CN113726166A - Flyback converter and control method thereof - Google Patents

Flyback converter and control method thereof Download PDF

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Publication number
CN113726166A
CN113726166A CN202110496801.1A CN202110496801A CN113726166A CN 113726166 A CN113726166 A CN 113726166A CN 202110496801 A CN202110496801 A CN 202110496801A CN 113726166 A CN113726166 A CN 113726166A
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time
flyback converter
tube
synchronous rectifier
signal
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CN113726166B (en
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许祥勇
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Joulwatt Technology Co Ltd
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Joulwatt Technology Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements
    • H02M3/33576—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer
    • H02M3/33592—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements having at least one active switching element at the secondary side of an isolation transformer having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

The invention discloses a flyback converter and a control method thereof, wherein the method comprises the following steps: judging the working mode of the flyback converter based on the limited time and the zero current time of the flyback converter in the same switching period; in a critical mode, controlling a synchronous rectifier tube to be turned off after delaying first time; under the intermittent mode, controlling the synchronous rectifier tube to be switched on again for a second time; and starting the power switching tube after delaying the second time at the zero-crossing detection moment so as to realize zero voltage switching of the power switching tube, wherein when the limited moment is positioned before the zero current moment, the flyback converter can be judged to work in a critical mode, and when the limited moment is positioned after the zero current moment, the flyback converter can be judged to work in an intermittent mode. The control method can not additionally increase the intermittent interval of the secondary side current when the zero voltage of the primary side power switch tube is switched on in the BCM mode, and the system efficiency is optimized.

Description

Flyback converter and control method thereof
Technical Field
The invention relates to the technical field of converters, in particular to a flyback converter and a control method thereof.
Background
A flyback converter is an insulated power converter that is commonly used for current-isolated ac-to-dc and dc-to-dc conversion between an input and one or more outputs. More precisely, the flyback converter is a step-up/step-down converter with an inductor split, constituting a transformer, so that the voltage ratio is multiplied by the additional advantage of the insulation. In a conventional flyback converter, a synchronous rectifier is usually used to replace a diode rectifier, so as to improve efficiency.
A typical configuration for a flyback converter includes a primary power switch coupled to a primary (or primary) transformer winding of a transformer, and a synchronous rectifier coupled to a secondary (or secondary) transformer winding of the transformer. The input voltage is provided by the primary winding and the primary power switch tube. The primary side driving voltage controls the on and off of the primary side power switch tube and conducts primary side current. The secondary side switch and the synchronous rectifier tube are used as supplements in operation. The conduction periods of the power switch tube and the synchronous rectifier tube are not overlapped, one switch is switched on, and the other switch is switched off. The current flow in the secondary portion of the transformer is referred to as the secondary current, which charges the output capacitor and provides the output voltage. When the switching conversion of the flyback converter occurs at a non-zero voltage of the power switch, the flyback converter may suffer power loss. Furthermore, a Zero Voltage Switching (ZVS) needs to be configured in the flyback converter to complete the switching conversion under zero voltage, so as to obtain high efficiency. In some cases, an active clamp may be configured on the secondary side of the flyback converter to clamp the voltage at the drain of the primary power switch tube when the primary power switch tube is off.
In the conventional zero-voltage switching technology ZVS, an auxiliary switching tube needs to be added in the flyback converter to realize ZVS. However, if the low-voltage auxiliary switch tube is adopted, a set of auxiliary winding is added, and the efficiency is not optimal; if the high-voltage auxiliary switch tube is adopted, a corresponding high-voltage driving circuit is added, and the cost and the volume are not optimal. In order to eliminate the above-mentioned auxiliary switching tube, the prior art proposes to implement ZVS by additionally switching on the secondary synchronous rectifier once.
However, in the existing scheme of implementing ZVS by additionally turning on the secondary synchronous rectifier once, on/off control of the primary power switching tube and control of the additional turning on once are both completed on the primary side, and a corresponding driving signal is transmitted to the secondary side, so that an additional isolation device is required when the synchronous rectifier is additionally turned on.
In addition, in a BCM mode (critical conduction mode) of the flyback converter, the existing control method can make the waveform of the exciting current discontinuous, the flyback converter will first enter into discontinuity, and then turn on the second pulse driving synchronous rectifier tube. As shown in fig. 1, in the ZVS control waveform in the BCM mode in the conventional scheme, an interrupted state of a DCM interval is additionally added to the flyback converter, an interrupted current period is additionally inserted into the flyback converter in the BCM mode, and when the flyback converter needs low-voltage input and full-load output, conduction losses of a main switching tube and a transformer are increased, which affects system efficiency.
Therefore, there is a need to provide an improved technical solution to overcome the above technical problems in the prior art.
Disclosure of Invention
In order to solve the technical problem, the invention provides a flyback converter and a control method thereof, zero voltage switching-on of a primary side power switching tube in different modes can be realized only by detecting and driving a secondary side part of the flyback converter, and a discontinuous interval of secondary side current cannot be additionally increased when zero voltage switching-on of the primary side power switching tube is realized in a BCM mode, so that system efficiency is optimized.
According to a first aspect of the present disclosure, a method for controlling a flyback converter is provided, where the flyback converter includes a primary winding, a secondary winding, a power switching tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, where the method includes:
detecting a limit moment when the switching frequency of the flyback converter reaches a preset limit switching frequency and a zero current moment of a secondary side current of the flyback converter in the same switching period;
judging the working mode of the flyback converter based on the limited time and the zero current time;
under the condition that the working mode of the flyback converter is judged to be a critical mode, the synchronous rectifier tube is controlled to be turned off after delaying for the first time;
under the condition that the working mode of the flyback converter is judged to be the discontinuous mode, controlling the synchronous rectifier tube to be turned on again for a second time;
starting to switch on the power switch tube after delaying a third time at a zero-crossing detection moment to realize zero-voltage switching on of the power switch tube, wherein the zero-crossing detection moment is the moment when the voltages at two power ends of the synchronous rectifier tube rise to the output voltage of the flyback converter,
wherein when the limited time is before the zero current time, the operation mode of the flyback converter can be determined as a critical mode,
when the limited time is located after the zero current time, the operation mode of the flyback converter can be determined to be an intermittent mode.
Optionally, after the power switching tube is turned on, the control method further includes:
detecting the change rate of the voltage of the two power ends of the synchronous rectifier tube within preset time, wherein the preset time is set as the time period before and after the power switch tube is switched on;
and judging whether the change rate is larger than a first threshold value, if so, increasing the first time or the second time by a first step time length in the next switching period, otherwise, decreasing the first time or the second time by a second step time length in the next switching period.
Optionally, after the power switching tube is turned on, the control method further includes:
detecting the change values of voltages at two power ends of the synchronous rectifier tube within a preset time length, wherein the preset time length is set as a time period before and after the power switch tube is switched on;
and judging whether the change value is larger than a second threshold value, if so, increasing the first time or the second time by a first step time length in the next switching period, otherwise, decreasing the first time or the second time by a second step time length in the next switching period.
Optionally, the controlling the synchronous rectifier tube to turn off after delaying the first time includes:
receiving a first turn-off signal for controlling the synchronous rectifier tube to turn off;
transmitting a second turn-off signal obtained by delaying the first turn-off signal for the first time to a control end of the synchronous rectifier tube; or
And acquiring a first turn-off threshold adjustment value corresponding to the first time, and increasing the turn-off threshold of the synchronous rectifier tube based on the first turn-off threshold adjustment value.
Optionally, increasing the first time or the second time by a first further length of time comprises:
delaying a turn-off signal for controlling the turn-off of the synchronous rectifying tube for a first superposition time, and then providing the delayed turn-off signal to a control end of the synchronous rectifying tube, wherein the first superposition time is the sum of the first time or the second time and the first step time; or
And acquiring a second turn-off threshold adjustment value corresponding to the first step time length, and increasing the turn-off threshold of the synchronous rectifier tube based on the second turn-off threshold adjustment value.
Optionally, reducing the first time or the second time by a second step length comprises:
delaying a turn-off signal for controlling the turn-off of the synchronous rectifier tube for a first superposition time, and then providing the delayed turn-off signal to a control end of the synchronous rectifier tube, wherein the first superposition time is the difference between the first time or the second time and a second stepping time; or
And acquiring a third turn-off threshold adjustment value corresponding to the second stepping time length, and reducing the turn-off threshold of the synchronous rectifier tube based on the third turn-off threshold adjustment value.
Optionally, the third time is equal to a resonant cycle time of the flyback converter multiplied by x, where x includes, but is not limited to, one of 1/5, 9/40, 1/4, 7/24, 1/3.
Optionally, an on signal for controlling the power switching tube to be turned on and an off signal for controlling the power switching tube to be turned off are both generated at the secondary side portion of the flyback converter and transmitted to the primary side portion of the flyback converter through the isolation element.
According to a second aspect of the present disclosure, a control method of a flyback converter is provided, where the flyback converter includes a primary winding, a secondary winding, a power switching tube connected to the primary winding, and a synchronous rectifier tube connected to the secondary winding, where the control method includes:
detecting the load size of the flyback converter;
judging the working mode of the flyback converter based on the load;
under the condition that the working mode of the flyback converter is judged to be a critical mode, the synchronous rectifier tube is controlled to be turned off after delaying for the first time;
under the condition that the working mode of the flyback converter is judged to be the discontinuous mode, controlling the synchronous rectifier tube to be turned on again for a second time;
under the condition that the working mode of the flyback converter is judged to be the auxiliary mode, the synchronous rectifier tube is controlled to be turned off after delaying the first time in the first N-1 switching cycles in each N switching cycles, the synchronous rectifier tube is controlled to be turned on again for the second time in the Nth switching cycle in each N switching cycles,
wherein, when the load is greater than or equal to a third threshold, the operating mode of the flyback converter can be determined as a critical mode,
in the case where the load is less than or equal to a fourth threshold value, it may be determined that the operation mode of the flyback converter is a discontinuous mode,
in the case where the load is greater than a fourth threshold and less than a third threshold, it may be determined that the operation mode of the flyback converter is an auxiliary mode,
wherein N is an integer greater than 1.
According to a third aspect of the present disclosure, there is provided a flyback converter including: a transformer including a primary winding and a secondary winding;
the power switch tube and the voltage input circuit are connected with the primary winding;
the synchronous rectifier tube and the voltage output circuit are connected with the secondary winding;
the driver is connected with the power switch tube;
the feedback control module is respectively connected with the synchronous rectifier tube and the output end of the flyback converter; and
an isolation element connected to the driver and the feedback control module, respectively,
wherein the feedback control module comprises:
the mode detection unit is used for detecting a limit moment corresponding to the fact that the switching frequency of the flyback converter reaches a preset limit switching frequency and a zero current moment of secondary side current of the flyback converter in the same switching period, and generating a first control signal when the limit moment is detected to be located before the zero current moment, or generating a second control signal when the limit moment is detected to be located after the zero current moment;
and the control unit is used for controlling the synchronous rectifier tube to be turned off after delaying the first time under the condition of receiving the first control signal, or controlling the synchronous rectifier tube to be turned on again for the second time under the condition of receiving the second control signal, so that the power switch tube is turned on, and the zero voltage of the power switch tube is turned on.
Optionally, the feedback control module further comprises:
the self-adaptive adjusting unit is used for detecting the voltage of the two-power-end voltage of the synchronous rectifier tube within preset time, generating a first adjusting signal under the condition that the change rate of the voltage of the two-power-end voltage of the synchronous rectifier tube is larger than a first threshold value, setting the preset time as a time period before and after the power switch tube is switched on,
generating a second adjusting signal under the condition that the change rate of the voltage of the two power ends of the synchronous rectifier tube is less than or equal to a first threshold value,
the first adjusting signal is used for controlling to increase the first time or the second time in the next switching period, and the second adjusting signal is used for controlling to decrease the first time or the second time in the next switching period.
Optionally, the adaptive adjustment unit includes:
the first end of the first capacitor is connected with the drain electrode of the synchronous rectifying tube;
the cathode of the clamping diode is connected with the second end of the first capacitor, and the anode of the clamping diode is connected with the reference ground;
the first resistor is connected between the second end of the first capacitor and the reference ground;
and a drain of the first switching tube is connected with a first end of the first capacitor, a gate of the first switching tube is connected with a second end of the first capacitor, and a source of the first switching tube outputs one of the first adjustment signal and the second adjustment signal.
Optionally, the feedback control module further comprises:
the self-adaptive adjusting unit is used for generating a first adjusting signal under the condition that the variation value of the voltages at the two power ends of the synchronous rectifier tube in a preset time length is greater than a second threshold value, wherein the preset time length is set as a time period before and after the power switch tube is switched on;
generating a second adjusting signal under the condition that the variation value of the voltages at the two power ends of the synchronous rectifier tube in the preset time length is less than or equal to a second threshold value,
the first adjusting signal is used for controlling to increase the first time or the second time in the next switching period, and the second adjusting signal is used for controlling to decrease the first time or the second time in the next switching period.
Optionally, the adaptive adjustment unit includes:
the first end of the second resistor is connected with the drain electrode of the synchronous rectifying tube;
a first end of the second capacitor is connected with a second end of the second resistor, and a second end of the second capacitor is connected with the reference ground;
a first end of the second switch tube is connected with a first end of the second resistor, a second end of the second switch tube is connected with a second end of the second resistor, and a control end of the second switch tube receives a control signal;
and the non-inverting input end of the error amplifier is connected with the first end of the second resistor, the inverting input end of the error amplifier is connected with the second end of the second resistor, and the output end of the error amplifier outputs one of the first adjusting signal and the second adjusting signal.
Optionally, the control feedback control module further includes:
the primary side control signal generating unit is used for providing a primary side switching-on signal and a primary side switching-off signal;
and the secondary side control signal generating unit is used for providing a secondary side on signal and a secondary side off signal.
The invention has the beneficial effects that: the invention discloses a flyback converter and a control method thereof, which can judge the working mode of the flyback converter by detecting and judging the limit time corresponding to the preset limit switching frequency and the zero current time of secondary side current when the switching frequency of the flyback converter reaches the preset limit switching frequency in each switching period, and then select the corresponding control method of a synchronous rectifier tube according to different modes. Meanwhile, in each switching period, the conducting time of the synchronous rectifier tube in the next switching period is adaptively adjusted according to the change condition of the voltages of the two power ends of the synchronous rectifier tube in a certain time period, so that the zero voltage switching-on of the primary side power switching tube in each switching period is closer to an ideal state.
On the other hand, on-off control signals of the primary power tube and the secondary synchronous rectifier tube are generated only in the secondary part, so that complete on-off control can be realized by arranging only one isolation element in the flyback converter, and the cost and the size are optimized.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
Drawings
The above and other objects, features and advantages of the present invention will become more apparent from the following description of the embodiments of the present invention with reference to the accompanying drawings.
Fig. 1 is a timing waveform diagram illustrating a conventional flyback converter implementing ZVS control in BCM mode;
fig. 2 illustrates a schematic structural diagram of a flyback converter provided according to an embodiment of the present disclosure;
FIG. 3 illustrates a block diagram of a feedback control module provided in accordance with an embodiment of the present disclosure;
fig. 4a and 4b respectively show schematic structural diagrams of adaptive adjustment units provided according to different embodiments of the present disclosure;
fig. 5 illustrates a timing waveform diagram of a flyback converter provided according to an embodiment of the present disclosure to implement ZVS control in DCM mode;
fig. 6 illustrates an operation waveform diagram of a ZVS detection method of a flyback converter provided according to an embodiment of the present disclosure;
fig. 7 illustrates a timing waveform diagram of a flyback converter provided according to an embodiment of the present disclosure implementing ZVS control in BCM mode;
fig. 8 illustrates a timing waveform diagram of a flyback converter provided according to an embodiment of the present disclosure to implement ZVS control in a BUR mode;
fig. 9 illustrates a timing waveform diagram of a flyback converter provided in accordance with an embodiment of the present disclosure to adjust a turn-off threshold in BCM mode to implement ZVS control;
fig. 10 shows a flowchart of a control method of a flyback converter provided according to an embodiment of the present disclosure.
Detailed Description
To facilitate an understanding of the invention, the invention will now be described more fully with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the drawings. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.
As shown in fig. 2, in an implementation of the present disclosure, a flyback converter includes: comprising a primary winding NPAnd secondary winding NSIs connected to the primary winding NPVoltage input circuit of (2) connected to the secondary winding NSAnd the power switch SW, the driver 3, the synchronous rectifier SR feedback control module 5 and the isolation element 51.
The voltage input circuit comprises a rectifier 2 and an input capacitor C1, wherein the rectifier 2 can be connected with a power supply through a first connection port 1, so that the power supply can provide electric energy to the flyback converter conveniently. The power source may include, but is not limited to, a power grid, a generator, a transformer, a battery, a solar panel, a wind turbine, a regenerative braking system, a hydraulic or wind generator, or any other form of device capable of providing electrical power to a flyback converter.
Further, the voltage input circuit further includes a primary winding N in the transformer TRPA third resistor R1, a third capacitor C2 and a first diode D1 are further provided between the same-name terminal and the different-name terminal. Wherein, the third resistor R1 and the third capacitor C2 are connected in parallel with each other and then connected to the primary winding NPBetween the different name terminal of (a) and the cathode of the first diode D1, the anode of the first diode D1 and the primary winding NPAre connected. Thus, the primary winding N can be absorbedPThe leakage inductance current improves the performance of the transformer.
The voltage output circuit comprises an output capacitor Co, which can be connected to a load via the second connection port 4, the load receiving the electrical energy (e.g. voltage and current) converted by the flyback converter. In some examples, the power converted by the flyback converter also passes through a filter before reaching the load. In some examples, the filter is a subcomponent of the flyback converter, an external component of the flyback converter, and/or a subcomponent of the load. In any case, the load may perform a function using filtered or unfiltered power from the flyback converter. Alternatively, the load may include, but is not limited to, a computing device and associated components, such as a microprocessor, electrical components, circuitry, laptop computer, desktop computer, tablet computer, mobile phone, battery, speaker, lighting unit, automotive/marine/aeronautical/train associated components, motor, transformer, or any other type of electrical device and/or circuitry that receives a voltage or current from a flyback converter.
In fig. 2, the load is equivalently represented as a load resistance RLAnd is indicated by a dashed line.
First power end of power switch tube SW and primary winding NPAnd a second power terminal thereof is connected to a reference ground. In one possible embodiment, the power switch SW is, for example, an NMOS field effect transistor, the first power terminal of which is the drain of the NMOS field effect transistor, the second power terminal of which is the source of the NMOS field effect transistor, and the control terminal of which is the gate of the NMOS field effect transistor.
The driver 3 includes a DRV (control signal output) pin, a GND (ground) pin, and a DRV _ in (drive signal input) pin. The DRV pin of the driver 3 is connected to the control terminal of the power switch SW, the GND pin is connected to the ground reference, and the DRV _ in pin is connected to the output terminal of the isolation element 51.
The synchronous rectifier SR is connected to the secondary winding NSBetween the synonym end of (c) and the reference ground. In one possible embodiment, the synchronous rectifier SR is a field effect transistor with NMOS, the drain and the secondary winding NSIs connected to the alias terminal and has its source connected to a reference ground.
First input end of feedback control module 5 and secondary winding NSThe second input terminal of the synchronous rectifier SR is connected to the reference ground, i.e., the source of the synchronous rectifier SR, the third input terminal of the synchronous rectifier SR is connected to the output terminal of the flyback converter, the first output terminal of the synchronous rectifier SR is connected to the gate of the synchronous rectifier SR, and the second output terminal of the synchronous rectifier SR is connected to the input terminal of the isolation device 51.
Alternatively, the isolation element 51 may be any one of an isolation transformer, an optical coupler, an isolation capacitor, and an isolation chip. The signal transmission from the second output terminal of the feedback control module 5 to the Drv _ in pin of the driver 3 can be realized through the isolation element 51.
As shown in fig. 3, in the present disclosure, the feedback control module 5 is provided with a primary side control signal generating unit 51 and a secondary side control signal generating unit 52. The primary side control signal generating unit 51 is configured to provide a corresponding primary side control signal PWM1 (including a primary side on signal for controlling the switching on of the primary side power switching tube SW and a primary side off signal for controlling the switching off of the power switching tube SW); the secondary control signal generating unit 52 is configured to provide a corresponding secondary control signal PWM2 (including a secondary on signal for controlling the turn-on of the synchronous rectifier SR and a secondary off signal for controlling the turn-off of the synchronous rectifier SR). The primary side control signal generating unit 51 may generate a corresponding primary side on signal through a working mode (fixed frequency or variable frequency) of the flyback converter, and may further obtain a voltage to be compared according to the voltage Vds _ SR at the two ends of the synchronous rectifier SR and the output voltage Vo of the flyback converter in each switching period, and output a primary side off signal according to a comparison result of the voltage to be compared and the reference voltage. The secondary control signal generating unit 52 may generate the secondary on signal and the secondary off signal by sampling and detecting the voltage Vds _ SR between the two power terminals of the synchronous rectifier SR, and the specific method and the circuit structure thereof may be implemented by the prior art, and will not be described in detail here.
The feedback control module 5 arranged on the secondary side of the flyback converter can simultaneously realize on/off control on the power switch tube SW and the synchronous rectifier tube SR, and the driver 3 on the primary side only needs to realize a level conversion function to convert a primary side on/off signal generated by the feedback control module 5 into a proper voltage signal capable of controlling on/off of the power switch tube SW actually, so that only one isolation device 51 is needed to realize transmission of a primary side driving signal (comprising a primary side on signal and a primary side off signal) from the secondary side to the primary side in the driving process. When the driving interlocking of the primary side and the secondary side is realized, the corresponding driving interlocking unit 53 can be directly arranged in the feedback control module 5 to realize the driving of the primary side and the secondary side, the transmission of a primary side driving signal from the primary side to the secondary side can be effectively prevented from being shared, the driving voltage of the synchronous rectifier tube SR is not required to be reduced in the aspect of control, the system efficiency and the reliability are further improved, and meanwhile, the design requirement on the primary side driver 3 is simplified to a certain degree.
Further, the feedback control module 5 is also provided with a mode detection unit 54, a control unit 55 and an adaptive adjustment unit 56.
The mode detection unit 54 is configured to detect a limit time (e.g., time t23 in fig. 7) corresponding to the switching frequency of the flyback converter reaching a preset limit switching frequency and a zero-current time (e.g., time t24 in fig. 7) of the secondary side current I _ Lm of the flyback converter in the same switching period, and generate the first control signal if the limit time is detected to be before the zero-current time, or generate the second control signal if the limit time is detected to be after the zero-current time.
The limited switching frequencies corresponding to different working modes of the flyback converter are different, and further, the time Tlimit required from the opening of one switching cycle to the reaching of the limited switching frequency of the flyback converter is also different in different working modes. In the BCM mode (critical mode), the time Tlimit required for the switching frequency of the flyback converter to reach the limited switching frequency in one switching period is short, and the limited time corresponding to the limited switching frequency is earlier than the zero current time of the secondary side current I _ Lm of the flyback converter; in the DCM mode (discontinuous mode), the time Tlimit required for the switching frequency of the flyback converter to reach the limited switching frequency in one switching period is relatively long, and the limited time when the switching frequency reaches the limited switching frequency is later than the zero current time of the secondary side current I _ Lm. Therefore, the working mode of the flyback converter in the current switching period can be determined by detecting the sequence of the limited time and the zero current time of the flyback converter in one switching period, and then corresponding control signals are output to trigger the flyback converter to enter different control stages for realizing the Zero Voltage Switching (ZVS) of the power switching tube SW.
It should be noted that, the case where the limited time coincides with the zero current time belongs to a critical case, and in one of the embodiments of the present disclosure, when the mode detection unit 54 detects that the limited time coincides with the zero current time, the second control signal may be generated; in other embodiments of the present disclosure, the mode detection unit 54 may also generate the first control signal when detecting that the defined time coincides with the zero current time, and in practical applications, one of the first control signal and the second control signal may be selected according to specific situations, which is not specifically limited by the present invention.
The control unit 55 is configured to control the synchronous rectifier SR to turn off after delaying the first time when receiving the first control signal, or to control the synchronous rectifier SR to turn on twice when receiving the second control signal, and then turn on the power switch SW, so as to implement zero-voltage turn-on of the power switch SW.
In the BCM mode, the mode detecting unit 54 generates a first control signal and transmits the first control signal to the control unit 55, and the control unit 55 can control the synchronous rectifier SR to turn off after delaying the first time under the trigger of the first control signal, that is, to prolong the on-time of the synchronous rectifier SR, referring to fig. 7, the amount of the on-time of the synchronous rectifier SR (i.e., the first time) corresponds to the time period between t24 and t 25. By increasing the on-time of the synchronous rectifier SR, the flyback converter can generate a reverse excitation current again after the follow current of the secondary current I _ Lm is finished, as shown by a dotted line portion of an I _ Lm waveform in fig. 7, when the synchronous rectifier SR is turned off, the reverse excitation current is transferred to the primary side, and then the power-saving capacitor of the primary-side power switch tube SW is discharged, so that the drain-source voltage Vds _ pri when the power switch tube SW is turned on is reduced, and zero-voltage turn-on is realized.
In the BCM mode, the flyback converter works in one switching cycle as follows:
in a time period from t20 to t21, the gate-source voltage Vgs _ pri of the power switch tube SW is at a high level, the power switch tube SW is in a conducting state, the source-drain voltage Vds _ pri of the power switch tube SW is at a low level, and the primary winding N of the transformer TR is connected with the primary winding N of the transformer TRPThe upper current rises linearly from zero and the transformer TR stores energy, while the secondary current I _ Lm rises linearly from zero. In the time period, the gate-source voltage Vgs _ SR of the synchronous rectifier SR is at a low level, the synchronous rectifier SR is in an off state, and the source-drain voltage Vds _ SR of the synchronous rectifier SR is at a high level and is greater than the output voltage Vo of the flyback converter.
At time t21, the gate-source voltage Vgs _ pri of the power switch SW goes low, the power switch SW turns off, the drain-source voltage Vds _ pri starts to rise, and the drain-source voltage Vds _ SR of the synchronous rectifier SR starts to fall.
In the time period from t21 to t22, the secondary side current I _ Lm of the flyback converter starts to decrease after rising to the peak value. Meanwhile, after a time delay of t 21-t 22, at time t22, the gate-source voltage Vgs _ SR of the synchronous rectifier SR becomes high, the synchronous rectifier SR is turned on, and at this time, the drain-source voltage Vds _ pri of the power switch SW rises to the maximum value, and the drain-source voltage Vds _ SR of the synchronous rectifier SR falls to zero.
At time t24, the secondary current I _ Lm is reduced to zero, but in the BCM mode, the first control signal generated by the mode detecting unit 54 triggers the control unit 55 to prolong the on-time of the synchronous rectifier SR, so that the secondary off signal, which is originally provided to the gate of the synchronous rectifier SR at time t24, is provided to the gate of the synchronous rectifier SR after time t 25. During this time period, the secondary side current I _ Lm again generates a reverse field current after the freewheeling ends.
At time t25, the gate-source voltage Vgs _ SR of the synchronous rectifier SR becomes low, the synchronous rectifier SR turns off, and at this time, the drain-source voltage Vds _ pri of the power switch SW starts to decrease and the drain-source voltage Vds _ SR of the synchronous rectifier SR starts to increase.
At time t26, i.e., at the time of Zero Crossing Detection (ZCD), the drain-source voltage Vds _ SR of the synchronous rectifier SR rises to be equal to the output voltage Vo at this time, and the feedback control module 5 starts to generate the pulse signal Drv _ in representing the primary side turn-on signal, i.e., the feedback control module 5 generates the rising edge of the pulse signal Drv _ in at time t 26. Then, the feedback control module 5 outputs the falling edge of the pulse signal Drv _ in to the driver 3 at time t27 after a preset delay time (third time Td) elapses, and the driver 3 controls the primary side power switching tube SW to be turned on after receiving the falling edge of the pulse signal Drv _ in. For example, the duration of the third time Td may be set according to circumstances, for example, the third time Td may be set to be equal to the resonant period time of the flyback converter multiplied by x, where x includes, but is not limited to, one of 1/5, 9/40, 1/4, 7/24, 1/3, and preferably 1/4.
At time t27, the drain-source voltage Vds _ pri of the power switch tube SW is reduced to zero, and zero-voltage turn-on of the power switch tube SW is achieved.
As can be seen from fig. 7, in the BCM mode, by prolonging the on-time of the synchronous rectifier SR, the zero-voltage turn-on of the power switch SW is realized, and meanwhile, the discontinuous interval of the secondary current is not additionally increased, so that the system efficiency is optimized.
Further, the control unit 55 may not only extend/shorten the on time of the synchronous rectifier SR by delaying/advancing the time for which the secondary-side off signal is supplied to the gate of the synchronous rectifier SR (as in fig. 7), but also extend/shorten the on time of the synchronous rectifier SR by adjusting the off threshold of the synchronous rectifier SR (as in fig. 9). For example, in the BCM mode, the turn-off threshold of the synchronous rectifier SR is normally a negative threshold (e.g., -3mV), when the turn-on time of the synchronous rectifier SR needs to be prolonged to achieve zero-voltage turn-on, the turn-off threshold of the synchronous rectifier SR can be increased to a positive threshold (e.g., +20mV), and when the turn-off threshold of the synchronous rectifier SR is positive, the current I on the synchronous rectifier SR is positiveSRA reverse stage may occur, and then a reverse exciting current may be generated in the transformer TR to control and realize zero voltage turn-on of the power switch tube SW. In FIG. 9, VDSIndicating the drain-source voltage, V, of the synchronous rectifier SRGTWhich represents the gate voltage of the synchronous rectifier SR. It is to be understood that the foregoing enumerated values are exemplary only, and are not intended as limitations on the presently disclosed technology.
In the DCM mode, the mode detecting unit 54 generates a second control signal and transmits the second control signal to the control unit 55, and the control unit 55 can control the synchronous rectifier SR to turn on twice in one switching cycle under the trigger of the second control signal, referring to fig. 5, the second turn-on time of the synchronous rectifier SR is a second time, which corresponds to a time period between t4 and t 5. In this time period, the flyback converter can generate a reverse excitation current again after the follow current of the secondary side I _ Lm is finished, as shown by a dotted line portion of an I _ Lm waveform in fig. 5, after the synchronous rectifier SR is turned off, the reverse excitation current is transferred to the primary side, and then the power-saving capacitor of the power switch tube SW on the primary side is discharged, so that the drain-source voltage Vds _ pri when the power switch tube SW is turned on is reduced, and zero-voltage turn-on is realized.
In the DCM mode, a working process of the flyback converter in one switching period is substantially the same as a working process of the flyback converter in one switching period in the BCM mode, and thus, the operation can be understood with reference to the BCM mode, which is not described herein again. The difference between the two is that in DCM, the synchronous rectifier SR is turned off at time t3 and turned on again at time t 4. In the time period T3-T4, the waveforms of the secondary current I _ Lm, the drain-source voltage Vds _ pri of the power switch SW, and the drain-source voltage Vds _ SR of the synchronous rectifier SR can be understood by referring to the prior art, and are not described herein again.
According to the method and the device, the working mode of the flyback converter in the current switching period can be rapidly and accurately determined to be the BCM mode or the DCM mode by judging the sequence of the limited time and the zero current time of the flyback converter in the switching period, the conducting time of the synchronous rectifier tube SR in the switching period is prolonged in different control modes according to different modes, the SW zero voltage switching of the power switching tube is realized, meanwhile, the extra discontinuous interval cannot be increased in the BCM mode, and the purpose of optimizing the system efficiency is achieved.
Further, in order to ensure that ZVS of the power switch tube SW at the turn-on time tends to be more ideal, the feedback control module of the present disclosure is further provided with an adaptive adjustment unit 56.
In one possible embodiment of the present disclosure, the adaptive adjusting unit 56 may be configured to detect the voltage Vds _ SR across the synchronous rectifier within a preset time, and generate a first adjusting signal if a change rate (denoted as dV/dt) of the voltage Vds _ SR across the synchronous rectifier is greater than a first threshold, or generate a second adjusting signal if the change rate dV/dt of the voltage Vds _ SR across the synchronous rectifier is less than or equal to the first threshold. The preset time can be set as a time period before and after the power switch tube SW is turned on.
In this embodiment, the adaptive adjustment unit 56 compares the change rate dV/dt of the voltage Vds _ SR between the two power terminals of the synchronous rectifier with the first threshold to determine whether the switching mode of the primary power switch SW in the current switching period is zero-voltage switching, and further dynamically adjusts the time length of the extended on-time of the synchronous rectifier SR in the next switching period according to the determination result, so that the ZVS of the power switch SW at the on-time can be more ideal.
In this embodiment, as an example, the circuit structure of the adaptive adjusting unit 56 is shown in fig. 4a, and includes: the circuit comprises a first capacitor C3, a clamping diode D2, a first resistor R2 and a first switch tube Q1. Wherein, the first end of the first capacitor C3 is connected with the drain of the synchronous rectifier SR; the cathode of the clamping diode D2 is connected with the second end of the first capacitor C3, and the anode of the clamping diode D2 is connected with the reference ground; the first resistor R2 is connected between the second end of the first capacitor C3 and the ground reference; the drain of the first switch Q1 is connected to the first end of the first capacitor C3, the gate of the first switch Q1 is connected to the second end of the first capacitor C3, and the source of the first switch Q1 outputs one of the first adjustment signal and the second adjustment signal.
Alternatively, the first switch Q1 may be an NMOS transistor.
As shown in fig. 4a, when the primary power switch SW is turned on hard in the current switching period, the voltage Vds _ SR across the synchronous rectifier has a positive change rate, and the current flowing through the first capacitor C3 in fig. 4 raises the gate voltage of the first switch Q1 to turn on the first switch Q1, at this time, the source of the first switch Q1 outputs the first adjustment signal having the first level state. When the primary power switch tube SW is turned on at zero voltage in the current switching period, the change rate of the voltage Vds _ SR between the two power terminals of the synchronous rectifier tube is zero or very small, which is not enough to set the gate potential of the first switch tube Q1 high to turn on the first switch tube Q1, and at this time, the source of the first switch tube Q1 outputs the second adjustment signal with the second level state. Furthermore, by determining the level state at the test1 interface connected to the source of the first switch Q1, the dynamic adjustment of the duration of the extended on-time of the synchronous rectifier SR (i.e., the first or second on-time) in the next switching cycle can be realized, and the circuit structure is simple.
In another possible embodiment of the present disclosure, the adaptive adjustment unit disposed in the feedback control module 5 may be further configured to generate a first adjustment signal when a variation value of the voltage Vds _ SR across the synchronous rectifier tube within the preset time period is greater than a second threshold, or generate a second adjustment signal when a variation value of the voltage Vds _ SR across the synchronous rectifier tube within the preset time period is less than or equal to the second threshold. The preset time length can be set as a time period before and after the power switch tube SW is switched on.
In this embodiment, the adaptive adjustment unit 56 compares the variation value of the voltage Vds _ SR between the two power terminals of the synchronous rectifier with the second threshold to determine whether the switching mode of the primary power switching tube SW in the current switching period is zero-voltage switching, and further dynamically adjusts the time length of the extended conduction time of the synchronous rectifier SR in the next switching period according to the determination result, so that the ZVS of the power switching tube SW at the switching-on time can be more ideal.
In this embodiment, as an example, the circuit structure of the adaptive adjusting unit 56 is shown in fig. 4b, and includes: a second resistor R3, a second capacitor C4, a second switch tube Q2 and an error amplifier U1. The first end of the second resistor R3 is connected with the drain of the synchronous rectifier SR; a first end of the second capacitor C4 is connected with a second end of the second resistor R3, and a second end of the second capacitor C4 is connected with the reference ground; a first end of the second switch tube Q2 is connected with a first end of the second resistor R3, a second end of the second switch tube Q2 is connected with a second end of the second resistor R3, and a control end of the second switch tube Q2 receives a control signal; the non-inverting input terminal of the error amplifier U1 is connected to the first terminal of the second resistor R3, the inverting input terminal of the error amplifier U1 is connected to the second terminal of the second resistor R3, and the output terminal of the error amplifier U1 outputs one of the first adjustment signal and the second adjustment signal.
Alternatively, the second switch Q2 may be an NMOS transistor or an NPN transistor.
At the switching-on time (e.g., time t7 in fig. 6) of the power switching tube SW, sampling and recording two power terminal voltages Vds _ SR of the synchronous rectifier tube as Vds1, delaying for a certain time (e.g., at any time between t7 and t8 in fig. 6), sampling and recording two power terminal voltages Vds _ SR of the synchronous rectifier tube as Vds2, calculating a difference Vds2-Vds1 therebetween, if the difference is greater than a second threshold, the primary power switching tube SW can be considered to be switched on hard in the current switching period, and otherwise, the primary power switching tube SW can be considered to be switched on at zero voltage in the current switching period. With reference to fig. 4b and fig. 6, during a time period from t0 to t7 in one switching cycle, the gate-source voltage Vgs _ Q2 of the second switching tube Q2 is at a high level, the second switching tube Q2 is in a conducting state, at a time t7, based on the filtering action of the second resistor R3 and the second capacitor C4, the voltage V1 at the inverting input terminal of the error amplifier U1 is very close to the voltage Vds _ SR across the synchronous rectifier at the time t7, that is, the voltage V1 at the inverting input terminal of the error amplifier U1 at the time t7 is equivalent to the sampling voltage Vds1 at the time to the voltage Vds _ SR across the synchronous rectifier. In a time period from t7 to t8, the gate-source voltage Vgs _ Q2 of the second switch Q2 is at a low level, the second switch Q2 is in an off state, and a voltage V2 at the non-inverting input terminal of the error amplifier U1 at any time in the time period corresponds to a sampling voltage Vds2 for the voltage Vds _ SR across the synchronous rectifier at the corresponding time. Therefore, when the primary side power switch SW is turned on hard in the current switching period, a signal output by the output terminal of the error amplifier U1 at any time in the time period t 7-t 8 is the first adjustment signal with the first level state. When the primary side power switch tube SW is turned on at zero voltage in the current switching period, a signal output by the output end of the error amplifier U1 at any time within a time period t 7-t 8 is a second adjustment signal with a second level state. Furthermore, by judging the level state at the test2 interface connected to the output end of the error amplifier U1, the dynamic adjustment of the duration (i.e., the first time or the second time) of the prolonged conduction time of the synchronous rectifier SR in the next switching period can be realized, and the circuit structure is simple.
Further, the first adjustment signal is used for controlling to increase the first time (e.g. t 24-t 25 in fig. 7) or the second turn-on time (i.e. the second time, t 4-t 5 in fig. 5) of the synchronous rectifier SR according to the first step duration in the next switching cycle, and the second adjustment signal is used for controlling to decrease the first time or the second time according to the second step duration in the next switching cycle. Optionally, the first time and the second time may be equal or unequal, and the first step time and the second step time may be equal or unequal, which is not limited in the present invention.
Further, in the foregoing embodiment, the mode detection unit 54 in the feedback control module 5 determines whether the flyback converter is in the DCM mode or the BCM mode by detecting the occurrence sequence of the limited time and the zero current time of the flyback converter in one switching cycle. In other embodiments of the present disclosure, the mode detecting unit 54 may further determine the mode of the flyback converter according to the size of the load, for example, when the mode detecting unit 54 detects that the load is greater than or equal to the third threshold, it may determine that the flyback converter at this time is in the BCM mode; when the load is detected to be smaller than or equal to the fourth threshold, the flyback converter at the moment can be determined to be in a DCM mode; when the load is detected to be greater than the fourth threshold and less than the third threshold, the flyback converter at the moment can be determined to be in the BUR mode. The BUR mode is an auxiliary mode added between the BCM mode and the DCM mode, and is to reduce the influence on the system efficiency when the synchronous rectifier SR needs to be turned on for the second time as much as possible (because the synchronous rectifier SR is also turned on for the second time, the system efficiency is affected).
Specifically, the control method for the synchronous rectifier in the BCM mode and the DCM mode is the same as that described above, and is not described herein again. In the BUR mode, referring to fig. 8, the BUR mode in the present disclosure circulates with N switching cycles as one BUR cycle, wherein the synchronous rectifier SR is controlled to be turned off after delaying for the first time in the first N-1 switching cycles of each N switching cycles (the working principle of the flyback converter is the same as the working principle of the flyback converter in the BCM mode), and the synchronous rectifier SR is controlled to be turned on twice in the nth switching cycle of each N switching cycles (the working principle of the flyback converter is the same as the working principle of the flyback converter in the DCM mode). Therefore, the purpose of optimizing the system efficiency can be achieved by reducing the second turn-on times of the synchronous rectifier tube SR in every N switching periods. Wherein N is an integer greater than 1.
Referring to fig. 10, fig. 10 shows a flowchart of a control method of a flyback converter provided according to an embodiment of the present disclosure, which can be used for the flyback converters described in the foregoing fig. 2 to 9.
As shown in fig. 10, in the present disclosure, the method of controlling the flyback converter includes performing steps S1 to S5.
Specifically, in step S1, a limit time when the switching frequency of the flyback converter reaches a preset limit switching frequency and a zero current time of the secondary side current of the flyback converter are detected in the same switching period.
The defined time and the zero current time are detected by the mode detection unit 54 in the feedback control module 5. For example, the switching frequency of the flyback converter starts to be detected and timed at the starting moment of each switching period, and when the switching frequency is detected to reach a preset limited switching frequency, the timing is stopped, and the timing duration at the moment is recorded; and simultaneously, sampling the current of the secondary side current I _ Lm and timing, stopping timing when the secondary side current I _ Lm becomes zero current, and recording the timing duration at the moment. Therefore, the starting time of each switching period is taken as a reference, and the limit time when the flyback converter reaches the corresponding limit switching frequency and the zero current time of the secondary side current of the flyback converter are determined according to the corresponding timing duration.
It should be understood that other conventional methods readily apparent to those skilled in the art may be employed in the present disclosure to determine the defined time and the zero current time, and will not be described in detail herein.
In step S2, the operation mode of the flyback converter is determined based on the limit time and the zero-current time. If it is determined that the operation mode of the flyback converter is the critical mode, step S3 is executed, and if it is determined that the operation mode of the flyback converter is the discontinuous mode, step S4 is executed.
In this embodiment, the time duration corresponding to the corresponding time may be compared to determine whether the limited time is before the zero current time. In the operation process, when one mark is detected, the time corresponding to the mark is judged to be before the other time, and the detection of the other time is stopped.
Further, when it is detected that the defined time is before the zero current time, indicating that the flyback converter is currently in the BCM mode, the mode detection unit 54 generates the first control signal, triggering execution of step S3. When it is detected that the defined time instant is after the zero current time instant, indicating that the flyback converter is currently in DCM mode, a second control signal is generated by the mode detection unit 54, triggering the execution of step S4.
It should be noted that, the case where the limited time coincides with the zero current time belongs to a critical case, and in one of the embodiments of the present disclosure, when the mode detection unit 54 detects that the limited time coincides with the zero current time, the second control signal may be generated; in other embodiments of the present disclosure, the mode detection unit 54 may also generate the first control signal when detecting that the defined time coincides with the zero current time, and in practical applications, one of the first control signal and the second control signal may be selected according to specific situations, which is not specifically limited by the present invention.
In step S3, the synchronous rectifier is turned off after a delay of a first time.
In this embodiment, controlling the synchronous rectification transistor to turn off after delaying the SR for the first time includes: receiving a first turn-off signal for controlling the synchronous rectifier tube SR to turn off; transmitting a second turn-off signal obtained by delaying the first turn-off signal for the first time to a control end of the synchronous rectifier tube SR; or acquiring a first turn-off threshold adjustment value corresponding to the first time, and increasing the turn-off threshold of the synchronous rectifier tube SR based on the first turn-off threshold adjustment value. For a specific method, reference may be made to the foregoing description of fig. 7 and fig. 9, which is not repeated herein. After that, step S5 is executed.
In step S4, the synchronous rectifier is controlled to be turned on again for a second time.
After the synchronous rectifier SR is turned off for the first time, the secondary on-signal may be provided to the control terminal of the synchronous rectifier SR again after a period of time to control the synchronous rectifier SR to be turned on for the second time, and the second on-time may be set to be the second time. For a specific method, reference may be made to the foregoing description of fig. 5, which is not repeated herein. After that, step S5 is executed.
In step S5, the power switch is turned on after the zero-crossing detection time starts to delay for a third time, so as to implement zero-voltage turn-on of the power switch.
In the extended on-time (the first time or the second time) of the synchronous rectifier SR, the flyback converter can generate a reverse exciting current again after the follow current of the secondary side current I _ Lm is finished, and after the synchronous rectifier SR is turned off, the reverse exciting current can be transferred to the primary side to discharge the power-saving capacitor of the primary side power switch tube SW, so that the drain-source voltage Vds _ pri when the power switch tube SW is turned on is reduced, and the zero-voltage turn-on is realized.
Meanwhile, after the synchronous rectifier SR is turned off, the drain-source voltage Vds _ SR starts to rise, and when the drain-source voltage Vds _ SR rises to be equal to the output voltage Vo in potential, corresponding to the zero-crossing detection time of the flyback converter, the feedback control module 5 starts to generate a pulse signal Drv _ in representing the primary side turn-on signal, that is, the feedback control module 5 generates a rising edge of the pulse signal Drv _ in at the time t 26. Then, the feedback control module 5 outputs the falling edge of the pulse signal Drv _ in to the driver 3 at time t27 after a preset delay time (third time Td) elapses, and the driver 3 controls the primary side power switching tube SW to be turned on after receiving the falling edge of the pulse signal Drv _ in. For example, the duration of the third time Td may be set according to circumstances, for example, the third time Td may be set to be equal to the resonant period time of the flyback converter multiplied by x, where x includes, but is not limited to, one of 1/5, 9/40, 1/4, 7/24, 1/3, and preferably 1/4.
Further, after the power switch tube SW is turned on, the control method further includes: detecting the change rate of voltage at two power ends of a synchronous rectifier tube within preset time; and judging whether the change rate is greater than a first threshold value, if so, increasing the first time or the second time by a first step time length in the next switching period, otherwise, decreasing the first time or the second time by a second step time length in the next switching period. The preset time can be set as the time period before and after the power switch tube is switched on.
Or detecting the change values of the voltages of the two power ends of the synchronous rectifier tube within a preset time length; and judging whether the change value is larger than a second threshold value, if so, increasing the first time or the second time by a first step time length in the next switching period, otherwise, decreasing the first time or the second time by a second step time length in the next switching period. The preset time length can be set as the time period before and after the power switch tube is switched on.
In this embodiment, the change rate dV/dt of the voltage Vds _ SR across the two power terminals of the synchronous rectifier is compared with a first threshold, or the change value of the voltage Vds _ SR across the two power terminals of the synchronous rectifier within a period of time is compared with a second threshold, so as to determine whether the switching mode of the primary power switching tube SW in the current switching period is zero-voltage switching, and further dynamically adjust the duration of the extended on-time of the synchronous rectifier SR in the next switching period according to the determination result, so that the ZVS of the power switching tube SW at the on-time can be more ideal. The detailed process and principle can be understood by referring to the foregoing description of the adaptive adjustment unit 56 and fig. 4a and 4b, which are not described herein again.
Further, increasing the first time or the second time by a first further length of time comprises: delaying a turn-off signal for controlling the turn-off of the synchronous rectifier tube for a first superposition time and then providing the delayed turn-off signal to a control end of the synchronous rectifier tube; or acquiring a second turn-off threshold adjustment value corresponding to the first step time length, and increasing the turn-off threshold of the synchronous rectifier tube based on the second turn-off threshold adjustment value. And the first superposition time is the sum of the first time or the second time and the first further time.
Further, reducing the first time or the second time by a second step length comprises: delaying a turn-off signal for controlling the turn-off of the synchronous rectifier tube for a first superposition time and then providing the delayed turn-off signal to a control end of the synchronous rectifier tube; or acquiring a third turn-off threshold adjustment value corresponding to the second stepping time length, and reducing the turn-off threshold of the synchronous rectifier tube based on the third turn-off threshold adjustment value. Wherein the first overlap-subtract time is a difference between the first time or the second time and the second stepping time.
Optionally, the first time and the second time may be equal or unequal, and the first step time and the second step time may be equal or unequal.
Further, the present disclosure also provides another control method of a flyback converter, including: detecting a load size of the flyback converter; judging the working mode of the flyback converter based on the load; under the condition that the working mode of the flyback converter is judged to be a critical mode, the synchronous rectifier tube is controlled to be turned off after delaying for the first time; under the condition that the working mode of the flyback converter is judged to be the discontinuous mode, controlling the synchronous rectifier tube to be turned on again for the second time; under the condition that the working mode of the flyback converter is judged to be the auxiliary mode, the synchronous rectifying tube is controlled to be turned off after delaying for the first time in the first N-1 switching periods in every N switching periods, the synchronous rectifying tube is controlled to be turned on again for the second time in the Nth switching period in every N switching periods, wherein under the condition that the load is larger than or equal to the third threshold value, the working mode of the flyback converter can be judged to be the critical mode, under the condition that the load is smaller than or equal to the fourth threshold value, the working mode of the flyback converter can be judged to be the discontinuous mode, and under the condition that the load is larger than the fourth threshold value and smaller than the third threshold value, the working mode of the flyback converter can be judged to be the auxiliary mode. Wherein N is an integer greater than 1. Specifically, the execution principle of the control method can be understood by referring to the foregoing description of fig. 8, and is not described herein again.
In summary, according to the present disclosure, zero voltage switching on of the primary power switching tube in different modes can be achieved only by detecting and driving the secondary side portion of the flyback converter, and when zero voltage switching on of the primary power switching tube is achieved in the BCM mode (critical mode), a discontinuous interval of the secondary current is not additionally increased, so that system efficiency is optimized. Meanwhile, based on the self-adaptive adjustment of the conduction time of the synchronous rectifier tube, the zero voltage of the primary side power switch tube in each switching period can be switched on more nearly to an ideal state.
On the other hand, the on-off control signals of the power tube and the secondary synchronous rectifier tube of the flyback converter are only generated at the secondary part, complete switch control can be realized by only arranging one isolation element, and cost and size are optimized.
Finally, it should be noted that: it should be understood that the above examples are only for clearly illustrating the present invention and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. And obvious variations or modifications of the invention may be made without departing from the scope of the invention.

Claims (14)

1. A control method of a flyback converter comprises a primary winding, a secondary winding, a power switch tube connected with the primary winding and a synchronous rectifier tube connected with the secondary winding, wherein the control method comprises the following steps:
detecting a limit moment when the switching frequency of the flyback converter reaches a preset limit switching frequency and a zero current moment of a secondary side current of the flyback converter in the same switching period;
judging the working mode of the flyback converter based on the limited time and the zero current time;
under the condition that the working mode of the flyback converter is judged to be a critical mode, the synchronous rectifier tube is controlled to be turned off after delaying for the first time;
under the condition that the working mode of the flyback converter is judged to be the discontinuous mode, controlling the synchronous rectifier tube to be turned on again for a second time;
starting to switch on the power switch tube after delaying a third time at a zero-crossing detection moment to realize zero-voltage switching on of the power switch tube, wherein the zero-crossing detection moment is the moment when the voltages at two power ends of the synchronous rectifier tube rise to the output voltage of the flyback converter,
wherein when the limited time is before the zero current time, the operation mode of the flyback converter can be determined as a critical mode,
when the limited time is located after the zero current time, the operation mode of the flyback converter can be determined to be an intermittent mode.
2. The control method of claim 1, wherein after the power switching tube is turned on, the control method further comprises:
detecting the change rate of the voltage of the two power ends of the synchronous rectifier tube within preset time, wherein the preset time is set as the time period before and after the power switch tube is switched on;
and judging whether the change rate is larger than a first threshold value, if so, increasing the first time or the second time by a first step time length in the next switching period, otherwise, decreasing the first time or the second time by a second step time length in the next switching period.
3. The control method of claim 1, wherein after the power switching tube is turned on, the control method further comprises:
detecting the change values of voltages at two power ends of the synchronous rectifier tube within a preset time length, wherein the preset time length is set as a time period before and after the power switch tube is switched on;
and judging whether the change value is larger than a second threshold value, if so, increasing the first time or the second time by a first step time length in the next switching period, otherwise, decreasing the first time or the second time by a second step time length in the next switching period.
4. The control method of claim 1, wherein controlling the synchronous rectifier to turn off after a first time delay comprises:
receiving a first turn-off signal for controlling the synchronous rectifier tube to turn off;
delaying the first turn-off signal for a first time to obtain a second turn-off signal, and transmitting the second turn-off signal to the control end of the synchronous rectifier tube; or
And acquiring a first turn-off threshold adjustment value corresponding to the first time, and increasing the turn-off threshold of the synchronous rectifier tube based on the first turn-off threshold adjustment value.
5. The control method according to any one of claims 2 and 3, wherein increasing the first time or the second time by a first further period comprises:
delaying a turn-off signal for controlling the turn-off of the synchronous rectifier tube for a first superposition time, and then providing the delayed turn-off signal to a control end of the synchronous rectifier tube, wherein the first superposition time is the sum of the first time or the second time and the first step time; or
And acquiring a second turn-off threshold adjustment value corresponding to the first step time length, and increasing the turn-off threshold of the synchronous rectifier tube based on the second turn-off threshold adjustment value.
6. The control method according to any one of claims 2 and 3, wherein reducing the first time or the second time by a second step length includes:
delaying a turn-off signal for controlling the turn-off of the synchronous rectifier tube for a first superposition time, and then providing the delayed turn-off signal to a control end of the synchronous rectifier tube, wherein the first superposition time is the difference between the first time or the second time and a second stepping time; or
And acquiring a third turn-off threshold adjustment value corresponding to the second stepping time length, and reducing the turn-off threshold of the synchronous rectifier tube based on the third turn-off threshold adjustment value.
7. The control method according to claim 1, wherein an on signal for controlling the power switching tube to be turned on and an off signal for controlling the power switching tube to be turned off are generated in a secondary side portion of the flyback converter and transmitted to a primary side portion of the flyback converter through an isolation element.
8. A control method of a flyback converter comprises a primary winding, a secondary winding, a power switch tube connected with the primary winding and a synchronous rectifier tube connected with the secondary winding, wherein the control method comprises the following steps:
detecting the load size of the flyback converter;
judging the working mode of the flyback converter based on the load;
under the condition that the working mode of the flyback converter is judged to be a critical mode, the synchronous rectifier tube is controlled to be turned off after delaying for the first time;
under the condition that the working mode of the flyback converter is judged to be the discontinuous mode, controlling the synchronous rectifier tube to be turned on again for a second time;
under the condition that the working mode of the flyback converter is judged to be the auxiliary mode, the synchronous rectifier tube is controlled to be turned off after delaying the first time in the first N-1 switching cycles in each N switching cycles, the synchronous rectifier tube is controlled to be turned on again for the second time in the Nth switching cycle in each N switching cycles,
wherein, when the load is greater than or equal to a third threshold, the operating mode of the flyback converter can be determined as a critical mode,
in the case where the load is less than or equal to a fourth threshold value, it may be determined that the operation mode of the flyback converter is a discontinuous mode,
in the case where the load is greater than a fourth threshold and less than a third threshold, it may be determined that the operation mode of the flyback converter is an auxiliary mode,
wherein N is an integer greater than 1.
9. A flyback converter, comprising:
a transformer including a primary winding and a secondary winding;
the power switch tube and the voltage input circuit are connected with the primary winding;
the synchronous rectifier tube and the voltage output circuit are connected with the secondary winding;
the driver is connected with the power switch tube;
the feedback control module is respectively connected with the synchronous rectifier tube and the output end of the flyback converter; and
an isolation element connected to the driver and the feedback control module, respectively,
wherein the feedback control module comprises:
the mode detection unit is used for detecting a limit moment corresponding to the fact that the switching frequency of the flyback converter reaches a preset limit switching frequency and a zero current moment of secondary side current of the flyback converter in the same switching period, and generating a first control signal when the limit moment is detected to be located before the zero current moment, or generating a second control signal when the limit moment is detected to be located after the zero current moment;
and the control unit is used for controlling the synchronous rectifying tube to be turned off after delaying the first time under the condition of receiving the first control signal, or controlling the synchronous rectifying tube to be turned on again for the second time under the condition of receiving the second control signal so as to realize zero-voltage turning-on of the power switching tube.
10. The flyback converter of claim 9, wherein the feedback control module further comprises:
the self-adaptive adjusting unit is used for detecting the voltage of the two-power-end voltage of the synchronous rectifier tube within a preset time, and generating a first adjusting signal under the condition that the change rate of the voltage of the two-power-end voltage of the synchronous rectifier tube is larger than a first threshold value, wherein the preset time is set as a time period before and after the power switch tube is switched on,
generating a second adjusting signal under the condition that the change rate of the voltage of the two power ends of the synchronous rectifier tube is less than or equal to a first threshold value,
the first adjusting signal is used for controlling to increase the first time or the second time in the next switching period, and the second adjusting signal is used for controlling to decrease the first time or the second time in the next switching period.
11. The flyback converter of claim 10, wherein the adaptive adjustment unit comprises:
the first end of the first capacitor is connected with the drain electrode of the synchronous rectifying tube;
the cathode of the clamping diode is connected with the second end of the first capacitor, and the anode of the clamping diode is connected with the reference ground;
the first resistor is connected between the second end of the first capacitor and the reference ground;
and a drain of the first switching tube is connected with a first end of the first capacitor, a gate of the first switching tube is connected with a second end of the first capacitor, and a source of the first switching tube outputs one of the first adjustment signal and the second adjustment signal.
12. The flyback converter of claim 9, wherein the feedback control module further comprises:
the self-adaptive adjusting unit is used for generating a first adjusting signal under the condition that the variation value of the voltages at the two power ends of the synchronous rectifier tube in a preset time length is greater than a second threshold value, wherein the preset time length is set as a time period before and after the power switch tube is switched on;
generating a second adjusting signal under the condition that the variation value of the voltages at the two power ends of the synchronous rectifier tube in the preset time length is less than or equal to a second threshold value,
the first adjusting signal is used for controlling to increase the first time or the second time in the next switching period, and the second adjusting signal is used for controlling to decrease the first time or the second time in the next switching period.
13. The flyback converter of claim 12, wherein the adaptive adjustment unit comprises:
the first end of the second resistor is connected with the drain electrode of the synchronous rectifying tube;
a first end of the second capacitor is connected with a second end of the second resistor, and a second end of the second capacitor is connected with the reference ground;
a first end of the second switch tube is connected with a first end of the second resistor, a second end of the second switch tube is connected with a second end of the second resistor, and a control end of the second switch tube receives a control signal;
and the non-inverting input end of the error amplifier is connected with the first end of the second resistor, the inverting input end of the error amplifier is connected with the second end of the second resistor, and the output end of the error amplifier outputs one of the first adjusting signal and the second adjusting signal.
14. The flyback converter of claim 10, wherein the feedback control module further comprises:
the primary side control signal generating unit is used for providing a primary side switching-on signal and a primary side switching-off signal;
and the secondary side control signal generating unit is used for providing a secondary side on signal and a secondary side off signal.
CN202110496801.1A 2021-05-07 2021-05-07 Flyback converter and control method thereof Active CN113726166B (en)

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