Synchronous rectification control circuit and switching power supply

Abstract

The present invention discloses a synchronous rectification control circuit and a switching power supply. In the process of a pull-down signal pulling down the gate-source voltage of a synchronous rectification switch tube, when it is detected that the voltage across the drain and source of the synchronous rectification switch tube reaches a preset first threshold voltage, the pull-down action of the pull-down signal is quickly stopped through a fast adjustment circuit. The gate-source voltage of the synchronous rectification switch tube can be controlled not to be pulled too low, so that the drain-source voltage of the synchronous rectification switch tube is maintained near the preset first threshold voltage. In the scheme of the present invention, the conduction loss of the switch tube is small, and the system efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the field of switching power supplies, and more particularly to a synchronous rectification control circuit and a switching power supply. Background Art

[0002] Flyback switching power supplies are often used in AC-DC, DC-DC and other power conversion situations, such as Figure 1 The figure shows a schematic block diagram of a flyback switching power supply. The flyback switching power supply includes an isolation transformer T, a primary main switch tube M1, an input capacitor C1, and an output capacitor C2. In order to improve the conversion efficiency of the switching power supply, a synchronous rectifier switch tube is usually used on the secondary side to replace the rectifier diode, such as Figure 1 The secondary side rectifier switch tube M1. During the operation of the flyback switching power supply, in order to prevent the switch tube from shutting down prematurely when the secondary side current decreases, thereby causing low efficiency, a control method commonly used is to connect a pull-down circuit to the gate of the rectifier switch tube M1. When the voltage drop V DS When the preset first threshold voltage Vref is reached, the gate-source voltage of the rectifier switch tube M1 is pulled down by the pull-down circuit, and the voltage across the drain and source is reduced accordingly.

[0003] In the prior art, the pull-down circuit is usually implemented by an operational amplifier and a pull-down switch tube. Due to the speed limitation of the operational amplifier, when the voltage drop V DS When the preset first threshold voltage Vref is touched, the output of the operational amplifier will have a delay time, and the pull-down switch tube will not be turned off immediately. Since the pull-down current of the rectifier switch tube M1 is large at this time, the gate-source voltage of the rectifier switch tube M1 is pulled down more during the delay period, the on-resistance becomes too large, and the voltage drop across the drain and source V DS Excessive deviation from the first threshold voltage Vref. The existing synchronous rectification control method has the problem that due to the speed and accuracy of the operational amplifier, the voltage drop across the drain and source often deviates significantly from the preset voltage value Vref during the adjustment process, resulting in reduced power supply efficiency. Summary of the Invention

[0004] In view of this, an object of the present invention is to provide a synchronous rectification control circuit and a switching power supply to solve the technical problem of low efficiency caused by slow operation speed in the prior art.

[0005] The technical solution of the present invention is to provide a synchronous rectification control circuit for controlling the synchronous rectification switch tube in the switching power supply, comprising:

[0006] A pull-down circuit receives a voltage signal across the drain and source of the synchronous rectifier switch tube, and outputs a pull-down signal to adjust the gate-source voltage of the synchronous rectifier switch tube when the voltage across the drain and source of the synchronous rectifier switch tube rises to a preset first threshold voltage.

[0007] A fast adjustment circuit is connected to the pull-down circuit, and the fast adjustment circuit receives the voltage signal across the drain and source of the synchronous rectifier switch tube and the preset first threshold voltage, and quickly controls the pull-down action of the pull-down signal according to the comparison result of the voltage signal across the drain and source and the first threshold voltage.

[0008] Preferably, when the voltage signal across the drain and source drops to the first threshold voltage, the fast adjustment circuit stops the pull-down action of the pull-down signal.

[0009] Preferably, the fast adjustment circuit includes a comparison circuit and a switch circuit.

[0010] The input end of the comparison circuit receives the voltage signal at the drain and source of the synchronous rectifier switch tube and the preset first threshold voltage respectively, and outputs a comparison signal.

[0011] The switch circuit receives the comparison signal and controls the on / off state of the switch circuit according to the comparison signal.

[0012] Preferably, the pull-down circuit includes an operational amplifier and a pull-down switch tube.

[0013] The input end of the operational amplifier receives the voltage signal at the drain and source of the synchronous rectifier switch tube and the preset first threshold voltage respectively, so as to output the operational amplified signal to the control end of the pull-down switch tube.

[0014] The switch circuit includes a first switch tube, a control terminal of the first switch tube receives the comparison signal, a first power terminal is connected between the output terminal of the operational amplifier and the control terminal of the pull-down switch tube, and a second power terminal is grounded.

[0015] The first power terminal of the pull-down switch is connected to the gate of the synchronous rectifier switch, and the second power terminal is grounded. Preferably, the voltage value of the first threshold voltage is set to be smaller than the corresponding drain-source voltage value when the synchronous rectifier switch is turned off.

[0016] Preferably, the pull-down circuit further includes a pull-down resistor, and the pull-down resistor and the pull-down switch are connected in series between the gate and the source of the synchronous rectification switch.

[0017] In a second aspect, a switching power supply is disclosed, comprising a primary-side main power switch tube and a secondary-side synchronous rectification switch tube, and also comprising the above-mentioned synchronous rectification control circuit, wherein the synchronous rectification control circuit is used to control the switching state of the synchronous rectification switch tube.

[0018] By adopting the synchronous rectification circuit structure and switching power supply of the present invention, during the process of pulling down the gate-source voltage of the synchronous rectification switch tube by the pull-down signal, when it is detected that the voltage across the drain and source of the synchronous rectification switch tube reaches a preset first threshold voltage, the pull-down action of the pull-down signal is quickly stopped through the fast adjustment circuit, thereby controlling the gate-source voltage of the synchronous rectification switch tube from being pulled too low, so that the drain-source voltage of the synchronous rectification switch tube is maintained near the preset first threshold voltage. Compared with the prior art, the solution of the present invention has low conduction loss and improved system efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic block diagram of a flyback switching power supply in the prior art;

[0020] Figure 2 1 is a circuit structure diagram of a synchronous rectification control circuit of the present invention;

[0021] Figure 3 Based on Figure 2 Working waveform of the synchronous rectification control circuit. DETAILED DESCRIPTION

[0022] 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.

[0023] 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.

[0024] 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.

[0025] like Figure 2 As shown in FIG. 1 , a circuit structure diagram of a synchronous rectification control circuit of the present invention is shown. The synchronous rectification control circuit can be used as follows: Figure 1In the flyback switching power supply shown, M1 is a synchronous rectifier switch tube, where the synchronous rectifier switch tube M1 is a transistor. The synchronous rectifier control circuit includes a pull-down circuit 1 and a fast adjustment circuit 2. The pull-down circuit 1 includes an operational amplifier OP and a pull-down switch tube M0. The input end of the operational amplifier OP receives the drain-source voltage signal V of the synchronous rectifier switch tube. DS and the preset first threshold voltage Vref, where the positive input terminal of the operational amplifier receives the drain-source voltage signal V DS The inverting input terminal receives the first threshold voltage Vref to output the operational amplified signal Vf to the pull-down switch M0. In the embodiment of the present invention, the source of the synchronous rectifier switch M1 is grounded, the drain of the pull-down switch is connected to the gate of the synchronous rectifier switch, and the source is connected to the source of the synchronous rectifier switch M1, that is, grounded.

[0026] Here, the value of the first threshold voltage Vref is set to a voltage value that is smaller than the drain-source voltage value of the synchronous rectifier switch tube when it reaches the shutdown state. For example, when the drain-source voltage of the synchronous rectifier switch tube reaches the shutdown threshold voltage Voff-10mv, the synchronous rectifier switch tube reaches the shutdown state, and the first threshold voltage Vref is set to a value between -40mv and -60mv.

[0027] The fast adjustment circuit 2 includes a comparison circuit and a switch circuit. Here, the comparison circuit includes a comparator CMP, and the switch circuit includes a switch tube S2. The switch tube S2 can be a transistor or other switch tube with a switching function. The input end of the comparator CMP receives the drain-source voltage signal V of the synchronous rectifier switch tube. DS and the preset first threshold voltage Vref, the reverse terminal of the comparator CMP in this embodiment inputs the drain-source voltage signal V DS The positive end inputs a first threshold voltage Vref and outputs a comparison signal Vc. The control end of the switch tube S2 receives the comparison signal Vc and controls the on / off of the switch tube according to the comparison signal.

[0028] The following combination Figure 2 Circuit diagram and Figure 3 The working waveform diagram in the figure illustrates the embodiment of the present invention. During the period from t0 to t2, the secondary current flows through the body diode D2 of the rectifier switch tube M1. When the logic control circuit detects the voltage drop V DS Small enough to reach the on-state voltage Von (negative value here), the output pulse signal controls the switch S0 to turn on (corresponding to Figure 2 During the period t0 to t2, the rectifier switch tube M1 is turned on, and the current flows through the rectifier switch tube M1. The drain-source current I DSLarge, at t3, the voltage across the drain and source V DS Rising to the first threshold voltage Vref, generally speaking, the first threshold voltage Vref is set to be at time t3, the on-resistance and the drain-source current I DS The product of the output of the operational amplifier OP will have a delay time, the pull-down switch tube is not turned on immediately, the on-resistance of the rectifier switch tube M1 becomes larger, and the voltage across the drain and source V DS Increase, the voltage across the drain and source V DS After exceeding the first threshold voltage Vref for a period of time, the pull-down switch tube M0 is turned on, generating a pull-down current to lower the gate voltage of the rectifier switch tube M1, and the power supply V DS Decreases, when the drain-source voltage V DS When the voltage drops to the first threshold voltage Vref, the comparison signal Vc output by the comparator CMP becomes a high-level active state to control the first switch tube S2 to turn on. The first switch tube S2 quickly pulls down the gate voltage of the pull-down switch tube M0, and the pull-down switch tube M0 is turned off. Figure 3 At time t5, since the pull-down switch tube M0 is kept closed, the voltage across the drain and source of the synchronous rectifier switch tube M1 is basically maintained near the first threshold voltage Vref. DS The voltage across the drain and source decreases again to V DS When the voltage is slightly greater than the first threshold voltage Vref, the pull-down switch tube M0 generates a pull-down current again to pull down the gate voltage of the synchronous rectifier switch tube M1, increasing the on-resistance Rdson. In this way, the on-resistance Rdson is continuously adjusted so that the voltage across the drain and source V DS Maintain near the first threshold voltage Vref, and so on until time t14, the voltage across the drain and source V DS When the turn-off threshold voltage Voff (e.g. -10mv) is touched, the logic control circuit controls the synchronous rectifier switch M1 to turn off directly ( Figure 2 Not shown, for example, a switch tube may be connected between the rectifier switch tube M1 and the ground, and the logic control circuit controls the switch tube to be turned on to control the synchronous rectifier switch tube M1 to be directly turned off).

[0029] exist Figure 3 In the figure, the solid line represents the working waveform corresponding to the solution of the present invention, and the dotted line represents the working waveform corresponding to the solution in the prior art. It can be seen that in the prior art, due to the delay of the operational amplifier, the pull-down switch tube cannot be turned off immediately at time t5, resulting in the gate voltage of the synchronous switch tube M1 being pulled down too much, the on-resistance being large, and the drain-source voltage V DS The deviation from the first threshold voltage is large, which affects the system efficiency. In the embodiment of the present invention, the comparator can quickly respond to the comparison result to achieve the effect of quickly turning off the pull-down switch tube, so that the voltage across the drain and source VDS By maintaining the voltage near the first threshold, system loss is small and efficiency is improved.

[0030] Preferably, the pull-down circuit may further include a pull-down resistor connected in series with the pull-down switch between the gate and source of the synchronous rectifier switch. This allows the pull-down current to be controlled based on the resistor's value, preventing excessive pull-down current in the early stages of the pull-down process, which could cause the gate voltage of the synchronous switch to drop too much and affect system efficiency.

[0031] Finally, the present invention discloses a switching power supply, comprising a primary side main power switch tube and a secondary side synchronous rectifier switch tube, and also comprising the above-mentioned synchronous rectifier control circuit, wherein the synchronous rectifier control circuit is used to control the synchronous rectifier switch tube. Similarly, the switching power supply of the present invention has a fast response speed and can also achieve a drain-source voltage V DS Maintaining the voltage near the first threshold voltage has the beneficial effect of reducing system loss and improving efficiency.

[0032] Those skilled in the art will appreciate that the synchronous rectification control circuit of the present invention is not limited to the switching power supply mode in the above embodiment, and the synchronous rectification control circuit of the present invention can be applied to the switching power supply field with the same technical problem.

[0033] 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 synchronous rectification control circuit for controlling a synchronous rectification switch tube in a switching power supply, characterized in that: include: A pull-down circuit receives a voltage signal across the drain and source of the synchronous rectifier switch tube, and outputs a pull-down signal to adjust the gate-source voltage of the synchronous rectifier switch tube when the voltage across the drain and source of the synchronous rectifier switch tube rises to a preset first threshold voltage. A fast adjustment circuit is connected to the pull-down circuit, the fast adjustment circuit receives the voltage signal across the drain and source of the synchronous rectifier switch tube and the preset first threshold voltage, and quickly controls the pull-down action of the pull-down signal based on the comparison result of the voltage signal across the drain and source and the first threshold voltage, wherein when the voltage signal across the drain and source drops to the first threshold voltage, the fast adjustment circuit terminates the pull-down action of the pull-down signal.

2. The control circuit according to claim 1, wherein: The fast adjustment circuit includes a comparison circuit and a switch circuit. The input end of the comparison circuit receives the voltage signal at the drain and source of the synchronous rectifier switch tube and the preset first threshold voltage respectively, and outputs a comparison signal. The switch circuit receives the comparison signal and controls the on / off state of the switch circuit according to the comparison signal.

3. The control circuit according to claim 2, characterized in that: The pull-down circuit includes an operational amplifier and a pull-down switch tube. The input end of the operational amplifier receives the voltage signal at the drain and source of the synchronous rectifier switch tube and the preset first threshold voltage respectively, so as to output the operational amplified signal to the control end of the pull-down switch tube. The switch circuit includes a first switch tube, a control terminal of the first switch tube receives the comparison signal, a first power terminal is connected between the output terminal of the operational amplifier and the control terminal of the pull-down switch tube, and a second power terminal is grounded. The first power terminal of the pull-down switch tube is connected to the gate of the synchronous rectification switch tube, and the second power terminal is grounded.

4. The control circuit according to claim 1, wherein: The voltage value of the first threshold voltage is set to be smaller than the drain-source voltage value corresponding to when the synchronous rectification switch tube is turned off.

5. The control circuit according to claim 3, characterized in that: The pull-down circuit further includes a pull-down resistor, which is connected in series with the pull-down switch tube between the gate and the source of the synchronous rectification switch tube.

6. A switching power supply comprising a primary side main power switch tube and a secondary side synchronous rectification switch tube, characterized in that: It also includes the synchronous rectification control circuit according to any one of claims 1 to 5, wherein the synchronous rectification control circuit is used to control the switching state of the synchronous rectification switch tube.

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