Voltage-limiting conduction synchronous rectification control method and control chip

Abstract

The invention provides a voltage-limiting conduction synchronous rectification control method and a control chip, a synchronous detection comparison circuit detects drain-source voltage of a synchronous rectification switch tube, and the synchronous rectification switch tube is fully conducted when the drain-source voltage is smaller than a turn-on threshold value; and when the drain-source voltage is greater than the turn-off threshold value, the synchronous rectification switch tube is turned off. The voltage-limiting conduction trigger circuit detects drain-source voltage after the synchronous rectification switch tube is fully conducted, and when the voltage-limiting conduction condition is met, a voltage-limiting conduction effective signal is output to the logic circuit and the voltage-limiting conduction control circuit to trigger voltage-limiting conduction control. According to the invention, by detecting the drain-source voltage value of the synchronous rectification switch tube, the synchronous rectification switch tube is controlled to be fully turned on, and turn-on and turn-off are limited. The circuit can be compatible with a DCM mode and a CCM mode at the same time, and in the DCM mode, the synchronous rectification switch tube is turned off only when the current is very small; and in the CCM mode, the turn-off delay is very fast.

Description

Technical Field

[0001] The present invention relates to the technical field of synchronous rectification control, and in particular to a voltage-limited conduction synchronous rectification control method and a control chip. Background Art

[0002] The output rectification circuit of the switching power supply generally adopts two methods: diode rectification and synchronous rectification. Figure 1 As shown in the figure, when the PWM switch tube Q1 on the primary side of the switching power supply is turned on, the rectifier diode D OUT In the cut-off state, when Q1 changes from on to off, the magnetic energy storage element T1 releases energy, and the rectifier diode D OUT The anode voltage is greater than the cathode voltage, and the rectifier diode D OUT conduction, producing Figure 1 As shown in I F Current. This is the use of the diode's natural characteristics of forward conduction and reverse cutoff, which is simple, convenient and highly reliable. However, due to the inherent junction voltage drop of the diode, the conduction loss is large. The synchronous rectification method uses MOS tubes for rectification, which has no diode junction voltage drop and can greatly improve the rectification efficiency. Figure 2 As shown, Q SR It is a synchronous rectifier switch tube. Current can pass through its body diode. However, in order to reduce conduction loss, it is necessary to turn on the MOS tube Q when the magnetic energy storage element releases energy. SR , so that the current passes through the MOS tube channel instead of its body diode, thereby reducing the conduction voltage drop and thus reducing losses. SR It does not have the characteristics of natural opening and closing. It is necessary to detect the voltage between its drain and source to determine whether it is opened, that is, synchronous rectification control technology, which includes two aspects: opening technology and closing technology.

[0003] In order to deeply understand the shutdown technology of the synchronous rectification switch tube, we first need to understand the two different working modes of the switching power supply: discontinuous conduction mode (DCM) and continuous conduction mode (CCM): after the energy of the magnetic energy storage element is completely released, if the primary PWM switch tube is not turned on at this time, this situation is often called discontinuous conduction DCM mode. When the energy stored in the magnetic energy storage element is not completely released, the primary PWM switch tube is turned on again, and the magnetic energy storage element stores energy again. This situation is often called continuous conduction CCM mode.

[0004] In addition, we can divide the turn-off delay of synchronous rectification into two parts: detection delay and drive delay: detection delay refers to the time from the end of energy release of magnetic energy storage element to the time when comparator flips, that is, the time when the turn-off signal is issued; drive delay refers to the time from the time when the turn-off signal is issued to the time when the gate voltage of synchronous rectification switch tube is pulled down to below the conduction threshold voltage, that is, the time when the synchronous rectification switch tube is in the cut-off state. In DCM mode, since the primary PWM switch is in the cut-off state when the synchronous rectification is turned off, the turn-off delay requirement is not high. The main consideration is to turn off the rectification switch tube when the current is very small to avoid the current passing through the body diode of the synchronous rectification MOS tube and reducing the efficiency. In CCM mode, since the primary PWM switch is in the turn-on process when the synchronous rectification MOS tube is turned off, the turn-off delay requirement is very high. Rapid detection causes the comparator to flip and quickly pulls down the gate voltage of the synchronous rectification switch tube to avoid the common phenomenon. Once the common phenomenon occurs, it will not only lead to a rapid decrease in efficiency, but also easily cause a turn-off spike voltage, damaging the synchronous rectification switch tube or the primary PWM switch tube.

[0005] In some solutions, a transconductance amplifier is often used to control the opening of the synchronous rectifier switch. Its working principle is to convert the drain-source voltage V DS The amplified voltage is compared with a DC bias voltage and provides a driving voltage for the gate of the synchronous rectifier switch tube. The gate voltage varies with V DS The changing waveform is as follows Figure 3 The DC bias voltage is a negative voltage, for example -40mV. DS When the synchronous rectifier switch is less than -40mV, the gate voltage of the synchronous rectifier switch is at its maximum value, fully turning on the synchronous rectifier switch until V DS When it is greater than -40mV, the gate voltage of the synchronous rectifier switch tube begins to decrease, and the internal resistance of the synchronous rectifier switch tube increases, V DS The negative pressure increases, preventing V DS The voltage increases, so under the action of the transconductance amplifier, as V DS The current gradually decreases, controlling the gate voltage of the synchronous rectifier switch to gradually decrease, so that V DS Stable at -40mV, such as Figure 3 The current of the synchronous rectifier switch tube is as small as close to zero, and the gate voltage decreases to below the turn-on threshold voltage, entering the cut-off state. In CCM mode, when the primary PWM switch tube is turned on, the synchronous rectifier switch tube still has a large current, so the synchronous rectifier switch is not turned off, so it will be common. Using a transconductance amplifier alone to turn on synchronous rectification is not suitable for CCM, nor is it suitable for switching power supply applications that are compatible with DCM and CCM.

[0006] The transconductance amplifier output is used to drive the synchronous rectifier switch tube.DS When the gate voltage is greater than the DC bias voltage, it drops in advance, increasing the on-resistance of the synchronous rectifier switch tube and generating a larger negative voltage drop. This has two advantages in DCM mode. On the one hand, it is easy to turn off the synchronous rectifier switch tube when the current is very small, avoiding large current passing through the synchronous rectifier switch tube body diode and improving rectification efficiency; on the other hand, it reduces the driving delay. However, this is very harmful in CCM mode, such as Figure 3 As shown, the gate voltage decreases in advance so that V DS Stable at -40mV, such as Figure 3 V in the middle DF stage DS As shown by the solid line of the voltage, if the gate voltage does not drop in advance, V DS The voltage should gradually approach the shutdown threshold V as the current decreases. th(off) , then if Figure 3 V in the middle DF stage DS In comparison, the gate voltage is lowered in advance, which makes the body diode of the synchronous rectifier switch under a larger forward bias voltage. In the process of the body diode voltage drop recovering to the synchronous rectifier turn-off threshold, it is necessary to extract or recombine minority carriers, so a relatively longer reverse recovery time is required, as shown in Figure 2. Figure 3 Medium V DS As shown in the FG stage with a slower rising slope, the detection delay is directly increased. In CCM mode, although the drop in gate voltage reduces the drive delay, it increases the detection delay. For some synchronous rectifier switches with slow body diode reverse recovery, the increase in detection delay may be greater than the decrease in drive delay, which generally increases the turn-off delay.

[0007] In the patent application number 201010504140.4, "Intelligent drive circuit and drive method for flyback converter", it is proposed to use a transconductance amplifier and a comparator to control the switch of the synchronous rectifier switch tube. It solves the compatibility problem of CCM and DCM, but does not solve the problem of increased detection delay caused by the transconductance amplifier reducing the gate voltage in advance. Therefore, it is often necessary to reduce the opening speed of the primary PWM switch tube more to avoid the common phenomenon. However, the lower the opening speed of the primary PWM switch tube, the lower the efficiency of the switching power supply.

[0008] Therefore, the industry is in urgent need of a synchronous rectification shutdown method that must meet at least the following requirements: 1. Compatible with DCM mode and CCM mode; 2. In DCM mode, the synchronous rectification switch tube is turned off only when the current is very small; 3. In CCM mode, it has an extremely fast shutdown delay. Summary of the invention

[0009] The purpose of the present invention is to provide a voltage-limited conduction synchronous rectification control method and control chip, which can solve the problem in the prior art that the transconductance amplifier reduces the gate voltage in advance, resulting in increased detection delay, thereby reducing the efficiency of the switching power supply.

[0010] The objective of the present invention is achieved through the following technical solutions:

[0011] In a first aspect, the present invention provides a voltage-limited conduction synchronous rectification control method, comprising the following steps:

[0012] Step S1, the synchronous detection comparison circuit continuously detects the drain-source voltage of the synchronous rectification switch tube until the drain-source voltage is less than the turn-on threshold and the synchronous rectification switch tube is fully turned on;

[0013] Step S2, the synchronous detection comparison circuit continuously detects whether the drain-source voltage after the synchronous rectification switch tube is fully turned on is greater than the turn-off threshold, and if the drain-source voltage is greater than the turn-off threshold, the process goes to step S4; at the same time, the voltage-limiting turn-on trigger circuit continuously detects the drain-source voltage after the synchronous rectification switch tube is fully turned on, and when the voltage-limiting turn-on condition is met, the voltage-limiting turn-on valid signal is output to the logic circuit and the voltage-limiting turn-on control circuit, triggering the voltage-limiting turn-on control, and the process goes to step S3;

[0014] Step S3, the voltage-limiting conduction control circuit controls the gate voltage of the synchronous rectification switch tube to reduce the gate voltage of the synchronous rectification switch tube to the resistance-increasing gate voltage, and returns to step S2;

[0015] Step S4: pull down the gate voltage of the synchronous rectification switch tube to make it in a cut-off state.

[0016] Furthermore, the method for determining the voltage-limited conduction condition is at least one of the following three methods:

[0017] The first method is voltage comparison method: set the voltage limit turn-on threshold voltage, compare the drain-source voltage when the synchronous rectifier switch is fully turned on with the voltage limit turn-on threshold voltage, and trigger the voltage limit turn-on control when the drain-source voltage is greater than the voltage limit turn-on threshold voltage;

[0018] The second method is the fixed timing method: the timing circuit starts timing when the synchronous rectifier switch tube is fully turned on, and the voltage limiting conduction control is triggered when the timing ends;

[0019] The third method is the time comparison method: the conduction time of the synchronous rectification switch tube in each cycle is saved. The current cycle starts from when the synchronous rectification switch tube is fully turned on. When the timing reaches 1 / a of the conduction time of the previous cycle, the voltage limiting conduction control is triggered, a>1.

[0020] Further, when the drain-source voltage is less than the turn-on threshold, the synchronous detection and comparison circuit 401 outputs a first valid signal to the logic circuit 402;

[0021] When the logic circuit 402 receives the first valid signal, the output pin VG of the synchronous rectification control chip is turned on by the voltage limiting circuit. SR The voltage of the synchronous rectifier rises rapidly to the maximum driving voltage, so that the synchronous rectifier switch tube is fully turned on; at the same time, the logic circuit 402 outputs a fast shutdown signal to the voltage-limiting turn-on trigger circuit 403;

[0022] When the voltage-limiting conduction trigger circuit 403 receives the fast shutdown signal, the level of its control terminal changes from high level to low level, and the voltage-limiting conduction trigger circuit 403 starts to detect whether the voltage-limiting conduction condition is met. When the voltage-limiting conduction condition is met, the third valid signal is output to the logic circuit 402 and the voltage-limiting conduction control circuit 404;

[0023] When the logic circuit 402 receives the third valid signal, it outputs a low-level fully-on signal to disconnect the output pin VG SR Maximum driving voltage;

[0024] When the voltage limiting conduction control circuit 404 receives the third valid signal, it detects and controls the output pin VG SR The voltage of the synchronous rectifier drops to the resistance-increasing gate voltage, controlling the synchronous rectifier switch tube to be turned on with voltage limiting; when the drain-source voltage of the synchronous rectifier switch tube is greater than the turn-off threshold, the synchronous detection comparison circuit 401 outputs a second valid signal to the logic circuit 402; when the logic circuit 402 receives the second valid signal, it outputs a low level to the output pin VG SR , controlling the synchronous rectification switch tube to turn off quickly.

[0025] In a second aspect, the present invention provides a voltage-limited conduction synchronous rectification control chip, including a synchronous detection comparison circuit 401, a logic circuit 402, a voltage-limited conduction trigger circuit 403, a voltage-limited conduction control circuit 404, a first switch S1 and a second switch S2; the input end of the synchronous detection comparison circuit 401 and the input end of the voltage-limited conduction trigger circuit 403 are respectively connected to the drain detection input pin VD of the voltage-limited conduction synchronous rectification control chip, and the ground of the synchronous detection comparison circuit 401, the ground of the voltage-limited conduction trigger circuit 403 and the ground of the voltage-limited conduction control circuit 404 are respectively connected The output end of the synchronous detection comparison circuit 401 is connected to the input end of the logic circuit 402; the first output end of the logic circuit 402 is connected to the control end of the first switch S1, and the second output end of the logic circuit 402 is connected to the control end of the second switch S2 and the control end of the voltage-limited conduction trigger circuit 403; one end of the first switch S1 inputs the power supply voltage, and the other end is respectively connected to one end of the second switch S2, the voltage-limited conduction control circuit 404 and the output pin VG of the voltage-limited conduction synchronous rectification control chip 40 SRThe other end of the second switch S2 is grounded; the output end of the voltage limiting conduction trigger circuit 403 is respectively connected to the logic circuit 402 and the voltage limiting conduction control circuit 404.

[0026] Furthermore, the synchronous detection comparison circuit 401 includes: a second comparator CMP2 and a third comparator CMP3, the negative input terminal of the second comparator CMP2 and the positive input terminal of the third comparator CMP3 are respectively connected to the drain detection input pin VD; the positive input terminal of the second comparator CMP2 inputs the turn-on threshold voltage of the synchronous rectification switch tube, and the negative input terminal of the third comparator CMP3 inputs the turn-off threshold voltage of the synchronous rectification switch tube; the output terminal of the second comparator CMP2 outputs a first valid signal, and the output terminal of the third comparator CMP3 outputs a second valid signal; the ground of the synchronous detection comparison circuit 401 is connected to the source detection input pin VS.

[0027] Furthermore, the logic circuit 402 includes: an OR gate OR, a first RS trigger RS1 and a second RS trigger RS2; the set input terminal S of the first RS trigger RS1 and the reset input terminal R of the second RS trigger RS2 are respectively connected to the output terminal of the second comparator CMP2, one input terminal of the OR gate OR and the set input terminal S of the second RS trigger RS2 are respectively connected to the output terminal of the third comparator CMP3; the other input terminal of the OR gate OR is connected to the output terminal of the voltage-limiting conduction trigger circuit 403; the output terminal of the OR gate OR is connected to the reset input terminal R of the first RS trigger RS1; the output terminal Q of the first RS trigger RS1 outputs a fully conductive signal to control the closing or opening of the first switch S1; the output terminal Q of the second RS trigger RS2 outputs a fast shutdown signal to the voltage-limiting conduction trigger circuit 403, and at the same time outputs a fast shutdown signal to control the closing or opening of the second switch S2.

[0028] Furthermore, the voltage-limiting conduction trigger circuit 403 is one of the following, or two or more of them are connected in parallel:

[0029] One: comprising a first comparator CMP1, wherein the positive input terminal of the first comparator CMP1 is connected to the drain detection input pin VD, the negative input terminal is input with the voltage limiting conduction threshold voltage, the control terminal is connected to the second output terminal of the logic circuit 402, and the output terminal of the first comparator CMP1 is used as the output terminal of the voltage limiting conduction trigger circuit 403;

[0030] 2: including a first comparator CMP1, a first current source I REF , a fourth switch S4 and a first capacitor C1; a first current source I REFThe output end of is respectively connected to one end of the fourth switch S4, one end of the first capacitor C1 and the positive input end of the first comparator CMP1; the other end of the fourth switch S4 and the other end of the first capacitor C1 are grounded; the control end of the first comparator CMP1 and the control end of the fourth switch S4 are respectively connected to the second output end of the logic circuit 402; the negative input end of the first comparator CMP1 inputs the first reference voltage, and the output end serves as the output end of the voltage limiting conduction trigger circuit 403;

[0031] Three: including the first comparator CMP1, the first current source I REF , a second current source I3, a fourth switch S4, a fifth switch S5, a sixth switch S6, a seventh switch S7, a first capacitor C1, a second capacitor C2, a third capacitor C3, a NOT gate NOT1 and a monostable trigger; the input end of the NOT gate NOT1, the control end of the sixth switch S6 and the control end of the first comparator CMP1 are respectively connected to the second output end of the logic circuit 402; the output end of the NOT gate NOT1 is respectively connected to the input end of the monostable trigger and the control end of the fifth switch S5; the output end of the monostable trigger is respectively connected to the control end of the fourth switch S4 and the control end of the seventh switch S7; the first current source I REF The output end of the second current source I3 is connected to one end of the fifth switch S5, and the other end of the fifth switch S5 is respectively connected to one end of the fourth switch S4, one end of the first capacitor C1 and the other end of the sixth switch S6; one end of the sixth switch S6 is respectively connected to one end of the second capacitor C2 and the negative input end of the first comparator CMP1; the output end of the second current source I3 is respectively connected to the positive input end of the first comparator CMP1, one end of the seventh switch S7 and one end of the third capacitor C3; the other end of the fourth switch S4, the other end of the first capacitor C1, the other end of the second capacitor C2, the other end of the third capacitor C3 and the other end of the seventh switch S7 are respectively grounded; the output end of the first comparator CMP1 serves as the output end of the voltage limiting conduction trigger circuit 403.

[0032] Furthermore, the voltage limiting conduction control circuit 404 includes a first N-channel MOS tube N1, a third switch S3, a first resistor R1 and a second resistor R2; the control end of the third switch S3 is connected to the output end of the voltage limiting conduction trigger circuit 403, and one end of the third switch S3 is respectively connected to one end of the second resistor R2 and the output pin VG of the voltage limiting conduction synchronous rectification control chip. SR The other end of the third switch S3 is connected to the drain of the first N-channel MOS transistor N1; the other end of the second resistor R2 is respectively connected to one end of the first resistor R1 and the gate of the first N-channel MOS transistor N1; the other end of the first resistor R1 and the source of the first N-channel MOS transistor N1 are grounded.

[0033] Furthermore, the voltage limiting conduction control circuit 404 includes a first N-channel MOS transistor N1, a third switch S3, a second resistor R2, a first resistor R1, a first P-channel MOS transistor P1 and a third current source I B ; The control end of the third switch S3 is connected to the output end of the voltage-limiting conduction trigger circuit 403; one end of the second resistor R2 and one end of the third switch S3 are connected to the output pin VG of the voltage-limiting conduction synchronous rectification control chip SR The other end of the third switch S3 is connected to the drain of the first N-channel MOS transistor N1, and the other end of the second resistor R2 is connected to the source of the first P-channel MOS transistor P1 and one end of the first resistor R1; the gate of the first P-channel MOS transistor P1 is input with a control voltage V B The drain of the first P-channel MOS transistor P1 is connected to the gate of the first N-channel MOS transistor N1 and the third current source I B The input terminal of the first N-channel MOS tube N1, the source of the third current source I B The output end of the first resistor R1 and the other end of the first resistor R1 are grounded.

[0034] Furthermore, the voltage-limited conduction synchronous rectification control chip also includes a voltage-limited conduction setting pin VLC, the first resistor R1 is externalized, one end of the first resistor R1 and the other end of the second resistor R2 are respectively connected to the voltage-limited conduction setting pin VLC, and the other end of the first resistor R1 is grounded.

[0035] Furthermore, the voltage-limited conduction synchronous rectification control chip further includes an adaptive voltage-limited driving circuit 601, and the adaptive voltage-limited driving circuit 601 includes a bias current source I BQ , a first NPN transistor Q1 and a second NPN transistor Q2; the collector of the second NPN transistor Q2 is connected to the other end of the second resistor R2, and the emitter of the second NPN transistor Q2 is connected to the drain detection input pin VD; a bias current source I BQ The output end is respectively connected to the base of the second NPN transistor Q2, the base and the collector of the first NPN transistor Q1; the emitter of the first NPN transistor Q1 is grounded.

[0036] The voltage-limited conduction synchronous rectification control method and control chip of the present invention can detect the drain-source voltage of the synchronous rectification switch tube, and control the synchronous rectification switch tube to fully conduct, limit conduction and shut down according to the different values ​​of the drain-source voltage. The present invention is compatible with both DCM mode and CCM mode. In DCM mode, the synchronous rectification switch tube is turned off only when the current is very small; in CCM mode, it has an extremely fast shut-off delay. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A simplified schematic diagram of a prior art diode rectifier flyback switching power supply;

[0038] Figure 2 A simplified schematic diagram of a synchronous rectifier flyback switching power supply in the prior art;

[0039] Figure 3 It is a timing waveform diagram of key nodes of a synchronous rectification flyback switching power supply in the prior art;

[0040] Figure 4 A schematic diagram of the circuit structure of a synchronous rectification control chip with voltage limiting conduction according to the present invention applied to a flyback switching power supply;

[0041] Figure 5 The circuit schematic diagram of the first embodiment of the voltage-limited conduction synchronous rectification control chip of the present invention;

[0042] Figure 6 It is a circuit principle diagram of a second embodiment of the voltage-limiting conduction trigger circuit of the present invention;

[0043] Figure 7 The third embodiment of the circuit schematic diagram of the voltage-limiting conduction trigger circuit of the present invention;

[0044] Figure 8 It is a circuit principle diagram of a second embodiment of the voltage-limiting conduction control circuit of the present invention;

[0045] Fig. 9 It is a working waveform diagram of key nodes of the voltage-limited conduction synchronous rectification control chip of the present invention;

[0046] Fig.10 A circuit schematic diagram of a second embodiment of a voltage-limited conduction synchronous rectification control chip of the present invention;

[0047] Fig.11 This is a circuit schematic diagram of a third embodiment of the voltage-limited conduction synchronous rectification control chip of the present invention. DETAILED DESCRIPTION

[0048] The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

[0049] The following is an explanation of the embodiments of the present disclosure by specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the contents disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, not all of the embodiments. The present disclosure can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present disclosure. It should be noted that, in the absence of conflict, the following embodiments and the features in the embodiments can be combined with each other. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present disclosure. In the description of the present application document, the positive input terminal of the comparator is connected to the reference voltage, and the negative input terminal is connected to the voltage detection terminal. Of course, it can also be connected in reverse, as long as the comparator outputs the reverse logic, and it does not affect the normal function. In addition, the NMOS tube can also be replaced by an NPN transistor. It should be understood that when an element described in the present invention is "connected" to another element, it can be directly connected or coupled to another element or there can be an intermediate element. Such simple logic reversal and device replacement, or simple connection relationship changes in the circuit structure that do not deviate from the purpose of the present invention, cannot be used to circumvent the present invention.

[0050] The circuit provided by the present invention is integrated inside the chip. In order to facilitate intuitive understanding, in the embodiments of the invention, a flyback synchronous rectification switching power supply circuit is taken as an example to explain in detail its working principle. However, this is not a limitation on the scope of application. It can also be applied to control chips of various power supply topologies such as BUCK step-down chips and forward switching power supplies.

[0051] In order to solve the problem of incompatibility of gate drive voltage of synchronous rectifier switch tube in DCM and CCM modes, Figure 4 The schematic diagram shows the structure of the synchronous rectification control chip with voltage limiting conduction of the present invention applied to the synchronous rectification flyback switching power supply circuit. Figure 4 As shown, the synchronous rectification flyback switching power supply circuit includes: the intelligent voltage-limited conduction synchronous rectification control chip 40 of the present invention, and the magnetic energy storage element 41, the primary pulse width modulation chip 42, the primary PWM switch tube 43, the synchronous rectification switch tube 44, and the capacitive energy storage element 45 of the prior art.

[0052] Figure 4 The working principle of the circuit is: when the primary pulse width modulation chip 42 controls the PWM switch tube 43 to turn on, the current flows from the input terminal V INThe current enters the primary winding 411 of the magnetic energy storage element 41, and the magnetic energy storage element 41 stores energy; after the primary pulse width modulation chip 42 controls the PWM switch tube 43 to be turned off, the magnetic energy storage element 41 releases energy, and the current flows from the secondary winding 412 of the magnetic energy storage element 41 through the synchronous rectification switch tube 44 to the output terminal V of the switching power supply circuit. OUT The magnetic energy storage element 41 continuously stores and releases energy. The energy is transferred from the V IN end to the output end V OUT .

[0053] The magnetic energy storage element 41 includes a primary winding 411 and a secondary winding 412 , and can be physically implemented in a variety of ways, such as winding with copper wire or winding with metal traces on a PCB board.

[0054] The primary pulse width modulation chip 42 controls the duty cycle of the PWM switch tube 43, that is, the ratio of the on time to the switching cycle, which can be achieved by adjusting the on time, the off time, the switching cycle, or a combination thereof.

[0055] The primary PWM switch tube 43 mainly functions as a switch, and may be an N-channel silicon MOS tube or a silicon carbide MOS tube, an NPN triode, a thyristor, or an insulated gate bipolar transistor.

[0056] The synchronous rectification switch tube 44 mainly functions as a switch, which is closed when the magnetic energy storage element 41 stores energy and opened when diluting energy. It can be an N-channel silicon MOS tube, an N-channel silicon carbide MOS tube, an insulated gate bipolar transistor, etc.

[0057] The output capacitive energy storage element 45 is a capacitor device, which can be an electrolytic capacitor, a ceramic capacitor, a film capacitor, etc.

[0058] The key point of the present invention is to propose a voltage-limited conduction synchronous rectification control chip and a voltage-limited conduction synchronous rectification control method.

[0059] In a first aspect, the present invention discloses a voltage-limited conduction synchronous rectification control method, comprising the following steps:

[0060] Step S1: The synchronous detection comparison circuit continuously detects the drain-source voltage V of the synchronous rectifier switch tube. DS , until the drain-source voltage V DS When the current is less than the turn-on threshold, the synchronous rectification switch is fully turned on, and the process goes to step S2.

[0061] The said fully conducting synchronous rectification switch tube means that the gate voltage of the synchronous rectification switch tube reaches the maximum level voltage of the driving circuit, so that the conduction resistance is minimized. The conduction mode of reducing the gate voltage of the synchronous rectification switch tube to increase its conduction resistance will be mentioned later, and the two are different conduction modes.

[0062] Step S2. On the one hand, the synchronous detection comparison circuit continuously detects whether the drain-source voltage after the synchronous rectification switch tube is fully turned on is greater than the shutdown threshold. If the drain-source voltage is greater than the shutdown threshold, it directly goes to step S4. On the other hand, the voltage-limiting conduction trigger circuit continuously detects the drain-source voltage after the synchronous rectification switch tube is fully turned on. When the voltage-limiting conduction condition is met, it outputs a voltage-limiting conduction valid signal to the logic circuit and the voltage-limiting conduction control circuit, triggers the voltage-limiting conduction control, and goes to step S3.

[0063] Further, at least one of the following three methods is used to determine whether the voltage-limited conduction condition is met:

[0064] The first method is voltage comparison method: set the voltage limit turn-on threshold voltage, and set the drain-source voltage V when the synchronous rectifier switch is fully turned on. DS Compared with the voltage-limiting turn-on threshold voltage, when the drain-source voltage V DS When it is greater than the voltage-limiting conduction threshold voltage, the voltage-limiting conduction control is triggered;

[0065] The second method is the fixed timing method: the timing circuit starts timing when the synchronous rectifier switch tube is fully turned on, and the voltage limiting conduction control is triggered when the timing ends;

[0066] The third method is the time comparison method: the conduction time of the synchronous rectification switch tube in each cycle is saved. The current cycle starts from when the synchronous rectification switch tube is fully turned on. When the timing reaches 1 / a of the conduction time of the previous cycle, a>1, the voltage limiting conduction control is triggered.

[0067] In practical applications, only one of the above three methods can be used, or multiple methods can be used at the same time. When multiple methods are used, multiple methods coexist, which does not mean that the voltage-limiting conduction control will be triggered only when all three methods are judged to meet the voltage-limiting conduction conditions, but when any one of them first judges that the voltage-limiting conduction conditions are met, the voltage-limiting conduction control will be entered.

[0068] Step S3, the voltage-limiting conduction control circuit controls the gate voltage of the synchronous rectification switch tube to reduce the gate voltage of the synchronous rectification switch tube to the resistance-increasing gate voltage.

[0069] Under the premise of not causing the synchronous rectification switch to shut down prematurely, the resistance-increasing gate voltage should be increased as much as possible, so that the conduction voltage drop of the synchronous rectification switch tube can be as close to the shutdown threshold as possible, thereby accelerating the recovery speed of the parasitic body diode of the synchronous rectification switch tube, making the detection delay the shortest and the efficiency of synchronous rectification the highest. Because different performance synchronous rectification switches require different driving voltages, in practical applications, the resistance-increasing gate voltage can be set according to the performance parameters of the synchronous rectification switch tube.

[0070] Step S4: The synchronous detection circuit detects the drain-source voltage V when the synchronous rectifier switch is turned on. DS >When the turn-off threshold is reached, the gate voltage of the synchronous rectifier switch tube is quickly lowered to put it in the cut-off state, that is, the synchronous rectifier switch tube is turned off.

[0071] The synchronous detection and comparison circuit controls the rapid opening and rapid conduction of the synchronous rectification switch tube. The drain-source voltage V DS When the synchronous rectifier switch is turned on quickly, the drain-source voltage V DS When the voltage is higher than the turn-off threshold, the synchronous rectifier switch is quickly turned off. This is the basic principle of synchronous rectification. In order to prevent the synchronous rectifier switch from being turned on by mistake, the drain-source voltage V is detected in the prior art. DS The slope of the drain-source voltage V DS The time width of the platform voltage, etc., and the technology used to prevent false triggering should not be used to circumvent the present invention.

[0072] In a second aspect, the present invention discloses a voltage-limited conduction synchronous rectification control chip, which integrates an intelligent voltage-limited conduction synchronous rectification control circuit, such as Figure 4 As shown, it includes a synchronous detection comparison circuit 401, a logic circuit 402, a voltage-limited conduction trigger circuit 403, a voltage-limited conduction control circuit 404, a first switch S1 and a second switch S2. These circuits are integrated on a wafer to form a voltage-limited conduction synchronous rectification control chip 40. The voltage-limited conduction synchronous rectification control chip 40 includes at least three pins: a source detection input pin VS, a drain detection input pin VD and an output pin VG SR The source detection input pin VS is connected to the source of the synchronous rectification switch tube 44, and the drain detection input pin VD is connected to the drain of the synchronous rectification switch tube 44. VD and VS provide a detection voltage signal V for the internal circuit of the voltage-limited conduction synchronous rectification control chip 40. DS , output pin VG SRThe output drive voltage controls the full conduction, voltage-limited conduction or shutdown of the synchronous rectifier switch tube 44. The input end of the synchronous detection comparison circuit 401 and the input end of the voltage-limited conduction trigger circuit 403 are connected to the drain detection input pin VD, and the ground of the synchronous detection comparison circuit 401, the ground of the voltage-limited conduction trigger circuit 403 and the ground of the voltage-limited conduction control circuit 404 are respectively connected to the source detection input pin VS. The output end of the synchronous detection comparison circuit 401 is connected to the input end of the logic circuit 402. The first output end of the logic circuit 402 is connected to the control end of the first switch S1, and the second output end of the logic circuit 402 is connected to the control end of the second switch S2 and the control end of the voltage-limited conduction trigger circuit 403. One end of the first switch S1 is input with a supply voltage V DD The other end is connected to one end of the second switch S2, the voltage limiting conduction control circuit 404 and the output pin VG of the voltage limiting conduction synchronous rectification control chip 40. SR The other end of the second switch S2 is grounded. The output end of the voltage limiting conduction trigger circuit 403 is connected to the logic circuit 402 and the voltage limiting conduction control circuit 404 respectively.

[0073] The synchronous detection comparison circuit 401 and the voltage limiting conduction trigger circuit 403 detect the drain-source voltage V of the synchronous rectifier switch tube in real time. DS , when the drain-source voltage V DS <Opening threshold V th(on) When the synchronous detection and comparison circuit 401 outputs the first valid signal T on to the logic circuit 402; when the logic circuit 402 receives the first valid signal T on When the voltage-limited conduction synchronous rectification control chip is turned on, the high-level full conduction signal FON is output to control the first switch S1 to close, the low-level fast turn-off signal OFF is output to control the second switch S2 to open, and the fast turn-off signal OFF is output to the voltage-limited conduction trigger circuit 403. SR The voltage rises rapidly to the maximum driving voltage V DD , so that the synchronous rectifier switch tube is fully turned on. When the voltage-limiting conduction trigger circuit 403 receives the fast shutdown signal OFF, the level of its control terminal changes from high level to low level, indicating that the synchronous rectifier switch tube begins to enter the fully turned-on stage, and the voltage-limiting conduction trigger circuit 403 begins to detect whether the voltage-limiting conduction condition is met. When the voltage-limiting conduction condition is met, the third valid signal T is output. VLC to the logic circuit 402 and the voltage limiting conduction control circuit 404; the logic circuit 402 receives the third valid signal T VLC When the voltage limiting conduction control circuit 404 receives the third valid signal T VLC When the output pin VG is detected and controlledSR The voltage of the synchronous rectifier switch drops to the resistance-increasing gate voltage, controlling the synchronous rectifier switch to conduct with voltage limiting. DS > Turn-off threshold V th(off) When the synchronous detection and comparison circuit 401 outputs the second valid signal T off to the logic circuit 402; when the logic circuit 402 receives the second valid signal T off When the output is low level, it will be turned on to the output pin VG of the voltage-limited synchronous rectification control chip. SR , controlling the synchronous rectification switch tube to turn off quickly.

[0074] Further, as an embodiment of the present invention, the synchronous detection comparison circuit 401 is as follows: Figure 5 As shown, it includes: a second comparator CMP2 and a third comparator CMP3, the negative input terminal of the second comparator CMP2 and the positive input terminal of the third comparator CMP3 are respectively connected to the drain detection input pin VD. The positive input terminal of the second comparator CMP2 inputs the turn-on threshold voltage V th(on) The negative input terminal of the third comparator CMP3 inputs the turn-off threshold voltage V of the synchronous rectifier switch tube. th(off) The output terminal of the second comparator CMP2 outputs a first valid signal T on , the output terminal of the third comparator CMP3 outputs a second valid signal T off The ground of the synchronous detection comparison circuit 401 is connected to the source detection input pin VS.

[0075] The second comparator CMP2 detects the drain-source voltage V DS Lower than the synchronous rectification turn-on threshold V th(on) When the first valid signal T is output on ; The third comparator CMP3 detects V DS The voltage is higher than the synchronous rectification turn-off threshold V th(off) When the second valid signal T is output off .

[0076] Further, as an embodiment of the present invention, the logic circuit 402 is as follows: Figure 5As shown, it includes: an OR gate OR, a first RS trigger RS1 and a second RS trigger RS2. The setting input terminal S of the first RS trigger RS1 and the reset input terminal R of the second RS trigger RS2 are respectively connected to the output terminal of the second comparator CMP2, and one input terminal of the OR gate OR and the setting input terminal S of the second RS trigger RS2 are respectively connected to the output terminal of the third comparator CMP3. The other input terminal of the OR gate OR is connected to the output terminal of the voltage-limiting conduction trigger circuit 403. The output terminal of the OR gate OR is connected to the reset input terminal R of the first RS trigger RS1. The output terminal Q of the first RS trigger RS1 outputs a fully conducting signal FON to control the closing or opening of the first switch S1. The output terminal Q of the second RS trigger RS2 outputs a fast turn-off signal OFF to the voltage-limiting conduction trigger circuit 403, and at the same time outputs a fast turn-off signal OFF to control the closing or opening of the second switch S2.

[0077] Upon receiving the first valid high level signal T on When the input terminal S of the first RS trigger RS1 is set to a high level, the first RS trigger RS1 is set to "1", and the output full conduction signal FON becomes a high level, controlling the first switch S1 to close. Due to the register function of the RS trigger, even if the high level of the input terminal S of the first RS trigger RS1 disappears, the output full conduction signal FON still remains at a high level. off or T VLC When the first valid signal T1 is high, the OR gate outputs a high level, sets the first RS flip-flop RS1 to "0", and the full conduction signal FON becomes a low level, controlling the first switch S1 to be disconnected. on When the reset input terminal R of the second RS flip-flop RS1 is high, the second RS flip-flop RS2 is set to "0", and the output fast shutdown signal OFF becomes low, controlling the second switch S2 to be turned off. Due to the register function of the RS flip-flop, even if the high level of the reset input terminal R of the second RS flip-flop RS1 disappears, the fast shutdown signal OFF still remains low. off When it becomes a high level, the second RS trigger RS2 is set to "1", the fast shutdown signal OFF becomes a high level, and the second switch S2 is controlled to be closed.

[0078] Further, as an embodiment of the present invention, when the voltage comparison method is used to determine whether the voltage limiting conduction condition is met, the voltage limiting conduction trigger circuit 403 is as follows: Figure 5 As shown, it includes: a first comparator CMP1, the positive input terminal of the first comparator CMP1 is connected to the drain detection input pin VD; the negative input terminal inputs the voltage-limiting turn-on threshold voltage V th(VLC) ; The control end inputs the fast shutdown signal OFF; the output end outputs the voltage limiting conduction effective signal T VLCThe ground of the voltage limiting conduction trigger circuit 403 is connected to the source detection input pin VS.

[0079] Figure 5 The working principle of the voltage limiting turn-on trigger circuit 403 is as follows: when the fast turn-off signal OFF is at a low level, which is also the full turn-on stage of the synchronous rectifier switch tube, the first comparator CMP1 converts the synchronous rectifier switch tube drain-source voltage V DS The voltage limit turn-on threshold voltage V th(VLC) For comparison, when V DS >V th(VLC) When the first comparator CMP1 outputs a voltage limiting conduction valid signal T VLC When the fast shutdown signal OFF is at a high level, the first comparator CMP1 outputs a voltage limiting conduction valid signal T VLC This is the first basic method of the three triggering voltage limiting conduction methods proposed by the present invention, that is, using voltage comparison for triggering.

[0080] Further, as another embodiment of the voltage-limiting conduction trigger circuit 403, when a fixed timing method is used to determine whether the voltage-limiting conduction condition is met, the voltage-limiting conduction trigger circuit 403 is as follows: Figure 6 As shown, it includes: a first comparator CMP1, a first current source I REF , a fourth switch S4 and a first capacitor C1. The first current source I REF The output end of is respectively connected to one end of the fourth switch S4, one end of the first capacitor C1 and the positive input end of the first comparator CMP1. The other end of the fourth switch S4 and the other end of the first capacitor C1 are grounded. The control end of the first comparator CMP1 and the control end of the fourth switch S4 are respectively input with the fast shutdown signal OFF. The negative input end of the first comparator CMP1 is input with the first reference voltage V REF(VLC) , the output terminal outputs the voltage-limited conduction effective signal T VLC The ground of the voltage limiting conduction trigger circuit 403 is connected to the source detection input pin VS.

[0081] Figure 6 The working principle of the voltage limiting turn-on trigger circuit 403 is as follows: the fast turn-off signal OFF changes from a high level to a low level, the synchronous rectifier switch tube is fully turned on, the fourth switch S4 changes from closed to open, and the first current source I REF The first capacitor C1 is charged. When the voltage across the first capacitor C1 reaches the first reference voltage V REF(VLC) When the first comparator CMP1 outputs a voltage limiting conduction valid signal T VLC becomes a high level. The charging time of the first capacitor C1 is Tcharge =C1*V REF(VLC) / I REF Therefore, after the synchronous rectifier switch is turned on, a fixed time T is delayed. charge After that, it enters the voltage limiting conduction control stage. This is the specific implementation circuit proposed by the present invention to use a fixed timing method to trigger the voltage limiting conduction control. The specific implementation circuit is not limited to this. As long as it can achieve the function of flipping the voltage limiting conduction effective signal level output after a fixed time, it can be regarded as the protection scope of the present invention.

[0082] Further, as a third embodiment of the voltage-limiting conduction trigger circuit 403, when the time comparison method is used to determine whether the voltage-limiting conduction condition is met, the voltage-limiting conduction trigger circuit 403 is as follows: Figure 7 As shown, it includes: a first comparator CMP1, a first current source I REF , a second current source I3, a fourth switch S4, a fifth switch S5, a sixth switch S6, a seventh switch S7, a first capacitor C1, a second capacitor C2, a third capacitor C3, a NOT gate NOT1 and a monostable trigger. The input end of the NOT gate NOT1, the control end of the sixth switch S6 and the control end of the first comparator CMP1 are respectively input with a fast shutdown signal OFF. The output end of the NOT gate NOT1 is respectively connected to the input end of the monostable trigger and the control end of the fifth switch S5. The output end of the monostable trigger is respectively connected to the control end of the fourth switch S4 and the control end of the seventh switch S7. The first current source I REF The output end of the second current source I3 is connected to one end of the fifth switch S5, and the other end of the fifth switch S5 is respectively connected to one end of the fourth switch S4, one end of the first capacitor C1 and the other end of the sixth switch S6. One end of the sixth switch S6 is respectively connected to one end of the second capacitor C2 and the negative input end of the first comparator CMP1. The output end of the second current source I3 is respectively connected to the positive input end of the first comparator CMP1, one end of the seventh switch S7 and one end of the third capacitor C3. The other end of the fourth switch S4, the other end of the first capacitor C1, the other end of the second capacitor C2, the other end of the third capacitor C3 and the other end of the seventh switch S7 are respectively grounded. The output end of the first comparator CMP1 outputs a voltage limiting conduction effective signal T VLC To the other input terminal of the voltage limiting conduction control circuit 404 and the OR gate OR.

[0083] Figure 7 The working principle of the voltage limiting conduction trigger circuit 403 shown is: the fast shutdown signal OFF changes from a high level to a low level, and the synchronous rectifier switch tube is fully turned on. The fifth switch S5 is closed, the sixth switch S6 is opened, and the monostable trigger sends a short high level pulse to control the fourth switch S4 and the seventh switch S7 to close and then open, so that the voltage on the first capacitor C1 and the third capacitor C3 is reset to 0V in a short time. After that, the voltage of the first capacitor C1 is at the first current source IREF The voltage of the third capacitor C3 gradually increases under the charging effect of the second current source I3. 3上 When the voltage of the first comparator CMP1 is equal to the voltage of the second capacitor C2, the first comparator CMP1 outputs a voltage limiting conduction effective signal T VLC becomes a high level, and when the fast shutdown signal OFF becomes a high level, the fifth switch S5 is opened and the sixth switch S6 is closed, and the voltage of the second capacitor C2 is equal to the voltage of the first capacitor C1.

[0084] The voltage on the second capacitor C2 represents the time port of the fast shutdown signal OFF in the previous cycle being at a low level. Assuming that the time T of the fast shutdown signal OFF in the previous cycle being at a low level n-1 , the voltage of C2 is:

[0085]

[0086] The third capacitor C3 rises from 0V to V C2 The time is:

[0087]

[0088] As long as the design I REF / I3 and the proportionality coefficient of C3 / C1 can achieve T D =T n-1 / a, a>1, then the fast shutdown signal OFF starts timing from the high level to the low level, and then delays the fast shutdown signal OFF to be low level for one-tenth of the time width a of the previous cycle, and then the voltage-limited conduction effective signal T VLC That is, after the synchronous rectification is turned on, the voltage limiting conduction control is triggered after a delay of one-tenth of the previous cycle opening time, which is the third way of triggering the voltage limiting conduction proposed by the present invention.

[0089] The above three methods for judging the voltage-limiting conduction conditions correspond to three different voltage-limiting conduction trigger circuits. When two or more methods are used at the same time to judge whether the voltage-limiting conduction conditions are met, the corresponding two or more voltage-limiting conduction trigger circuits can be connected in parallel.

[0090] Further, as an embodiment of the present invention, the voltage limiting conduction control circuit 404 is as follows: Figure 5 As shown, it includes a first N-channel MOS transistor N1, a third switch S3, a first resistor R1 and a second resistor R2. The control end of the third switch S3 inputs a voltage-limiting conduction effective signal T VLC One end of the third switch S3 is respectively connected to one end of the second resistor R2, the other end of the first switch S1 and the output pin VG of the voltage-limited conduction synchronous rectification control chip 40. SRThe other end of the third switch S3 is connected to the drain of the first N-channel MOS transistor N1. The other end of the second resistor R2 is respectively connected to one end of the first resistor R1 and the gate of the first N-channel MOS transistor N1. The other end of the first resistor R1 and the source of the first N-channel MOS transistor N1 are grounded.

[0091] Figure 5 The working principle of the voltage limiting conduction control circuit shown is: when the voltage limiting conduction effective signal T VLC When it becomes high level, the third switch S3 is closed, VG SR The voltage of the pin starts to drop, and stops dropping when it drops to the resistance-increasing gate voltage. The value of the resistance-increasing gate voltage can be set according to actual needs. The specific value of the resistance-increasing gate voltage is determined by adjusting the resistance values ​​of the first resistor R1 and the second resistor R2. The value calculation formula of the resistance-increasing gate voltage is:

[0092]

[0093] Where V th1 is the conduction threshold of the first N-channel MOS transistor N1.

[0094] Further, as another embodiment of the voltage limiting conduction control circuit 404, as Figure 8 As shown, the first N-channel MOS transistor N1, the third switch S3, the second resistor R2, the first resistor R1, the first P-channel MOS transistor P1 and the third current source I B The control terminal of the third switch S3 receives the voltage-limiting conduction effective signal T outputted from the voltage-limiting conduction trigger circuit 403. VLC One end of the second resistor R2 and one end of the third switch S3 are connected to the output pin VG of the voltage-limited conduction synchronous rectification control chip 40. SR The other end of the third switch S3 is connected to the drain of the first N-channel MOS transistor N1, and the other end of the second resistor R2 is connected to the source of the first P-channel MOS transistor P1 and one end of the first resistor R1. The gate input control voltage V B The drain of the first P-channel MOS transistor P1 is connected to the gate of the first N-channel MOS transistor N1 and the third current source I B The source of the first N-channel MOS tube N1, the third current source I B The output end of the first resistor R1 and the other end of the first resistor R1 are grounded.

[0095] Figure 8 The working principle of the voltage limiting conduction control circuit shown is: when the voltage limiting conduction effective signal T VLC When it becomes high level, the third switch S3 is closed, VG SRThe voltage of the pin starts to drop, and stops dropping when it drops to the resistance-increasing gate voltage. The value of the resistance-increasing gate voltage can be adjusted by adjusting the control voltage V B The numerical value is obtained, and the calculation formula is:

[0096]

[0097] Where V SGP is the voltage difference between the source and gate of the P-channel MOS tube P1, so V B or I B The resistance-increasing gate voltage can be adjusted.

[0098] like Fig. 9 The figure shows a timing diagram of synchronous rectification control of voltage-limited conduction of the present invention. The working principle of the present invention is further described in detail below in conjunction with the timing diagram:

[0099] At time B, the drain-source voltage V DS The synchronous rectification turn-on condition is met, thus triggering the synchronous rectification turn-on process. Due to the existence of detection and drive delay, until t1, VG SR The voltage of the pin rises;

[0100] At t1, VG SR The voltage of the pin rises quickly to the maximum voltage value V DD , so that the synchronous rectification switch tube 44 is fully turned on. DD It is the maximum voltage that the control chip can increase, and there is no special limit on VG SR The voltage of the pin is such that the conduction voltage drop of the synchronous rectifier switch tube is minimized;

[0101] At time t2, the voltage limiting turns on the effective signal T VLC It becomes a high-level effective signal and enters the voltage-limited conduction process. SR The voltage of the pin drops rapidly to the resistance-increasing gate voltage V LC , the on-resistance of the synchronous rectifier switch increases, and the on-voltage drop increases. Compared with the case of no voltage limiting, V DS The voltage is far away from the synchronous rectification turn-off threshold V th(off) , preventing the synchronous rectification from shutting down prematurely;

[0102] From t3 to t4, VG SR The voltage of the pin is basically stable at the resistance-increasing gate voltage V LC , rather than further decreasing, not only prevents V DS The voltage is too far away from V th(off) , and as the synchronous rectifier switch current decreases, V DS The voltage is getting closer to V th(off) , minimizing the synchronous rectification detection delay and ultimately reducing the shutdown delay;

[0103] At t4, V DS Greater than the synchronous rectification turn-off threshold V th(off) , VG SR The voltage of the pin is quickly pulled to a low voltage to put the synchronous rectification switch tube in the cut-off state, that is, to turn off the synchronous rectification switch tube.

[0104] Furthermore, as an embodiment of the present invention, the present invention also provides a voltage-limited conduction synchronous rectification control chip 50, such as Fig.10 Compared with the voltage-limited conduction synchronous rectification control chip 40, a voltage-limited conduction setting pin VLC is added, the first resistor R1 in the voltage-limited conduction control circuit 404 is externalized, one end of the first resistor R1 and the other end of the second resistor R2 are respectively connected to the voltage-limited conduction setting pin VLC, and the other end of the first resistor R1 is grounded.

[0105] The first resistor R1 can be placed externally on the periphery of the control chip to design the resistance value of the first resistor R1 according to actual application requirements to set the resistance-increasing gate voltage V LC The specific value of achieves design flexibility. Figure 8 The first resistor R1 in the embodiment can also be externally arranged, and the value of the resistance-increasing gate voltage can be set by designing the resistance value of the first resistor R1.

[0106] The present invention proposes three triggering modes for entering the voltage-limited conduction control stage, according to the drain-source voltage V DS The voltage value and conduction time of the synchronous rectifier switch tube are adaptively kept in a fully conductive state for a longer time, reducing the conduction voltage drop to improve the rectification efficiency, and only enter the voltage-limited conduction control when the conduction voltage drop is small. In order to further enhance the adaptability, it is not necessary to design the resistance-increasing gate voltage according to the parameters of the synchronous rectifier tube, and the drain-source voltage V can be detected in real time. DS , according to its value, the gate voltage of the synchronous rectification switch tube is gradually reduced from the maximum voltage to the increased resistance gate voltage.

[0107] Furthermore, as an embodiment of the present invention, the present invention also provides a voltage-limited conduction synchronous rectification control chip 60, such as Fig.11 As shown. Figure 5 The control chip 40 shown, Figure 8 The voltage limiting conduction control circuit 404 shown or Fig.10 On the basis of the control chip 50 shown, an adaptive voltage limiting driving circuit 601 is added. Here, only the circuit schematic diagram of the adaptive voltage limiting driving circuit 601 added on the basis of the control chip 50 is shown, which is used to illustrate the connection relationship and working principle of the adaptive voltage limiting driving circuit 601. The remaining working principles and connection relationships are the same and are not repeated here.

[0108] like Fig.11As shown, the adaptive voltage limiting driving circuit 601 includes a bias current source I BQ , a first NPN transistor Q1 and a second NPN transistor Q2. The collector of the second NPN transistor Q2 is connected to the other end of the second resistor R2, and the emitter of the second NPN transistor Q2 is connected to the drain detection input pin VD. Bias current source I BQ The output end of is connected to the base of the second NPN transistor Q2, the base and the collector of the first NPN transistor Q1 respectively. The emitter of the first NPN transistor Q1 is grounded.

[0109] The working principle of the adaptive voltage limiting driving circuit 601 is:

[0110]

[0111] I C2 is the current through the collector of transistor Q2, I S is the saturation drift current, V BE1 and V BE2 are the voltage differences between the gate and emitter of transistors Q1 and Q2, V T =kT / q, which is a constant that changes with temperature.

[0112] From equations ⑤ and ⑥, we can get:

[0113]

[0114] Combined with formula ③, the increased resistance gate voltage after adding the adaptive voltage limiting driving circuit 601 can be calculated as:

[0115]

[0116] From formula ⑧, it can be seen that the gate voltage of the resistance increase increases with the conduction voltage drop of the synchronous rectifier switch tube (V DS The absolute value of V LC1 +I BQ *R2, which is the V set by resistor R1 LC1 It is the minimum driving voltage for turning on the synchronous rectifier tube. When the current of the synchronous rectifier switch tube is large, it is best to keep it under a higher driving voltage to improve the rectification efficiency and reduce the detection delay. At the same time, it increases the adaptability of the synchronous rectifier switch tube and is not easily affected by the device parameters of the synchronous rectifier switch tube itself.

[0117] In the present invention, the current source can be replaced by a resistor to obtain a similar effect. The current source is used in the implementation because it is a basic component commonly used in integrated circuits and for the convenience of calculation, so as to more clearly explain the principles of the present invention, but it is not used to limit the scope of rights of the present invention.

[0118] The above is only for explaining the implementation mode of the present invention and is not intended to limit the present invention. For those skilled in the art, any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention without creative work should be included in the protection scope of the present invention.

Claims

1. A voltage-limited conduction synchronous rectification control method, characterized in that: The following steps are involved: Step S1, the synchronous detection comparison circuit continuously detects the drain-source voltage of the synchronous rectification switch tube, and fully turns on the synchronous rectification switch tube when the drain-source voltage is less than the turn-on threshold; Step S2, the synchronous detection comparison circuit continuously detects whether the drain-source voltage after the synchronous rectification switch tube is fully turned on is greater than the turn-off threshold, and if the drain-source voltage is greater than the turn-off threshold, the process goes to step S4; at the same time, the voltage-limiting conduction trigger circuit continuously detects the drain-source voltage after the synchronous rectification switch tube is fully turned on, and when the voltage-limiting conduction condition is met, the voltage-limiting conduction effective signal is output to the logic circuit and the voltage-limiting conduction control circuit, triggering the voltage-limiting conduction control, and the process goes to step S3; Step S3, the voltage-limiting conduction control circuit controls the gate voltage of the synchronous rectification switch tube to reduce the gate voltage of the synchronous rectification switch tube to the resistance-increasing gate voltage, and returns to step S2; Step S4: pull down the gate voltage of the synchronous rectification switch tube to make it in a cut-off state.

2. The voltage-limited conduction synchronous rectification control method according to claim 1, characterized in that: The method for determining the voltage-limiting conduction condition is at least one of the following three methods: The first method is voltage comparison method: set the voltage limit turn-on threshold voltage, compare the drain-source voltage when the synchronous rectifier switch is fully turned on with the voltage limit turn-on threshold voltage, and trigger the voltage limit turn-on control when the drain-source voltage is greater than the voltage limit turn-on threshold voltage; The second method is the fixed timing method: the timing circuit starts timing when the synchronous rectifier switch tube is fully turned on, and the voltage limiting conduction control is triggered when the timing ends; The third method is the time comparison method: the conduction time of the synchronous rectification switch tube in each cycle is saved. The current cycle starts from when the synchronous rectification switch tube is fully turned on. When the timing reaches 1 / a of the conduction time of the previous cycle, the voltage limiting conduction control is triggered, a>1.

3. The voltage-limited conduction synchronous rectification control method according to claim 1, characterized in that: When the drain-source voltage is less than the turn-on threshold, the synchronous detection and comparison circuit outputs a first valid signal to the logic circuit; When the logic circuit receives the first valid signal, it controls the voltage of the output pin of the voltage-limited conduction synchronous rectification control chip to rise rapidly to the maximum driving voltage, so that the synchronous rectification switch tube is fully turned on; at the same time, the logic circuit outputs a fast shutdown signal to the voltage-limited conduction trigger circuit; When the voltage-limiting conduction trigger circuit receives the fast shutdown signal, the level of its control terminal changes from a high level to a low level, and the voltage-limiting conduction trigger circuit starts to detect whether the voltage-limiting conduction condition is met. When the voltage-limiting conduction condition is met, the third valid signal is output to the logic circuit and the voltage-limiting conduction control circuit; When the logic circuit receives the third valid signal, it outputs a low-level fully-on signal to disconnect the maximum driving voltage of the output pin; When the voltage-limiting conduction control circuit receives the third valid signal, it detects and controls the voltage of the output pin to drop to the increased-resistance gate voltage, thereby controlling the synchronous rectifier switch tube to be voltage-limited and turned on; when the drain-source voltage of the synchronous rectifier switch tube is greater than the turn-off threshold, the synchronous detection and comparison circuit outputs the second valid signal to the logic circuit; when the logic circuit receives the second valid signal, it outputs a low level to the output pin, thereby controlling the synchronous rectifier switch tube to be quickly turned off.

4. Voltage-limited conduction synchronous rectification control chip, characterized in that: The invention comprises a synchronous detection comparison circuit (401), a logic circuit (402), a voltage-limited conduction trigger circuit (403), a voltage-limited conduction control circuit (404), a first switch (S1) and a second switch (S2); the input end of the synchronous detection comparison circuit (401) and the input end of the voltage-limited conduction trigger circuit (403) are respectively connected to the drain detection input pin of the voltage-limited conduction synchronous rectification control chip, and the ground of the synchronous detection comparison circuit (401), the ground of the voltage-limited conduction trigger circuit (403) and the ground of the voltage-limited conduction control circuit (404) are respectively connected to the source detection input pin of the voltage-limited conduction synchronous rectification control chip; the output end of the synchronous detection comparison circuit (401) The first output end of the logic circuit (402) is connected to the control end of the first switch (S1), and the second output end of the logic circuit (402) is connected to the control end of the second switch (S2) and the control end of the voltage-limiting conduction trigger circuit (403); one end of the first switch (S1) inputs the supply voltage, and the other end is respectively connected to one end of the second switch (S2), the voltage-limiting conduction control circuit (404) and the output pin of the voltage-limiting conduction synchronous rectification control chip; the other end of the second switch (S2) is grounded; and the output end of the voltage-limiting conduction trigger circuit (403) is respectively connected to the logic circuit (402) and the voltage-limiting conduction control circuit (404).

5. The voltage-limited conduction synchronous rectification control chip according to claim 4, characterized in that: The synchronous detection comparison circuit (401) comprises: a second comparator (CMP2) and a third comparator (CMP3); the negative input terminal of the second comparator (CMP2) and the positive input terminal of the third comparator (CMP3) are respectively connected to the drain detection input pin; the positive input terminal of the second comparator (CMP2) inputs the turn-on threshold voltage of the synchronous rectification switch tube, and the negative input terminal of the third comparator (CMP3) inputs the turn-off threshold voltage of the synchronous rectification switch tube; the output terminal of the second comparator (CMP2) outputs a first valid signal, and the output terminal of the third comparator (CMP3) outputs a second valid signal; and the ground of the synchronous detection comparison circuit (401) is connected to the source detection input pin.

6. The voltage-limited conduction synchronous rectification control chip according to claim 4, characterized in that: The logic circuit (402) comprises: an OR gate (OR), a first RS trigger (RS1) and a second RS trigger (RS2); the set input terminal (S) of the first RS trigger (RS1) and the reset input terminal (R) of the second RS trigger (RS2) are respectively connected to the output terminal of the second comparator (CMP2), and one input terminal of the OR gate (OR) and the set input terminal (S) of the second RS trigger (RS2) are respectively connected to the output terminal of the third comparator (CMP3); the other input terminal of the OR gate (OR) is connected to the output terminal of the voltage-limiting conduction trigger circuit (403); the output terminal of the OR gate (OR) is connected to the reset input terminal (R) of the first RS trigger (RS1); the output terminal (Q) of the first RS trigger (RS1) outputs a fully conducting signal to control the closing or opening of the first switch (S1); the output terminal (Q) of the second RS trigger (RS2) outputs a fast shut-off signal to the voltage-limiting conduction trigger circuit (403), and at the same time outputs a fast shut-off signal to control the closing or opening of the second switch (S2).

7. The voltage-limited conduction synchronous rectification control chip according to claim 4, characterized in that: The voltage-limiting conduction trigger circuit (403) is one of the following, or two or more of them are connected in parallel: One: comprising a first comparator (CMP1), wherein the positive input terminal of the first comparator (CMP1) is connected to the drain detection input pin, the negative input terminal is input with the voltage limiting conduction threshold voltage, the control terminal is connected to the second output terminal of the logic circuit (402), and the output terminal of the first comparator (CMP1) serves as the output terminal of the voltage limiting conduction trigger circuit (403); 2: It includes a first comparator (CMP1), a first current source (I REF ), a fourth switch (S4) and a first capacitor (C1); a first current source (I REF ) are respectively connected to one end of a fourth switch (S4), one end of a first capacitor (C1) and a positive input end of a first comparator (CMP1); the other end of the fourth switch (S4) and the other end of the first capacitor (C1) are grounded; the control end of the first comparator (CMP1) and the control end of the fourth switch (S4) are respectively connected to the second output end of the logic circuit (402); the negative input end of the first comparator (CMP1) inputs a first reference voltage, and the output end serves as the output end of a voltage-limiting on trigger circuit (403); Three: including a first comparator (CMP1), a first current source (I REF ), a second current source (I3), a fourth switch (S4), a fifth switch (S5), a sixth switch (S6), a seventh switch (S7), a first capacitor (C1), a second capacitor (C2), a third capacitor (C3), a NOT gate (NOT1) and a monostable trigger; the input end of the NOT gate (NOT1), the control end of the sixth switch (S6) and the control end of the first comparator (CMP1) are respectively connected to the second output end of the logic circuit (402); the output end of the NOT gate (NOT1) is respectively connected to the input end of the monostable trigger and the control end of the fifth switch (S5); the output end of the monostable trigger is respectively connected to the control end of the fourth switch (S4) and the control end of the seventh switch (S7); the first current source (I REF ) is connected to one end of a fifth switch (S5), and the other end of the fifth switch (S5) is respectively connected to one end of a fourth switch (S4), one end of a first capacitor (C1) and the other end of a sixth switch (S6); one end of the sixth switch (S6) is respectively connected to one end of a second capacitor (C2) and a negative input end of a first comparator (CMP1); the output end of a second current source (I3) is respectively connected to a positive input end of the first comparator (CMP1), one end of a seventh switch (S7) and one end of a third capacitor (C3); the other end of the fourth switch (S4), the other end of the first capacitor (C1), the other end of the second capacitor (C2), the other end of the third capacitor (C3) and the other end of the seventh switch (S7) are respectively grounded; the output end of the first comparator (CMP1) serves as the output end of a voltage limiting conduction trigger circuit (403).

8. The voltage-limited conduction synchronous rectification control chip according to claim 4, characterized in that: The circuit structure of the voltage-limiting conduction control circuit (404) is one of the following: The invention comprises a first N-channel MOS transistor (N1), a third switch (S3), a first resistor (R1) and a second resistor (R2); the control end of the third switch (S3) is connected to the output end of the voltage-limiting conduction trigger circuit (403); one end of the third switch (S3) is respectively connected to one end of the second resistor (R2) and the output pin of the voltage-limiting conduction synchronous rectification control chip; the other end of the third switch (S3) is connected to the drain of the first N-channel MOS transistor (N1); the other end of the second resistor (R2) is respectively connected to one end of the first resistor (R1) and the gate of the first N-channel MOS transistor (N1); the other end of the first resistor (R1) and the source of the first N-channel MOS transistor (N1) are grounded; or The invention comprises a first N-channel MOS transistor (N1), a third switch (S3), a second resistor (R2), a first resistor (R1), a first P-channel MOS transistor (P1) and a third current source (I B ); the control end of the third switch (S3) is connected to the output end of the voltage-limiting conduction trigger circuit (403); one end of the second resistor (R2) and one end of the third switch (S3) are connected to the output pin of the voltage-limiting conduction synchronous rectification control chip; the other end of the third switch (S3) is connected to the drain of the first N-channel MOS tube (N1), and the other end of the second resistor (R2) is respectively connected to the source of the first P-channel MOS tube (P1) and one end of the first resistor (R1); the gate of the first P-channel MOS tube (P1) inputs a control voltage, and the drain of the first P-channel MOS tube (P1) is respectively connected to the gate of the first N-channel MOS tube (N1) and the third current source (I B ) input terminal; the source of the first N-channel MOS tube (N1), the third current source (I B ) and the other end of the first resistor (R1) are grounded.

9. The voltage-limited conduction synchronous rectification control chip according to claim 8, characterized in that: It also includes a voltage limiting conduction setting pin, the first resistor (R1) is externally placed, one end of the first resistor (R1) and the other end of the second resistor (R2) are respectively connected to the voltage limiting conduction setting pin, and the other end of the first resistor (R1) is grounded.

10. The voltage-limited conduction synchronous rectification control chip according to claim 8 or 9, characterized in that: The invention also comprises an adaptive voltage limiting driving circuit (601), wherein the adaptive voltage limiting driving circuit (601) comprises a bias current source (I BQ ), a first NPN transistor (Q1) and a second NPN transistor (Q2); the collector of the second NPN transistor (Q2) is connected to the other end of the second resistor (R2), and the emitter of the second NPN transistor (Q2) is connected to the drain detection input pin; a bias current source (I BQ ) are respectively connected to the base of the second NPN transistor (Q2), the base and the collector of the first NPN transistor (Q1); the emitter of the first NPN transistor (Q1) is grounded.

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