CN113765395B - Control system and control method for improving primary side feedback sampling precision of active clamp flyback converter - Google Patents

Control system and control method for improving primary side feedback sampling precision of active clamp flyback converter Download PDF

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CN113765395B
CN113765395B CN202111008767.5A CN202111008767A CN113765395B CN 113765395 B CN113765395 B CN 113765395B CN 202111008767 A CN202111008767 A CN 202111008767A CN 113765395 B CN113765395 B CN 113765395B
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switching tube
clamping
voltage
capacitor
signal
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CN113765395A (en
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徐申
钱毅杰
孙乾坤
李凌昀
陈明刚
孙伟锋
时龙兴
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Southeast University-Wuxi Institute Of Integrated Circuit Technology
Southeast University
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Southeast University-Wuxi Institute Of Integrated Circuit Technology
Southeast University
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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
    • 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
    • H02M3/33507—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 with automatic control of the output voltage or current, e.g. flyback converters
    • H02M3/33523—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 with automatic control of the output voltage or current, e.g. flyback converters with galvanic isolation between input and output of both the power stage and the feedback loop
    • 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

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Dc-Dc Converters (AREA)

Abstract

The invention discloses a control system and a control method for improving the primary feedback sampling precision of an active clamp flyback converter, wherein the control system is characterized in that a sampling hold module is used for sampling the voltage of an auxiliary winding when a clamp switching tube is turned off, so that the accurate value of an output voltage is obtained, the accurate value is compared with a reference voltage to obtain an error signal of the output voltage, the error signal and an input voltage signal obtained by an input voltage sampling module are input into a PWM control module, so that the proper switching tube conduction time is obtained, and a clamp switching tube driving signal and a main switching tube driving signal are generated.

Description

Control system and control method for improving primary side feedback sampling precision of active clamp flyback converter
Technical Field
The invention relates to the technical field of active clamp flyback converter control, in particular to a control system and a control method for improving primary side feedback sampling precision of an active clamp flyback converter.
Background
The flyback converter has a simple circuit structure and relatively high conversion efficiency. Compared with a Buck/Boost circuit, the output voltage of the Buck/Boost circuit is less influenced by the input voltage, and the introduced transformer structure further realizes grid isolation. Flyback power supplies are very widely used in current low power adapters.
Most of chargers of portable intelligent devices such as mobile phones and tablet computers which are widely used currently use flyback power supply topology. At present, mobile intelligent devices are still rapidly developed, and as people use mobile intelligent devices such as mobile phones and the like more frequently, the electric quantity of the devices has become one of main bottlenecks for limiting the development of the mobile devices. While increasing battery capacity and density, businesses and consumers also place higher demands on the rate at which batteries are charged, i.e., the power charged. Meanwhile, the enhancement of environmental awareness also requires that the power efficiency reach an unprecedented level. These demands have made improving the structure and control algorithms of flyback converters one of the focus of research for low-power adapters today.
As the power supply is advanced toward higher power and higher frequency, the switching loss caused by the voltage and current changes between the source and drain of the switching transistor Guan Shunjian becomes a problem to be solved. In order to achieve high frequency and prevent electromagnetic interference, the flyback power supply must achieve zero voltage switching (Zero Voltage Switching, ZVS for short). For this purpose, an active clamp flyback converter (Active Clamp Flyback Converter, abbreviated as ACF) is proposed, as shown in fig. 1. By adding a clamp switching tube and a clamp capacitor to replace the traditional passive clamp circuit, the active clamp can absorb leakage inductance energy in an ideal state without damage. Since the clamp capacitance is large, the clamp effect of the main switching tube voltage is better, there is almost no high frequency harmonic, and zero-voltage switching of the main switching tube and the clamp tube is realized, which makes an increase in switching frequency possible. At present, most of high-frequency low-power gallium nitride power supplies are in power supply topology using ACF, so that the problem of electromagnetic interference can be avoided while high efficiency is realized.
The topology of ACF is basically shaped, and the focus of research is mainly to realize stronger performance through optimization of control algorithm and sampling mode. Typical control modes sample directly at the output end, and require the use of an isolation device such as an optocoupler for sampling. In order to remove the isolation device to improve the system integration level and reduce the cost, some researches in recent years use a primary side feedback control mode, and indirect sampling is performed by using an auxiliary winding of a transformer, so that the isolation device in the system is omitted.
At present, sampling of output voltage by a primary side feedback control mode often uses a sampling scheme of successive approximation by a comparator. The sampling scheme often uses a comparator to compare the auxiliary winding voltage to the predicted output voltage. The comparator output is low when the auxiliary winding voltage is lower than the predicted output voltage, and high when the auxiliary winding voltage is higher than the predicted output voltage. Judging the relation between the time of outputting the high level by the comparator and the reference time, if the time of outputting the high level is smaller than the reference time, estimating the output voltage to be reduced by one unit amount, if the time of outputting the high level is larger than the reference time, estimating the output voltage to be increased by one unit amount, and if the time of outputting the high level is equal to the reference time, estimating the output voltage to be unchanged. The specific increase or decrease is usually preset empirically. And finally, estimating the output voltage to obtain an approximate value of the output voltage through linear conversion.
However, this sampling method can only obtain an approximate value of the output voltage, but cannot obtain an accurate value of the output voltage. This limits further improvement in the accuracy of the primary side feedback control.
Disclosure of Invention
In view of the above, the present invention aims to provide a control system and a control method for improving the primary feedback sampling precision of an active clamp flyback converter, which are used for solving the technical problems mentioned in the background art.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
a control system for improving primary side feedback sampling precision of an active clamp flyback converter comprises a primary side of a transformer and a secondary side of the transformer; the primary side comprises a primary winding, an excitation inductance, a leakage inductance, a main switching tube, a clamping switching tube and a clamping capacitor; the secondary side includes secondary winding, rectifier diode, output capacitor and load resistance, the secondary side still includes auxiliary winding, and with auxiliary winding series connection's first bleeder resistor and second bleeder resistor, control system includes: the device comprises a sample hold module, an error amplifier, an input voltage sampling module, a PWM control module and a driving module, wherein,
the sampling and holding module is used for obtaining an output voltage signal by sampling and holding the auxiliary winding voltage when the falling edge of the driving signal of the clamp switching tube arrives, and outputting the output voltage signal to the error amplifier;
the error amplifier is used for acquiring the output voltage signal and comparing the output voltage signal with the reference voltage of the output voltage to obtain an error signal of the output voltage;
the input voltage sampling module is used for sampling the auxiliary winding voltage in the on period of the main switching tube to obtain a voltage signal proportional to the input voltage, and converting the voltage signal into an input voltage signal;
the PWM control module calculates the on time of a main switching tube of the next period according to the error signal of the output voltage and the input voltage signal input by the error amplifier, generates a main switching tube driving signal and a clamping switching tube driving signal, and transmits the main switching tube driving signal and the clamping switching tube driving signal to the driving module;
the driving module is used for controlling the switching and the switching of the main switching tube and the clamping switching tube to form a closed loop system according to the driving signal of the main switching tube and the driving signal of the clamping switching tube;
and the capacitance value of the clamping capacitor is changed, so that the moment when the clamping capacitor and the leakage inductance resonance period are ended coincides with the moment when the clamping switch tube is turned off.
Further, the sample-and-hold module specifically includes:
a first buffer having a negative input terminal connected to an output terminal thereof, a positive input terminal connected over the auxiliary winding, and acquiring the auxiliary winding voltage;
one end of the first switch is connected with the output end of the first buffer, and the other end of the first switch is connected with the holding capacitor and the positive input end of the second buffer, wherein the first switch is controlled by the clamp switching tube driving signal;
the negative input end of the second buffer is connected to the output end of the second buffer, the positive input end of the second buffer is connected with the holding capacitor, and the output end of the second buffer transmits the output voltage signal to the error amplifier;
the first switch is triggered to be closed by the falling edge of the driving signal of the clamping switching tube, the first buffer samples the voltage of the auxiliary winding, the sampled voltage signal is kept through the holding capacitor, and the kept output voltage signal is output to the error amplifier through the second buffer.
Further, the driving module controls the switching and the switching of the main switching tube and the clamping switching tube to form a closed loop system according to the driving signal of the main switching tube and the driving signal of the clamping switching tube, and specifically includes:
when the driving signal of the main switching tube is at a high level, controlling the main switching tube to be conducted;
when the driving signal of the main switching tube is at a low level, the main switching tube is controlled to be turned off;
when the driving signal of the clamping switching tube is at a high level, controlling the clamping switching tube to be conducted;
and when the driving signal of the clamping switching tube is at a low level, controlling the clamping switching tube to be turned off.
Further, by changing the capacitance value of the clamping capacitor, the moment when the clamping capacitor and the leakage inductance resonance period are ended coincides with the moment when the clamping switch tube is turned off, which specifically includes:
the capacitance value of the clamping capacitor is changed to control the voltage drop rate of the two ends of the clamping capacitor, so that the resonance period of the clamping capacitor and leakage inductance is changed; the capacitance value of the clamping capacitor is increased, the voltage drop rate at two ends of the clamping capacitor is reduced, and the resonance period of the clamping capacitor and leakage inductance is prolonged; the capacitance value of the clamping capacitor is reduced, the voltage drop rate of the two ends of the clamping capacitor is increased, and the resonance period of the clamping capacitor and leakage inductance is shortened.
A control method for improving primary side feedback sampling precision of an active clamp flyback converter comprises a primary side of a transformer and a secondary side of the transformer; the primary side comprises a primary winding, an excitation inductance, a leakage inductance, a main switching tube, a clamping switching tube and a clamping capacitor; the secondary side comprises a secondary winding, a rectifier diode, an output capacitor, a load resistor, an auxiliary winding, a first voltage dividing resistor and a second voltage dividing resistor which are connected in series with the auxiliary winding, and the control method comprises the following steps:
step S01, when the falling edge of a driving signal of a clamping switching tube arrives, an output voltage signal is obtained by sampling and holding the auxiliary winding voltage;
step S02, comparing the output voltage signal obtained in the step S01 with the reference voltage of the output voltage to obtain an error signal of the output voltage;
step S03, sampling the auxiliary winding voltage during the conduction period of the main switching tube to obtain a voltage signal proportional to the input voltage, and converting the voltage signal into the input voltage signal;
step S04, calculating the on time of a main switching tube of the next period according to the error signal of the output voltage obtained in the step S02 and the input voltage signal obtained in the step S03, and generating a main switching tube driving signal and a clamping switching tube driving signal;
step S05, controlling the switching and closing of the main switching tube and the clamping switching tube to form a closed loop system according to the driving signals of the main switching tube and the clamping switching tube;
and the capacitance value of the clamping capacitor is changed, so that the moment when the clamping capacitor and the leakage inductance resonance period are ended coincides with the moment when the clamping switch tube is turned off.
The beneficial effects of the invention are as follows:
aiming at the problems of insufficient sampling precision and larger error in primary side feedback control of an active clamp flyback converter, the invention properly prolongs the resonance period of a clamp capacitor and a leakage inductance through proper design of system parameters, and samples the voltage of an auxiliary winding when a clamp switching tube is turned off through a sample hold module, thereby improving the sampling precision of output voltage.
Drawings
FIG. 1 is a circuit block diagram of an active clamp flyback converter as referred to in the background;
fig. 2 is a waveform diagram of main signals of the inverter provided in embodiment 1 in a steady operation state;
FIG. 3 is a schematic structural diagram of a control system for improving primary feedback sampling accuracy of an active clamp flyback converter according to embodiment 1;
fig. 4 is a schematic structural view of a sample-and-hold module provided in embodiment 1;
FIG. 5 is a waveform schematic diagram of the sample-and-hold module provided in embodiment 1 sampling the auxiliary winding voltage;
in the accompanying drawings:
np is a primary winding, lm is an excitation inductance, lr is a leakage inductance, Q1 is a main switching tube, Q2 is a clamping switching tube, and Cr is a clamping capacitor;
ns is a secondary winding, do is a rectifier diode, co is an output capacitor, and Ro is a load resistor;
na is an auxiliary winding, R1 is a first voltage dividing resistor, and R2 is a second voltage dividing resistor;
a1 is a first buffer, A2 is a second buffer, S1 is a first switch, and C1 is a holding capacitor;
vaux1 is the auxiliary winding voltage, vos is the output voltage signal, vref is the reference voltage of the output voltage, voer is the error signal of the output voltage, vin is the input voltage signal, DUTYL is the main switching tube driving signal, DUTYH is the clamp switching tube driving signal, VCr is the voltage across the clamp capacitor, vf is the forward conduction voltage drop of the rectifier diode, ILm is the magnetizing inductance current, and ILr is the leakage inductance current.
Detailed Description
For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments of the present invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
Referring to fig. 2-5, the present embodiment provides a control system for improving primary feedback sampling accuracy of an active clamp flyback converter, where the active clamp flyback converter includes a primary side of a transformer and a secondary side of the transformer;
the primary side comprises a primary winding Np, an excitation inductance Lm, a leakage inductance Lr, a main switching tube Q1, a clamping switching tube Q2 and a clamping capacitor Cr;
the secondary side comprises a secondary winding Ns, a rectifier diode Do, an output capacitor Co and a load resistor Ro;
the secondary side also comprises an auxiliary winding Na, and a first voltage dividing resistor R1 and a second voltage dividing resistor R2 which are connected in series with the auxiliary winding Na;
the control system comprises: the device comprises a sample hold module, an error amplifier, an input voltage sampling module, a PWM control module and a driving module, wherein,
the sampling and holding module is used for obtaining an output voltage signal Vos by sampling and holding the auxiliary winding voltage Vaux1 when the falling edge of the clamp switching tube driving signal DUTYH comes and outputting the output voltage signal Vos to the error amplifier;
the error amplifier is used for acquiring an output voltage signal Vos and comparing the output voltage signal Vos with a reference voltage Vref of the output voltage to obtain an error signal Voer of the output voltage;
the input voltage sampling module is used for sampling the auxiliary winding voltage Vaux1 in the on period of the main switching tube to obtain a voltage signal proportional to the input voltage, and converting the voltage signal into an input voltage signal Vin;
the PWM control module calculates the on time of a main switching tube in the next period according to an error signal Voer of output voltage input by the error amplifier and an input voltage signal Vin, generates a main switching tube driving signal DUTYL and a clamping switching tube driving signal DUTYH, and transmits the main switching tube driving signal DUTYL and the clamping switching tube driving signal DUTYH to the driving module;
the driving module is used for controlling the switching and closing of the main switching tube Q1 and the clamping switching tube Q2 to form a closed loop system according to the main switching tube driving signal DUTYL and the clamping switching tube driving signal DUTYH;
wherein, by changing the capacitance value of the clamping capacitor Cr, the moment when the clamping capacitor Cr and the leakage inductance resonance period are ended coincides with the moment when the clamping switch tube Q2 is turned off.
Specifically, in this embodiment, the sample-and-hold module specifically includes:
a first buffer A1 having a negative input terminal connected to an output terminal thereof, a positive input terminal connected over the auxiliary winding Na, and acquiring an auxiliary winding voltage Vaux1;
a first switch S1 having one end connected to an output end of the first buffer A1 and the other end connected to the holding capacitor C1 and a positive input end of the second buffer A2, wherein the clamp switching tube driving signal DUTYH controls the first switch S1;
a second buffer A2 having a negative input terminal connected to an output terminal thereof, a positive input terminal connected to the holding capacitor C1, and an output terminal transmitting an output voltage signal Vos to the error amplifier;
the falling edge of the clamp switching tube driving signal DUTYH triggers the first switch S1 to be closed, the first buffer A1 samples the auxiliary winding voltage Vaux1, the sampled voltage signal is held by the holding capacitor, and the held output voltage signal Vos is output to the error amplifier by the second buffer A2.
Specifically, in the present embodiment, when the main switching tube driving signal DUTYL is at a high level, the main switching tube Q1 is controlled to be turned on; when the main switching tube driving signal DUTYL is at a low level, the main switching tube Q1 is controlled to be turned off; when the clamp switching tube driving signal DUTYH is at a high level, controlling the clamp switching tube Q2 to be conducted; when the clamp switching tube driving signal DUTYH is at a low level, the clamp switching tube Q2 is controlled to be turned off.
Specifically, in this embodiment, the capacitance value of the clamp capacitor Cr is changed to control the rate of voltage VCr at two ends of the clamp capacitor, so as to change the resonance period of the clamp capacitor Cr and leakage inductance; the capacitance value of the clamping capacitor Cr is increased, the rate of voltage VCr at two ends of the clamping capacitor is reduced, and the resonance period of the clamping capacitor Cr and leakage inductance is prolonged; the capacitance value of the clamp capacitor Cr is reduced, the rate of voltage VCr at two ends of the clamp capacitor is increased, and the resonance period of the clamp capacitor Cr and leakage inductance is shortened.
Specifically, in order to facilitate explanation of the working principle of the control system provided by the present embodiment, a working cycle is divided into 4 different phases, specifically:
stage t0-t 1:
at time t0, clamp switching tube driving signal DUTYH becomes low level, clamp switching tube Q2 is turned off, main switching tube driving signal DUTYL becomes high level, and main switching tube is turned on. The input voltage signal Vin is directly applied to the primary excitation inductance Lm and the leakage inductance Lr, and the excitation inductance current ILm and the leakage inductance current ILr linearly increase.
This state continues until the main switching tube driving signal DUTYL becomes low level at time t1, and the main switching tube Q1 is turned off. A dead time may be added between when DUTYH goes low and when DUTYL goes high to achieve zero voltage switching.
The voltage across the primary side transformer in this phase is equal to the input voltage signal Vin, and the value of the auxiliary winding voltage Vaux1 can be expressed as:
Figure BDA0003238063010000071
stage t1-t 2:
at time t1, the main switching tube driving signal DUTYL becomes low level, the main switching tube is turned off, the clamp switching tube driving signal DUTYH becomes high level, and the clamp switching tube Q2 is turned on. At this time, the voltage VCr < Np (vo+vf)/Ns across the clamp capacitor, the clamp capacitor Cr starts resonating with the excitation inductors Lm and Lr, and the clamp capacitor Cr is charged. At the same time the auxiliary winding voltage Vaux1 starts to increase due to the charging of the clamping capacitance Cr. A dead time may be added between when DUTYL goes low and when DUTYH goes high to achieve zero voltage switching.
At time t2, the voltage VCr value at two ends of the clamping capacitor reaches N p (V o +V f )/N s At this time, the rectifying diode Do at the output end is turned on to supply power to the load, and the voltage value at the two ends of the exciting inductance Lm is clamped at-N p (V o +V f )/N s And (3) upper part.
Stage t2-t 3:
at time t2, the diode at the output end is turned on, and the voltage across the primary winding is clamped at-N p (V o +V f )/N s The clamp capacitor Cr resonates with the leakage inductance Lr. In this stage, the voltage VCr across the clamp capacitor is always higher than N p (V o +V f )/N s And will rise and fall first.
By time t3, the value of VCr drops to N p (V o +V f )/N s At this time, the diode at the output is turned off. In this stage, the voltage value across the excitation inductance Lm is still clamped at-N p (V o +V f )/N s As a result, the exciting inductance current ILm linearly decreases, and the difference between the exciting inductance current ILm and the leakage inductance current ILr is transmitted from the primary winding Np to the secondary through the transformer, thereby supplying power to the load.
Stage t3-t 4:
the diode at the output end at the moment t3 is turned off, the resonance period of the clamping capacitor Cr and the leakage inductance Lr is ended, and the excitation inductance Lm is not clamped to-N by the output voltage any more p (V o +V f )/N s . At this time, the clamp capacitance Cr starts resonating with the excitation inductances Lm and Lr.
At time t4, clamp switching transistor driving signal DUTYH becomes low level, and clamp switching transistor Q2 is turned off. A new duty cycle is started.
By changing the capacitance value of the clamp capacitor Cr, the rate of decrease of the voltage VCr across the clamp capacitor can be controlled, thereby changing the resonance period of the clamp capacitor Cr and the leakage inductance Lr. The capacitance value of the clamping capacitor Cr is increased, the rate of voltage VCr at two ends of the clamping capacitor is reduced, and the resonance period of the clamping capacitor Cr and leakage inductance Lr is prolonged; the capacitance value of the clamp capacitor Cr is reduced, the rate of voltage VCr at two ends of the clamp capacitor is increased, and the resonance period of the clamp capacitor Cr and the leakage inductance Lr is shortened.
Therefore, by selecting the clamping capacitor Cr with proper parameters, the moment when the resonance period of the clamping capacitor Cr and the leakage inductance Lr is ended coincides with the moment when the clamping switch tube Q2 is turned off, that is, t3 and t4 coincide. At this time, the clamp switching tube driving signal DUTYH changes from a high level to a low level to form a falling edge, the voltage across the secondary side transformer is equal to the output voltage Vo, and the relationship between the auxiliary winding voltage Vaux1 and the output voltage Vo can be expressed as:
Figure BDA0003238063010000081
specifically, in the present embodiment, the capacitance of the clamp capacitor Cr is selected to be larger, so that the resonance period between the clamp capacitor Cr and the leakage inductance Lr is properly prolonged to be as close as possible to or coincide with the turn-off time of the clamp switching tube Q2.
Specifically, in this embodiment, when the falling edge of the clamp switching tube driving signal DUTYH comes, the sample-hold module samples and holds the auxiliary winding voltage Vaux1 to obtain the output voltage signal Vos, where the expression of Vos can be obtained according to the following formula:
Figure BDA0003238063010000082
vos is held and output to an error amplifier.
Specifically, in this embodiment, the error amplifier compares the output voltage signal Vos input by the sample-and-hold module with the reference voltage Vref of the output voltage to obtain an error value between the current output voltage and the reference voltage, that is, the error signal Voer of the output voltage, and outputs the error value to the PWM control module from the output end; the value of Vref is: vref= (Na/Ns) Voref, where Voref is the output voltage that the power system expects to reach.
Specifically, in the present embodiment, the input voltage sampling module samples the auxiliary winding voltage Vaux1 in the t0-t1 stage, and calculates the formula V of the input voltage signal Vin in ＝-(N p /N a )((R 1 +R 2 )/R 2 )V aux1 Thereby converting the sampling voltage signal, obtaining the value of the input voltage signal Vin and outputting the value to the PWM control module.
Specifically, in this embodiment, the PWM control module calculates the peak current Ipk according to the error signal Voer of the output voltage input by the error amplifier, and then calculates the on time Ton of the main switching tube in the next period according to the input voltage signal Vin input by the input voltage sampling module by the formula ton= (Ipk/Vin) Lm, so as to generate the driving signal DUTYL of the main switching tube and the driving signal DUTYH of the clamp switching tube.
In the next switching cycle, DUTYL is first high, DUTYH is low, and this state is maintained for Ton. After which the DUTYL goes low and the DUTYH goes high until the switching period ends. And when the switching period is finished, the PWM control module recalculates the on time Ton of the main switching tube in the next period according to the updated Voer and Vin values.
Fig. 5 is a schematic waveform diagram of a sample-and-hold module sampling and holding the auxiliary winding voltage Vaux1, when the falling edge of the first clamp switching tube driving signal DUTYH arrives, the sample-and-hold module samples Vaux1 and outputs a sampled voltage signal Vos, and the voltage signal Vos is held until the next falling edge of DUTYH arrives, and the sample-and-hold module resamples the updated voltage signal Vos.
Example 2
The embodiment provides a control method for improving primary side feedback sampling precision of an active clamp flyback converter, wherein the active clamp flyback converter comprises a primary side of a transformer and a secondary side of the transformer; the primary side comprises a primary winding, an excitation inductance, a leakage inductance, a main switching tube, a clamping switching tube and a clamping capacitor; the secondary side comprises a secondary winding, a rectifier diode, an output capacitor and a load resistor; the secondary side also comprises an auxiliary winding, and a first voltage dividing resistor and a second voltage dividing resistor which are connected in series with the auxiliary winding, and the method comprises the following steps:
step S01, when the falling edge of a driving signal of a clamping switching tube arrives, an output voltage signal is obtained by sampling and holding the voltage of an auxiliary winding;
step S02, comparing the output voltage signal obtained in the step S01 with the reference voltage of the output voltage to obtain an error signal of the output voltage;
step S03, sampling the voltage of the auxiliary winding in the on period of the main switching tube to obtain a voltage signal proportional to the input voltage, and converting the voltage signal into the input voltage signal;
step S04, calculating the on time of a main switching tube of the next period according to the error signal of the output voltage obtained in the step S02 and the input voltage signal obtained in the step S03, and generating a main switching tube driving signal and a clamping switching tube driving signal;
step S05, controlling the switching and the switching of the main switching tube and the clamping switching tube to form a closed loop system according to the driving signals of the main switching tube and the clamping switching tube;
the capacitance value of the clamping capacitor is changed, so that the moment when the clamping capacitor and the leakage inductance resonance period are ended coincides with the moment when the clamping switch tube is turned off.
The present invention is not described in detail in the present application, and is well known to those skilled in the art.
The foregoing describes in detail preferred embodiments of the present invention. It should be understood that numerous modifications and variations can be made in accordance with the concepts of the invention by one of ordinary skill in the art without undue burden. Therefore, all technical solutions which can be obtained by logic analysis, reasoning or limited experiments based on the prior art by the person skilled in the art according to the inventive concept shall be within the scope of protection defined by the claims.

Claims (5)

1. A control system for improving primary side feedback sampling precision of an active clamp flyback converter comprises a primary side of a transformer and a secondary side of the transformer; the primary side comprises a primary winding, an excitation inductance, a leakage inductance, a main switching tube, a clamping switching tube and a clamping capacitor; the secondary side includes secondary winding, rectifier diode, output capacitor and load resistance, the secondary side still includes auxiliary winding, and with auxiliary winding series connection's first bleeder resistor and second bleeder resistor, its characterized in that, control system includes: the device comprises a sample hold module, an error amplifier, an input voltage sampling module, a PWM control module and a driving module, wherein,
the sampling and holding module is used for obtaining an output voltage signal by sampling and holding the auxiliary winding voltage when the falling edge of the driving signal of the clamp switching tube arrives, and outputting the output voltage signal to the error amplifier;
the error amplifier is used for acquiring the output voltage signal and comparing the output voltage signal with the reference voltage of the output voltage to obtain an error signal of the output voltage;
the input voltage sampling module is used for sampling the auxiliary winding voltage in the on period of the main switching tube to obtain a voltage signal proportional to the input voltage, and converting the voltage signal into an input voltage signal;
the PWM control module calculates the on time of a main switching tube of the next period according to the error signal of the output voltage and the input voltage signal input by the error amplifier, generates a main switching tube driving signal and a clamping switching tube driving signal, and transmits the main switching tube driving signal and the clamping switching tube driving signal to the driving module;
the driving module is used for controlling the switching and the switching of the main switching tube and the clamping switching tube to form a closed loop system according to the driving signal of the main switching tube and the driving signal of the clamping switching tube;
and the capacitance value of the clamping capacitor is changed, so that the moment when the clamping capacitor and the leakage inductance resonance period are ended coincides with the moment when the clamping switch tube is turned off.
2. The control system for improving primary feedback sampling accuracy of an active clamp flyback converter of claim 1, wherein the sample-and-hold module specifically comprises:
a first buffer having a negative input terminal connected to an output terminal thereof, a positive input terminal connected over the auxiliary winding, and acquiring the auxiliary winding voltage;
one end of the first switch is connected with the output end of the first buffer, and the other end of the first switch is connected with the holding capacitor and the positive input end of the second buffer, wherein the first switch is controlled by the clamp switching tube driving signal;
the negative input end of the second buffer is connected to the output end of the second buffer, the positive input end of the second buffer is connected with the holding capacitor, and the output end of the second buffer transmits the output voltage signal to the error amplifier;
the first switch is triggered to be closed by the falling edge of the driving signal of the clamping switching tube, the first buffer samples the voltage of the auxiliary winding, the sampled voltage signal is kept through the holding capacitor, and the kept output voltage signal is output to the error amplifier through the second buffer.
3. The control system for improving primary feedback sampling precision of an active clamp flyback converter according to claim 1, wherein the driving module controls switching and closing of the main switching tube and the clamp switching tube to form a closed loop system according to the main switching tube driving signal and the clamp switching tube driving signal, and specifically comprises:
when the driving signal of the main switching tube is at a high level, controlling the main switching tube to be conducted;
when the driving signal of the main switching tube is at a low level, the main switching tube is controlled to be turned off;
when the driving signal of the clamping switching tube is at a high level, controlling the clamping switching tube to be conducted;
and when the driving signal of the clamping switching tube is at a low level, controlling the clamping switching tube to be turned off.
4. The control system for improving primary feedback sampling precision of an active clamp flyback converter according to claim 1, wherein the control system is characterized by changing the capacitance value of the clamp capacitor so that the ending time of the resonance period of the clamp capacitor and the leakage inductance coincides with the closing time of the clamp switching tube, and specifically comprises:
the capacitance value of the clamping capacitor is changed to control the voltage drop rate of the two ends of the clamping capacitor, so that the resonance period of the clamping capacitor and leakage inductance is changed; the capacitance value of the clamping capacitor is increased, the voltage drop rate at two ends of the clamping capacitor is reduced, and the resonance period of the clamping capacitor and leakage inductance is prolonged; the capacitance value of the clamping capacitor is reduced, the voltage drop rate of the two ends of the clamping capacitor is increased, and the resonance period of the clamping capacitor and leakage inductance is shortened.
5. A control method for improving primary side feedback sampling precision of an active clamp flyback converter comprises a primary side of a transformer and a secondary side of the transformer; the primary side comprises a primary winding, an excitation inductance, a leakage inductance, a main switching tube, a clamping switching tube and a clamping capacitor; the secondary side comprises a secondary winding, a rectifier diode, an output capacitor, a load resistor, an auxiliary winding, a first voltage dividing resistor and a second voltage dividing resistor which are connected in series with the auxiliary winding, and the control method is characterized by comprising the following steps:
step S01, when the falling edge of a driving signal of a clamping switching tube arrives, an output voltage signal is obtained by sampling and holding the auxiliary winding voltage;
step S02, comparing the output voltage signal obtained in the step S01 with the reference voltage of the output voltage to obtain an error signal of the output voltage;
step S03, sampling the auxiliary winding voltage during the conduction period of the main switching tube to obtain a voltage signal proportional to the input voltage, and converting the voltage signal into the input voltage signal;
step S04, calculating the on time of a main switching tube of the next period according to the error signal of the output voltage obtained in the step S02 and the input voltage signal obtained in the step S03, and generating a main switching tube driving signal and a clamping switching tube driving signal;
step S05, controlling the switching and closing of the main switching tube and the clamping switching tube to form a closed loop system according to the driving signals of the main switching tube and the clamping switching tube;
and the capacitance value of the clamping capacitor is changed, so that the moment when the clamping capacitor and the leakage inductance resonance period are ended coincides with the moment when the clamping switch tube is turned off.
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