CN111200365A - Control method and control circuit of flyback converter - Google Patents
Control method and control circuit of flyback converter Download PDFInfo
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- CN111200365A CN111200365A CN202010136126.7A CN202010136126A CN111200365A CN 111200365 A CN111200365 A CN 111200365A CN 202010136126 A CN202010136126 A CN 202010136126A CN 111200365 A CN111200365 A CN 111200365A
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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
- H02M3/33507—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
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Abstract
A control method and a control circuit of a flyback converter belong to the technical field of switching power supplies. Firstly, extracting a control signal in direct proportion to the secondary current information or the secondary power information of the flyback converter, and then adjusting the switching frequency of the flyback converter according to the control signal, wherein the larger the control signal is, the higher the switching frequency of the flyback converter is; the invention provides a method for obtaining secondary current information by monitoring the voltage on a primary current detection resistor and combining the follow current feedback of an auxiliary winding, and in addition, the secondary power information can be obtained by introducing secondary voltage feedback on the basis. Compared with the traditional control scheme that the adjustment has limitation due to the fact that the feedback signal of the output voltage of the flyback converter is used as the basis, the switching frequency of the flyback converter is adjusted by using the secondary current information or the secondary power information of the flyback converter as the basis, and the switching frequency can be accurately adjusted in real time according to the load condition.
Description
Technical Field
The invention belongs to the technical field of switching power supplies, and relates to a control method of a flyback converter and a control circuit of the flyback converter, which can realize the control method.
Background
Flyback converters are also known as single-ended Flyback or Buck-Boost converters, and are named because the output end of the Flyback converter obtains energy when the primary winding is disconnected from a power supply. The flyback converter has a simple circuit structure and low cost, and is widely applied to low-power supplies and various power adapters.
There are various control methods for the energy transmission of the flyback converter, for example, a common closed-loop control is implemented by adjusting the voltage of a feedback signal FB of the output voltage of the flyback converter through an optical coupler at the secondary side, that is, the voltage of the FB determines the energy transmission. Besides, the method also comprises a plurality of control modes, such as a continuous mode (CCM), a quasi-resonant mode (QR), a discontinuous mode (DCM) and the like. Each control mode corresponds to different switching frequencies, and the switching frequencies correspond to switching losses, which directly affect the conversion efficiency of the flyback converter. Especially in the quasi-resonant mode, the switching frequency increases with decreasing load, and the switching loss ratio also increases.
Therefore, according to the energy transfer, the switching frequency needs to be further controlled, and a frequency reduction control strategy is necessary to be added. Fig. 7 shows a down-conversion control strategy, in which the switching frequency is adjusted according to the value of the feedback signal FB of the output voltage of the flyback converter. Although the value of the feedback signal FB has a direct relationship with the transfer of energy, it is monotonous. However, due to the influence of factors such as wired voltage compensation and ramp compensation, the feedback signal FB and the load magnitude are not in a linear relationship, and the feedback signal FB is used as a frequency reduction condition, so that the feedback signal FB has limitations and cannot be accurately adjusted in real time.
Disclosure of Invention
Aiming at the limitation problem existing in the traditional flyback converter switching frequency control method that the switching frequency is adjusted by utilizing the feedback signal FB, the invention provides a frequency control mode based on energy detection feedback for adjusting the switching frequency of the flyback converter, and the switching frequency is adjusted in real time according to the secondary current information or the secondary power information of the flyback converter.
The technical scheme of the invention is as follows:
a control method of a flyback converter comprises the following steps:
step one, extracting a control signal which is in direct proportion to the secondary current information or the secondary power information of the flyback converter;
and step two, adjusting the switching frequency of the flyback converter according to the control signal obtained in the step one, wherein the larger the control signal is, the higher the switching frequency of the flyback converter is.
Specifically, the flyback converter comprises a primary winding unit, a secondary winding unit and an auxiliary winding unit, wherein the primary winding unit comprises a primary winding, a switching tube and a primary current detection resistor, one end of the primary winding is connected with a power supply voltage, the other end of the primary winding is connected with one end of the switching tube, and the other end of the switching tube is grounded after passing through the primary current detection resistor; the auxiliary winding unit comprises an auxiliary winding, a first resistor and a second resistor, one end of the auxiliary winding is grounded, and the other end of the auxiliary winding is grounded through a series structure of the first resistor and the second resistor;
the specific method for extracting the control signal in proportion to the flyback converter secondary current information in the first step is as follows:
step 1.1, when a switching tube is closed, sampling and holding a voltage signal on a primary side current detection resistor to obtain the peak voltage of the voltage signal on the primary side current detection resistor;
step 1.2, after the switching tube is turned on, sampling and holding a voltage signal on the primary side current detection resistor to obtain a valley voltage of the voltage signal on the primary side current detection resistor;
step 1.3, summing the peak voltage and the valley voltage of the voltage signal on the primary side current detection resistor obtained in the step 1.1 and the step 1.2, and multiplying the sum by a coefficient K to obtain a first intermediate signal;
and step 1.4, in a secondary freewheeling stage, obtaining the control signal which is in direct proportion to the information of the secondary current of the flyback converter by performing RC (resistance-capacitance) filtering on the first intermediate signal.
Specifically, the specific method for extracting the control signal proportional to the flyback converter secondary power information in the step one is as follows: in step 1.3, the peak voltage and the valley voltage of the voltage signal on the primary side current detection resistor are summed, multiplied by a coefficient K and then multiplied by a feedback signal of a secondary voltage to obtain a second intermediate signal; step 1.4, in a secondary freewheeling stage, obtaining the control signal which is in direct proportion to the flyback converter secondary power information by performing RC (remote control) filtering on the second intermediate signal;
and the feedback signal of the secondary voltage is obtained by sampling and holding a signal at the series point of the first resistor and the second resistor in the auxiliary winding unit when the switching tube is closed.
Specifically, the method for determining the freewheel time in the secondary freewheel phase in step 1.4 includes: and the follow current time obtained by performing follow current detection on signals at the series point of the first resistor and the second resistor in the auxiliary winding unit is the follow current time of the secondary follow current stage.
Specifically, the specific method for adjusting the switching frequency of the flyback converter in the step two is as follows: charging a first capacitor by using a voltage control voltage source controlled by the control signal during the closing period of a switch tube in the flyback converter, and enabling the switch tube in the flyback converter to be opened and controlling the first capacitor to discharge when the voltage on the first capacitor is greater than a reference voltage; the larger the control signal is, the faster the first capacitor is charged, the smaller the switching period of the switching tube is, and the larger the switching frequency is.
The invention also provides a technical scheme of the control circuit for realizing the control method, which comprises the following steps:
the control circuit of the flyback converter comprises a control signal generation module and a switching frequency control module, wherein the control signal generation module is used for generating a control signal which is in direct proportion to the secondary current information or the secondary power information of the flyback converter, the switching frequency control module is used for adjusting the switching frequency of the flyback converter according to the control signal, and the larger the control signal is, the higher the switching frequency of the flyback converter is.
Specifically, the flyback converter comprises a primary winding unit, a secondary winding unit and an auxiliary winding unit, wherein the primary winding unit comprises a primary winding, a switching tube and a primary current detection resistor, one end of the primary winding is connected with a power supply voltage, the other end of the primary winding is connected with one end of the switching tube, and the other end of the switching tube is grounded after passing through the primary current detection resistor; the auxiliary winding unit comprises an auxiliary winding, a first resistor and a second resistor, one end of the auxiliary winding is grounded, and the other end of the auxiliary winding is grounded through a series structure of the first resistor and the second resistor;
the control signal generating module comprises a follow current detecting unit, a first sampling and holding unit, a second sampling and holding unit, an adder, a multiplier, a first switch, a second switch, a third resistor and a second capacitor,
the input end of the follow current detection unit is connected with the series point of the first resistor and the second resistor, and the output end of the follow current detection unit generates a follow current time signal;
the control end of the first sampling and holding unit is connected with a grid driving signal of a switching tube, the sampling end of the first sampling and holding unit is connected with a voltage signal on the primary side current detection resistor, and the output end of the first sampling and holding unit is connected with the first input end of the adder;
the control end of the second sampling and holding unit is connected with the inverted signal of the grid drive signal of the switching tube, the sampling end of the second sampling and holding unit is connected with the voltage signal on the primary side current detection resistor, and the output end of the second sampling and holding unit is connected with the second input end of the adder;
the first input end of the multiplier is connected with the output end of the adder, the second input end of the multiplier is connected with the coefficient K, and the output end of the multiplier is connected with one end of the first switch;
the other end of the first switch is connected with one end of the third resistor and one end of the second switch, and the control end of the first switch is connected with the follow current time signal;
the other end of the second switch is grounded, and the control end of the second switch is connected with the inverted signal of the follow current time signal;
the other end of the third resistor generates a control signal which is in direct proportion to the information of the secondary current of the flyback converter, and the control signal is grounded after passing through the second capacitor.
Specifically, the control signal generating module further includes a third sample-and-hold unit, a control end of the third sample-and-hold unit is connected to an inverted signal of the gate driving signal of the switching tube, a sampling end of the third sample-and-hold unit is connected to a series point of the first resistor and the second resistor, an output end of the third sample-and-hold unit is connected to a third input end of the multiplier, and at this time, the control signal generated by the control signal generating module is in direct proportion to the information of the secondary power of the flyback converter.
Specifically, the switching frequency control module comprises an voltage-controlled voltage source, a first capacitor, a third switch and a comparator,
the voltage-controlled voltage source charges the first capacitor under the control of the control signal, and the larger the control signal is, the faster the first capacitor is charged;
the positive input end of the comparator is connected with a voltage signal on the first capacitor, the negative input end of the comparator is connected with reference voltage, and the output end of the comparator generates a grid enable signal of the switching tube;
the third switch is connected with two ends of the first capacitor, and the control end of the third switch is connected with the grid drive signal of the switch tube.
The invention has the beneficial effects that: the switching frequency is adjusted through the control signal which is in direct proportion to the secondary current information or the secondary power information of the flyback converter, so that the switching frequency is accurately adjusted in real time according to the load condition of the flyback converter, the power density is considered, and the conversion efficiency is optimized.
Drawings
Fig. 1 is a circuit diagram of a flyback converter.
Fig. 2 is a schematic diagram of an implementation structure for obtaining the secondary freewheel time and the secondary voltage feedback in the control circuit of the flyback converter according to the present invention.
Fig. 3 is a schematic diagram of an implementation structure for obtaining secondary current information in a control circuit of a flyback converter according to the present invention.
Fig. 4 is a schematic diagram of an implementation structure for obtaining secondary power information in a control circuit of a flyback converter according to the present invention.
Fig. 5 is a schematic diagram of an implementation structure of a control circuit of a flyback converter according to the present invention, which utilizes a signal CCsg to perform frequency control.
Fig. 6 is a waveform diagram of relative nodes in a control method of a flyback converter and a control circuit thereof according to the present invention.
Fig. 7 is a schematic diagram of a conventional downconversion control strategy.
Fig. 8 is a graph comparing the conversion efficiency of the control circuit and the conversion efficiency of the flyback converter with the conversion efficiency of the conventional control method.
Fig. 9 is a graph of frequency versus load magnitude, which shows a comparison between the control method of the flyback converter and the control circuit thereof according to the present invention and the conventional control method.
Detailed Description
The technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.
As shown in fig. 1, the flyback converter includes a primary winding unit, a secondary winding unit, and an auxiliary winding unit, where the primary winding unit includes a primary winding, a switching tube Q1, and a primary current detection resistor R17, one end of the primary winding is connected to a supply voltage VBUS, the other end is connected to one end of a switching tube Q1, the other end of the switching tube Q1 is grounded after passing through the primary current detection resistor R17, a voltage signal on the primary current detection resistor R17 is CS, and a gate driving signal of the switching tube Q1 is DRVi. The auxiliary winding unit comprises an auxiliary winding, a first resistor R9 and a second resistor R11, one end of the auxiliary winding is grounded, the other end of the auxiliary winding is grounded through a series structure of a first resistor R9 and a second resistor R11, and a signal at the series point of the first resistor R9 and the second resistor R11 is a signal ZCD.
The existing frequency control is generally based on the voltage of a feedback signal FB of the output voltage of a flyback converter, but the feedback signal FB and the load size have a nonlinear relation, so that the feedback signal FB is used as the down-conversion condition, and the frequency control has a limitation. Different from the traditional control scheme in which a feedback signal FB of the output voltage of the flyback converter is taken as a basis, the invention provides a frequency control mode based on energy detection feedback, the switching frequency of the flyback converter is adjusted through a control signal in direct proportion to the secondary current information or the secondary power information of the flyback converter, and the switching frequency can be accurately adjusted in real time according to the load condition.
Two specific embodiments are provided below, in the first embodiment, the secondary current information can be obtained by monitoring the voltage on the primary side current detection resistor R17 in combination with the freewheeling feedback of the auxiliary winding, and the switching frequency is adjusted by using the secondary current information, and in the second embodiment, the secondary power information is obtained by combining the secondary voltage feedback on the basis of the first embodiment, and the switching frequency can also be adjusted by using the secondary power information.
First embodiment, adjusting the switching frequency according to the output current
In an embodiment, an implementation scheme for obtaining a control signal CCsg proportional to information of a secondary current of a flyback converter by monitoring a voltage across a primary current detection resistor R17 in combination with a freewheeling feedback of an auxiliary winding includes the following steps:
1.1, when a gate drive signal DRVi of a switching tube Q1 controls a switching tube Q1 to be closed, sampling and holding a voltage signal CS on a primary side current detection resistor R17 to obtain a peak voltage signal CSpk of the voltage signal CS on the primary side current detection resistor.
1.2, after the gate drive signal DRVi of the switching tube Q1 effectively controls the switching tube Q1 to be turned on, sampling and holding the voltage signal CS on the primary side current detection resistor R17 to obtain a valley voltage signal CSvl of the voltage signal CS on the primary side current detection resistor R17.
1.3, summing a peak voltage signal CSpk and a valley voltage signal CSvl of a voltage signal CS on the primary current detection resistor R17, and multiplying the sum by a coefficient K to obtain a first intermediate signal.
And 1.4, obtaining a control signal CCsg by carrying out RC filtering on the first intermediate signal in the secondary freewheeling stage, and grounding the KCS signal in other time periods.
After the switching tube Q1 in the flyback converter is turned off, the current of the transformer flows to the secondary through the freewheeling diode D1 in the secondary winding unit, the current flowing through the freewheeling diode D1 is decreased from high to low, and the secondary is in a freewheeling period as long as the current is not decreased to 0. The first intermediate signal is applied to the RC structure during the secondary freewheeling stage, i.e. the square wave of the amplitude of the first intermediate signal during the secondary freewheeling period is filtered by the RC, resulting in a control signal CCsg that is proportional to the information of the secondary current of the flyback converter.
The signal ZCD at the series point of the first resistor R9 and the second resistor R11 in the auxiliary winding unit can be subjected to freewheel detection to obtain the freewheel time, which is the freewheel time of the secondary freewheel phase. As shown in fig. 2, wherein the resistors 101 and 102 correspond to the first resistor R9 and the second resistor R11 of the auxiliary winding unit in fig. 1, the freewheel detection unit 104 performs freewheel detection on the signal ZCD to obtain the freewheel time signal Tfwt indicating the secondary freewheel time information.
As shown in fig. 3, a circuit for obtaining a control signal CCsg proportional to information of a secondary current of a flyback converter is provided, which includes a first sample-and-hold unit 202, a second sample-and-hold unit 203, an adder 204, a multiplier 205, a first switch 206, a second switch 207, a third resistor 209, and a second capacitor 210, wherein a control end of the first sample-and-hold unit 202 is connected to a gate driving signal DRVi of a switching tube, a sampling end of the first sample-and-hold unit is connected to a voltage signal CS on a primary side current detection resistor, and an output end of the first sample-and-hold unit is connected to a first input end of the adder 204; the control end of the second sample-and-hold unit 203 is connected to the inverted signal DRVo of the gate driving signal of the switching tube, the sampling end thereof is connected to the voltage signal CS on the primary current detection resistor, and the output end thereof is connected to the second input end of the adder 204; the multiplier 205 has a first input terminal connected to the output terminal of the adder 204, a second input terminal connected to the coefficient K, and an output terminal connected to one end of the first switch 206; the other end of the first switch 206 is connected with one end of a third resistor 209 and one end of a second switch 207, and the control end of the first switch is connected with a freewheeling time signal Tfwt; the other end of the second switch 207 is grounded, and the control end thereof is connected with the inverted signal Tfwto of the freewheel time signal; the other end of the third resistor 210 generates a control signal CCsg proportional to the flyback converter secondary current information and is grounded through the second capacitor 210.
The specific working process is as follows: the gate driving signal DRVi of the switching tube Q1 is used to control the first sampling unit 202, and the signal DRVo of the negation of the gate driving signal DRVi of the switching tube Q1 is used to control the second sampling unit 203, that is, when the switching tube Q1 is turned off, the first sampling unit 202 is controlled to sample and hold the voltage signal CS on the primary side current detection resistor to obtain the peak signal CSpk; after the switching tube Q1 is turned on, the second sampling unit 203 is controlled to sample and hold the voltage signal CS on the primary current detection resistor to obtain a valley signal CSvl; the adder 204 sums the peak value signal CSpk and the valley value signal CSvl of the voltage signal CS on the primary current detection resistor; the multiplier 205 multiplies the sum signal of the adder 204 by a coefficient K to obtain a first intermediate signal; during the secondary freewheeling, the first switch 206 is opened, the second switch 207 is closed, and the first intermediate signal output by the multiplier 205 is filtered through the RC to obtain a control signal CCsg, wherein the control signal CCsg at this time represents the secondary current information; during the secondary freewheel discontinuity the first switch 207 is open and the second switch 206 is closed, connecting the first intermediate signal output by the multiplier 205 to ground.
The value of the control signal CCsg can be adjusted by adjusting the value of the coefficient K, and if the value of the coefficient K is too small, the value of the control signal CCsg is too small, so that the control precision is difficult to ensure; if the coefficient K is too large, the value of the control signal CCsg is too large to be used in an actual situation, so that a person skilled in the art needs to adjust the value of the coefficient K according to needs, and in the embodiment, a preferable value of the coefficient K is 0.1 to 10, but is not limited to a range of 0.1 to 10, as long as the accuracy requirement and the actual use can be met.
Besides the solutions provided in this embodiment, the current transformer may be used to obtain the secondary current information, and it should be noted that the present invention is not limited to obtaining the secondary current information by using the solutions of this embodiment, and all solutions capable of obtaining the secondary current information should be within the scope of the present invention.
After the control signal CCsg is obtained, the switching frequency of the flyback converter can be adjusted by using the control signal CCsg, as shown in fig. 5, a frequency control implementation is provided, but the implementation is not limited to only realizing frequency control by using the structure shown in fig. 5, in this embodiment, the switching frequency control module includes a voltage-controlled voltage source 300, a first capacitor 302, a third switch 303 and a comparator 301, the voltage-controlled voltage source 300 charges the first capacitor 302 under the control of the control signal CCsg, and the larger the control signal CCsg is, the faster the first capacitor 302 is charged; the positive input end of the comparator 301 is connected to the voltage signal on the first capacitor 302, the negative input end thereof is connected to the reference voltage Vref, and the output end thereof generates the gate enable signal DRVen of the switching tube; the third switch 303 is connected to both ends of the first capacitor 302, and a control end thereof is connected to the gate driving signal DRVi of the switching tube.
Fig. 6 is a waveform diagram of some key nodes in the present invention, and the specific working process of analyzing frequency control with reference to fig. 5 and 6 is as follows: when the gate driving signal DRVi of the switching tube Q1 is at a low level, the voltage-controlled voltage source 300 charges the first capacitor 302, when the voltage V302 of the first capacitor 302 is greater than the reference voltage Vref, the gate enable signal DRVen of the switching tube at a high level is generated, the gate driving signal DRVi of the switching tube Q1 is enabled, the gate driving signal DRVi of the switching tube Q1 is at a high level, the switching tube Q1 is turned on, and at the same time, the third switch 303 is controlled to be closed when DRVi is high, so that the first capacitor 302 is discharged, and then the first capacitor 302 starts to be charged again in the next cycle. Therefore, the voltage V302 on the first capacitor 302 is a sawtooth wave, and the frequency thereof is proportional to the control signal CCsg, the larger the control signal CCsg is, the faster the charging speed of the first capacitor 302 is, so the control signal CCsg can control the on-time of the switch Q1, and the on-time of the switch Q1 determines the start time of the next switching cycle, so the present embodiment is actually a controlled switching cycle, and the larger the control signal CCsg is, the smaller the switching cycle is, and the higher the switching frequency is. In the first embodiment, the control signal CCsg includes information of the output current of the flyback converter, and the larger the control signal CCsg is, the faster the start time of the switching transistor Q1 in the next switching period is, so the switching frequency of the flyback converter in the first embodiment is proportional to the output current.
Second embodiment, adjusting the switching frequency according to the output power
In the first embodiment, the control signal CCsg represents the secondary current information, and the frequency control is performed by using the secondary current information. In the second embodiment, on the basis that the voltage of the primary current detection resistor R17 is monitored and the follow current feedback of the auxiliary winding is combined, the feedback Vosg of the secondary voltage is introduced to obtain the control signal CCsg in proportion to the secondary power information of the flyback converter, and a scheme for controlling the switching frequency by using the control signal CCsg in proportion to the secondary power information of the flyback converter is provided. The detailed method comprises the following steps: in step 1.3 of the first embodiment, after summing the peak voltage signal CSpk and the valley voltage signal CSvl of the voltage signal CS on the primary current detection resistor, multiplying the sum by the coefficient K and introducing the feedback signal Vosg of the secondary voltage to obtain a second intermediate signal, that is, the first intermediate signal obtained in the first embodiment is (CSpk + CSvl) × K, the second intermediate signal obtained in the second embodiment is (CSpk + CSvl) × K × Vosg, the control signal CCsg proportional to the information of the secondary current of the flyback converter is obtained by RC filtering the first intermediate signal in the secondary freewheeling stage, the feedback signal Vosg of the secondary voltage is introduced to the second intermediate signal in the first intermediate signal, and since the product of the current and the voltage is power information, the control signal CCsg proportional to the information of the secondary power of the flyback converter is obtained by RC filtering the second intermediate signal in the secondary freewheeling stage.
As shown in fig. 2, the control terminal of the third sample-and-hold unit 103 is connected to the inverted signal DRVo of the gate driving signal of the switching tube, the sampling terminal thereof is connected to the series point signal ZCD of the first resistor and the second resistor, and the output terminal thereof generates the feedback signal Vosg of the secondary voltage. DRVo is a signal obtained by negating the gate driving signal DRVi of the switching tube Q1, and the negating of DRVi controls the third sampling unit 103 to sample and hold the signal ZCD to obtain the feedback Vosg of the secondary voltage, which is Na/Ns Vout, where Na is the number of turns of the auxiliary winding of the flyback converter, Ns is the number of turns of the output secondary winding of the flyback converter, and Vout is the output voltage of the flyback converter.
As shown in fig. 4, the feedback signal Vosg of the secondary voltage is applied to a third input terminal of the multiplier 205, and the multiplier 205 multiplies the output signal of the adder 204 by the coefficient K and the feedback signal Vosg of the secondary voltage to obtain a second intermediate signal; during the secondary freewheeling, the first switch 206 is opened, the second switch 207 is closed, and the second intermediate signal output by the multiplier 205 is filtered through RC to obtain the control signal CCsg, where the control signal CCsg represents the secondary power information. In the second embodiment, the switching transistor Q1 can also be controlled to be turned on by the control signal CCsg, and the switching frequency of the flyback converter is directly proportional to the output power in the second embodiment, thereby implementing a control strategy for adjusting the switching frequency according to the output power.
In summary, the present invention uses the secondary current information or the secondary power information of the flyback converter as the adjustment basis of the switching frequency, so as to implement a control method for accurately adjusting the switching frequency in real time according to the load condition of the flyback converter, and provide a specific implementation circuit with a simple circuit structure, as can be seen from the comparison graphs shown in fig. 8 and 9, compared with the existing control method, the present invention optimizes the conversion efficiency while considering the power density.
Those skilled in the art can make various other specific changes and combinations based on the teachings of the present invention without departing from the spirit of the invention, and these changes and combinations are within the scope of the invention.
Claims (9)
1. A control method of a flyback converter is characterized by comprising the following steps:
step one, extracting a control signal which is in direct proportion to the secondary current information or the secondary power information of the flyback converter;
and step two, adjusting the switching frequency of the flyback converter according to the control signal obtained in the step one, wherein the larger the control signal is, the higher the switching frequency of the flyback converter is.
2. The method of claim 1, wherein the flyback converter comprises a primary winding unit, a secondary winding unit and an auxiliary winding unit, wherein the primary winding unit comprises a primary winding, a switching tube and a primary current detection resistor, one end of the primary winding is connected to a supply voltage, the other end of the primary winding is connected to one end of the switching tube, and the other end of the switching tube is grounded after passing through the primary current detection resistor; the auxiliary winding unit comprises an auxiliary winding, a first resistor and a second resistor, one end of the auxiliary winding is grounded, and the other end of the auxiliary winding is grounded through a series structure of the first resistor and the second resistor;
the specific method for extracting the control signal in proportion to the flyback converter secondary current information in the first step is as follows:
step 1.1, when a switching tube is closed, sampling and holding a voltage signal on a primary side current detection resistor to obtain the peak voltage of the voltage signal on the primary side current detection resistor;
step 1.2, after the switching tube is turned on, sampling and holding a voltage signal on the primary side current detection resistor to obtain a valley voltage of the voltage signal on the primary side current detection resistor;
step 1.3, summing the peak voltage and the valley voltage of the voltage signal on the primary side current detection resistor obtained in the step 1.1 and the step 1.2, and multiplying the sum by a coefficient K to obtain a first intermediate signal;
and step 1.4, in a secondary freewheeling stage, obtaining the control signal which is in direct proportion to the information of the secondary current of the flyback converter by performing RC (resistance-capacitance) filtering on the first intermediate signal.
3. The method of claim 2, wherein the specific method of extracting the control signal proportional to the flyback converter secondary power information in the first step is as follows: in step 1.3, the peak voltage and the valley voltage of the voltage signal on the primary side current detection resistor are summed, multiplied by a coefficient K and then multiplied by a feedback signal of a secondary voltage to obtain a second intermediate signal; step 1.4, in a secondary freewheeling stage, obtaining the control signal which is in direct proportion to the flyback converter secondary power information by performing RC (remote control) filtering on the second intermediate signal;
and the feedback signal of the secondary voltage is obtained by sampling and holding a signal at the series point of the first resistor and the second resistor in the auxiliary winding unit when the switching tube is closed.
4. A method for controlling a flyback converter according to claim 2 or 3, wherein the method for determining the freewheel time of the secondary freewheel phase in step 1.4 is as follows: and the follow current time obtained by performing follow current detection on signals at the series point of the first resistor and the second resistor in the auxiliary winding unit is the follow current time of the secondary follow current stage.
5. The method of claim 1, wherein the specific method of adjusting the switching frequency of the flyback converter in the second step is: charging a first capacitor by using a voltage control voltage source controlled by the control signal during the closing period of a switch tube in the flyback converter, and enabling the switch tube in the flyback converter to be opened and controlling the first capacitor to discharge when the voltage on the first capacitor is greater than a reference voltage; the larger the control signal is, the faster the first capacitor is charged, the smaller the switching period of the switching tube is, and the larger the switching frequency is.
6. The control circuit of the flyback converter is characterized by comprising a control signal generation module and a switching frequency control module, wherein the control signal generation module is used for generating a control signal which is in direct proportion to the secondary current information or the secondary power information of the flyback converter, the switching frequency control module is used for adjusting the switching frequency of the flyback converter according to the control signal, and the larger the control signal is, the higher the switching frequency of the flyback converter is.
7. The control circuit of the flyback converter of claim 6, wherein the flyback converter comprises a primary winding unit, a secondary winding unit and an auxiliary winding unit, wherein the primary winding unit comprises a primary winding, a switching tube and a primary current detection resistor, one end of the primary winding is connected to the supply voltage, the other end of the primary winding is connected to one end of the switching tube, and the other end of the switching tube is grounded after passing through the primary current detection resistor; the auxiliary winding unit comprises an auxiliary winding, a first resistor and a second resistor, one end of the auxiliary winding is grounded, and the other end of the auxiliary winding is grounded through a series structure of the first resistor and the second resistor;
the control signal generating module comprises a follow current detecting unit, a first sampling and holding unit, a second sampling and holding unit, an adder, a multiplier, a first switch, a second switch, a third resistor and a second capacitor,
the input end of the follow current detection unit is connected with the series point of the first resistor and the second resistor, and the output end of the follow current detection unit generates a follow current time signal;
the control end of the first sampling and holding unit is connected with a grid driving signal of a switching tube, the sampling end of the first sampling and holding unit is connected with a voltage signal on the primary side current detection resistor, and the output end of the first sampling and holding unit is connected with the first input end of the adder;
the control end of the second sampling and holding unit is connected with the inverted signal of the grid drive signal of the switching tube, the sampling end of the second sampling and holding unit is connected with the voltage signal on the primary side current detection resistor, and the output end of the second sampling and holding unit is connected with the second input end of the adder;
the first input end of the multiplier is connected with the output end of the adder, the second input end of the multiplier is connected with the coefficient K, and the output end of the multiplier is connected with one end of the first switch;
the other end of the first switch is connected with one end of the third resistor and one end of the second switch, and the control end of the first switch is connected with the follow current time signal;
the other end of the second switch is grounded, and the control end of the second switch is connected with the inverted signal of the follow current time signal;
the other end of the third resistor generates a control signal which is in direct proportion to the information of the secondary current of the flyback converter, and the control signal is grounded after passing through the second capacitor.
8. The control circuit of the flyback converter of claim 7, wherein the control signal generating module further comprises a third sample-and-hold unit, a control end of the third sample-and-hold unit is connected to an inverted signal of the gate driving signal of the switching tube, a sampling end of the third sample-and-hold unit is connected to a series point of the first resistor and the second resistor, and an output end of the third sample-and-hold unit is connected to a third input end of the multiplier, where the control signal generated by the control signal generating module is proportional to the flyback converter secondary power information.
9. The control circuit of the flyback converter of any of claims 6-8, wherein the switching frequency control module comprises a voltage controlled voltage source, a first capacitor, a third switch, and a comparator,
the voltage-controlled voltage source charges the first capacitor under the control of the control signal, and the larger the control signal is, the faster the first capacitor is charged;
the positive input end of the comparator is connected with a voltage signal on the first capacitor, the negative input end of the comparator is connected with reference voltage, and the output end of the comparator generates a grid enable signal of the switching tube;
the third switch is connected with two ends of the first capacitor, and the control end of the third switch is connected with the grid drive signal of the switch tube.
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| CN115441702A (en) * | 2022-11-08 | 2022-12-06 | 成都智融微电子有限公司 | Self-adaptive shielding time generation system applied to flyback power supply circuit |
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