Self-powered circuit of synchronous rectifier, synchronous rectifier and flyback converter

Abstract

The invention provides a self-powered circuit of a synchronous rectifier, the synchronous rectifier and a flyback converter, and the self-powered circuit comprises a first power tube with a first end connected with the output end of the flyback converter and a second end connected with the power supply end of the synchronous rectifier; and the first driving unit generates control end voltage of the first power tube according to the voltage of a first node, so that the first power tube is conducted, and the first node is a connection point of the synchronous rectifier tube and the secondary winding. According to the self-powered circuit, the first power tube is driven to be conducted through the voltage of the first node, so that the situation that the power supply end cannot take electricity due to the fact that the first power tube cannot be driven to be conducted when the voltage of the output end is small is avoided. Meanwhile, in the power taking process of the power supply end, the first diode for preventing the current from flowing backwards is short-circuited, so that the voltage difference limit between the output end and the power supply end is further reduced.

Description

Technical Field

[0001] The invention belongs to the field of switching power supplies, and in particular relates to a self-powered circuit of a synchronous rectifier, a synchronous rectifier and a flyback converter. Background Art

[0002] In a flyback converter, the synchronous rectifier draws power from the low voltage output of the system to achieve higher efficiency. Figure 1 As shown, the prior art discloses a method such as Figure 1 The power supply circuit shown in the figure takes power from the low voltage output end of the system to charge the power supply capacitor of the synchronous rectifier. Figure 1 It can be seen that its working principle is: when DZ is not broken down and turned on, resistor R1 pulls up the gate voltage of M1 to turn on M1, and the power supply terminal VCC of the synchronous rectifier draws power from the low-voltage output terminal Vo. At the same time, during the power-drawing process, if the current is too large, T1 is turned on, which reduces the VGS of M1 and the current, thus achieving a current limiting effect. When DZ breaks down and turns on, the gate voltage of M1 is pulled down, M1 is turned off, and the power supply terminal VCC of the synchronous rectifier stops drawing power.

[0003] In summary, if the synchronous rectifier needs to draw power from the low-voltage output terminal, there is a minimum voltage difference between Vo and VCC. Specifically, Vo must be greater than the sum of VCC, the conduction voltage drop of D1, and the conduction voltage drop of M1, that is, Vo>VCC+VD1+Vth. Therefore, when the output voltage of the system is low, the synchronous rectifier will be unable to draw power from the output terminal Vo. Summary of the invention

[0004] In order to solve the technical problem in the prior art that the voltage difference between the output end voltage of the system and the power supply end voltage of the synchronous rectifier is small, resulting in the inability of the synchronous rectifier to obtain power from the output end, the present invention proposes a self-powered circuit of a synchronous rectifier, a synchronous rectifier and a flyback converter, wherein the self-powered circuit includes:

[0005] A first power tube, a first end of which is connected to the output end of the flyback converter, and a second end of which is connected to the power supply end of the synchronous rectifier.

[0006] a first driving unit, generating a control terminal voltage of the first power tube according to a voltage of the first node, so as to turn on the first power tube;

[0007] The first node is a connection point between the synchronous rectifier and the secondary winding.

[0008] Furthermore, it also includes: a first diode and a first switch, the positive electrode of the first diode is connected to the second end of the first power tube, and the negative electrode is connected to the power supply end, the first switch is connected in parallel with the first diode, and the first switch is closed when the first power tube is turned on.

[0009] Furthermore, it also includes: a second driving unit, which generates a control terminal voltage of the first power tube according to the voltage of the output terminal to turn on the first power tube.

[0010] Furthermore, it also includes: a current limiting unit, connected between the first power tube and the power supply end, when the current flowing through the first power tube is greater than a preset current, pulling down the control terminal voltage of the first power tube to reduce the current flowing through the first power tube.

[0011] Preferably, the first driving unit includes: a first pull-up resistor and a second switch, the first pull-up resistor connects the first node and the control end of the first power tube, and the second switch connects the control end of the first power tube and the second end of the first power tube.

[0012] Preferably, the second driving unit includes: a second pull-up resistor and a second switch, the second pull-up resistor connects the output end and the control end of the first power tube, and the second switch connects the control end of the first power tube and the second end of the first power tube.

[0013] Preferably, the current limiting unit includes: a second power tube and a current limiting resistor, the current limiting resistor is connected to the second end of the first power tube and the power supply end, the control end of the second power tube is connected to the second end of the first power tube, the first end is connected to the control end of the first power tube, and the second end is connected to the power supply end.

[0014] Further, when the synchronous rectifier is turned on, the voltage of the first node is greater than the voltage of the output end, and the first driving unit controls the first power tube to be turned on;

[0015] When the synchronous rectifier is turned off and the difference between the voltage at the output end and the voltage at the power supply end is greater than a preset voltage difference, the second driving unit controls the first power tube to be turned on.

[0016] A synchronous rectifier comprises the self-powered circuit described above.

[0017] A flyback converter comprises the synchronous rectifier mentioned above.

[0018] The self-powered circuit proposed by the present invention drives the first power tube to conduct through the voltage of the first node, so as to avoid the failure to drive the first power tube to conduct when the output terminal voltage is small, so that the power supply terminal cannot draw power. At the same time, in the process of drawing power at the power supply terminal, the first diode that prevents current backflow is short-circuited, so that the voltage difference limit between the output terminal and the power supply terminal is further reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1is a structural diagram of a synchronous rectifier in a flyback converter;

[0020] Figure 2 A self-powered circuit for a synchronous rectifier in the prior art;

[0021] Figures 3 to 5 A structural diagram of a self-powered circuit of a synchronous rectifier proposed by the present invention;

[0022] Figure 6 This is a circuit diagram of the self-powered circuit proposed by the present invention. DETAILED DESCRIPTION

[0023] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0024] like Figure 2 In the self-powered circuit of the synchronous rectifier shown, when the voltage difference between the output terminal voltage and the power supply terminal of the flyback converter is small, the synchronous rectifier cannot draw power from the output terminal. In order to solve this problem, the present invention proposes a self-powered circuit of the synchronous rectifier to achieve that the synchronous rectifier can still draw power from the output terminal when the voltage difference between the output terminal and the power supply terminal is small.

[0025] Specifically, Figure 3 As shown, the self-powered circuit of the synchronous rectifier proposed by the present invention includes:

[0026] A first power tube M1, wherein a first end of the first power tube M1 is connected to an output end Vo of the flyback converter, and a second end of the first power tube M1 is connected to a power supply end Vcc of the synchronous rectifier;

[0027] A first driving unit generates a control terminal voltage of the first power tube M1 according to a voltage of a first node SW to turn on the first power tube M1. The first node SW is a connection point between the synchronous rectifier tube and the secondary winding;

[0028] When the synchronous rectifier is turned on, the voltage of the first node SW is greater than the voltage of the output terminal Vo. If the power supply terminal Vcc needs to draw power, the first driving unit is controlled to drive the first power tube SW to turn on, and the power supply terminal Vcc draws power from the output terminal Vo.

[0029] In summary, the self-powered circuit proposed in the present invention utilizes the voltage of the first node in the flyback converter to bootstrap the control terminal voltage of the first power tube to a voltage higher than the output terminal voltage, thereby driving the first power tube to turn on. Therefore, the conduction of the first power tube is no longer limited by the voltage difference between the output terminal and the power supply terminal. Therefore, when the output terminal voltage is very small, the first power tube can still be turned on and the power supply terminal can still draw power from the output terminal.

[0030] Further, such as Figure 3 As shown, in order to prevent the power supply terminal Vcc from backflowing current to the output terminal Vo, the self-power supply circuit further includes a first diode D1 connected between the first power tube M1 and the power supply terminal Vcc, the cathode of the first diode D1 is connected to the power supply terminal Vcc, and the anode is connected to the first power tube M1. However, considering that when the power supply terminal Vcc is powered, the voltage difference limit between the first node SW and the power supply terminal Vcc is small, which is beneficial for the first driving unit to drive the first power tube M1 to turn on, the first diode D1 is also provided with a first switch K1 in parallel, and when the first power tube M1 is turned on, the first switch K1 is closed, so that the voltage difference limit between the first node SW and the power supply terminal Vcc is reduced when the power supply terminal Vcc is powered.

[0031] Further, such as Figure 4 As shown, the self-powered circuit proposed in the present invention also includes a second driving unit. When the synchronous rectifier is turned off and the voltage difference between the output terminal Vo and the power supply terminal Vcc is greater than the conduction voltage drop of the first power tube M1, if the power supply terminal Vcc needs to take power, the second driving unit is controlled to generate the control terminal voltage of the first power tube M1 according to the voltage of the output terminal Vo, so that the first power tube M1 is turned on. When the synchronous rectifier is turned off, the voltage of the first node SW is zero, so the first driving unit cannot drive the first power tube M1 to turn on during the period when the synchronous rectifier is turned off. During this period, the existing scheme can be retained, and the voltage of the output terminal Vo is used to drive the first power tube M1 to turn on. It should be noted that the second driving unit does not play a major role in controlling the power supply terminal Vcc to take power. In actual design, it is also possible not to retain the existing scheme, and the power supply terminal Vcc can meet the power supply terminal Vcc power demand. In addition, when the second driving unit in this scheme drives the first power tube M1 to turn on, the voltage difference between the output terminal Vo and the power supply terminal Vcc only needs to be greater than the conduction voltage drop of the first power tube M1. Compared with the prior art, in this solution, when the second driving unit drives the first power tube M1 to turn on, the first diode D1 is short-circuited, and the conduction voltage drop of the first diode D1 does not affect the power supply terminal Vcc from drawing power from the output terminal Vo, so the voltage difference limit for driving the first power tube M1 to turn on according to the output terminal Vo voltage will be smaller.

[0032] Further, such as Figure 5As shown, the self-powered circuit also includes a current limiting unit, which is connected between the first power tube M1 and the power supply terminal Vcc. When the current of the first power tube M1 is greater than the preset current, the current limiting unit pulls down the control terminal voltage of the first power tube M1 to reduce the current of the first power tube M1, thereby performing overcurrent protection in the process of drawing power from the power supply terminal Vcc.

[0033] Preferably, in one embodiment, Figure 6 As shown, the first driving unit includes a first pull-up resistor R1 and a second switch K2, the second driving unit includes a second pull-up resistor R2 and a second switch K2, and the current limiting unit includes a second power tube M2 and a current limiting resistor R3. The specific connections are shown in the figure and are not described here.

[0034] When the synchronous rectifier is turned on, that is, the voltage of the first node SW is greater than the output terminal voltage Vo. If the voltage of the power supply terminal Vcc is less than the preset voltage at this time, that is, the power supply terminal Vcc needs to draw power, so the second switch K2 is controlled to be disconnected and the first switch K1 is closed, then the first power tube M1 is driven to turn on by the voltage of the first node SW, and the power supply terminal Vcc draws power from the output terminal Vo. In the process of drawing power from the power supply terminal Vcc, if the current is too large, the second power tube M2 is turned on, so that the gate voltage of the first power tube M1 is pulled down and the power supply current is reduced. When the voltage of the power supply terminal Vcc gradually rises to be greater than or equal to the preset voltage, the self-power supply circuit controls the second switch K2 to be turned on and the first switch K1 to be disconnected, then the first power tube M1 is turned off due to the gate voltage being pulled down, and the power supply terminal Vcc is controlled to stop drawing power.

[0035] When the synchronous rectifier is turned off and the voltage difference between the output voltage Vo and the power supply end Vcc is greater than the conduction voltage drop of the first power tube M1, if the voltage of the power supply end Vcc is less than the preset voltage, that is, the power supply end Vcc needs to draw power, so the second switch K2 is controlled to be disconnected and the first switch K1 is closed, then the first power tube M1 is driven to turn on by the output end Vcc, and the power supply end Vcc draws power from the output end Vo. Similarly, in the process of drawing power from the power supply end Vcc, if the current is too large, the second power tube M2 is turned on, so that the gate voltage of the first power tube M1 is pulled down and the power supply current is reduced. When the voltage of the power supply end Vcc gradually rises to be greater than or equal to the preset voltage, the self-power supply circuit controls the second switch K2 to be turned on and the first switch K1 to be disconnected, then the first power tube M1 is turned off due to the gate voltage being pulled down, and the power supply end Vcc is controlled to stop drawing power.

[0036] In summary, the self-powered circuit proposed in the present invention drives the first power tube M1 to turn on through the voltage of the first node SW, so as to avoid the situation that the first power tube M1 cannot be driven to turn on when the voltage of the output terminal Vo is small, so that the power supply terminal Vcc cannot be powered. At the same time, in the process of powering the power supply terminal Vcc, the voltage difference limit between the output terminal Vo and the power supply terminal Vcc is further reduced by short-circuiting the first diode D1.

[0037] It should be noted that, in theory, a charge pump can be used to replace the voltage of the first node to drive the first power tube M1 to turn on. However, using the voltage of the first node to drive the first power tube does not require additional circuits, and only requires the existing settings in the flyback converter, which can effectively reduce costs.

[0038] The present invention also provides a synchronous rectifier, which includes the self-power supply circuit provided above.

[0039] The present invention further provides a flyback converter, which includes the synchronous rectifier mentioned above.

[0040] It should be noted that the specific implementation and corresponding illustrations given are merely a way of describing the implementation method of the present invention, and do not limit the specific structure of the implementation scheme of the present invention. Various changes or modifications can be made to these implementation schemes without departing from the principle and essence of the present invention, but these changes and modifications fall within the scope of protection of the present invention.

[0041] 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 the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0042] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. A self-powered circuit of a synchronous rectifier, characterized in that: include: A first power tube, a first end of which is connected to the output end of the flyback converter, and a second end of which is connected to the power supply end of the synchronous rectifier. a first driving unit, generating a control terminal voltage of the first power tube according to a voltage of the first node, so as to turn on the first power tube; The first node is a connection point between the synchronous rectifier and the secondary winding.

2. The self-powered circuit according to claim 1, characterized in that: Also includes: A first diode and a first switch, wherein the positive electrode of the first diode is connected to the second end of the first power tube, and the negative electrode is connected to the power supply end, the first switch is connected in parallel with the first diode, and the first switch is closed when the first power tube is turned on.

3. The self-powered circuit according to claim 1, characterized in that: Also includes: The second driving unit generates a control terminal voltage of the first power tube according to the voltage of the output terminal, so as to turn on the first power tube.

4. The self-powered circuit according to claim 1, characterized in that: Also includes: The current limiting unit is connected between the first power tube and the power supply end, and when the current flowing through the first power tube is greater than a preset current, the control terminal voltage of the first power tube is pulled down to reduce the current flowing through the first power tube.

5. The self-powered circuit according to claim 1, characterized in that: The first driving unit includes: a first pull-up resistor and a second switch, the first pull-up resistor connects the first node and the control end of the first power tube, and the second switch connects the control end of the first power tube and the second end of the first power tube.

6. The self-powered circuit according to claim 3, characterized in that: The second driving unit includes: a second pull-up resistor and a second switch, the second pull-up resistor is connected to the output end and the control end of the first power tube, and the second switch is connected to the control end of the first power tube and the second end of the first power tube.

7. The self-powered circuit according to claim 4, characterized in that: The current limiting unit includes: a second power tube and a current limiting resistor, the current limiting resistor is connected to the second end of the first power tube and the power supply end, the control end of the second power tube is connected to the second end of the first power tube, the first end is connected to the control end of the first power tube, and the second end is connected to the power supply end.

8. The self-powered circuit according to claim 3, characterized in that: When the synchronous rectifier is turned on, the voltage of the first node is greater than the voltage of the output end, and the first driving unit controls the first power tube to be turned on; When the synchronous rectifier is turned off and the difference between the voltage at the output end and the voltage at the power supply end is greater than a preset voltage difference, the second driving unit controls the first power tube to be turned on.

9. A synchronous rectifier, characterized in that: A self-powered circuit comprising any one of claims 1-8.

10. A flyback converter, characterized in that: Includes the synchronous rectifier as claimed in claim 9.

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