CN108462392A - Synchronous rectification control circuit and control method thereof - Google Patents

Abstract

The invention discloses a synchronous rectification control circuit and a control method thereof, which are used for controlling a switching power supply and comprise a transformer, a first switching transistor and a second switching transistor. The synchronous rectification control circuit comprises a conduction sensing module, a voltage averaging module, a volt-second balancing module and a logic control module. The conduction sensing module comprises a first reference potential and a conduction signal. The voltage averaging module comprises an averaging circuit and outputs a second reference potential. The volt-second balancing module comprises a first reference current, a second reference current, a volt-second balancing switch, a volt-second balancing comparator and a timing capacitor, and outputs a reset signal. The logic control module comprises a logic circuit for controlling the second switch transistor to be in a conducting state or a cut-off state.

Description

Synchronous rectification control circuit and its control method

technical field

The invention relates to a synchronous rectification control circuit and a control method thereof, in particular to a synchronous rectification control circuit and a control method thereof which utilize the volt-second balance principle to detect the cut-off time of a switching transistor.

Background technique

Referring to Figure 1, it is the basic circuit architecture of a buck-boost regulator. As shown in FIG. 2 , when the switching transistor SW ₁ is in the on state, the input power supply V _in stores energy in the inductor L, and the diode D ₁ is in the off state. As shown in FIG. 3 , after the switch transistor SW ₁ is turned off, the current stored in the inductor L generates an electromotive force opposite to the input power supply _Vin to charge the capacitor C, and makes the diode D ₁ turn on. In this way, the DC input power V _in can be briefly converted to AC power and the output voltage V _o is stepped down, and then the power required by the load R _L can be provided.

When the aforementioned rectifier circuit is turned on, the diode _{D 1 itself} will generate a barrier voltage, so that part of the power is wasted on the diode D ₁ . This kind of consumption is particularly obvious at low output voltages. For example, when the output voltage V _o is 1.5V, even if a Schottky diode is used at this time, a barrier voltage of 0.4V±0.1V will still be generated, causing conversion efficiency to deteriorate.

In order to solve the defects of the above-mentioned asynchronous rectification circuit, for example, the U.S. Publication No. US8526202 patent case provides a synchronous rectification circuit in which the diode _D1 is replaced by another switching transistor, the switching transistor can be a metal oxide semiconductor field effect transistor, etc. The transistor has a linear volt-ampere relationship when it is turned on, and the on-resistance is about 50mΩ. Assuming the conduction current is 2A, the resulting voltage drop is only 0.1V, which can improve the shortcoming of conversion loss caused by the high barrier voltage of the diode D ₁ in the existing circuit. This kind of circuit is called a synchronous rectification circuit because the gate voltage of the switching transistor must be in phase synchronization with the rectified voltage. The two switching transistors in the synchronous rectification circuit must be turned on or off alternately at each time point, otherwise, if the two switching transistors are turned on at the same time, it may cause damage to the switching power supply.

The aforementioned patent case uses the voltage difference between the two ends of the switching transistor to control the turn-on or cut-off, that is, a comparator with a shift voltage is designed at one end to form a structure similar to a zero-current detection circuit. Although this method can also achieve synchronous rectification purpose, but when this kind of circuit works in continuous conduction mode, the switching transistor on the secondary side will be burned. Therefore, how to accurately control the switching time and make the circuit work in the continuous conduction mode is a problem that the synchronous rectification circuit is eager to overcome.

Contents of the invention

The purpose of the present invention is to solve the problem that the prior art cannot work in the continuous conduction mode.

To achieve the above object, the present invention provides a synchronous rectification control circuit, which is used to control a switching power supply, the switching power supply includes a transformer, a first switching transistor coupled to the primary side of the transformer, and a coupling To the second switching transistor between the secondary side of the transformer and the load, the synchronous rectification control circuit includes a conduction sensing module, a voltage averaging module, and a volt-second balancing module. The conduction sensing module includes a first reference potential, and a conduction sensing comparator, the conduction sensing comparator outputs a conduction signal, and the conduction signal is based on the drain and source of the second switching transistor Whether the potential difference is at a high potential, and then configured as the first reference potential or ground potential. The voltage averaging module includes an averaging circuit coupled to the conduction signal, a second reference potential output by the averaging circuit, and a logic control module. The volt-second balance module includes a first reference current corresponding to the first reference potential, a second reference current corresponding to the second reference potential, and a second reference current interposed between the first reference current and the second reference current and guided by the A volt-second balance switch for switching on or off is controlled by a pass signal, a volt-second balance comparator, and a timing capacitor. The balance switch corresponds to the negative input end of one end of the second reference current and a reset signal coupled to the output end of the volt-second balance comparator, and the timing capacitor is connected between the positive input end and the negative input end. The logic control module includes a logic circuit coupled to the conduction signal and the reset signal, and the logic circuit is coupled to the gate of the second switch transistor, and controls the second switch transistor to be in an on or off state .

Further, the conduction sensing module includes a first threshold potential, and the conduction signal is configured as the first threshold potential according to whether the potential difference between the drain and the source of the second switching transistor is higher than the first threshold potential. A reference potential or ground potential.

Further, the volt-second balance module includes a second threshold potential coupled to the positive input terminal, and the reset signal is configured as pulse wave or ground potential according to whether the potential of the negative input terminal is lower than the second threshold potential .

Further, the averaging circuit includes at least one averaging resistor coupled to the conduction signal, and at least one averaging capacitor connecting the averaging resistor and ground potential.

Further, the volt-second balance module includes a pulse generator coupled to the conduction signal, and a reset switch connected in parallel to the timing capacitor, the pulse generator is switched from low to high when the conduction signal A pulse is generated at the potential and turns on the reset switch.

Another object of the present invention is to provide a synchronous rectification control method, which is used to control a switching power supply. The switching power supply includes a transformer, and a transformer is coupled to the primary side of the transformer and intermittently switches on and off. state of the first switching transistor, and a second switching transistor coupled between the secondary side of the transformer and the load, the synchronous rectification control method includes the following steps: (a) setting the first switching transistor to conduct The time of the on-state is the conduction time, and the time of the off-state is the off-time; (b) generate a conduction signal, which is configured to output a first reference potential during the on-time, and in the off-time Output the ground potential in the middle, and generate a second reference potential equal to the average value of the conduction signal; (c) convert the first reference potential to a first reference current, and convert the second reference potential to a second reference current; ( d) Provide a timing capacitor, which flows a timing current and generates an integral voltage; (e) sets the timing current to be configured as the first reference current minus the second during the on-time Reference current, within the cut-off time, the timing current is configured as the negative value of the second reference current; and (f) judging whether the integrated voltage is from a high potential to zero, and performing the following steps: (f1) if the When the integrated voltage goes from the high potential to zero, the second switching transistor is turned off.

Further, the step (f) further includes the following steps: (f2) clearing the voltage of the timing capacitor.

Therefore, the present invention has the following beneficial effects compared with prior art:

1. The present invention uses the voltage across the drain-drain-source-gate of the second switching transistor to calculate the conduction time of the secondary side of the transformer, and improves the disadvantage of the prior art that only works in the discontinuous conduction mode using a zero-current detection method.

2. The volt-second balance module of the present invention uses a combination of capacitance and current to calculate the precise switching time when the second switching transistor should be turned on or off, replacing the existing method of adding a control coil to the transformer to calculate the turn-on time, and can Effectively reduce the overall circuit size.

Description of drawings

Figure 1: A circuit diagram of an existing buck-boost regulator.

Figure 2: It is a schematic diagram of an existing buck-boost regulator when the switching transistor is turned on.

Figure 3: It is a schematic diagram of a conventional buck-boost regulator when the switching transistor is turned off.

Fig. 4: is the schematic diagram of the combination of the present invention and the switching power supply.

Fig. 5: is a circuit diagram of an embodiment of the present invention.

FIG. 6 is a timing diagram of an embodiment of the present invention.

Figure 7: Timing diagram for an existing regulator operating in continuous conduction mode.

Fig. 8: is the circuit diagram of the volt-second balance module of the present invention.

Fig. 9 is a timing diagram of the volt-second balance module of the present invention.

Fig. 10: is a flowchart of the control method of the present invention.

Among them, reference signs:

Synchronous rectification control circuit.100 Conduction sensing module.10

Conduction sensing comparator.11 Voltage averaging module.20

Average circuit.21 Average resistance.211

Average Capacitance.212 Volt-Second Balance Module.30

Volt-second balance switch.31 Volt-second balance comparator.32

Positive input.321 Negative input.322

Timing capacitor.33 Pulse generator.34

Reset switch.35 Logic control module.40

Logic circuit.41 Switching power supply.200

Transformer. 210 First switching transistor. 220

Second switching transistor. 230 First reference potential. V _ref1

Second reference potential. V _ref2 First threshold potential. V _A

Second threshold potential.V _B Integral voltage.V _C

First Gate Control Signal.V _G1 Second Gate Control Signal.V _G2

On signal.V ₁₁ Reset signal.V ₃₂

Duty Period.D Inductor Current.I _L

First reference current.I _ref1 Second reference current.I _ref2

Switching transistor. SW ₁ drain-source voltage. V _ds1 , V _ds2

Input Power.V _in Output Voltage.V _o

Inductor.L Diode.D ₁

Capacitance.C On-time.T _on

cut-off time.T _off cycle time.T _s

Load.R _L rising slope.m ₁

Declining slope.m ₂

Detailed ways

A few preferred implementations of the technical features and operation methods of this application are hereby given, and will be described later with illustrations, so as to provide reference for examination. Furthermore, the proportions of the drawings in the present invention may not be drawn according to the actual scale for the convenience of explanation, and the proportions in the drawings are not intended to limit the scope of protection claimed by the present invention.

"Circuit Structure Description"

Regarding the technology of the present invention, please refer to FIG. 4 , the present invention provides a synchronous rectification control circuit for controlling a switching power supply 200 . The switching power supply 200 includes a transformer 210, a primary circuit coupled to the primary side of the transformer 210, a second switching transistor 230 coupled between the secondary side of the transformer 210 and the load, and an output connected in parallel with the load capacitance. In this embodiment, the primary circuit includes a first switch transistor 220 coupled between the primary side of the transformer 210 and ground potential, and a first gate control signal coupled to the gate of the first switch transistor 220 V _G1 . The first gate control signal V _G1 is used to control the on or off state of the first switch transistor 220 , and the generation method of the first gate control signal V _G1 has been known by those skilled in the art of the present invention. Familiar, will not be described in detail here.

Please refer to FIG. 5 and FIG. 6 , specifically, the synchronous rectification control circuit of the present invention mainly includes a conduction sensing module, a voltage averaging module 20 , a volt-second balancing module 30 , and a logic control module 40 . The conduction sensing module is used for sensing the conduction state of the first switching transistor 220, which includes a first reference potential V _ref1 , and a conduction sensing comparator 11, the conduction sensing comparator 11 Outputting a conduction signal, the conduction signal is configured as the first reference potential Vref1 or the ground potential according to whether the potential difference between the drain and the source of the second switching transistor 230 is at a high potential. When the circuit is in a steady state and the first transistor is turned on, its drain-source voltage _Vds1 is at a low potential, and the second switching transistor 230 is turned off, its drain-source voltage Vds2 is at a high potential, at this moment the turn-on signal appears The first reference potential V _ref1 ; conversely, when the first transistor is turned off, the second switch transistor 230 is turned on and its drain-source voltage V _ds2 is low. In this embodiment, the negative input side of the conduction sensing comparator 11 is set with a first threshold potential V _A , and the conduction signal depends on whether the potential difference between the drain and the source of the second switching transistor 230 is higher than the The first threshold potential V _A is further configured as the first reference potential V _ref1 or the ground potential, thereby canceling the bias voltage in the circuit.

The voltage averaging module 20 includes an averaging circuit 21 coupled to the conduction signal, and a second reference potential V _ref2 output by the averaging circuit 21 . In this embodiment, the averaging circuit 21 is a series-parallel circuit of resistors and capacitors, whose function is to convert the time-varying conduction signal into an average voltage (that is, the second reference potential V _ref2 ). In the present invention, the averaging circuit 21 only needs to achieve the purpose of converting the conduction signal into an average voltage, and the specific circuit configuration of the average circuit 21 is not limited here.

The volt-second balance module 30 includes a first reference current I _ref1 corresponding to the first reference potential V _ref1 , a second reference current I _ref2 corresponding to the second reference potential V _ref2 (specifically, it can be provided by an existing voltage-to-current converter, etc. circuit implementation), a volt-second balance switch 31 (which can be a transistor component) interposed between the first reference current I _ref1 and the second reference current I _ref2 and controlled by the conduction signal to turn on or off, a volt-second A balance comparator 32, and a timing capacitor 33, the volt-second balance comparator 32 includes a positive input terminal 321 at a low potential, a negative terminal coupled to the volt-second balance switch 31 corresponding to one end of the second reference current I _ref2 Input terminal 322, and a reset signal V32 coupled to the output terminal of the volt-second balance comparator 32, and the timing capacitor 33 is connected between the positive input terminal 321 and the negative input terminal 322, and the timing capacitor 33 is connected between terminal is to generate an integral voltage V _C by the current. In this embodiment, the volt-second balance module 30 includes a pulse generator 34 coupled to the conduction signal, and a reset switch 35 connected in parallel to the timing capacitor 33. The pulse generator 34 is When the conduction signal turns from a low potential to a high potential, a pulse wave is generated, and the reset switch 35 is turned on. Since one end of the timing capacitor 33 is connected to the ground potential, when the reset switch 35 is turned on, the reset switch 35 is turned on. The charge on the timing capacitor 33 will move towards the ground potential, thereby ensuring that the integrated voltage V _C is completely cleared every time the first switching transistor 220 is turned on.

The logic control module 40 includes a logic circuit 41 coupled to the conduction signal and the reset signal _V32 , and the logic circuit 41 is coupled to the gate of the second switching transistor 230, and outputs a second gate control The signal V _G2 controls the second switch transistor 230 to turn on or off. In this embodiment, the logic circuit 41 is composed of a logic gate and an RS flip-flop, and the output of the RS flip-flop is reversed to the conduction signal, thereby controlling the second switch transistor 230 to be connected to the first switch. The transistors 220 are turned on alternately to achieve the purpose of switching synchronous rectification.

"Principle Explanation"

In the present invention, mainly based on the principle of volt-second balance, the characteristic that the ripple current of the inductor on the transformer 210 changes in each cycle is equal to zero in the continuous conduction mode is used to obtain the correct time point at the end of a cycle . Referring to the circuit shown in FIG. 1 , set the on-time T _on , the cycle time T _s , and set the off-time T _off , then the duty cycle D of the first switching transistor 220 is:

The aforementioned inductor current I _L flowing through the transformer 210 is shown in FIG. 7 . It can be seen from the figure that the inductor current IL includes two parts: the rising slope m1 (0-dT _s interval) and the falling slope m2 (dT _s -T _s interval) in a single cycle time, and then depends on the relationship between the voltage and current changes on the inductor , and the principle of volt-second balance, it can be seen that the relationship between the duty cycle D and the rising slope m ₁ and falling slope m ₂ is as follows:

Next, assume that the voltage acting on increasing the current is the first reference voltage V _ref1 , and the voltage acting on the average value of the current is the second reference voltage V _ref2 , then the first reference voltage V _ref1 and the second reference voltage V _ref2 The ratio is =(m ₁ -m ₂ ):(-m ₂ ), which can be substituted into formula 2 to get:

Since the second reference voltage V _ref2 is equal to the product of the first reference voltage V _ref1 and the duty cycle D, it can be understood that the second reference voltage V _ref2 is equal to the average value of the voltage of the first reference voltage V _ref1 and the duty cycle D .

Referring to Figure 8 and Figure 9 together, it can be known from the definition of capacitors:

Assume that the current used to increase the integrated voltage V _c is the first reference current I _ref1 , and the current that decreases the integrated voltage V _c is the second reference current I _ref2 , and substitute the on-time T _on and off-time T _off into Equation 4 got later and According to the principle of volt-second balance, it can be known that the voltage change is zero, that is, Δv ₍₊₎ = Δv _(-) . Let the ratios of the first reference current I _ref1 and the second reference current I _ref2 correspond to the first reference voltage and the second reference voltage be K1 and K2. To simplify the calculation, set K1=K2 and substitute it into Equation 4 to get:

It can be seen from the above that Equation 3 is the same as Equation 5, so the aforementioned physical circuit consisting of inductance to detect on-time T _on and on-time T _off can be replaced by a capacitor circuit. Since the current ripple flowing through the timing capacitor 33 will be The two ends of the timing capacitor 33 generate an integrated voltage V _C , so the present invention can obtain the correct time point for switching the second switching transistor 230 on or off through the voltage change at both ends of the timing capacitor 33 to achieve the purpose of synchronous rectification.

"Control Method Process Description"

The control method of this case is described in detail below, referring to FIG. 10, the described synchronous rectification control method includes the following steps:

Step (a) Set the time when the first switching transistor 220 is in the on state as the on time T _on , and the time in which the first switch transistor 220 is in the off state as the off time T _off . In this step, the on-time T _on and the off-time T _off can be determined according to user requirements.

Step (b) generates a turn-on signal, the turn-on signal is configured to output a first reference potential V _ref1 during the turn-on time T _on , output a ground potential during the turn-off time T _off , and generate a signal equal to the turn-on time The second reference potential V _ref2 of the signal mean value.

Step (c) converting the first reference potential V _ref1 into a first reference current I _ref1 , and converting the second reference potential V _ref2 into a second reference current I _ref2 . In this step, a voltage-to-current conversion circuit is specifically used, and the voltage-to-current ratios of the first reference current I _ref1 and the second reference current I _ref2 are consistent.

Step (d) provides a timing capacitor 33 , the timing capacitor 33 flows a timing current and generates an integrated voltage V _C .

Step (e) sets that during the on-time, the timing current is configured as the first reference current I _ref1 minus the second reference current I _ref2 , and during the off-time, the timing current is configured as The negative value of the second reference current I _ref2 . Thereby, when the first switching transistor 220 is turned on, the timing current gradually rises, and when the first switching transistor 220 is turned off, and when the second switching transistor 230 is turned on, the timing current gradually decreases, and the The voltage on the timing capacitor 33 then rises and falls.

Step (f) judging whether the integrated voltage V _C is from a high potential to zero, and performing the following steps: (f1) when the integrated voltage V _C is from a high potential to zero, turning off the second switching transistor 230, and performing the step ( f2) Clear the voltage of the timing capacitor 33 . This step is for continuous judgment. If the integrated voltage V _C has not yet reached zero from the high potential, the judgment of step (f) will be continued.

The content of the present invention has been described in detail above, and the above description is only a preferred embodiment of the present invention, and should not limit the scope of the present invention with this, that is, all equivalent changes and modifications made according to the patent scope of the present invention are all Should still belong to the protection scope of the claims of the present invention.

Claims (7)

1. a kind of synchronous commutating control circuit is used in one exchange type power of control, which includes a transformer, One be coupled to the first switch transistor of the transformer primary and one be coupled between the transformer secondary and load the Two switching transistors, which is characterized in that the synchronous commutating control circuit includes：

One conducting sensing module includes that one first reference potential and a conducting sensing comparator, the conducting sense comparator A Continuity signal is exported, whether which is in high electricity according to the potential difference of the second switch transistor drain and source electrode Position, and then be configured as first reference potential or ground potential；

One average voltage module includes that the average circuit of the coupling Continuity signal and one are exported by the average circuit One second reference potential；

One voltage-second balance module includes the first reference current of a correspondence first reference potential, and one corresponds to second reference Second reference current of current potential, one is situated between first reference current and second reference current and is cut by Continuity signal control The voltage-second balance switch of on or off state is changed, a voltage-second balance comparator and a timer capacitor, the voltage-second balance compare Device includes a positive input terminal for being in low potential, and one couples the negative of the corresponding second reference current one end of voltage-second balance switch Input terminal and one couple the voltage-second balance comparator output terminal reset signal, and the timer capacitor is connected across the positive input Between end and the negative input end；And

One Logic control module, includes the coupling Continuity signal and the logic circuit of the reset signal, and the logic circuit The grid of the second switch transistor is coupled, and it is on or off state to control the second switch transistor.

2. the synchronous commutating control circuit as described in claim the 1, which is characterized in that the conducting sensing module includes one Whether one threshold potential, the Continuity signal are higher than the first threshold according to the potential difference of the second switch transistor drain and source electrode Current potential, and then be configured as first reference potential or ground potential.

3. the synchronous commutating control circuit as described in claim the 1, which is characterized in that the voltage-second balance module includes a coupling The second threshold current potential of the positive input terminal is connect, whether the reset signal is electric less than the second threshold according to the current potential of the negative input end Position, and then be configured as pulse wave or ground potential.

4. the synchronous commutating control circuit as described in claim the 1, which is characterized in that the average circuit includes an at least coupling Connect the average resistance of the Continuity signal, and the average capacitance of at least one bridging average resistance and ground potential.

5. the synchronous commutating control circuit as described in claim the 1, which is characterized in that the voltage-second balance module includes a coupling Connect the pulse generator of the Continuity signal and Resetting Switching that one is connected in parallel on the timer capacitor, which leads at this Pulse wave is generated when messenger switchs to high potential by low potential and the Resetting Switching is made to be connected.

6. a kind of synchronous rectification control method is used in one exchange type power of control, which includes a transformer, One is coupled to the first switch transistor of the transformer primary and intermittently switched conductive and cut-off state and a coupling Second switch transistor between the transformer secondary and load, which is characterized in that the synchronous rectification control method packet Contain following steps：

(a) the first switch transistor is set in the time of conducting state as turn-on time, and the time in cut-off state is cut-off Time；

(b) Continuity signal is generated, which is configured to export one first reference potential in turn-on time, ending Ground potential is exported in time, and generates the second reference potential of an equivalent Continuity signal mean value；

(c) convert first reference potential into one first reference current, convert second reference potential as one second reference current；

(d) timer capacitor, the timer capacitor one chrono-amperometric of circulation are provided, and generate an integral voltage；

(e) be set in the turn-on time, the chrono-amperometric is configured and subtracts second reference current for first reference current, Within the deadline, the chrono-amperometric is configured as the negative value of second reference current；And

(f) whether the integral voltage is judged from high potential to zero, and executes following steps：(f1) if the integral voltage is from high potential To zero, then end the second switch transistor.

7. the method as described in claim the 6, which is characterized in that step (f) further includes following steps：(f2) timing is removed The voltage of capacitance.