CN113472210B - Hybrid control circuit for primary side feedback flyback converter - Google Patents

Hybrid control circuit for primary side feedback flyback converter Download PDF

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CN113472210B
CN113472210B CN202110757760.7A CN202110757760A CN113472210B CN 113472210 B CN113472210 B CN 113472210B CN 202110757760 A CN202110757760 A CN 202110757760A CN 113472210 B CN113472210 B CN 113472210B
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cycle
primary side
knee point
current
voltage
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CN113472210A (en
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周泽坤
艾雪
李世磊
王卓
张波
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University of Electronic Science and Technology of China
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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
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/08—Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

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

Abstract

The invention belongs to the technical field of power electronics, and particularly relates to a hybrid control circuit for a primary side feedback flyback converter. The invention utilizes the knee point detection and sampling hold circuit, the error amplifier, the BCM/DCM mixed control circuit formed by the cycle-by-cycle current limit generation circuit, the variable current generation circuit and the cycle clock generation circuit, can realize high-efficiency intelligent constant output voltage control in the primary side feedback flyback converter, and obviously improves the efficiency of the system under light load while expanding the load range by the working mode of the mixed variable frequency BCM and DCM.

Description

Hybrid control circuit for primary side feedback flyback converter
Technical Field
The invention belongs to the technical field of power electronics, and particularly relates to a hybrid control circuit for a primary side feedback flyback converter.
Background
Under the gradual promotion of power electronic technology, the application of the internet of things, industrial electronics, automotive electronics, portable equipment and the like which are full of opportunities and challenges is rapidly developed, and the research on power management technology matched with the internet of things, industrial electronics, automotive electronics and portable equipment is generally concerned in the field of integrated circuits. Meanwhile, with the diversification of functions and miniaturization of volumes of electronic products, the performance requirements of power management chips are increasing. In the power management chip, the switching power supply controls the periodic on and off of a power switching tube in the system, so that the transmission and conversion processes of energy are controlled to adjust the output voltage, and the power management chip has the remarkable application advantages of small volume, low power consumption, high efficiency, large loading capacity, wide load range and the like, and is favored by the market. The flyback converter has the advantages of simple structure, good electrical isolation between input and output voltages, no need of an output filter inductor and the like, can be well applied to wide input range, high voltage and low power and application of multiple groups of outputs, and becomes a research hotspot. Compared with the traditional secondary feedback flyback converter, the primary feedback flyback converter omits a network for outputting voltage feedback by an optical coupler device, TL431 and the like, avoids the problem caused by low environmental interference resistance of the optical coupler device, and reduces feedback network elements, so that the converter is more miniaturized.
To date, various control modes have been derived from the primary-side feedback flyback converter, and each parameter of the system is adjusted by a feedback means, so that the system can output stable and accurate current or voltage, and common operating modes of the system include a Continuous Conduction Mode (CCM) and a Discontinuous Conduction Mode (DCM). In CCM the primary inductor current of the transformer is not zero at the beginning of each cycle and the secondary inductor current is not zero at the end of each cycle, while in DCM there is a period of time when the current of the transformer is zero, which is completely powered by the output capacitor.
The primary side feedback flyback converter working in the CCM mode has the advantages that the primary side current and the secondary side current are not zero at the beginning and the end of each period, direct current components exist, the primary side peak current and the secondary side peak current are lower under the same load condition, stress bearing of a primary side power tube and a rectifier diode can be reduced, output ripples are relatively small, and the primary side feedback flyback converter can work under the no-load condition. However, in order to ensure that the flyback converter system can operate in the CCM mode, a larger inductor and transformer core area are required, which means that a larger transformer is required, which is not favorable for system miniaturization. Secondly, because the secondary side current is not zero in each period, for the primary side feedback type flyback converter, knee point voltage with special properties does not exist, the sampling of output voltage information is difficult, and finally, because the secondary side current is not zero, the problem of reverse recovery of a freewheeling diode exists at the beginning of each period. For the converter working in the DCM mode, firstly, the knee point voltage has resonance characteristics, so that the output sampling design is simple. And secondly, because the secondary side current of each period can reach zero, under different applications, the equivalent resistance of different secondary side coils can not cause errors on output adjustment, the adaptability is better, and meanwhile, the problem of reverse recovery of a freewheeling diode is avoided. Under the condition of consistent load, the size of the transformer required by the DCM mode is far smaller than that of the CCM mode. The same DCM mode has its drawbacks, that is, the peak current of the primary side and the peak current of the secondary side are both relatively high, which requires the power switch device to have higher breakdown voltage and bear larger conducting instantaneous current, and the loss caused by the same is larger, resulting in lower efficiency of heavy load. In addition, the output ripple is larger in the DCM state, a larger load capacitor is required for reducing the ripple, and finally, the system cannot be applied to the idle load application in the DCM state and must have a dummy load.
Based on the problem of CCM sampling difficulty and DCM inefficiency, researchers have proposed a Boundary Conduction Mode (BCM), which is a critical Conduction state between CCM and DCM, where at the beginning of each cycle, the primary inductor current gradually increases from zero with a fixed slope, and the secondary inductor current will just decrease to a zero current point at the end of each cycle, and then start the next cycle. Because the secondary side current can be reduced to zero in each period, knee point voltage is used for ensuring the realization of primary side feedback sampling, the influence of reverse recovery current of a fly-wheel diode is avoided, better load adjustment can be realized under the condition that no external load compensation component exists, subharmonic oscillation is ensured not to occur, and the problem of slope compensation is avoided. However, compared with the DCM mode, the BCM requires a larger inductor to limit the current reduction speed, the converter has a larger volume, and the BCM is a variable frequency control, and the switching frequency is continuously increased with the reduction of the load, so that the system loss is continuously increased under the light load condition, and the light load efficiency is not well represented.
Disclosure of Invention
Aiming at the problems of large size and low output efficiency of the converter in the single working mode, the invention provides a hybrid control circuit for a primary side feedback flyback converter.
In order to achieve the purpose, the technical scheme of the invention is as follows:
a hybrid control circuit for a primary side feedback flyback converter is characterized by comprising a knee point detection and sample hold circuit, an error amplifier, a cycle-by-cycle current limit generation circuit, a variable current generation circuit, a cycle clock generation circuit and an AND gate;
the knee point detection and sample hold circuit is used for collecting knee point voltage VS, and the knee point voltage VS is processed by the error amplifier to form a regulation voltage VC carrying load information;
the cycle-by-cycle current limit generating circuit is used for obtaining a cycle-by-cycle peak current limit voltage VPEAK which is positively correlated with VC according to the regulated voltage VC, controlling the maximum value which can be reached by the primary side inductive current by the cycle-by-cycle current limit generating circuit, and closing the power tube after the primary side current reaches a set peak value for controlling the conduction time of the primary side power tube;
the variable current generating circuit is used for generating a current IEA positively correlated with VC according to the regulated voltage VC, and then generating a variable delay signal Vary-T through the periodic clock generating circuit;
the Vary-T is connected with one input end of the AND gate, the knee point detection signal BOUN output by the knee point detection and sampling holding circuit is connected with the other input end of the AND gate, the AND gate outputs a primary side power tube switching signal, namely the variable delay signal Vary-T competes with the knee point detection signal BOUN, the starting time of the next period of the power tube is determined by the latter signal of the variable delay signal Vary-T and the knee point detection signal BOUN, and the switching period of the power tube is determined by the latter signal of the two signals.
The whole technical scheme of the invention is that the knee point detection and sampling hold circuit is used for sampling the output voltage, and then the output voltage VC of the error amplifier in the system is used for dividing the load into three ranges according to the extremely light load, the light load and the heavy load. The VC voltage generates a peak current limit through a cycle-by-cycle current limit circuit, and the peak current limit is used for turning off a primary side power tube and controlling the primary side conduction time; and meanwhile, the VC voltage generates variable delay through a variable delay timing circuit, the starting time of the power tube is determined by the competition of the knee point detection signal and the variable delay signal, the knee point detection signal starts the next period when the load is light, and the next period is started by depending on the variable delay signal when the load is heavy.
The invention has the beneficial effects that: the primary side feedback flyback converter can enable the system to work in a BCM mode during heavy load and forcibly work in a DCM mode during light load, forcibly reduce the frequency and return the frequency of the system, improve the light load efficiency and realize high-efficiency output in a wider load range.
Drawings
Fig. 1 is a schematic diagram of a primary feedback flyback converter system using a hybrid control circuit.
FIG. 2 the present invention proposes a hybrid control schematic.
FIG. 3 is a graph of peak current limit and frequency versus load for a system according to the present invention.
Fig. 4 is a timing diagram of BCM-DCM-BCM switching according to the present invention.
Detailed Description
The technical scheme of the invention is described in detail below with reference to the accompanying drawings:
the schematic diagram of the primary side feedback flyback converter system of the BCM and DCM hybrid control circuit provided by the invention is shown in FIG. 1. As shown in figure 2, the system utilizes knee point voltage VS acquired by knee point detection and sample hold circuit to form regulation voltage VC carrying load information after being processed by Error Amplifier (EA), and then obtains cycle-by-cycle peak current limit voltage V positively correlated with VC through cycle-by-cycle current limit generating circuitPEAK. The cycle-by-cycle current limit can control the maximum value of the primary side inductive current, the power tube is closed after the primary side current reaches a set peak value and is used for controlling the conduction time of the primary side power tube, and the conduction time of the primary side power tube is in direct proportion to VC. At the same time, VC controls the variable current generating circuit to generate a current I which is positively correlated with VCEAThe variable current generates a variable delay signal Vary-T through a periodic clock signal generating circuit, then the variable delay signal competes with a knee point detection signal, and the next period of the power tube is started by the variable delay signal and the latter signal of the knee point detection signal, so that the switching period of the power tube is determined by the latter signal of the two signals. The whole system is in a variable frequency regulation state under light load and heavy load, and the conduction time and the switching period of the system are changed along with the load, so that the stability of the output voltage is ensured. The output voltage of the overall system can be expressed as:
Figure GDA0003555623140000041
wherein, VOUTRepresenting the system output voltage, VINFor bus voltage, TONFor the power tube on time, RLRepresenting the size of the load, TSWFor switching period of power tube，LPThe primary side excitation inductance is IPKRepresenting the magnitude of the cycle-by-cycle peak current limit.
The mechanism of the constant voltage output of the system is to obtain output voltage information, namely R, according to system samplingLThe size of (2). The system then maintains the output constant by adjusting other variables through negative feedback. For conventional modulation schemes, two general categories can be identified: 1. pulse Width Modulation (PWM) by keeping the switching period of the power tube constant according to RLRegulating the peak current IPKI.e. the duty cycle of the on-time, to ensure VOUTIs constant; 2. pulse Frequency Modulation (PFM), by holding I as opposed to PWMPKInstead, the switching period is adjusted so that V is maintainedOUTIs constant. The relation between the peak current and the frequency of the system and the load is shown in fig. 3, and is different from the traditional PWM and PFM modulation, the modulation mode provided by the invention adopts a method of simultaneously adjusting the peak current and the switching period to realize the constant voltage output in both the DCM and BCM working modes, and the adjustment mode has a larger adjustment range compared with the traditional single variable adjustment method. And by combining the working mode of adopting DCM under light load and adopting BCM under heavy load, the efficiency of the system under light load is greatly improved.
The static power consumption of the system during operation can be expressed as
Figure GDA0003555623140000042
Wherein IINFor inputting current, D1Is the primary side on duty cycle, i.e. the ratio of the primary side on time to the switching period, D2Is the secondary on-duty ratio, i.e. the ratio of the secondary on-time to the switching period, RONIs the on-resistance of the primary power tube and acts as a sampling resistor in an internal sampling structure, NPSIs the turn ratio, V, of the primary and secondary transformersFThe conduction voltage drop of the secondary freewheeling diode.
Can be obtained according to the voltage-second conservation of the primary side inductance
Figure GDA0003555623140000043
Thus the static power consumption can be expressed as
Figure GDA0003555623140000044
The switching loss of system operation can be expressed as
Figure GDA0003555623140000051
Where VDS, max is the maximum drain voltage of the primary power tube when it is turned off, and its value is about (V) for simplicityIN+NPSVOUT+NPSVF)，trAnd tfRise and fall times, respectively, of the drive signal, COSSIs the output capacitance of the primary power tube.
The loss of the whole system consists of two parts of static loss and switching loss
Ploss＝Ploss_static+Ploss_switching (6)
The inductor size and the voltage and current stress borne by the power tube in the BCM state are a proper compromise selection, the system mainly works in the BCM conduction state, but the BCM is controlled by frequency conversion, if the system completely adopts the BCM working mode, the output frequency is completely controlled by a knee point detection signal, when the load is reduced, the output of EA is reduced, the switching frequency of the primary power tube is accelerated, according to the formula (4), the switching loss of the system is increased along with the increase of frequency, and therefore in order to improve the efficiency in light load, the DCM working mode is adopted to forcibly reduce the frequency and return the frequency. From the expressions (4) to (6), it can be seen that the lower the frequency is under the same load condition, the lower the static loss and the switching loss of the system are reduced, so that the efficiency of the system under light load is improved.
According to the principle of loop negative feedback, the output voltage VC of the error amplifier EA can reflect the load condition, and the higher the output voltage of the error amplifier is, the heavier the load of the system is represented. The load can be divided into 3 states as shown in fig. 3 by detecting the size of VC. When V2<VC<When the voltage at V3 is at a heavy load, VC is high, the peak current limit of a primary side is increased, so that the secondary side current after the turn ratio conversion is increased, the conduction time of the secondary side is long, which means that the turning time of a knee point detection signal needs to be long, because the VC value is large, the clock period determined by the variable delay chain is short, the two signals are subjected to AND logic operation, the knee point detection signal coming after the rising edge determines the closing time of the power tube, the system opens the power tube to open the next period after the knee point detection signal comes, and the system works in a quasi-resonance BCM mode. When V1<VC<At V2, the system is in a light load state, the VC signal becomes low, the secondary side conduction time is shortened, and the variable delay timing time is lengthened, so that the rising edge of the knee point detection signal will occur before the rising edge of the variable delay timing end, therefore, the variable delay timing signal controls the start logic of the power transistor, and the system is in a DCM conduction mode at this time. Finally, when VC is lower than V1, the system is in a limit working conduction state for very light load, and in order to maintain necessary energy transmission, the peak current limit voltage and the system switching frequency are respectively fixed at VPEAK,MINAnd fSW,MINWherein f isSW,MINThe setting of (c) requires a compromise between better load response and lower losses.
The switching sequence of the BCM and the DCM is shown in fig. 4, where, BOUN represents a knee point signal, when the primary power tube is closed, the BOUN signal is turned down, when the knee point arrives, the BOUN signal is turned up, and Vary-T represents a variable delay chain signal, when the primary power tube is closed, the primary power tube is turned down to start timing, and when the timing is ended, the primary power tube is turned up, PWM is a grid signal of the primary power tube, when PWM is high, the power tube is turned on, and otherwise, when PWM is low, the power tube is turned off. VC size determination primary side conduction time TONAnd variable delay timer time tDTime t of detecting the knee pointBThe larger of the two determines the closing time of the power tube to finish the system energyStorage and transfer procedures in BCM segment (T)S1And TS3) The switching frequency of the system rises with the reduction of the load, and the peak current limit is reduced with the reduction of the load; in DCM segment (T)S2) The switching frequency begins to fold back and the peak current limit continues to decrease, and after VC falls below V1, the system operates at the minimum switching frequency and the minimum peak current limit, at which time the minimum load that the system can accommodate is also reached. The frequency folding point means that the variable delay timing time and the knee point detection time are equal.
Figure GDA0003555623140000061
Wherein K1 and K2 are respectively the regulation and control coefficients generated by the peak current limit generation module in the system, tD,MAXMaximum secondary side conduction time, VPEAK,MINIs the minimum peak current limit voltage.
Therefore, V2 can be flexibly set by equation (7).
In summary, the BCM/DCM hybrid control circuit formed by the knee point detection and sample hold circuit, the error amplifier, the cycle-by-cycle current limit generation circuit, the variable current generation circuit, and the cycle clock generation circuit can realize efficient and intelligent constant output voltage control in the primary feedback flyback converter, and the working mode of the hybrid frequency conversion BCM and DCM expands the load range and simultaneously significantly improves the efficiency of the system under light load.

Claims (1)

1. A hybrid control circuit for a primary side feedback flyback converter is characterized by comprising a knee point detection and sample hold circuit, an error amplifier, a cycle-by-cycle current limit generation circuit, a variable current generation circuit, a cycle clock generation circuit and an AND gate;
the knee point detection and sampling holding circuit is used for collecting knee point voltage VS, and the knee point voltage VS is processed by the error amplifier to form a regulation voltage VC carrying load information;
the cycle-by-cycle current limit generation circuit is used for obtaining the current according to the regulated voltage VCTo a cycle-by-cycle peak current limit voltage V positively correlated with VCPEAKThe cycle-by-cycle current limit generating circuit controls the maximum value of the primary side inductive current, and the power tube is closed after the primary side current reaches a set peak value, so as to control the conduction time of the primary side power tube;
the variable current generating circuit is used for generating a current I which is positively correlated with VC according to the regulated voltage VCEAThen, a variable delay signal Vary-T is generated through a periodic clock generating circuit;
the Vary-T is connected with one input end of the AND gate, the knee point detection signal BOUN output by the knee point detection and sampling holding circuit is connected with the other input end of the AND gate, the AND gate outputs a primary side power tube switching signal, namely the variable delay signal Vary-T competes with the knee point detection signal BOUN, the starting time of the next period of the power tube is determined by the latter signal of the variable delay signal Vary-T and the knee point detection signal BOUN, and the switching period of the power tube is determined by the latter signal of the two signals.
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US7733672B2 (en) * 2006-11-29 2010-06-08 Linear Technology Corporation Programmable conduction mode control for switching circuits
EP2512021B1 (en) * 2011-04-14 2017-07-19 Nxp B.V. A controller for a switched mode power converter
CN206575376U (en) * 2017-02-14 2017-10-20 苏州大学 A kind of miniature photovoltaic grid-connected inverter
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