CN113315380B - Switching power supply control circuit, system and control method - Google Patents

Switching power supply control circuit, system and control method Download PDF

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Publication number
CN113315380B
CN113315380B CN202110573541.3A CN202110573541A CN113315380B CN 113315380 B CN113315380 B CN 113315380B CN 202110573541 A CN202110573541 A CN 202110573541A CN 113315380 B CN113315380 B CN 113315380B
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signal
switch
time
value
current
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CN113315380A (en
Inventor
郭艳梅
郜小茹
朱臻
陈一辉
李岳辉
缪海峰
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Shanghai Bright Power Semiconductor Co Ltd
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Shanghai Bright Power Semiconductor Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M3/00—Conversion of dc power input into dc power output
    • H02M3/22—Conversion of dc power input into dc power output with intermediate conversion into ac
    • H02M3/24—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters
    • H02M3/28—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac
    • H02M3/325—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal
    • H02M3/335—Conversion of dc power input into dc power output with intermediate conversion into ac by static converters using discharge tubes with control electrode or semiconductor devices with control electrode to produce the intermediate ac using devices of a triode or a transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M3/33507—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
    • 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
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • H02M1/44—Circuits or arrangements for compensating for electromagnetic interference in converters or inverters
    • 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
    • H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
    • H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
    • H02M7/219—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a bridge configuration
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
    • Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
    • Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

The invention provides a switching power supply control circuit which is used for driving a first switch in a switching power supply to convert input voltage into output voltage and comprises an on-time control unit, a first switching unit and a second switching unit, wherein the on-time control unit receives a first signal and a second signal, the first signal represents a current limiting value, and the second signal represents current flowing through the first switch so as to select a working mode of the switching power supply according to the first signal and the second signal; when the current peak value flowing through the first switch is smaller than the first reference current value, the switch power supply is in a first working mode, and the on-time control unit determines the off time of the first switch according to the time when the current flowing through the first switch is increased to the first reference current value, so that the current peak value flowing through the first switch can reach the first reference current value.

Description

Switching power supply control circuit, system and control method
Technical Field
The present invention relates to the field of electronic circuits, and in particular, to a control circuit, a control method, and a control system for a switching power supply.
Background
In the field of switching power supplies, the current commonly used control modes are mainly a voltage control mode, a current control mode and a Constant On Time (COT) control mode, wherein COT control has superior load transient response and a simple control circuit composition, and is widely used. In the conventional COT control method, the switching tube is turned on when the output voltage of the switching power supply is less than the reference signal, and the switching tube is turned off when the on-time reaches a preset value. For a conventional COT control switching power supply, the switching frequency thereof is gradually reduced as the load is reduced. As the load is relieved, the switching frequency may be reduced to fall within the audio frequency range (e.g., 200Hz-20kHz), causing audio noise; on the other hand, when the input voltage increases, the fixed on-time may increase the value of the current flowing through the switching tube, which may cause an overcurrent. When the load is further reduced to a light load or no-load state, for example, below 10% of the rated load, it is necessary to control the switching loss to improve the efficiency of the entire switching power supply and reduce the standby power consumption. In order to avoid the generation of audio noise and maintain good operation efficiency and safety performance, a new control circuit and a new control method are required.
Disclosure of Invention
The invention aims to provide a control circuit, a control method and a control system for a switching power supply, so as to avoid audible audio noise generated by human ears in the working process of the switching power supply when a load and an input voltage are changed, reduce the switching loss of the switching power supply, limit the peak current flowing through a switching tube and improve the performance of the switching power supply.
The switching power supply control circuit is used for driving a first switch in a switching power supply to convert an input voltage into an output voltage and is characterized by comprising an on-time control unit, a first switching unit and a second switching unit, wherein the on-time control unit receives a first signal and a second signal, the first signal represents a current limiting value, and the second signal represents current flowing through the first switch so as to select a working mode of the switching power supply according to the first signal and the second signal; the on-time control unit compares the current flowing through the first switch with a first reference current value generated according to the current limiting value, when the current flowing through the first switch is smaller than the first reference current value, the switching power supply is in a first working mode, and the on-time control unit determines the off time of the first switch according to the time when the current flowing through the first switch is increased to the first reference current value so as to maintain the current value flowing through the first switch not to be lower than the first reference current value.
In one embodiment, the on-time control unit further receives a third signal, where the third signal indicates an off time of the first switch, when the peak value of the current flowing through the first switch reaches the first reference value and is smaller than a second reference current value generated according to the current limit value, the switching power supply is in a second operating mode, and the on-time control unit determines the off time of the first switch according to the third signal.
The switching power supply control system is characterized by further comprising an on-time control unit, a current limiting unit and a current limiting unit, wherein the on-time control unit receives a first signal and a second signal, the first signal represents a current limiting value, and the second signal represents current flowing through the switching element so as to select the working mode of the switching power supply according to the first signal and the second signal; the on-time control unit compares the current flowing through the first switch with a first reference current value generated according to the current limiting value, when the current flowing through the first switch is smaller than the first reference current value, the switching power supply is in a first working mode, and the on-time control unit determines the off time of the switching element according to the time when the current flowing through the switching element is increased to the first reference current value so as to maintain the current value flowing through the switching element not lower than the first reference current value.
In one embodiment, the switching power supply is an isolated switching power supply, and includes a primary side circuit and a secondary side circuit, where the primary side circuit and the secondary side circuit are respectively connected to different ground terminals, the switching element and the on-time control unit are both electrically connected to the primary side circuit, and the freewheeling element is connected to the secondary side circuit.
In one embodiment, the on-time control unit further includes an on-time generation unit, a current limiting unit, and a second logic circuit, the switching power supply is an isolated switching power supply and includes a primary side circuit and a secondary side circuit, the primary side circuit and the secondary side circuit are respectively connected to different ground terminals, the switching element, the current limiting unit, and the second logic circuit are all electrically connected to the primary side circuit, and the on-time control circuit is electrically connected to the secondary side circuit.
According to an embodiment of the present invention, a control method for a switching power supply, for driving a first switch in the switching power supply to convert an input voltage into an output voltage, is characterized by comprising:
detecting a current flowing through the first switch;
when the current flowing through the first switch is smaller than a first reference current value, the switching power supply is in a first working mode, and under the first working mode, the turn-off time of the first switch is determined according to the time when the current flowing through the first switch is increased to the first reference current value, so that the current value flowing through the first switch is not lower than the first reference current value.
In one embodiment, when the peak value of the current flowing through the first switch reaches a second reference current value, the turn-off time of the first switch is determined according to the time when the peak value of the current reaches the second reference current value.
In one embodiment, the control method further comprises:
and when the current peak value flowing through the first switch reaches the first reference value and is smaller than a second reference current value generated according to the current limiting value, the switching power supply is in a second working mode, and the turn-off time of the first switch is determined according to an on-time signal.
According to the embodiment of the invention, in a working state with a heavier load, a frequency conversion control mode is adopted, the switching frequency is reduced along with the reduction of the load, the conducting time of the switch is determined by the detected current value flowing through the switching element/the energy storage element, when the switching frequency is reduced to an audio frequency range, the frequency locking working mode is entered, and the switching frequency is fixed at a preset value higher than the audio frequency range by adaptively adjusting the conducting time of the switch, so that the generation of audio noise is effectively avoided. In the frequency locking working mode, different output loads can be adapted by adjusting the conducting time of the switch, the switching frequency is kept basically constant, and the current flowing through the switch does not exceed a limit value. If the load is continuously lightened to a light load state, the minimum conduction time limit is reached, and then the frequency conversion control mode is returned, so that the switching loss is further reduced, and the efficiency is improved. Although the switching frequency may enter the audio frequency range, the switching power supply system has lower energy in a light load state, so that noise which can be perceived by human ears can be avoided, and the user experience is ensured. According to the control circuit and the control method provided by the invention, the generation of audio noise can be effectively avoided within the full load range of the working of the switching power supply, and meanwhile, higher efficiency is kept and the safe working of the switching power supply is ensured.
Drawings
FIG. 1 is a schematic diagram of a control circuit according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a control circuit according to another embodiment of the present invention;
FIG. 3 is a schematic diagram of a control circuit according to another embodiment of the present invention;
FIG. 4 is a schematic diagram of a control circuit according to another embodiment of the present invention;
FIG. 5 is a schematic diagram of a conducting time generating unit according to an embodiment of the present invention;
FIG. 6 is a schematic diagram of a circuit structure of an ON-time generating unit according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of a control system according to an embodiment of the present invention;
FIG. 8 is a schematic diagram of main waveforms according to an embodiment of the present invention.
Detailed Description
The overvoltage protection circuit and method, and the switching power supply according to the present invention will be described in detail with reference to the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description. It is to be noted that the drawings are in a very simplified form and are not to precise scale, which is merely for the purpose of facilitating and distinctly claiming the embodiments of the present invention. In addition, the "time", "on time length", and "on time" referred to in this specification are time periods, that is, the length of a period of time; "time", "when … …", "turn-off time" are all time points, that is, the time when a certain event occurs; "certain signal indicating … …" indicates that a non-zero level indicates that a certain element or a certain module is acting accordingly; "… … signal arrival" indicates that the active level of a signal is received by a subsequent module or unit.
Fig. 1 is a schematic diagram of a control circuit 20 for controlling a switching power supply 10, where the switching power supply 10 includes a first switch 11, and converts an input voltage Vin into an output voltage Vout by turning on and off the first switch 11 to drive a load (not shown), and the switching power supply 10 may be an isolated/non-isolated AC-DC converter, outputting a substantially constant output voltage Vout, or may be an isolated/non-isolated DC-DC converter, and also outputting a substantially constant output voltage Vout. The control circuit comprises an on-time control unit 21 for generating a signal Turnoff _ S indicating the first switch-off time, an off-time control unit 23 for generating a signal Turnon _ S indicating the first switch-on time, and a first logic circuit 22; wherein the on-time control unit 21 receives a first signal I limit And a second signal I CS The first signal I limit Characterizing a current limiting value, which may be generated by a reference inside the control circuit or sent from outside the control circuit, the second signal I CS And characterizing the current value flowing through the first switch and obtaining the current value through detection. According to the second signal I CS When the peak value of the current flowing through the first switch is smaller than a first reference current value, the switching power supply is in a first working mode, and the on-time control unit 21 determines the on-time according to the second signal I CS The moment of increasing to the first reference current value determines the moment of turning off the first switch to maintain the peak value of the current flowing through the first switch not to be lower than the first reference current value. In this embodiment, the first reference current value is ANDA reference value proportional to said current-limiting value may be derived from the first signal I limit Obtained after being processed by a voltage division circuit or a multiplier. For example, when the load is light (for example, the load current is less than 10% of the rated output current), the current flowing through the first switch 11 is also small, and the switching loss of the switching power supply is mainly caused by the switching action of the switch. In this operating mode, the on-time is extended to I by controlling the first switch 11 CS When the current reaches the first reference current value, the peak value of the current flowing through the first switch may reach the first reference current value, and as the on-time of the first switch 11 is prolonged, the switching frequency is properly reduced compared to the case where the first reference current value is not set, and the switching loss is reduced accordingly, thereby improving the efficiency of the switching power supply in the light load state. On the other hand, when the load is heavy (i.e. when the output current of the switching power supply is large), the current flowing through the first switch is also large, and at this time, the second signal I representing the current flowing through the first switch is detected CS The control circuit 20 can determine that the load is heavy and the load current needs to be limited, and when the load changes, a frequency conversion control mode is adopted to adapt to the load requirement, and when the load decreases, the switching frequency is reduced, so that the energy transmitted to the output side is reduced; when the load is reduced below the predetermined value, the corresponding switching frequency is reduced to the predetermined reference frequency, and the control circuit 20 fixes the switching frequency at the reference frequency, for example, a value in the range of 22-28kHz, thereby avoiding the generation of audio noise.
The signal Turnon _ S indicating the on-time of the first switch is determined by the off-time control unit 23 according to the feedback detection voltage signal Vfb reflecting the output voltage Vout and a preset reference voltage Vref. After the Turnon _ S signal and the Turnoff _ S signal are processed by the first logic circuit 22, a control signal Gate _ S of the first switch 11 is generated to control the on/off of the first switch 11. In some embodiments according to the present invention, the off-time control unit 23 can also determine the on-time of the first switch 11 according to other signals, such as a short pulse signal with a fixed frequency, or other detection signals capable of reflecting the output voltage Vout. In some embodiments according to the invention, the switching frequency signalNumber f SW The control signal Gate _ S that can be derived from the first switch 11 or the signal Turnon _ S indicating the moment the first switch is switched on, or a combination of the above signals.
The advantageous effects of the control circuit of the present invention will be more clearly explained in conjunction with expressions of circuit principles. Taking the BUCK circuit as an example, specifically, the current Ipeak flowing through the first switch can be expressed as:
Figure BDA0003083534660000071
where L is an inductance value of the energy storage in the switching power supply, and Ton is a conduction time length of one period of the first switch 11.
Expression for the energy delivered in connection with a switching power supply:
Figure BDA0003083534660000072
where Po is the energy provided by the switching power supply to the load, i.e. the product of the output voltage Vout and the load current Io, and f is the switching frequency.
It can be seen from the above expression that, when the switching frequency f is fixed, if the load changes, the load change can be responded quickly by adjusting the switch on-time Ton, and the generation of audio noise is effectively avoided.
Further, in another embodiment according to the present invention, in conjunction with fig. 2, how to adjust the switch on-time is described in detail. As shown in fig. 2, when the peak value of the current flowing through the first switch is smaller than a predetermined current value, the on-time control unit 21 controls the on-time according to the first signal I limit And a second signal I CS And changing the turn-off time of the first switch to maintain the peak value of the current flowing through the first switch not to be lower than the first reference current value. The first signal I limit Characterizing a current-limiting value, which may be a reference internal to the control circuitThe second signal I may be sent from the outside of the control circuit CS And characterizing the value of the current flowing through the first switch and obtaining the value through detection. The control circuit 20 comprises an on-time generating unit 26 outputting an on-time signal Vcot _ S indicating the switching-off action of the first switch 11. The on-time signal Vcot _ S may be generated according to a difference between a reference frequency value and a detected switching frequency value of the first switch 11, thereby adaptively changing the turn-off time of the first switch 11; the Vcot _ S signal may also be generated according to a fixed on-time, thus instructing the first switch 11 to turn off after the same on-time. The on-time control unit 21 further comprises a current limiting unit 24 and a second logic circuit 25, wherein the current limiting unit 24 receives the second signal I CS And a restriction value I limit . When the peak current flowing through the first switch is smaller than a first reference current value generated according to the current limiting value (i.e. a light load condition or a no-load standby condition), the switching power supply is in a first working mode, the current limiting unit 24 instructs the second logic circuit 25 through a Vcsmin _ S signal, and the turn-off time of the first switch is delayed, so that energy can be continuously transmitted to an output end, and the current in the converter is increased until the second signal I is obtained CS Rises to the first reference current value. As the on-time of the first switch 11 is prolonged, the switching frequency is properly reduced compared to the situation without setting the first reference current value, and the switching loss is reduced accordingly, thereby improving the efficiency of the switching power supply in the light load state. When the peak current flowing through the first switch is larger than a first reference current value generated according to the current limiting value and smaller than a second reference current value generated according to the current limiting value, the switching power supply is in a second working mode, wherein the first reference current value is smaller than the second reference current value. In the second operation mode, the second logic circuit 25 determines the turn-off time of the first switch 11 (i.e. determines the turn-on time of the first switch 11) according to the Vcot _ S signal generated by the turn-on time generating unit 26, so that the turn-on time of the first switch 11 is kept unchanged, or the turn-on time is adaptively changed to keep the switching frequency unchanged.
Specifically, in some embodiments of the present invention, the method of the on-time generating unit 26 changing the off time of the first switch 11 (i.e. the on time of the first switch 11) according to the difference between the reference frequency and the switching frequency may be to adjust the length of the on time of the first switch 11 cycle by cycle, for example, when the switching frequency is higher than the reference frequency, the on-time generating unit controls the off time of the first switch to be delayed by a unit time; when the switching frequency is lower than the reference frequency, the on-time generation unit controls the turn-off time of the first switch to be advanced by one unit time, so that the on-time is adjusted as smoothly as possible, and the working state of the switching power supply is stable and controllable. In other embodiments of the present invention, the on-time generating unit 26 may change the off-time of the first switch 11 (i.e. the on-time of the first switch 11) according to the difference between the reference frequency and the switching frequency, or may adjust the length of the on-time of the first switch 11 in proportion to the difference between the reference frequency and the switching frequency cycle by cycle, for example, when the switching frequency is higher than the reference frequency, the on-time generating unit controls the off-time of the first switch to be delayed, and the delayed time is proportional to the difference between the reference frequency and the switching frequency; when the switching frequency is lower than the reference frequency, the on-time generation unit controls the turn-off time of the first switch to be advanced, and the advanced time is proportional to the difference between the reference frequency and the switching frequency, so that the on-time of the switch is adjusted more quickly, and the performance of the switching power supply is improved. The method for adjusting the on-time of the switch by the on-time generating unit 26 may be performed at intervals, and may be set in the on-time control unit 21 according to the requirements of specific applications. Finally, when the switching power supply 10 is in steady-state operation, the switching frequency of the first switch is equal to the reference frequency, so that the generation of audio noise is effectively avoided.
As shown in fig. 2 and 3, the switching power supply operates in the first operation mode or the second operation mode, which may be indicated by a signal or signals. As shown in fig. 2, the current limiting unit 24 comparesTwo signals I CS And according to the current limit signal I limit Generating a first reference current value and a second reference current value and outputting a signal Vcsmin _ S, which may be a square wave signal with different levels, for example, and indicating the second signal I by setting a rising edge and a falling edge CS The time when the peak value (i.e. the detected current value flowing through the first switch 11) reaches the first reference current value and the second reference current value is sent to the second logic circuit 25 to determine whether the switching power supply operates in the first operating mode or the second operating mode. As shown in FIG. 3, the current limiting unit compares the second signal I CS And according to the current limit signal I limit Generating a first reference current value and a second reference current value, and outputting a signal Vcsmin _ S and a signal Vcsmax _ S respectively indicating the second signal I CS The time when the peak value (i.e. the detected current value flowing through the first switch 11) reaches the first reference current value and the second reference current value is sent to the second logic circuit 25 to determine whether the switching power supply operates in the first operating mode or the second operating mode.
With reference to fig. 3 and 4, a specific implementation manner of the current limiting unit 24 according to another embodiment of the present invention may be exemplarily described. The current limiting unit 24 may specifically be comparators 241 and 243, and the second signal I CS The positive terminals of the comparators 241 and 243 are switched in; the current limiting signal I limit The signal is processed by the proportional circuit 242 and then connected to the negative terminal of the comparator 241 as a first reference current value; the current limiting signal I limit The negative terminal of the comparator 243 is connected as the second reference current value. In other embodiments, the current limit signal I limit Or a first reference current value and a second reference current value can be obtained after different proportional circuits are processed, and are respectively sent to the comparators 241 and 243, so that the first reference current value is smaller than the second reference current value. If the second signal I CS The peak value (i.e., the detected value of the current flowing through the first switch 11) is smaller than the first reference current value, and the output signal Vcsmin _ S of the current limiting unit 24 is at a low level, and vice versa. If the second signal I CS (i.e., the detected value of the current flowing through the first switch 11) has a peak value smaller than the second reference current value, and the output signal Vc of the current limiting unit 24smax _ S is low and vice versa. The second logic circuit 25 may be a combination of a logic and gate 251 and a logic or gate 252, as shown in fig. 4, or a combination of a plurality of gate devices to achieve the same function.
Specifically, the second logic circuit 25 is configured to, when the peak value of the detected current value flowing through the first switch 11 is smaller than the first reference current value, change the output signal Turnoff _ S of the second logic circuit 25 not according to the Vcot _ S signal output by the on-time generating unit 26 but according to the time at which the detected current value flowing through the first switch 11 rises to the first reference current value, and keep the first switch on until this time comes, thereby ensuring that the current value flowing through the first switch 11 can reach the first reference current value. As the on-time of the first switch 11 is prolonged, the switching frequency is properly reduced compared to the case where the first reference current value and the related logic circuit are not set, and the switching loss of the first switch is reduced accordingly, thereby improving the efficiency of the switching power supply in the light load state and the no-load standby state. When the second signal I CS When the peak value reaches the first reference value and is smaller than a second reference current value generated according to the current limiting value, the switching power supply is in a second working mode, and the on-time control unit determines the turn-off time of the first switch according to the Vcot _ S signal indicating the turn-off action of the first switch 11. As shown in fig. 4, in one embodiment, the second logic circuit 25 includes a logic and gate 251 and a logic or gate 252. When the output signal Vcsmin _ S of the current limiting unit 24 is at a low level, that is, the peak value of the current flowing through the first switch does not reach the first reference current value, the and gate 251 outputs a low level; since the second reference current value is higher than the first current value, the output signal Vcsmin _ S of the comparator 243 is also at a low level, the or gate 252 outputs a low level indicating that the first switch is turned off, and the output signal of the or gate 252 cannot be changed by the Vcot _ S signal output by the on-time generating unit 26. That is, when the switching power supply is in the first operating mode, the signal Turnoff _ S indicating the first switch to be turned off does not change according to the Vcot _ S signal, but rises to the first reference value according to the detected value of the current flowing through the first switch 11The timing of the current value is changed. When the output signal Vcsmin _ S of the current limiting unit 24 is at a high level, that is, the peak value of the current flowing through the first switch is greater than the first reference current value, the output of the and gate 251 is controlled by Vcot _ S, and at this time, if the peak value of the current flowing through the first switch is less than the second reference current value, the output signal Vcsmax _ S of the current limiting unit 24 is at a low level, and at this time, the switch power supply is in the second working mode, and the signal Turnoff _ S indicating the turn-off of the first switch is changed according to the Vcot _ S signal, that is, the turn-off time of the first switch is determined by the Vcot _ S signal output by the on-time generating unit 26. When the output signal Vcsmax _ S of the current limiting unit 24 is at a high level, that is, the peak value of the current flowing through the first switch is greater than the second reference current value (meanwhile, the peak value of the current flowing through the first switch is also necessarily greater than the first reference current value), the signal Turnoff _ S indicating that the first switch is turned off is at a high level, which indicates that the first switch is turned off. Because the current detection signal is introduced, particularly in a heavy-load working state, the turn-off time of the first switch is controlled according to the current detection and comparison result, the function of current limiting in a switch-by-switch period is realized, devices such as a switch and the like can be protected, and damage is avoided.
Further, referring to fig. 5, in one embodiment, the Vcot _ S signal may be generated based on a difference between a reference frequency value and a detected switching frequency value of the first switch 11, thereby adaptively changing the turn-off timing of the first switch 11. The on-time generating unit 26 comprises a frequency error signal generating unit 261 receiving a switching frequency detection signal f of the first switch SW And a reference frequency signal f ref To generate a frequency error signal Δ f characterizing a difference between the reference frequency signal and the switching frequency detection signal; an on-time adjustment amount generating unit 262, receiving the frequency error signal Δ f, and generating an on-time adjustment amount Δ Ton according to the frequency error signal Δ f, representing a value of the on-time that needs to be adjusted in the period; the timer circuit 263 changes the on-time signal Vcot _ S according to the on-time adjustment amount Δ Ton.
When the on-time generation unit 26 generates the reference frequency signalNumber f ref And a switching frequency detection signal f of the first switch 11 SW When the on-time signal Vcot _ S is changed, the specific operation mode may be: when the switching frequency of the first switch is higher than the reference frequency, the on-time generating unit 26 controls the timer to delay the turn-off time of the first switch by one unit time; when the switching frequency of the first switch is lower than the reference frequency, the on-time generating unit 26 controls the timer to advance the turn-off timing of the first switch by one unit time. The unit time may be determined according to a counting step of a timer. Further, it may be so arranged that the time at which the off timing of the first switch 11 is advanced/retarded is positively correlated with the difference between the switching frequency detection value and the reference frequency.
In an exemplary embodiment of the present invention, a specific circuit implementation of the on-time generating unit 26 is provided as shown in fig. 6.
Fig. 6 provides a specific circuit implementation of the on-time generating unit 26, such that the time for advancing/delaying the turn-off timing of the first switch 11 is positively correlated with the difference between the switching frequency and the reference frequency. Specifically, as shown in fig. 6, the on-time generating unit 26 may include a timing circuit composed of a switch Q1, a capacitor C1 and a current source I1, the timing circuit detects a voltage signal corresponding to the switching frequency of the previous switching cycle, the voltage signal corresponding to the switching frequency and a voltage signal corresponding to the reference frequency signal generate a current signal proportional to the difference between the two input voltage signals through an OTA (operational transconductance amplifier), the current signal periodically charges a capacitor C3 through a switch Q4, and the voltage of the capacitor C3 is connected to the negative input terminal of the comparator Comp1, the voltage of the capacitor C3 represents the sum of the on-time adjustment quantity Δ Ton and the on-time Ton before adjustment, and the on-time adjustment quantity delta Ton is proportional to the difference value delta f between the switching frequency and the reference frequency, on the other hand, the first signal f. SW The timing circuit composed of the indication current source I2, the switch Q2 and the capacitor C2 generates a voltage signal corresponding to the switching frequency and outputs the voltage signal to the positive terminal of the comparator Comp1, and the signal output by the comparator Comp1 passes through an edgeAfter detection, a Vcot _ S signal is generated which is composed of a narrow pulse and corresponds to the adjusted turn-off time of the first switch 11.
The frequency error signal generating unit 261 and the timer circuit 263 may also be implemented by other known timing circuits, and the like, for example, a digitized timing circuit, and the on-time adjustment amount generating unit 262 may also be implemented by a combination of other known logic circuits, digital circuits, and the like. The on-time generating unit 26 may be configured to adjust the off-time of the first switch according to the difference between the switching frequency and the reference frequency, for example, when the timer reaches the on-time set according to the difference between the switching frequency and the reference frequency, the timer outputs a short pulse to indicate the off-time of the first switch. The switching frequency is fixed at the preset value by adaptively adjusting the conduction time of the switch, so that the generation of audio noise is effectively avoided.
With continued reference to fig. 1, the control circuit provided by the present invention further includes a turn-off time control unit 23, where the turn-off time control unit 23 may be a comparator as shown in fig. 4, a positive terminal of the comparator receives a feedback signal Vfb representing the output voltage Vout of the switching power supply, a negative terminal of the comparator receives a reference voltage signal Vref, and compares the feedback signal Vfb with the reference voltage signal Vref, and when the feedback signal Vfb is lower than the reference voltage signal Vref, the comparator 23 outputs a high level indicating that the first switch 11 is turned on, so that the output voltage rises back. The turn-off time control unit may also be implemented by a combination of other analog or logic circuits known to those skilled in the art, and may also receive other signals representing the output voltage Vout for judgment, so as to implement a function of instructing to turn on the first switch 11 when the output voltage Vout is lower than an expected value.
As shown in fig. 4, the first logic circuit 22 may be, for example, an RS flip-flop, specifically, a signal Turnon _ S indicating a time when the first switch is turned on is connected to an S terminal of the RS flip-flop, a signal Turnoff _ S indicating a time when the first switch is turned off is connected to an R terminal of the RS flip-flop, and a control signal Gate _ S of the first switch 11 is generated at an output terminal Q terminal of the RS flip-flop to indicate that the first switch 11 is turned on or off. It is noted that in the specific embodiments of the on-time generation unit 26 shown in fig. 5 and 6, an edge detection circuit may be provided at the input and/or output of each unit according to the choice of the specific implementation.
The control circuit according to the embodiments of the present invention may have different configurations when applied to the switching power supply, and forms a control system together with the switching power supply. The switching power supply control system includes a switching element 11, a free-wheeling element 12, and an energy storage element 13. For example, in an isolated flyback converter, the on-time control unit 21 (including its constituent units) and the first switch 11 are both electrically connected to a primary side circuit of the switching power supply 10, the off-time control unit 23 and the on-time generation unit 26 are electrically connected to a secondary side circuit of the switching power supply 10, a signal Turnon _ S generated by the off-time control unit 23 and indicating that the first switch is turned on is transmitted to the primary side control circuit in an isolated manner through an electromagnetic coupling transmission manner via a modulation manner such as OOK (on-off-keying), and cooperates with the primary on-time control unit 21 to control the first switch 11. In other embodiments, for example, in an isolated flyback converter, as shown in fig. 7, the primary side circuit and the secondary side circuit are separated by a dotted line in the figure, the switching element 11 is connected to the primary side circuit, the freewheeling element 12 is a diode or a switching tube connected to the secondary side circuit, and the energy storage element 13 is a transformer T1, and simultaneously provides isolation between the primary side circuit and the secondary side circuit. The current limiting unit 24 and the second logic circuit 25 are electrically connected to the primary side circuit, the turn-off time control unit 23 and the turn-on time generating unit 26 are electrically connected to the secondary side circuit, the turn-off time control unit 23 receives a Vfb signal representing the output voltage Vout from the secondary side circuit, and generates a signal Turnon _ S indicating that the switching element 11 is turned on, and the signal Vcot _ S generated by the turn-on time generating unit 26 is transmitted to the primary side control circuit in an isolated manner through an isolated transmission circuit 27 via a modulation manner such as OOK and a transmission manner such as electromagnetic coupling, and the signal Vcot _ S cooperates with the output signal Vcsmin _ S and/or Vcsmax _ S of the current limiting unit 24 under the control of the second logic circuit 25 to generate a signal Turnoff _ S indicating that the switching element 11 is turned off, and cooperates with the signal Turnon _ S indicating that the switching element 11 is turned on and transmitted to the primary side circuit in an isolated manner, the final control signal Gate _ S is generated by the processing of the first logic circuit 22, and the switching element 11 is controlled to operate by the drive circuit. The above arrangement makes the unit needing detection as close as possible to the detected signal source, so as to reduce the interference of noise to the signal and improve the reliability of the signal detection and transmission process. The isolation transmission circuit 27 may separately transmit the signal Turnon _ S generated by the off-time control unit 23 and used for indicating the first switch to be turned on and the signal Vcot _ S generated by the on-time generation unit 26 to the primary circuit after modulating the signals, and then demodulate the signals; or, the signal Turnon _ S generated by the off-time control unit 23 and used for instructing the first switch to turn on and the signal Vcot _ S generated by the on-time generation unit 26 may be logically processed and synthesized into one signal, and the signal is modulated and then transmitted to the primary side circuit in an isolated manner, and then demodulated, and corresponding signals are obtained on the primary side respectively by detecting the rising edge and the falling edge of the synthesized signal, so as to perform the next operation. According to the specific application requirements, for example, the connection mode of each component unit of the control circuit of the present invention in the isolated converter may be adjusted differently according to the selection of the detection signal of the off-time control unit 23, or the selection of the detection signal of the current limiting unit 24, and the like, and the electromagnetic coupling structure of the isolation transmission circuit 27, such as a coupling coil, a coupling capacitor, and the like, may also be selected, which are not described herein again. The aforementioned switching power supply 10 may be an isolated/non-isolated AC-DC converter outputting a substantially constant output voltage Vout, or may be an isolated/non-isolated DC-DC converter outputting a substantially constant output voltage Vout, and is not limited to a flyback topology or a Buck topology.
In order to improve the integration level, facilitate the user, and reduce the interference on the control signal, the control circuit provided in the foregoing embodiments may also form a control system together with the controlled switch device, and may further be packaged together, and the working mechanism thereof is the same as or similar to that described above, and is not described herein again.
FIG. 8 is a schematic diagram of main waveforms according to an embodiment of the present invention, referring to FIG. 8And fig. 4, the mechanism of action of the various primary signals in control circuit 20 can be more fully understood. In each of the operation modes shown in fig. 8, assuming that the input voltage is constant, the change in the load current causes the operation mode to be switched. When the feedback signal Vfb decreases to the reference voltage signal Vref, the output signal Turnon _ S of the off-time control unit 23 outputs a short pulse, which instructs the first switch 11 to turn on. The operation of the on-time control unit 21 can be divided into three types, as shown in fig. 8(a), which is a first operation mode in which the current detection signal I flowing through the first switch CS Always less than a first reference current, which may be, for example, 0.3 x I limit If the output signal Vcsmin _ S from the current limiting unit 24 is at a low level, the control signal Gate _ S is based on the current detection signal I CS Determines the turn-off time of the first switch, corresponding to the time when the control signal Gate _ S in the waveform one becomes low level; as shown in fig. 8(b), the second operation mode is when the current detection signal I CS Reaches a first reference current value (e.g., 0.3 x I) limit ) And is less than a second reference current value (e.g., I) limit ) At this time, when the Vcsmin _ S signal output by the current limiting unit 24 becomes a high level, and the Vcsmax _ S signal is still a low level, at this time, when the Vcot _ S signal becomes a high level, the Turnoff _ S signal output by the on-time control unit 21 becomes a high level, and further indicates the off time of the first switch, that is, in this working mode, the on-time signal Vcot _ S determines the off time of the first switch; when the current detection signal I is as shown in FIG. 8(c) CS Reaches a second reference current value (e.g., I) limit ) When the Vcsmax _ S signal goes high, the Turnoff _ S signal output by the on-time control unit 21 goes high, and indicates the turn-off time of the first switch. As can be seen from fig. 8(a), (b) and (c), as the load current increases, the switching frequency of the first switch changes in stages, and when the load is light or in an idle standby state, the switching frequency decreases as the load current decreases, so as to reduce the switching loss and improve the overall efficiency; when the load enters a medium load or heavy load state, the switch power supplyEntering a frequency locking working mode or adopting COT control to avoid audio noise and stabilize output voltage; when the load is too heavy, the turn-off time of the first switch is determined according to the current limiting control mode so as to protect circuit elements and control loss.
According to the embodiment of the invention, when the on-time of the switch is less than or equal to a preset minimum on-time (corresponding to the first reference current value), which usually indicates that the load is too light and the current in the switching power supply is very small, the frequency locking mode exits at this time, and a frequency conversion control mode is adopted, although the switching frequency may be reduced to an audio frequency range, because the energy in the switching power supply system is lower and the decibel value of the audio noise is lower in a light load state, noise which can be perceived by human ears can be avoided, and the user experience is ensured. And the loss in the switching process of the switch is more obvious compared with the conduction loss of the switch, so that the reduction of the switching frequency is also beneficial to reducing the loss and improving the efficiency.
The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. For the control system disclosed in the embodiment, the description is relatively simple because the control system corresponds to the control circuit disclosed in the embodiment, and the relevant points can be referred to the control circuit part for description. In addition, the control method disclosed in the embodiment can be referred to the description of the control circuit and the control system.
The above description is only for the purpose of describing some preferred embodiments of the present invention, and is not intended to limit the scope of the present invention, and it is the protection scope of the claims that any changes and modifications that may be made by those skilled in the art from the above disclosure without departing from the technical scope of the present invention.

Claims (17)

1. A switching power supply control circuit for driving a first switch in a switching power supply to convert an input voltage to an output voltage, the switching power supply control circuit comprising:
the conduction time control unit receives a first signal and a second signal, the first signal represents a fixed current limiting value, the second signal represents the current flowing through the first switch, and the working mode of the switching power supply is selected according to the first signal and the second signal; wherein the content of the first and second substances,
the on-time control unit compares the current flowing through the first switch with a fixed first reference current value generated according to the current limiting value, when the peak value of the current flowing through the first switch is smaller than the first reference current value, the switching power supply is in a first working mode, the on-time control unit determines the off time of the first switch according to the time when the current flowing through the first switch is increased to the first reference current value, and the first switch is kept on before the off time comes, so that the peak value of the current flowing through the first switch reaches the first reference current value;
the on-time control unit further receives a third signal, the third signal indicates an off-time of the first switch, when a current peak value flowing through the first switch reaches the first reference current value and is smaller than a second reference current value generated according to the current limiting value, the switching power supply is in a second working mode, and the on-time control unit determines the off-time of the first switch according to the third signal.
2. The control circuit of claim 1, wherein the on-time control unit comprises an on-time generation unit that receives a switching frequency detection signal and a reference frequency signal, and generates the third signal based on a difference between the reference frequency signal and the switching frequency detection signal such that the difference between the reference frequency signal and the switching frequency detection signal is reduced, wherein the switching frequency detection signal represents a switching frequency of the first switch, and the reference frequency signal corresponds to a target frequency value.
3. The control circuit of claim 1, wherein the on-time control unit includes an on-time generation unit that generates the third signal according to a preset reference value such that a switching frequency of the first switch is constant.
4. The control circuit of claim 1, wherein the first reference current value is less than the second reference current value.
5. The control circuit of claim 1, wherein the on-time control unit includes a current limiting unit that receives the first signal and the second signal and generates a first current limiting signal and a second current limiting signal, wherein the first current limiting signal indicates a time at which a value of current flowing through the first switch rises to the current limiting value, and wherein the second current limiting signal indicates a time at which the value of current flowing through the first switch rises to the first reference current value.
6. The control circuit of claim 5, wherein the current limiting unit comprises a first comparing unit and a second comparing unit, the first comparing unit receiving the first signal and the second signal and generating a first current limiting signal when a current value flowing through the first switch is greater than the current limiting value; the second comparison unit receives the first reference current value and the second signal, and generates a second current limiting signal when the current value flowing through the first switch is larger than the first reference current value.
7. The control circuit of claim 2, wherein the on-time generating unit comprises:
a frequency error signal generating unit receiving the switching frequency detection signal and the reference frequency signal to generate a frequency error signal representing a difference between the reference frequency signal and the switching frequency detection signal;
the on-time adjustment quantity generating unit is used for receiving the frequency error signal and generating an on-time adjustment quantity representing the value of the on-time required to be adjusted in the period according to the frequency error signal;
and the timer circuit generates the third signal according to the on-time adjustment quantity.
8. The control circuit of claim 7 wherein the on-time adjustment causes the timer circuit to delay the turn-off time of the first switch when the frequency error signal indicates that the switching frequency is above the target frequency value; when the frequency error signal indicates that the switching frequency is lower than the target frequency value, the on-time adjustment amount causes the timer circuit to advance the turn-off time of the first switch; the on-time adjustment is generated based on the frequency error signal.
9. The control circuit of claim 1, further comprising an off-time control unit that receives a fourth signal indicative of the output voltage and compares the fourth signal to a reference voltage signal to generate a signal indicative of the first switch on time.
10. The control circuit of claim 9, further comprising a first logic circuit that receives the signal indicative of the time the first switch is off and the signal indicative of the time the first switch is on and that generates the control signal for the first switch.
11. A switching power supply control system including a switching element, a freewheel element, and an energy storage element for converting an input voltage to an output voltage, the control system further comprising:
a conduction time control unit, receiving a first signal and a second signal, the first signal representing a fixed current limiting value, the second signal representing a current flowing through the switching element, so as to select a working mode of the switching power supply according to the first signal and the second signal; wherein the content of the first and second substances,
the on-time control unit compares the current flowing through the switching element with a fixed first reference current value generated according to the current limiting value, when the peak value of the current flowing through the switching element is smaller than the first reference current value, the switching power supply is in a first working mode, the on-time control unit determines the off time of the switching element according to the time when the current flowing through the switching element is increased to the first reference current value, and the switching element is kept on before the off time comes so as to maintain the peak value of the current flowing through the switching element not to be lower than the first reference current value;
the on-time control unit also receives a third signal, the third signal indicates the off-time of the switching element, when the peak value of the current flowing through the switching element reaches the first reference current value and is smaller than a second reference current value generated according to the current limiting value, the switching power supply is in a second working mode, and the on-time control unit determines the off-time of the switching element according to the third signal.
12. The control system of claim 11, wherein the switching power supply is an isolated switching power supply, and comprises a primary circuit and a secondary circuit, the primary circuit and the secondary circuit are respectively connected to different ground terminals, the switching element and the on-time control unit are both electrically connected to the primary circuit, and the follow current element is electrically connected to the secondary circuit.
13. The control system of claim 11, wherein the on-time control unit comprises:
the on-time generating unit generates a third signal, and the third signal indicates the turn-off time of the switching element;
a current limiting unit receiving the first signal and the second signal and generating a first current limiting signal indicating a timing at which a value of a current flowing through the switching element increases to the current limiting value and a second current limiting signal indicating a timing at which the value of the current flowing through the switching element increases to the first reference current value;
and the second logic circuit receives the first current limiting signal, the second current limiting signal and the third signal, and if the second logic circuit does not receive the first current limiting signal when receiving the third signal, the second logic circuit waits for the first current limiting signal to arrive and instructs the switching element to switch off.
14. The control system according to claim 13, wherein the switching power supply is an isolated switching power supply, and comprises a primary circuit and a secondary circuit, the primary circuit and the secondary circuit are respectively connected to different ground terminals, the switching element, the current limiting unit, and the second logic circuit are electrically connected to the primary circuit, and the on-time control circuit is electrically connected to the secondary circuit.
15. The control system of claim 11, wherein the switching power supply is a non-isolated switching power supply.
16. A switching power supply control method for driving a first switch in a switching power supply to alternately turn on and off to convert an input voltage to an output voltage, the control method comprising:
detecting a current flowing through the first switch;
when the peak value of the current flowing through the first switch is smaller than a fixed first reference current value generated according to a fixed current limiting value, the switching power supply is in a first working mode, under the first working mode, the turn-off moment of the first switch is determined according to the moment when the current flowing through the first switch is increased to the first reference current value, and the first switch is kept on before the turn-off moment comes so as to maintain the peak value of the current flowing through the first switch not to be lower than the first reference current value;
and when the current peak value flowing through the first switch reaches the first reference current value and is smaller than a second reference current value generated according to the current limiting value, the switching power supply is in a second working mode, and the turn-off time of the first switch is determined according to an on-time signal in the second working mode.
17. The control method according to claim 16, wherein when the value of the current flowing through the first switch reaches a second reference current value, the turn-off timing of the first switch is determined according to the timing at which the value of the current flowing through the first switch reaches the second reference current value.
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CN113315380A (en) 2021-08-27

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