阅读说明：本技术 一种反激变换器及其控制方法 (Flyback converter and control method thereof ) 是由 李樟红 于 2021-06-29 设计创作，主要内容包括：本发明涉及变换器设计领域,公开一种反激变换器的控制方法,该方法包括：根据反激变换器不同的工况条件对反激变换器的工作模式进行控制,使得反激变换器工作在第一工作模式或第二工作模式；当反激变换器工作在第一模式时,通过向次级侧开关单元输送相互间隔的一同步整流控制信号和一ZVS控制信号来控制次级侧开关单元在一个开关周期内开通两次,其中,同步整流控制信号和所述ZVS控制信号分别为脉冲信号,ZVS控制信号的脉冲宽度小于同步整流控制信号的脉冲宽度。本发明通过检测不同的信号来反映不同的工况条件,同时根据不同的检测信号使反激变换器工作在不同的工作状态,以使得在各种情况下反激变换器均具有最优工作性能。(The invention relates to the field of converter design, and discloses a control method of a flyback converter, which comprises the following steps: controlling the working mode of the flyback converter according to different working conditions of the flyback converter, so that the flyback converter works in a first working mode or a second working mode; when the flyback converter works in a first mode, the secondary side switching unit is controlled to be switched on twice in one switching period by transmitting a synchronous rectification control signal and a ZVS control signal which are mutually spaced, wherein the synchronous rectification control signal and the ZVS control signal are respectively pulse signals, and the pulse width of the ZVS control signal is smaller than that of the synchronous rectification control signal. The flyback converter can reflect different working conditions by detecting different signals, and simultaneously, the flyback converter can work in different working states according to different detection signals, so that the flyback converter has the optimal working performance under various conditions.)

1. A control method of a flyback converter, the converter circuit comprising a primary side switch unit, a secondary side switch unit, a transformer and an output capacitor, the secondary side switch unit comprising a first terminal and a second terminal, the first terminal being connected to the transformer and the second terminal being connected to the output capacitor, the control method comprising: controlling the working mode of the flyback converter according to the input voltage or the output feedback signal or the load of the flyback converter, so that the flyback converter works in a first working mode or a second working mode;

when the flyback converter works in a first mode, a synchronous rectification control signal and a ZVS control signal which are mutually spaced are transmitted to the secondary side switching unit to control the secondary side switching unit to be switched on twice in one switching period, wherein the synchronous rectification control signal and the ZVS control signal are respectively pulse signals, and the pulse width of the ZVS control signal is smaller than that of the synchronous rectification control signal;

when the flyback converter works in a second mode, the secondary side switch unit is controlled to be switched on once in one switching period by transmitting the ZVS control signal with smaller pulse width to the secondary side switch unit.

2. The method of controlling a flyback converter as in claim 1, wherein: and the interval time between the synchronous rectification control signal and the ZVS control signal is controlled according to the size of the load of the flyback converter, and the interval time is longer when the load is smaller.

3. The method of controlling a flyback converter as in claim 1, wherein: when the flyback converter works in a second mode, during the period that the secondary side switching unit is switched on, the current generated by the secondary side winding is negative current.

4. The method of controlling a flyback converter as in claim 1, wherein: the pulse width of the ZVS control signal is controlled according to the interpolar voltage between the drain and the source of the primary side switch unit;

when the interpolar voltage of the primary side switch is less than an interpolar voltage threshold, then in a next switching cycle, decreasing the pulse width of the ZVS control signal; when the inter-pole voltage of the primary side switch is greater than the inter-pole voltage threshold, increasing the pulse width of the ZVS control signal in a next switching cycle.

5. The method of controlling a flyback converter as in claim 1, wherein: the pulse width of the ZVS control signal is controlled according to the input voltage, and when the input voltage is increased, the pulse width of the ZVS control signal is increased in the next period; when the input voltage drops, the next cycle decreases the pulse width of the ZVS control signal.

6. The method of controlling a flyback converter as in claim 1, wherein: the pulse width of the ZVS control signal is controlled according to the input voltage and the output voltage;

the larger the input voltage is, the wider the pulse width of the ZVS control signal is, and the smaller the input voltage is, the narrower the pulse width of the ZVS control signal is;

the smaller the output voltage is, the wider the pulse width of the ZVS control signal is, and the larger the output voltage is, the narrower the pulse width of the ZVS control signal is.

7. The method of controlling a flyback converter as in claim 1, wherein: controlling the working mode of the flyback converter according to the input voltage of the flyback converter, wherein the control method comprises the following steps: when the input voltage is smaller than a first input voltage threshold and larger than a second input voltage threshold, controlling the converter to work in the first working mode; when the input voltage is larger than a first input voltage threshold value, controlling the converter to work in the second working mode; and when the input voltage is smaller than a second input voltage threshold value, controlling the converter to work in a third working mode, and when the converter works in the third working mode, controlling the secondary side switching unit to be switched on once in one switching period by transmitting the synchronous rectification control signal to the secondary side switching unit.

8. The method of controlling a flyback converter as in claim 1, wherein: controlling the working mode of the flyback converter according to the magnitude of the output feedback signal of the flyback converter, wherein the control method comprises the following steps: when the output feedback signal is greater than or equal to a first feedback signal threshold value, controlling the converter to work in the first mode; when the detected output feedback signal is smaller than the first feedback signal threshold value and larger than or equal to a second feedback signal threshold value, controlling the converter to work in the second mode; and when the output feedback signal is smaller than a second feedback signal threshold value, controlling the converter to work in a skip cycle mode.

9. The method of controlling a flyback converter as in claim 1, wherein: controlling the working mode of the flyback converter according to the size of the load of the flyback converter, specifically controlling the working mode of the flyback converter according to the output power reflecting the size of the load of the converter, and controlling the converter to work in a first working mode when the output power is greater than or equal to a first power threshold; when the output power is smaller than a first power threshold and larger than or equal to a second power threshold, controlling the converter to work in the second working mode; and when the output power is smaller than a second power threshold value, controlling the converter to work in a skip cycle mode.

10. A flyback converter for converting an input voltage to an output voltage for provision to a load, characterized by: the method comprises the following steps:

a transformer configured to include a primary side winding and a secondary side winding;

a primary side switching unit configured to be connected between the primary side winding and a ground terminal;

a secondary side switching unit configured to connect the secondary side winding;

the control device is configured to control the flyback converter to work in a first working mode or a second working mode according to the input voltage or the output feedback signal of the flyback converter or the size of a load;

when the flyback converter works in a first mode, a synchronous rectification control signal and a ZVS control signal which are mutually spaced are transmitted to the secondary side switching unit to control the secondary side switching unit to be switched on twice in one switching period, wherein the synchronous rectification control signal and the ZVS control signal are respectively pulse signals, and the pulse width of the ZVS control signal is smaller than that of the synchronous rectification control signal;

when the flyback converter works in a second mode, the secondary side switch unit is controlled to be switched on once in one switching period by transmitting the ZVS control signal to the secondary side switch unit.

11. The flyback converter of claim 10, wherein: when the flyback converter works in a second mode, the current generated by the secondary side winding only has negative current during the switching-on period of the secondary side switching unit.

12. A control method of a flyback converter, the flyback converter including a primary side switch unit, a secondary side switch unit and a transformer, the secondary side switch unit being connected to a primary side winding of the transformer, the secondary side switch unit being connected to a secondary side winding of the transformer, the control method comprising:

detecting the input voltage or the output feedback signal of the flyback converter or the size of a load;

controlling the flyback converter to work in any one of a first working mode and a second working mode according to the detected input voltage or the output feedback signal or the size of the load;

when the flyback converter works in a first mode, the secondary side switching unit is controlled to be switched on twice in one switching period by transmitting a wide pulse signal and a narrow pulse signal which are mutually spaced to the secondary side switching unit;

when the flyback converter works in a second mode, the narrow pulse signal is transmitted to the secondary side switch unit to control the secondary side switch unit to be switched on once in one switch period so as to realize zero voltage switching-on of the primary side switch unit.

13. The method of controlling a flyback converter as in claim 12, wherein: when the flyback converter works in a first mode, the synchronous rectification control signal is transmitted to the secondary side switch unit after the primary side switch unit is turned off for a set delay time to control the secondary side switch unit to be turned on, and the turn-on is finished when the excitation current of the flyback converter is reduced to zero.

14. A control method of a flyback converter circuit is disclosed, wherein the flyback converter comprises a primary side switch unit, a secondary side switch unit, a transformer and an output capacitor, the secondary side switch unit comprises a first end and a second end which are respectively electrically connected with the transformer and the output capacitor; the control method comprises the following steps:

detecting the input voltage or the output feedback signal of the flyback converter or the size of a load;

outputting a mode selection signal according to the detected input voltage or the output feedback signal or the size of the load;

selecting the flyback converter to work in a first working mode or a second working mode according to the mode selection signal;

when the flyback converter works in a first mode, the secondary side switch unit is controlled to be switched on twice in one switch period by transmitting a synchronous rectification control signal and a ZVS control signal which are mutually spaced to the secondary side switch unit;

when the flyback converter works in a second mode, the secondary side switch unit is controlled to be switched on once in one switching period by transmitting the ZVS control signal to the secondary side switch unit.

Technical Field

The invention relates to the field of converter design, in particular to a flyback converter and a control method thereof.

Background

The flyback converter is widely applied to medium and small power switching power supplies due to the advantages of low cost, simple topology and the like. In general, in order to improve the working efficiency of the flyback converter, the secondary side adopts a synchronous rectification method, and meanwhile, because the valley bottom switching of the primary side power switch tube can be realized, the synchronous rectification quasi-resonant flyback converter is often adopted, so that the switching loss can be obviously reduced. However, under the working condition of high-voltage input, although the valley bottom is conducted, the problem of larger conduction loss still exists.

Referring to fig. 1, fig. 1 is a circuit diagram of a conventional flyback converter with synchronous rectification on the secondary side, which includes a primary side controller U1 and a secondary side controller U2, and controls a primary side switching unit through the primary side controller U1 and a secondary side switching unit through the secondary side controller U2.

Fig. 2 is a working waveform diagram of a secondary side control strategy for implementing ZVS of a primary side power switching unit, a reverse current is generated in a secondary side coil through conduction delay of a secondary side rectifier tube, after the secondary side rectifier tube is turned off, the primary side power tube is turned on after a preset dead time, and zero voltage turn-on (ZVS) of a primary side main power tube is implemented through resonance of a current participating excitation inductor and a parasitic capacitor of the primary side power switching tube in the dead time.

However, the operation state requires that the circuit must operate in critical conduction mode in the whole input voltage range and the whole load range; according to the working principle of the critical conduction mode of a resonant flyback converter (QRFLck), under the same load condition, the higher the input voltage is, the higher the working frequency is; also for input voltage, the lighter the load, the higher its operating frequency. Therefore, the operating frequency under light load with high voltage input becomes very high, and the switching loss caused thereby seriously affects the efficiency. Therefore, the mode is suitable for working under the working conditions of heavy load or full load and the like with large load, and the current control method cannot ensure that the flyback converter can reach the optimal working state within the full-voltage full-load range.

Therefore, how to ensure that the flyback converter can achieve the optimal working state in the full-voltage full-load range so as to improve the working efficiency of the converter is a technical problem to be solved urgently in the industry.

Disclosure of Invention

In view of the defects of the prior art, the invention provides a flyback converter capable of realizing zero-voltage switching on a primary side and a control method thereof, so as to solve the problem that the conventional flyback converter can reach the optimal working state under the condition of wide-range working conditions.

The invention provides a control method of a flyback converter, a converter circuit comprises a primary side switch unit, a secondary side switch unit, a transformer and an output capacitor, the secondary side switch unit comprises a first end and a second end, the first end is connected with the transformer, the second end is connected with the output capacitor, the control method comprises the following steps: controlling the working mode of the flyback converter according to the input voltage or the output feedback signal of the flyback converter or the size of a load, so that the flyback converter works in a first working mode or a second working mode;

when the flyback converter works in a first mode, a synchronous rectification control signal and a ZVS control signal which are mutually spaced are transmitted to the secondary side switching unit to control the secondary side switching unit to be switched on twice in one switching period, wherein the synchronous rectification control signal and the ZVS control signal are respectively pulse signals, and the pulse width of the ZVS control signal is smaller than that of the synchronous rectification control signal;

when the flyback converter works in the second mode, the secondary side switch unit is controlled to be switched on once in one switching period by transmitting a ZVS control signal with smaller pulse width to the secondary side switch unit.

In one embodiment, the interval time between the synchronous rectification control signal and the ZVS control signal is controlled according to the size of the load of the flyback converter, and the interval time is longer when the load is smaller.

In one embodiment, when the flyback converter operates in the second mode, the current generated by the secondary side winding is a negative current during the turn-on period of the secondary side switching unit.

In one embodiment, the pulse width of the ZVS control signal is controlled according to an inter-electrode voltage between the drain and the source of the primary-side switching unit;

when the interpolar voltage of the primary side switch is smaller than an interpolar voltage threshold, reducing the pulse width of the ZVS control signal in the next switching period; and when the interpolar voltage of the primary side switch is greater than the interpolar voltage threshold, increasing the pulse width of the ZVS control signal in the next switching period.

In one embodiment, the pulse width of the ZVS control signal is controlled according to the input voltage, and when the input voltage increases, the pulse width of the ZVS control signal is increased in the next period; when the input voltage drops, the next cycle decreases the pulse width of the ZVS control signal.

In one embodiment, the pulse width of the ZVS control signal is controlled according to the input voltage and the output voltage;

the larger the input voltage is, the wider the pulse width of the ZVS control signal is, and the smaller the input voltage is, the narrower the pulse width of the ZVS control signal is;

the smaller the output voltage, the wider the pulse width of the ZVS control signal, and the larger the output voltage, the narrower the pulse width of the ZVS control signal.

In one embodiment, the operation mode of the flyback converter is controlled according to the magnitude of the input voltage of the flyback converter, and specifically, when the input voltage is smaller than a first input voltage threshold and larger than a second input voltage threshold, the converter is controlled to operate in a first operation mode; when the input voltage is larger than the first input voltage threshold value, controlling the converter to work in a second working mode; and when the input voltage is smaller than the second input voltage threshold value, controlling the converter to work in a third working mode, and when the converter works in the third working mode, controlling the secondary side switching unit to be switched on once in one switching period by transmitting a synchronous rectification control signal to the secondary side switching unit.

In one embodiment, the operation mode of the flyback converter is controlled according to the magnitude of the output feedback signal of the flyback converter, and specifically, when the output feedback signal is greater than or equal to a first feedback signal threshold value, the converter is controlled to operate in a first mode; when the detected output feedback signal is smaller than the first feedback signal threshold value and is larger than or equal to the second feedback signal threshold value, controlling the converter to work in a second mode; and when the output feedback signal is smaller than the second feedback signal threshold value, controlling the converter to work in a cycle skipping mode.

In one embodiment, the operating mode of the flyback converter is controlled according to the size of the load of the flyback converter, specifically, the operating mode of the flyback converter is controlled according to the output power reflecting the size of the load of the converter, and when the output power is greater than or equal to a first power threshold, the converter is controlled to operate in a first operating mode; when the output power is smaller than the first power threshold and larger than or equal to the second power threshold, controlling the converter to work in a second working mode; and when the output power is smaller than the second power threshold value, controlling the converter to work in a skip cycle mode.

The present invention also provides a flyback converter for converting an input voltage into an output voltage for supply to a load, comprising:

a transformer configured to include a primary side winding and a secondary side winding;

a primary side switching unit configured to be connected between the primary side winding and a ground terminal;

a secondary side switching unit configured to connect the secondary side winding;

the control device is configured to control the flyback converter to work in a first working mode or a second working mode according to the input voltage or the output feedback signal of the flyback converter or the size of a load;

when the flyback converter works in a first mode, a synchronous rectification control signal and a ZVS control signal which are mutually spaced are transmitted to the secondary side switching unit to control the secondary side switching unit to be switched on twice in one switching period, wherein the synchronous rectification control signal and the ZVS control signal are respectively pulse signals, and the pulse width of the ZVS control signal is smaller than that of the synchronous rectification control signal;

when the flyback converter works in the second mode, the secondary side switch unit is controlled to be switched on once in one switch period by transmitting a ZVS control signal to the secondary side switch unit.

In one embodiment, when the flyback converter operates in the second mode, the current generated by the secondary side winding is a negative current during the turn-on period of the secondary side switching unit.

The invention also provides a control method of the flyback converter, which comprises the following steps: the flyback converter comprises a primary side switch unit, a secondary side switch unit and a transformer, wherein the secondary side switch unit is connected with a primary side winding of the transformer, the secondary side switch unit is connected with a secondary side winding of the transformer, and the control method comprises the following steps:

detecting the input voltage or the output feedback signal of the flyback converter or the size of a load;

controlling the flyback converter to work in any one of a first working mode and a second working mode according to the detected input voltage or the output feedback signal or the size of the load;

when the flyback converter works in a first mode, the secondary side switching unit is controlled to be switched on twice in one switching period by transmitting a wide pulse signal and a narrow pulse signal which are mutually spaced to the secondary side switching unit;

when the flyback converter works in the second mode, the narrow pulse signal is transmitted to the secondary side switch unit to control the secondary side switch unit to be switched on once in one switch period so as to realize zero voltage switching-on of the primary side switch unit.

In one embodiment, when the flyback converter operates in the first mode, the synchronous rectification control signal is transmitted to the secondary side switching unit after the primary side switching unit is turned off for a set delay time to control the secondary side switching unit to be turned on, and the turning on is finished when the excitation current of the flyback converter drops to zero.

The invention also provides a control method of the flyback converter circuit, the flyback converter comprises a primary side switch unit, a secondary side switch unit, a transformer and an output capacitor, the secondary side switch unit comprises a first end and a second end which are respectively electrically connected with the transformer and the output capacitor; the control method comprises the following steps:

detecting the input voltage or the output feedback signal of the flyback converter or the size of a load;

outputting a mode selection signal according to the detected input voltage or the output feedback signal or the size of the load;

selecting the flyback converter to work in a first working mode or a second working mode according to the mode selection signal;

when the flyback converter works in a first mode, the secondary side switching unit is controlled to be switched on twice in one switching period by transmitting a synchronous rectification control signal and a ZVS control signal which are mutually spaced to the secondary side switching unit;

when the flyback converter works in the second mode, the secondary side switch unit is controlled to be switched on once in one switch period by transmitting a ZVS control signal to the secondary side switch unit.

Compared with the prior art, the invention has the beneficial effects that:

the flyback converter can reflect different working conditions by detecting different signals (input voltage or output feedback signals or loads), and simultaneously, the flyback converter can work in different working states according to different detection signals so as to realize the optimal working performance of the flyback converter under various conditions.

Drawings

FIG. 1 is a schematic diagram of a conventional converter with a secondary side synchronous rectifier circuit;

fig. 2 is a control sequence of a conventional flyback converter;

fig. 3 is a schematic diagram of a flyback converter according to a first embodiment of the present invention;

fig. 4 is a waveform diagram illustrating operation of the flyback converter in the first operation mode according to the first embodiment of the present invention;

fig. 5 is a waveform diagram illustrating operation of the flyback converter in the second operation mode according to the first embodiment of the present invention;

fig. 6 is a control flowchart of a control method of the flyback converter according to the first embodiment of the present invention;

fig. 7 is a control flowchart of a control method of a flyback converter according to a second embodiment of the present invention;

FIG. 8 is a diagram illustrating a skip cycle operating mode according to a second embodiment of the present invention;

fig. 9 is a schematic diagram of a flyback converter according to a third embodiment of the present invention;

fig. 10 is a control flowchart of a control method of a flyback converter according to a third embodiment of the present invention;

fig. 11 is a diagram illustrating the selection of an operation mode according to a feedback signal according to a third embodiment of the present invention.

Detailed Description

Exemplary embodiments that embody features and advantages of the present disclosure will be described in detail in the following description in conjunction with the accompanying drawings. It is to be understood that the disclosure is capable of various modifications in various embodiments without departing from the scope of the disclosure, and that the description and drawings are to be taken as illustrative of the modifications in nature, and not as limiting the disclosure.

Furthermore, the drawings of the present disclosure are merely schematic representations, not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and thus, a repetitive description thereof will be omitted. Some of the block diagrams shown in the figures are functional entities and do not necessarily correspond to physically or logically separate entities. These functional entities may be implemented in software, or in one or more hardware modules or integrated circuits, or in different networks and/or processor devices and/or microcontroller devices.

First embodiment

Referring to fig. 3, fig. 3 is a schematic diagram of a flyback converter (hereinafter, referred to as a converter) for converting an input voltage Vin into an output voltage, which includes a transformer TX1, a primary side switch unit SM, a secondary side switch unit SR, a clamping unit 33, an output capacitor Co, and a control device.

The transformer TX1 has a primary side winding P1 and a secondary side winding S1; the drain of the primary side switching unit SM is connected to the primary side winding P1, and the source of the primary side switching unit SM is connected to ground; one end of the clamping unit 33 is connected to the drain of the primary side switching unit SM, and the other end is connected to the input voltage Vin, and the clamping unit 33 may be a common RCD clamping circuit or an active clamping circuit.

The source 114 (first terminal) of the secondary side switch unit SR is connected to the secondary side winding P1 including the transformer TX1, and the drain 115 (second terminal) of the secondary side switch unit SR is connected to one terminal of the output capacitor Co.

The control device comprises a primary side control unit 11 and an isolation driver unit 22, wherein the primary side control unit 11 is used for controlling the working mode of the converter according to the magnitude of the input voltage Vin, so that the converter works in a first working mode, a second working mode or a third working mode; controlling the secondary side switching unit SR to be turned on twice in one switching period by supplying a synchronous rectification control signal and a ZVS control signal, which are spaced from each other, to the secondary side switching unit SR when the converter operates in the first mode; controlling the secondary side switching unit SR to turn on once in one switching cycle by supplying a ZVS control signal to the secondary side switching unit SR when the converter operates in the second mode; when the converter operates in the third operation mode, the secondary side switching unit SR is controlled to be turned on once in one switching period by supplying the synchronous rectification control signal to the secondary side switching unit SR.

Referring to fig. 4, fig. 4 is a waveform diagram illustrating the operation of the converter in the first mode, wherein G _ SM is a driving control signal of the primary side switching unit SM; g _ SR is a driving control signal of the secondary side switching unit SR, which includes a wide pulse signal and a narrow pulse signal with a narrower width, the wide pulse signal is a synchronous rectification control signal, and the narrow pulse signal is a ZVS control signal; l _ P is the current (i.e., excitation current) of the primary side winding P1; l _ S is the current of the secondary winding S1; SM _ Vds is the interpolar voltage of the drain and source of the primary side switching unit SM; SR _ Vds is the inter-electrode voltage between the drain and the source of the secondary side switching unit SR.

[ T0-T1 ] stage: the primary side control signal controls the switching-on of the primary side switch unit SM, the exciting current rises, and the primary side control signal controls the switching-off of the primary side switch unit SM when the exciting current reaches a preset value at T1;

[ T1-T2 ] stage: this phase is a first dead time in which both the primary side switching unit SM and the secondary side switching unit SR are turned off;

[ T2-T3 ] stage: at the time of T2, the synchronous rectification signal controls the secondary side switching unit SR to turn on, the exciting current decreases, and decreases to 0 at the time of T3, at this time, the synchronous rectification signal controls the secondary side switching unit SR to turn off;

[ T3-T4 ] stage: the interval time of the synchronous rectification control signal and the ZVS control signal is determined according to the load size, and the smaller the load is, the longer the interval time is;

[ T4-T5 ] stage: at the time of T4, the ZVS control signal controls the secondary side switch unit SR to turn on again, and at the time of T5, the ZVS control signal controls the secondary side switch unit SR to turn off;

[ T5-T6 ] stage: after the preset second dead time, at time T6, the primary side switching unit SM is controlled to turn on again, at which time the primary side switching unit SM just achieves zero voltage turn-on (ZVS).

At time T4, the ZVS control signal controls the secondary side switch unit SR to turn on again, which can be implemented by one of the following methods:

the first method is as follows: detects the inter-pole voltage SM _ vds of the primary-side switching unit SM resonates to the peak appearance timing, when the peak appears, the primary side control unit 11 outputs a ZVS control signal to control the secondary side switching unit SR to be turned on, specifically, when the primary side switch unit SM is turned off, the secondary side switch unit SR realizes synchronous rectification through a synchronous rectification control signal (or realizes diode rectification through a parasitic diode of a rectifying transistor), when the current l _ S (i.e., the magnetizing current) of the secondary winding S1 drops to 0, the inter-pole voltage SM _ Vds of the primary switching unit SM and the inter-pole voltage SR _ Vds of the secondary switching unit SR respectively enter into resonance, and at this time, it is detected whether the voltage at the drain 113 of the primary switching unit SM reaches a preset nth resonance peak, and if the voltage reaches the preset nth resonance peak, a ZVS control signal is output to control the conduction of the secondary switching unit SR.

The second method comprises the following steps: detects the inter-pole voltage SR _ vds of the secondary-side switching unit SR resonates to the valley occurrence timing, when the trough occurs, the primary side control unit 11 outputs the ZVS control signal to control the secondary side switch unit SR to be turned on, specifically, when the primary side switch unit SM is turned off, the secondary side switch unit SR realizes synchronous rectification through a synchronous rectification control signal (or realizes diode rectification through a parasitic diode of a rectifying transistor), when the current l _ S (i.e., the exciting current) of the secondary winding S1 decreases to 0, the inter-pole voltage SM _ Vds of the primary switching unit SM and the inter-pole voltage SR _ Vds of the secondary switching unit SR enter resonance respectively, and at this time, whether the inter-pole voltage SR _ Vds between the source 114 and the drain 115 of the secondary switching unit SR reaches the predetermined nth resonance trough is detected, and if the inter-pole voltage SR _ Vds reaches the predetermined nth resonance trough, the ZVS control signal is output to control the conduction of the secondary switching unit SR.

Wherein the value of n is determined according to the size of the load of the converter, n is a positive integer, and wherein the lighter the load, the larger the value of n. The detection of the magnitude of the load of the converter can be achieved by detecting the current flowing through the secondary-side switching unit SR or the current flowing through the primary-side switching unit SM (the magnitude of the current is proportional to the magnitude of the load).

And the ZVS control signal controls the secondary side switch unit SR to be switched off by the following method:

formula of calculation based on secondary exciting negative currentSetting a reference current reference I_{neg_ref}Wherein Vin is input voltage, Vo is output voltage, and Nps is the turn ratio of primary and secondary windings of the transformer; lm is the inductance of the exciting inductor; ceq is the capacitance value of the resonance equivalent capacitor, namely, the reference current reference is set according to the input voltage Vin and the output voltage Vo, and when the output voltage Vin is certain, the reference current reference I can be set only according to the input voltage Vin_{neg_ref}。

During the period that the ZVS control signal controls the conduction of the secondary side switch unit SR, the secondary exciting negative current I is collected at the source 114 of the secondary side switch unit SR through the resistor or the mutual inductor_{neg_sense}Signal and collected secondary exciting negative current I_{neg_sense}Signal and reference current reference I_{neg_ref}Comparing when the collected secondary exciting negative current I_{neg_sense}Signal greater than reference current reference I_{neg_ref}When the temperature of the water is higher than the set temperature,the ZVS control signal is turned off to control the secondary side switching unit SR to turn off. Wherein, the secondary exciting negative current I_{neg_sense}The current l _ S of the secondary winding S1 is a negative value.

In this embodiment, the secondary side switch unit SR includes a synchronous rectification switch tube with a large specification parameter and a small specification parameter, and only the synchronous rectification switch tube with the small specification is turned on when the secondary side switch unit SR needs to be turned on for the second time.

Referring to fig. 5, fig. 5 is a waveform diagram of the operation of the converter in the second mode, wherein when the converter is in the second mode, the secondary side switching unit SR is controlled to turn on once in one switching period by supplying the ZVS control signal with a smaller pulse width to the secondary side switching unit SR. As shown in fig. 5, at time T4-T5, the ZVS control signal controls the secondary side switch unit SR to turn on, and during the turn-on of the secondary side switch unit SR, the current l _ S generated by the secondary side winding S1 has only negative current; at time T6, the primary side switching unit SM is switched on after a preset dead time, at which time the primary side switching unit SM just achieves Zero Voltage Switching (ZVS).

A control method of the inverter according to the first embodiment of the present invention is described below with reference to fig. 3 and 6, and the control method includes the following steps:

step 1: obtaining the input voltage V of the converter_in(ii) a Wherein the input voltage Vin can be obtained by any one of the following methods: the method comprises the steps that firstly, the voltage at the anode 111 of the input voltage Vin is directly sampled through resistance voltage division to obtain the input voltage Vin; in the second method, the inter-electrode voltage of the source 114 and the drain 115 of the primary side switching unit SM is sampled during the conduction period of the primary side switching unit SM, and the input voltage Vin is reversely deduced through the turn ratio relationship of the converter; method three, obtain the input voltage Vin by sampling the voltage at the drain 113 of the primary side switching unit SM using the winding; and the method IV is obtained by detecting the rising slope of the exciting current or based on the ratio of the rising time and the falling time of the exciting current of the transformer.

Step 2: comparing the input voltage Vin with a first voltage threshold and a second voltage threshold, respectively; the first voltage threshold value is larger than or equal to nVout, Vout is the output voltage of the converter, and n is the turn ratio of a primary side winding P1 and a secondary side winding S1 of the converter; the first voltage threshold is greater than the second voltage threshold.

When the input voltage Vin is smaller than the first voltage threshold and larger than the second voltage threshold, the primary side control unit generates a first working mode selection signal to control the converter to work in the first working mode, and at the moment, a synchronous rectification control signal and a ZVS control signal exist simultaneously. That is, when the converter operates in the first operation mode, the primary side control unit controls the secondary side switching unit SR to be turned on twice in one switching period by supplying the synchronous rectification control signal and the ZVS control signal, which are spaced apart from each other, to the secondary side switching unit SR.

When the input voltage Vin is greater than or equal to the first voltage threshold, the primary side control unit generates a second operation mode selection signal to control the converter to operate in the second operation mode, and only the ZVS control signal exists at this time. That is, when the converter operates in the second operation mode, the primary side control unit controls the secondary side switching unit SR to be turned on only once in one switching period by supplying the ZVS control signal to the secondary side switching unit SR.

When the input voltage Vin is smaller than the second voltage threshold, the primary side control unit generates a third operating mode selection signal to control the converter to operate in the third operating mode, and only the synchronous rectification control signal exists at the moment. That is, when the converter operates in the third operating mode, the primary side control unit controls the secondary side switching unit SR to be turned on only once in one switching period by supplying the synchronous rectification control signal to the secondary side switching unit SR. Since the inter-pole voltage of the primary side switching unit SM may naturally resonate to 0 when the input voltage Vin is less than the second voltage threshold (i.e., at the time of a low input voltage), the primary side switching unit SM may realize zero-voltage turn-on even without outputting a ZVS control signal when the converter operates in the third mode.

Wherein, the pulse width of the ZVS control signal is controlled by one of the following methods:

the method comprises the steps that control is carried out according to an input voltage Vin, specifically, when the input voltage Vin rises, the working time of a ZVS control signal is increased in the next period, namely, the pulse width of the ZVS control signal is increased; when the input voltage Vin is reduced, the pulse width of the ZVS control signal is reduced in the next period, so that zero-voltage switching-on of the primary side switching tube under the full input voltage Vin is realized.

A second method, controlling according to the inter-electrode voltage between the drain and the source of the primary side switch unit SM, specifically, presetting an initial value for the working time of the ZVS control signal in the primary side controller unit, and sampling whether the voltage of the drain 113 of the primary side switch unit SM has dropped to 0 at the turn-on time of the primary side switch unit SM after the ZVS control signal stops working, and if the voltage of the drain 113 of the primary side switch unit SM has dropped to 0, reducing the pulse width of the ZVS control signal in the next period; on the contrary, if the voltage at the drain 113 of the primary side switching unit SM does not drop to 0 at the turn-on time of the primary side switching unit SM, the next cycle increases the pulse width of the ZVS control signal.

Second embodiment

Referring to fig. 7, fig. 7 is a control flow chart of a control method of an inverter according to a second embodiment of the present invention, which is different from the control method of the first embodiment in that: in the second embodiment, the operation mode of the converter is controlled according to the magnitude of the load of the converter, which is reflected by the magnitude of the output power of the converter.

In the present embodiment, the output power is obtained by sampling the current flowing through the primary side switching unit SM by connecting a sampling resistor or transformer to the drain 112 of the primary side switching unit SM, and in other implementations, the output power can also be obtained by sampling the current flowing through the source 114 of the secondary side switching unit SR or sampling the current at the sampling point 116 of the converter output end by using a resistor or transformer (refer to fig. 3).

When the output power of the converter is larger than a first output power threshold value, the converter works in a first working mode; when the output power of the converter is smaller than the first output power threshold value and larger than the second output power threshold value, the converter works in a second working mode; when the output power of the converter is smaller than the second output power threshold, the converter operates in a skip cycle mode (skip mode), wherein the operation timings of the converter operating in the first mode and the second mode are respectively the same as those in the first embodiment, as shown in fig. 4 to 5; and the timing chart of the working state of the converter working in the skip cycle mode is shown in fig. 8, when the converter works in the skip cycle mode, after a preset time elapses, a preset number of primary side switch control signals exist to control the primary side switch unit SM to work.

A control method of an inverter according to a second embodiment of the present invention is described below with reference to fig. 7 to 8, the control method including the steps of:

step 1: obtaining the output power of the converter; the method for detecting the output power comprises the following steps: detecting the output power by detecting a current flowing through the primary side switching unit SM; detecting the output power by detecting the current flowing through the secondary side switching unit SR; or output power is detected by detecting the output current.

Step 2: comparing the output power to a first power threshold; wherein the first power threshold is less than the full load power.

And step 3: when the output power is greater than or equal to the first power threshold, a first working mode selection signal is generated, the mode selection signal controls the converter to work in the first working mode, namely, the synchronous rectification signal and the ZVS control signal work in the same switching period, and the secondary side switching unit SR is switched on twice.

If the output power is greater than or equal to the first power threshold and the input voltage Vin is less than the second voltage threshold in the first embodiment, the synchronous rectification control signal is only output to control the secondary side switch unit SR to be turned on once.

And 4, step 4: when the output power is smaller than the first power threshold and larger than the second power threshold, generating a second working mode selection signal, wherein the mode selection signal controls the converter to work in a second working mode, namely, the ZVS control signal controls the secondary side switch unit SR to work, and the secondary side switch unit SR is switched on once;

and 5: when the output power is less than the second power threshold, the converter operates in a skip cycle mode.

Third embodiment

Referring to fig. 9, fig. 9 is a schematic diagram of a converter in a third embodiment, and the present embodiment is different from the first embodiment in that: the converter in the implementation is provided with a feedback loop with an optical coupler, the primary side control unit 11 obtains an output feedback signal by sampling the voltage at a sampling point 114 of the feedback loop with the optical coupler, the primary side control unit 11 selects a working mode of the converter according to the detected output feedback signal, and when the output feedback signal of the converter is greater than a first feedback signal threshold value, the converter is controlled to work in a first working mode; and when the feedback signal of the converter is smaller than the threshold value of the second feedback signal, the converter is controlled to work in a second working mode. Wherein, the working time sequence of the converter working in the first mode and the second mode is the same as the first embodiment respectively (refer to fig. 3-4); the converter operates in the skip cycle mode substantially the same as that in the second embodiment, that is, after a predetermined time elapses, a predetermined number of primary side switch control signals exist to control the primary side switch unit SM to operate.

A control method of an inverter according to a third embodiment of the present invention is described below with reference to fig. 10, the control method including the steps of:

step 1: obtaining an output feedback signal of the converter; wherein the output feedback signal magnitude is obtained from the feedback loop formed by combining TL 431.

Step 2: comparing the output feedback signal to a first feedback signal threshold; wherein the first feedback threshold signal is less than a feedback signal voltage reflected by the full load power of the converter.

And step 3: when the output feedback signal is greater than or equal to the first feedback signal threshold, a first working mode selection signal is generated, the mode selection signal controls the converter to work in the first working mode, namely, the synchronous rectification signal and the ZVS control signal work in the same switching period, and the secondary side switching unit SR is switched on twice.

And 4, step 4: when the output feedback signal is smaller than the first feedback signal threshold and larger than the second feedback signal threshold, generating a second working mode selection signal, wherein the mode selection signal controls the converter to work in a second working mode, namely the secondary side switch unit SR is switched on once, and only the ZVS control signal controls the secondary side switch unit SR to work;

and 5: when the output feedback signal is less than the second feedback signal threshold, the converter operates in a skip cycle mode.

FIG. 11 is a diagram illustrating the switching process of different working states of the output feedback signal in FIG. 10 during the variation process of the output feedback signal, when the output feedback signal is greater than or equal to V₀When the converter is operating in a skip cycle mode; when the output feedback signal increases to V₂The converter starts to enter the second operating mode (the operating sequence thereof refers to fig. 4); when the output feedback signal increases to V₄At this point, the converter begins to enter the first mode of operation (the timing of its operation is with reference to fig. 3).

Conversely, when the output feedback signal level is from V₅Down to V₃When the converter enters a second working mode from the first working mode; when the output feedback signal falls to V₁At that moment, the converter enters skip cycle (skip) mode from the first mode.

While the present invention has been described with reference to several exemplary embodiments, it is understood that the terminology used is intended to be in the nature of words of description and illustration, rather than of limitation. As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalences of such meets and bounds are therefore intended to be embraced by the appended claims.