CN112087146B - Control method and circuit of asymmetric half-bridge flyback converter - Google Patents

Control method and circuit of asymmetric half-bridge flyback converter Download PDF

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CN112087146B
CN112087146B CN202010973709.5A CN202010973709A CN112087146B CN 112087146 B CN112087146 B CN 112087146B CN 202010973709 A CN202010973709 A CN 202010973709A CN 112087146 B CN112087146 B CN 112087146B
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CN112087146A (en
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江波
张法旺
王启羽
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Mornsun Guangzhou Science and Technology Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33569—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only having several active switching elements

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Abstract

The invention discloses a control method and a circuit of an asymmetric half-bridge flyback converter, which comprises a current detection module CS and a third winding LA which are electrically connected with a main circuit of the asymmetric half-bridge flyback converter, a peak current sampling moment capture module TS, a sampling hold module SS, an output voltage sampling moment capture module VoT, an output voltage sampling module VoS, a pre-clamping moment calculation module CTC, an output voltage isolation sampling module FB and a PWM generation and Mode switching module, wherein the pre-clamping time is obtained by switching in a CAHBF Mode in an AHBF Mode, the conduction duration of an auxiliary switch tube is controlled by the pre-clamping time in the first period of the CAHBF Mode, the negative current value flowing through a clamping tube is ensured to be at a lower level, the problems that the transition process of switching in the AHBF Mode into the CAHBF Mode is slow, the current tolerance value margin is overlarge during the one-way tube selection is high and the cost is caused by the overlarge are effectively solved, meanwhile, the optimized unidirectional tube selection can improve the efficiency of the converter in the CAHBF Mode and reduce the loss.

Description

Control method and circuit of asymmetric half-bridge flyback converter
Technical Field
The invention relates to the field of asymmetric half-bridge flyback converters, in particular to a control method and a circuit of an asymmetric flyback converter.
Background
Conversion efficiency is an important index of the switching power supply, and the development of the soft switching technology further improves the conversion efficiency of the switching power supply. In practical application environments, the switching power supply converter can work in a full-load state and can also work in a light-load or even no-load state, and therefore high efficiency of the switching power supply in various load states is required to be considered by switching power supply designers. At present, the automatic switching of the working modes of the switching power supply following the load condition proves to be an effective means for ensuring the high efficiency of the switching power supply under various load states.
The asymmetric half-bridge flyback converter has the characteristic of soft switching due to the topology, and becomes a research hotspot of the high-efficiency application occasions of the existing switching power supply. When the main switch of the asymmetric half-bridge flyback converter is fully loaded and heavily loaded to just realize zero voltage switching, the power level parameter design of the converter is considered to be better, the asymmetric half-bridge flyback converter shown in fig. 1 generally has higher conversion efficiency when being fully loaded and heavily loaded, but the negative peak value of the exciting inductive current is increased along with the reduction of the load and exceeds the requirement of the main switch of the converter to realize zero voltage switching, and invalid loss is generated, so that the efficiency is reduced, and the converter has low light load efficiency and large no-load power consumption.
Chinese patent No. 201911352361.1, "switching power supply device", proposes to adopt an asymmetric half-bridge flyback converter and a controller as shown in fig. 1, and to control the converter to operate in an asymmetric half-bridge flyback Mode (AHBF Mode) or a clamped asymmetric half-bridge flyback Mode (CAHBF Mode) according to different load currents by adding a unidirectional clamp module connected in parallel with the primary side of the transformer and adopting the Mode switching curve as shown in fig. 2, so as to ensure optimal efficiency during heavy load or full load, and to realize effective control of the excitation inductance current peak value during light load, thereby greatly improving the converter light load efficiency, reducing the no-load loss, and making the converter system efficiency better in the full load range.
When the working Mode of the clamping asymmetric half-bridge flyback converter (CAHBF converter) is switched from an asymmetric half-bridge flyback Mode (AHBF Mode, the subsequent AHBF Mode refers to an asymmetric half-bridge flyback Mode) to a clamping asymmetric half-bridge flyback Mode (CAHBF Mode, the subsequent CAHBF Mode refers to a clamping asymmetric half-bridge flyback Mode), negative current flows through the one-way clamping module. As shown in FIG. 3, the current shown by the dotted line is the excitation inductor current ILm waveform, where IN1Is the negative peak value of the magnetic inductance current in AHBF Mode Mode, IN2And the current is a clamped excitation inductance current negative peak value in the CAHBF Mode, and the current flows through the unidirectional clamping module and is maintained until the unidirectional clamping module is switched off.
A prototype is built by adopting the system parameters shown in the table 1, and the actually measured clamping negative current value required by the main switching tube Q1 to realize ZVS is shown in the table below.
TABLE 1
Figure BDA0002685004530000021
When the converter is switched into the CAHBF Mode from the AHBF Mode, a transition Mode switching control method is adopted, the auxiliary switch tube Q2 moves the turn-off edge of the auxiliary switch tube in a smaller step length in each switching period after the auxiliary switch tube is switched into the CAHBF Mode, so that the purpose of increasing and decreasing the magnitude of negative current is achieved, and the main switch Q1 is ensured to just realize ZVS. In the process that the auxiliary switching tube Q2 moves the turn-off edge in a certain step length, the clamped negative current value gradually transits from a larger value to a smaller value for realizing ZVS of the main switching tube Q1.
When the clamping switch tube Q3 works, namely works in CAHBF Mode, the negative current value I flowing through the clamping switch tube Q3N2It is not necessary to be very large, and it is sufficient to ensure that the main switching tube Q1 realizes ZVS, as shown in table 1. In consideration of the reliability of the switching converter, the clamping switching tube Q3 is selected to ensure that the maximum withstand current value of the device is larger than the maximum flowing current value. Therefore, if the control method for mode switching is not processed, the clamping negative current value flowing through the clamping switch tube Q3 is almost equal to the current value flowing through the auxiliary switch tube Q2 under the condition that the unidirectional clamping module clamps for a short time. The type selection of the clamp switch Q3 needs to be the same as that of the auxiliary switch Q2, which means that the clamp switch Q3 needs to select a switch with a large current specification, and the parasitic parameters are larger than those of a switch with a small current specification, so the switching loss is also larger, and the converter efficiency in the CAHBF Mode is lower. Therefore, from a cost and efficiency perspective, the selection of the clamp switching transistor Q3 may be optimized by improving the control strategy for mode switching.
Disclosure of Invention
In view of this, the present invention provides a control method and a circuit for an asymmetric half-bridge flyback converter, which can effectively increase the mode switching speed, optimize the type selection of the clamp switching transistor Q3, reduce the cost of the converter, and improve the efficiency in the clamp state.
The first purpose of the invention is to provide a control method of an asymmetric half-bridge flyback converter, which comprises the following steps:
when the converter is switched from the AHBF Mode to the CAHBF Mode, the current output voltage and excitation peak current are sampled, meanwhile, the turn-off time point of the auxiliary switching tube Q2 is calculated according to the original secondary side turn ratio N of the transformer, the excitation inductance Lm and the negative current value during expected clamping, and Mode switching control is carried out in the next period according to the calculated turn-off time point of the auxiliary switching tube Q2.
The second improved idea of the control method provided by the invention is as follows: when the converter is switched from the AHBF Mode to the CAHBF Mode;
in the first period, the turn-off time of the auxiliary switch tube Q2 is determined by the pre-clamping time value calculated by the pre-clamping time calculation module, that is, the auxiliary switch tube Q2 is turned off earlier than the clamping switch tube Q3, and the amount of time ahead is determined by the pre-clamping time value;
in the second period, the turn-off time of the auxiliary switching tube Q2 is determined by the control logic in the clamp asymmetric half-bridge flyback mode, that is, the auxiliary switching tube Q2 adjusts the turn-off edge according to the ZVS condition of the main switching tube Q1, and each switching period moves forward or backward by a specific step length until the ZVS is realized by the main switching tube Q1.
The third improved idea of the control method provided by the invention is as follows: and adjusting the working Mode of the converter according to the load condition, and when the load is reduced to the point that the converter needs to be switched from the AHBF Mode to the CAHBF Mode, presetting a clamp value, namely controlling the auxiliary switch tube Q2 to be switched off at a preset time value in advance in the first period after the AHBF Mode is switched into the CAHBF Mode, enabling the clamp switch tube Q3, and then starting the second period to formally enter the CAHBF Mode and generate corresponding control action.
A second object of the present invention is to provide a control circuit of an asymmetric half-bridge flyback converter, which is characterized in that: the device comprises a current detection module CS and a third winding LA which are electrically connected with a main circuit of the asymmetric half-bridge flyback converter, and further comprises a peak current sampling moment capture module TS, a sample hold module SS, an output voltage sampling moment capture module VoT, an output voltage sampling module VoS, a pre-clamping moment calculation module CTC, an output voltage isolation sampling module FB and a PWM generation and mode switching module which are connected with the current detection module CS; the current detection module CS is connected with the sampling holding module SS, the sampling holding module SS is respectively connected with the peak current sampling time capture module TS and the pre-clamping time calculation module CTC, the peak current sampling time capture module TS is connected with the PWM generation and mode switching module, the PWM generation and mode switching module is respectively connected with the output voltage sampling time capture module VoT, the output voltage isolation sampling module FB and the pre-clamping time calculation module CTC, the output voltage sampling time capture module VoT is connected with the output voltage sampling module VoS, and the output voltage sampling module VoS is respectively connected with the pre-clamping time calculation module CTC and the third winding LA.
As a first specific implementation manner of the control circuit of the asymmetric half-bridge flyback converter, the positive electrode of the current detection module CS is connected to the source electrode of the auxiliary switching tube Q2, the source electrode of the clamping switching tube Q3, and the different name end of the primary winding of the transformer; the negative electrode of the current detection module CS is connected with the ground and the negative input end; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
As a second specific implementation manner of the control circuit of the asymmetric half-bridge flyback converter, an anode of the current detection module Cs is connected to the input terminal + Vin; the negative electrode of the current detection module CS is connected with the drain electrode of the main switching tube Q1; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
As a third specific implementation manner of the control circuit of the asymmetric half-bridge flyback converter, the positive electrode of the current detection module Cs is connected to the source of the clamping tube Q3 and the synonym terminal of the primary winding of the transformer; the negative electrode of the current detection module Cs is connected with the source electrode of the auxiliary switching tube Q2 and the input end-Vin; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
The invention conception of the application is as follows: namely, when the AHBF Mode is switched into the CAHBF Mode, a clamp value is preset to ensure that a large current value does not flow through the clamp switching transistor Q3, then the converter enters the CAHBF Mode and generates a corresponding control action, the clamp switching transistor Q3 can select a MOS transistor with a small current according to the control idea, the parasitic parameters of the MOS transistor are relatively small, the drive loss and the switching loss are both reduced, and the drive loss and the switching loss under clamping can be reduced.
Interpretation of terms:
asymmetric half-bridge flyback mode: in a switching cycle period, the main switching tube and the auxiliary switching tube are complementarily conducted, and the controller controls the one-way clamping module to be always in an off state, which is called AHBF Mode for short.
Clamped asymmetric half-bridge flyback mode: in a switching cycle period, a main switching tube, an auxiliary switching tube and a clamping switching tube are switched on or off alternately, and specifically, each cycle period comprises five stages: an excitation stage, an auxiliary switch zero voltage switching-on stage, a demagnetization stage, a current clamping stage and a main switch zero voltage switching-on stage; in the excitation stage and the auxiliary switch zero voltage switching-on stage, the clamping switch tube is switched off; in the demagnetization stage, the auxiliary switch tube is switched on, the clamping switch tube can be switched on or off, and no current flows through the clamping switch tube; at the end of the period, the exciting inductance current reaches a set value, the auxiliary switch tube is turned off, the clamping switch tube is in a conducting state, and the clamping current flows through the clamping switch tube; in the current clamping stage, the clamping switch tube is switched on, the clamping current flows through the clamping switch tube, the clamping switch keeps the clamping current basically unchanged, and the clamping switch tube is switched off at the end moment of the current clamping stage; at the stage of zero voltage switching-on of the main switch, the clamping switch tube is turned off, the clamping current is released, the voltage of the main switch tube is reduced to zero or close to zero, and at the moment, the main switch tube is controlled to be switched on, so that zero voltage switching-on of the main switch tube is realized, and the CAHBF Mode is abbreviated as CAHBF Mode.
Transition mode switching: when the Mode of the converter is switched from the AHBF Mode to the CAHBF Mode, the turn-off time of the auxiliary switch Q2 is determined by the control logic in the CAHBF Mode, that is, the auxiliary switch Q2 adjusts the turn-off edge according to the ZVS condition of the main switch Q1, and each switching cycle is shifted forward or backward by a specific step length until the ZVS is realized by the main switch Q1.
Pre-clamping: when the Mode of the converter is switched into the CAHBF Mode from the AHBF Mode in the first switching period, the turn-off time of the auxiliary switch tube Q2 is determined by the pre-clamping time value calculated by the pre-clamping time calculation module, and the turn-off time of the auxiliary switch tube Q2 is represented as being earlier than the turn-off time of the clamping switch tube Q3, and the earlier time quantity is determined by the pre-clamping time value. From the second switching cycle to enter CAHBF Mode, the turn-off timing of the auxiliary switch Q2 is determined by the control logic in CAHBF Mode.
The working principle of the invention is analyzed by combining with the specific embodiment, which is not described herein, and the beneficial effects of the invention are as follows:
1. a time value is clamped in advance, so that the current flowing through the unidirectional clamping module is always in a smaller level, and a diode D3 and a clamping switch tube Q3 on the unidirectional clamping module can select a switch tube with a smaller current specification than an auxiliary switch tube, so that the system cost is reduced;
2. because the parasitic parameters of the device with the small current specification are small, the loss of the CAHBF Mode can be reduced by using the device with the small current specification through the unidirectional clamping module, and the efficiency of the converter is improved;
3. the method has the advantages that a time value is clamped in advance, so that transition from an AHBF Mode to a CAHBF Mode can be accelerated, the Mode switching speed is increased, and the dynamic response performance of the converter can be remarkably improved.
Drawings
Fig. 1 is a circuit block diagram of an asymmetric half-bridge flyback converter and a controller in the prior art;
fig. 2 is a schematic diagram of a prior art asymmetric half-bridge flyback converter and controller mode switching;
fig. 3 is a waveform diagram of an asymmetric half-bridge flyback converter and a controller using a transition mode switching method according to the prior art;
fig. 4 is a circuit block diagram of a control circuit of the asymmetric half-bridge flyback converter of the present invention;
fig. 5 is a schematic diagram of the operating waveforms and mode processing of the asymmetric half-bridge flyback converter and the controller using the pre-clamping control scheme according to the present invention;
fig. 6 is a waveform diagram illustrating an exemplary operation of the control circuit of the asymmetric half-bridge flyback converter in the CAHBF Mode.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
DETAILED DESCRIPTION OF EMBODIMENT (S) OF INVENTION
As shown in fig. 4, a control circuit of an asymmetric half-bridge flyback converter includes a current detection module CS and a third winding LA electrically connected to a main circuit of the asymmetric half-bridge flyback converter, and further includes a peak current generation time capture module TS, a sample-and-hold module SS, an output voltage sampling time capture module VoT, an output voltage sampling module VoS, a pre-clamping time calculation module CTC, and a PWM generation and mode switching module connected to the current detection module CS.
The current detection module CS is used for detecting the current peak value of the transformer excitation inductor during the conduction period of the main switching tube Q1, and the positive electrode of the current detection module CS is connected with the source electrode of the auxiliary switching tube Q2, the source electrode of the clamping switching tube Q3 and the synonym terminal of the primary winding of the transformer; the negative electrode of the current detection module CS is connected with the ground and the negative input end; output terminal I of current detection module CSoutAnd input terminal I of sample-and-hold module SSinAnd (4) connecting.
The current detection module CS further includes the following two connection modes in the main circuit of the asymmetric half-bridge flyback converter:
(1) the positive electrode of the current detection module Cs is connected with the input end + Vin; the negative electrode of the current detection module CS is connected with the drain electrode of the main switching tube Q1; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
(2) The positive electrode of the current detection module Cs is connected with the source electrode of the clamping tube Q3 and the synonym end of the primary winding of the transformer; the negative electrode of the current detection module Cs is connected with the source electrode of the auxiliary switching tube Q2 and the input end-Vin; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
And the third winding LA is used for detecting an output voltage value during the conduction period of the auxiliary switching tube Q2, the third winding LA shares a magnetic core with the transformer, the homonymous end of the third winding LA is the same as the homonymous end of the secondary winding, the homonymous end of the third winding LA is connected with the ground, and the synonym end VA of the third winding LA is connected with the input end VoS2 of the output voltage sampling module VoS.
A peak current generation time capturing module TS for capturing the time of the generation of the negative peak value and the positive peak value of the exciting inductive current and outputting the corresponding trigger level and T +, and sending the level to a sampling and holding module SS for extracting the information of the positive peak value and the negative peak value from the exciting inductive current, and simultaneously, the module collects an input voltage signal and sends the input voltage signal to a PWM generation and mode switching module as one of criteria for mode switching; an input end V1 of the peak current generation time capture module TS is connected to an output end GQ1 of the PWM generation and mode switching module, an output end T + of the peak current generation time capture module TS is connected to an output end T + of the sample hold module SS, and a GND end of the peak current generation time capture module TS is connected to ground.
The peak current generation time capture module TS can be implemented in different ways, including but not limited to the following two ways:
(1) generating a sampling signal of a positive peak value of exciting inductance current by judging the moment when the voltage of the drain and the source of a main switching tube rises from zero to a certain voltage value;
(2) and generating an excitation inductance positive peak current sampling signal by judging the falling edge moment of the main switch grid driving signal.
The sampling and holding module SS is used for sampling the exciting inductance current value during the conduction period of the main switching tube Q1, extracting the positive peak value of the exciting inductance current according to the peak current generation time, specifically tracking the exciting inductance current when a positive peak value sampling trigger signal T + is at a high level, and holding and outputting the exciting inductance current corresponding to the falling edge time of the trigger signal, namely the positive peak value of the exciting inductance current when the positive peak value sampling trigger signal T + is at a low level; input terminal I of sample-and-hold module SSinAnd the output end I of the current detection module CSoutInput terminal T of connection, sampling and holding module SS+Output end T of catching module TS at time of generating peak current+Output terminal I of connection, sampling and holding module SS+Input end I of pre-clamping time calculation module CTCPAnd (4) connecting.
The output voltage sampling time capturing module VoT is configured to generate a trigger level signal required for triggering sampling of the output voltage by the third winding LA, and specifically, when the trigger level signal output by the output voltage sampling time capturing module VoT is at a high level, the module tracks the voltage of the third winding LA, and the voltage of the third winding LA is in a turn ratio relationship with the output voltage Vo at the time when the trigger signal is at the high level. Preferably, when the output voltage detection module circuit is designed as an integrated circuit, the resistance voltage division can be carried out on the third winding LA, so that the requirement on the withstand voltage process of the integrated circuit is reduced; the input end VoT1 of the output voltage sampling time capturing module VoT is connected to the output end GS2 of the PWM generating and mode switching module, the output end VoT2 of the output voltage sampling time capturing module VoT is connected to the input end VoS1 of the output voltage sampling module VoS, and the GND end of the output voltage sampling time capturing module VoT is connected to ground.
The output voltage sampling module VoS is used for sampling the output voltage and calculating the pre-clamping time value, the input end VoS1 of the output voltage sampling module VoS is connected with the output end VoT2 of the output voltage sampling time capturing module VoT, the input end VoS2 of the output voltage sampling module VoS is connected with the synonym end VA of the third winding LA, and the output end VoS3 of the output voltage sampling module VoS and the input end V of the pre-clamping time calculating module CTC are connectedoutAnd the GND end of the output voltage sampling module VoS is connected with the ground.
And the pre-clamping time calculation module CTC is used for calculating the time length of the auxiliary switching tube Q2 before the clamping switching tube Q3 is turned off according to the excitation inductance current peak value Ip, the output voltage Vo, the excitation inductance value Lm and the primary and secondary side turn ratio N when the AHBF Mode is switched into the CAHBF Mode, namely the pre-clamping time length, so that the PWM generation and Mode switching module is provided with the turn-off control information for controlling the auxiliary switching tube Q2. Input end V of pre-clamping time calculation module CTCoutConnected with the output end VoS3 of the output voltage sampling module VoS, and the input end I of the pre-clamping time calculation module CTCPAnd the CT is a signal input end and is connected with an output end I + of the sampling and holding module SS.
The implementation of the pre-clamp time calculation module CTC can also be implemented in different ways, including but not limited to the following two ways:
(1) for an asymmetric half-bridge flyback converter adopting peak current control, a positive peak value of exciting inductance current is not sampled and kept, and an FB signal is used as a signal corresponding to the positive peak value current of the exciting inductance current;
(2) the calculation of the pre-clamping time is not carried out in real time, a sequence table formed by calculation results is stored in a data storage container, the pre-clamping time results are imported in a query mode when the converter works, the calculation results can be one or more, the calculation formula is not limited to the pre-clamping time calculation formula, and the calculation formula can be an estimated pre-clamping time sequence.
The PWM generation and Mode switching module mainly comprises two parts, namely, an output voltage isolation sampling module FB is used for carrying out closed-loop voltage stabilization control on output voltage, and the turn-off time of Q2 when the AHBF Mode is switched into the CHABF _ Mode is controlled according to a pre-clamping time CT value calculated by a pre-clamping pre-time calculation module CTC, so that control signals of a first period auxiliary switch tube Q2 and a clamping switch tube Q3 when the AHBF Mode is switched into the CAHBF Mode and control signals of a subsequent period after the AHBF Mode is formally switched into the CAHBF Mode are generated. The input end FB of the PWM generation and Mode switching module is connected with a feedback signal FB of the output voltage isolation sampling module, the output end of the PWM generation and Mode switching module outputs GQ1, GQ2 and GQ3 signals for controlling a main switching tube, an auxiliary switching tube and a clamping switching tube, meanwhile, the GQ1 signal is also connected with the input end V1 of the peak current generation time capturing module TS, the GQ2 signal is also connected with the input end VoT1 of the output voltage sampling time capturing module VoT, in the PWM generation and Mode switching module, the FB signal is mainly used for controlling the stability of output voltage, the CT signal is mainly used for controlling the turn-off of the auxiliary switching tube Q2, and the pre-clamping time length when the AHBF Mode is switched into the CAHBF Mode is determined so as to control the turn-off time length of the auxiliary switching tube in advance of the clamping switching tube.
The output voltage isolation sampling module FB is used for isolating and sampling the secondary output voltage and the load condition and is used for closed-loop feedback of the converter.
The working principle of the invention is as follows: the output voltage sampling moment capture module VoT and the output voltage sampling module VoS detect the output voltage value through the third winding LA; the pre-clamping time calculation module CTC calculates an initial value of clamping when the AHBF Mode is switched to the CAHBF Mode according to the current positive peak value of the exciting inductor, the primary and secondary turn ratio of the transformer, the primary exciting inductor and the output voltage value, the pre-clamping time is obtained by calculating when the AHBF Mode is switched into the CAHBF Mode, the conducting duration of the auxiliary switch tube Q2 is controlled by the pre-clamping time in the first period of the CAHBF Mode, and the current value flowing through the clamping switch tube Q3 is ensured to be at a lower level.
The working principle of each module in the embodiment of the present invention is further described below with reference to fig. 4, 5, and 6, specifically as follows:
the moment of generating the negative peak value of the exciting inductor current of the peak current generation moment capture module TS can be regarded as the moment when the drain-source voltage becomes zero when the main switching tube Q1 is switched on, the module samples the input voltage and the drain voltage of the main switching tube Q1, judges whether the drain-source voltage of the main switching tube Q1 is zero or not through comparison, and then outputs a negative peak value sampling trigger signal; as shown in fig. 6, the timing of the positive peak of the magnetizing inductor current is considered to be the falling edge timing of the gate drive signal of the main switching transistor Q1, so this module samples the gate drive signal GQ1 of Q1, and triggers the positive peak sampling signal T + by its falling edge.
Each cycle period in fig. 6 comprises five phases: the excitation stage, the auxiliary switch zero voltage switching-on stage, the demagnetization stage, the current clamping stage and the main switch zero voltage switching-on stage. The working principle of each cycle period can be referred to a control method and a circuit of an asymmetric half-bridge flyback circuit in a Chinese patent with the application number of 201910513578.X, which is not described herein again. By taking the working principle of the demagnetization stage into consideration that the turn ratio of the transformer is N (the ratio of the number of turns of the primary side of the transformer to the number of turns of the secondary side of the transformer), the primary side excitation inductor Lm and the output voltage Vo of the converter, the slope of the demagnetization current should satisfy the following relation:
Figure BDA0002685004530000081
considering the demagnetization starting point as the peak value Ip of the exciting inductor current, and determining the expected clamping current I according to the negative current required for realizing zero-voltage switching-on ZVS of the main switching tube Q1N2If the auxiliary switching tube is turned on, namely the pre-clamping time T _ CT, the on-state duration of the auxiliary switching tube meets the following relational expression:
Figure BDA0002685004530000091
when the converter is switched into the CAHBF Mode from the AHBF Mode, the pre-clamping time T _ CT calculated according to the formula is assigned to the conduction duration of the auxiliary switch tube Q2 in the first period of the CAHBF Mode, the excitation inductance current peak value Ip switched into the first period of the CAHBF Mode cannot be suddenly changed due to the delay of a closed loop feedback loop, and therefore the clamping current value obtained by controlling the conduction duration of the auxiliary switch tube Q2 through the T _ CT theoretically meets the negative clamping current value I expected in the calculation formulaN2. As shown in fig. 5, from the second switching cycle in the CAHBF Mode, the control logic of Q1, Q2, Q3 is controlled by the control logic of the CAHBF Mode.
Since the length of time that the auxiliary switch Q2 of the first period after the AHBF Mode is switched into the CAHBF Mode is earlier than the clamp switch Q3 (i.e., the pre-clamp time value), the system requirements can be met only within the current specification of the clamp switch Q3, and the control of the auxiliary switch Q2 after the AHBF Mode is formally switched into the CAHBF Mode is controlled by the CAHBF Mode control logic. Preferably, a fixed pre-clamping value is set (for example, the switching frequency of the converter is 300KHz, i.e., the period is 3.33us, and C _ TC is set to 1us) to save the operation resources of the control system.
Through the analysis, the asymmetric half-bridge flyback converter can calculate a reasonable pre-clamping time value by detecting the current peak value and the output voltage value of the primary side exciting inductor, so that the control of the auxiliary switching tube Q2 when the AHBF Mode is switched into the CAHBF Mode is completed, the negative current value flowing through the clamping switching tube Q3 when the control Mode is switched is achieved, devices with small current specifications can be selected by the clamping switching tube Q3 and the diode D3 in the unidirectional clamping module, and the beneficial effects of reducing the cost, reducing the loss in a clamping state, improving the Mode switching speed and improving the dynamic performance of the converter are achieved.
The 240W asymmetric half-bridge flyback converter real model machine adopting the pre-clamping value calculation and mode switching scheme of the invention is designed and manufactured according to the input and output specifications listed in table 2.
TABLE 2
Input voltage range 170VAC-264VAC (bus voltage range is about 240VDC-370VDC)
Output specification Vo＝12V、Io＝20A、Po＝240W
Switching frequency range 30 kHz-300 kHz (full load 300kHz)
Table 3 shows the comparison between the conduction time of the auxiliary switch Q2 calculated in the CAHBF Mode of the 240W model of the asymmetric half-bridge flyback converter and the conduction time of the auxiliary switch Q2 of the main switch Q1, which is actually measured to realize ZVS. The comparison condition of the calculated value of the clamping current when the pre-clamping of the scheme is carried out under different voltages, different loads and different working frequencies and the clamping current when the pre-clamping is tested by an actual prototype is shown, the calculation error of the clamping current value obtained by the pre-clamping time scheme of the scheme is within +/-10 percent, and the practical application requirement is met.
TABLE 3
Figure BDA0002685004530000101
It should be noted that the current detection circuit and the mode switching method of the asymmetric half-bridge flyback converter according to the embodiments of the present invention are still within the scope of protection of the present invention by changing the resonant cavity position of the asymmetric half-bridge flyback converter, the connection manner of the unidirectional clamping module and the transformer, the position of the current detection module, the peak current generation time capture module, and the implementation method of the pre-clamping time calculation module.
The resonant cavity position of the asymmetric half-bridge flyback converter, the connection mode of the one-way clamping module and the transformer can be combined in various ways, and a large number of embodiments are provided in the Chinese patent with the application number of 201911352361.1 and the Chinese patent application with the application number of 201910513578.X, which belong to the scope of the asymmetric half-bridge flyback converter.
The above is only a preferred embodiment of the present invention, and it should be noted that the above preferred embodiment should not be considered as limiting the present invention, and it will be apparent to those skilled in the art that several modifications and decorations can be made without departing from the spirit and scope of the present invention, and these modifications and decorations should also be considered as the protection scope of the present invention, which is not described herein by way of example, and the protection scope of the present invention should be subject to the scope defined by the claims.

Claims (6)

1. When the converter is switched from an AHBF Mode to a CAHBF Mode, an auxiliary switch tube Q2 is switched off in advance of a clamping switch tube Q3 for a time quantum to realize pre-clamping, the time quantum is sampling current output voltage and excitation peak current, meanwhile, according to the original secondary side turn ratio N of the transformer, an excitation inductor Lm and a negative current value during expected clamping, the time point of switching off an auxiliary switch tube Q2 is calculated, and Mode switching control is carried out in the next period according to the calculated switching off time point of the auxiliary switch tube Q2, wherein the AHBF Mode represents an asymmetric half-bridge flyback Mode, and the CAHBF Mode represents a clamping asymmetric half-bridge flyback Mode.
2. When the converter is switched from an AHBF Mode to a CAHBF Mode, the AHBF Mode represents an asymmetric half-bridge flyback Mode, and the CAHBF Mode represents a clamping asymmetric half-bridge flyback Mode;
in the first period, the turn-off time of the auxiliary switch tube Q2 is determined by a pre-clamping time value calculated by a pre-clamping time calculation module, namely the auxiliary switch tube Q2 is turned off in advance of the clamping switch tube Q3, the amount of time in advance is determined by the pre-clamping time value, and the pre-clamping time value is calculated according to the excitation inductance current peak value Ip, the output voltage Vo, the excitation inductance Lm and the original secondary side turn ratio N;
in the second period, the turn-off time of the auxiliary switching tube Q2 is determined by the control logic in the clamp asymmetric half-bridge flyback mode, that is, the auxiliary switching tube Q2 adjusts the turn-off edge according to the ZVS condition of the main switching tube Q1, and each switching period moves forward or backward by a specific step length until the ZVS is realized by the main switching tube Q1.
3. A control circuit of an asymmetric half-bridge flyback converter is characterized in that: the circuit comprises a current detection module CS and a third winding LA which are electrically connected with a main circuit of an asymmetric half-bridge flyback converter, and further comprises a peak current sampling time capture module TS, a sample hold module SS, an output voltage sampling time capture module VoT, an output voltage sampling module VoS, a pre-clamping time calculation module CTC, an output voltage isolation sampling module FB and a PWM generation and mode switching module which are connected with the current detection module CS, wherein the current detection module CS is connected with the sample hold module SS, the sample hold module SS is respectively connected with the peak current sampling time capture module TS and the pre-clamping time calculation module CTC, the peak current sampling time capture module TS is connected with the PWM generation and mode switching module, and the PWM generation and mode switching module is respectively connected with the output voltage sampling time capture module VoT, the output voltage isolation sampling module FB, The pre-clamping time calculation module CTC is connected, the output voltage sampling moment capture module VoT is connected with the output voltage sampling module VoS, and the output voltage sampling module VoS is respectively connected with the pre-clamping time calculation module CTC and the third winding LA.
4. The control circuit of an asymmetric half-bridge flyback converter according to claim 3, wherein: the positive electrode of the current detection module CS is connected with the source electrode of the auxiliary switching tube Q2, the source electrode of the clamping switching tube Q3 and the synonym end of the primary winding of the transformer; the negative electrode of the current detection module CS is connected with the ground and the negative input end; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
5. The control circuit of an asymmetric half-bridge flyback converter according to claim 3, wherein: the positive electrode of the current detection module CS is connected with the input end + Vin; the negative electrode of the current detection module CS is connected with the drain electrode of the main switching tube Q1; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
6. The control circuit of an asymmetric half-bridge flyback converter according to claim 3, wherein: the positive electrode of the current detection module CS is connected with the source electrode of the clamping tube Q3 and the synonym end of the primary winding of the transformer; the negative electrode of the current detection module CS is connected with the source electrode of the auxiliary switching tube Q2 and the input end-Vin; the output terminal Iout of the current detection module CS is connected to the input terminal Iin of the sample-and-hold module SS.
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