurs in the flyback circuit.

3. The flyback circuit according to claim 1, wherein: The protection circuit includes: a timing circuit, configured to start timing when a fault trigger signal is detected, and end timing when the timing time reaches a threshold time, during which the main power tube stops switching; The pull-down circuit pulls down the supply voltage to the shutdown threshold when the timing ends.

4. The flyback circuit according to claim 3, wherein: The pull-down circuit includes: a first switch tube, having a first end connected to an output end of a power supply circuit, the power supply circuit outputting the power supply voltage, a second end connected to a ground, and a control end connected to an output end of the timing circuit; when the timing circuit ends, the timing circuit outputs a conduction trigger signal to control the first switch tube to conduct; A first operational amplifier has a first input terminal connected to the output terminal of the power supply circuit, a second input terminal receiving the shutdown threshold, and an output signal used to reset the timing circuit.

5. The flyback circuit according to claim 1, wherein: If, at the end of the timing, the input end of the flyback circuit receives the input power normally, the input power charges the power supply circuit so that the supply voltage rises to reach the start-up threshold.

6. A control method for a flyback circuit, the flyback circuit comprising a main power transistor connected to a primary winding, characterized in that: receiving an input power supply to generate a supply voltage to power a switch control circuit of the flyback circuit; After a period of time after detecting the fault trigger signal, the supply voltage is pulled down to the shutdown threshold; When a fault trigger signal is detected, the main power tube is controlled to stop switching, and when the supply voltage rises to the turn-on threshold again, the main power tube is controlled to switch normally.

7. The control method of the flyback circuit according to claim 6, wherein: The fault trigger signal is a trigger signal generated when input power failure, overvoltage, undervoltage, overcurrent, overpower or overtemperature occurs in the flyback circuit.

8. The control method of the flyback circuit according to claim 6, wherein: When a fault trigger signal is detected, timing starts, and when the timing time reaches a threshold time, timing ends. During the timing period, the main power tube stops switching, and the supply voltage is greater than the shutdown threshold; When the timing ends, the supply voltage is pulled down to the shutdown threshold.

9. The control method of the flyback circuit according to claim 6, wherein: If, at the end of the timing, the input end of the flyback circuit receives the input power normally, the supply voltage rises until it reaches the turn-on threshold.

10. The control method of the flyback circuit according to claim 6, wherein: If the input end of the flyback circuit is not connected to the input power supply at the end of the timing, the supply voltage drops from the shutdown threshold.

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