CN103151943A - Dual-threshold control system and method for switch power supply - Google Patents
Dual-threshold control system and method for switch power supply Download PDFInfo
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- CN103151943A CN103151943A CN2013101102807A CN201310110280A CN103151943A CN 103151943 A CN103151943 A CN 103151943A CN 2013101102807 A CN2013101102807 A CN 2013101102807A CN 201310110280 A CN201310110280 A CN 201310110280A CN 103151943 A CN103151943 A CN 103151943A
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Abstract
The invention provides a dual-threshold control system and a dual-threshold control method for a switch power supply. The system comprises a rectifying circuit, a converting circuit and a control circuit, wherein the converting circuit is connected with the rectifying circuit, the control circuit is connected with the converting circuit and is used for controlling the output voltage of the converting circuit, and a dual-threshold selector is arranged in the control circuit. According to the dual-threshold control system and the dual-threshold control method for the switch power supply, through the additional arrangement of the dual-threshold selector, the selector can select different output thresholds according to the output load weight of the system. When the system is changed from the light load into the heavy load, the dual-threshold selector can timely regulate the output threshold, further, the switching-on time of a switch tube Q1 is timely regulated, the dynamic response speed of the system is accelerated, the output voltage ripple of the system is reduced, and the transformer noise caused by sudden load changes cannot be heard by people.
Description
Technical field
The present invention relates to the switch power technology field, relate in particular to a kind of dual threshold control system and method for Switching Power Supply.
Background technology
The electricity that electric field sends can not directly use under many circumstances, must carry out the electric energy conversion to it and just can use.For example; use transformer, frequency converter a kind of electric energy of form can be converted to the electric energy of another form; the circuit of realizing this electric energy conversion is exactly Switching Converter Topologies, and wherein, the Switching Converter Topologies with closed-loop control and link of protection is exactly Switching Power Supply.
Figure 1 shows that a kind of traditional pulse frequency modulated (Pulse frequency modulation, PFM) inverse-excitation type AC-DC switching power converters circuit comprises: rectification circuit 300, the translation circuit 400 that is connected with rectification circuit 300 and the control circuit 100 that is used for control change circuit 400 output voltages that is connected with translation circuit.
Wherein, translation circuit 400 comprises: transformer T1, the former limit circuit 401 of transformer T1, the secondary output circuit 402 of transformer T1 and the compole feedback circuit 403 of transformer T1.The former limit circuit 401 of transformer T1 comprises: former limit winding Np, the diode D1 of transformer T1, capacitor C 1, resistance R 1, switching tube Q1, resistance R cs, former limit winding Np and diode D1, capacitor C 1 are connected mutually, capacitor C 1 and resistance R 1 parallel connection, the collector electrode of switching tube Q1 is connected with the positive pole of diode D1, base stage is connected with control circuit 100, emitter is by resistance R cs ground connection, and emitter also is connected with control circuit 100 simultaneously; The secondary output circuit 402 of transformer T1 comprises: the secondary winding Ns of the transformer T1 that is one another in series, diode D2 and capacitor C 2, and the resistance R 2 in parallel with capacitor C 2; The compole feedback circuit 403 of transformer T1 comprises: compole winding Na, diode D3 and the capacitor C 3 of the transformer T1 that is one another in series, and the resistance R 3 that is one another in series and resistance R 4, in resistance R 3 after series connection and the two ends of resistance R 4, one end of resistance R 3 is connected with the positive pole of diode D3, an end ground connection of resistance R 4.Resistance R 3 and the middle tie point f that takes out of resistance R 4 from series connection as signal FB, make it be connected with control circuit 100, and namely signal FB inputs to control circuit 100.
Control circuit 100 comprises: sample circuit 101, error amplifier 102, demagnetization testing circuit 103, index maker 104, comparator 105, comparator 106, trigger 107 and driver 108.In actual applications, generally control circuit 100 can be designed to a control chip.the signal FB that exports in the compole feedback circuit inputs to sample circuit 101 and demagnetization testing circuit 103 simultaneously, the output of sample circuit 101 is connected with an input of error amplifier 101, the output of demagnetization testing circuit is connected with the input of index maker 104, the output of index maker 104 is connected with another input of error amplifier 101, the output of error amplifier 104 is connected with the R input of trigger 107, the output of comparator 106 is connected with the S input of trigger 107, the output of trigger 107 is connected with the input of driver 108, the output of driver 108 is connected on the base stage of switching tube Q1.One input of comparator 106 is connected with the emitter of switching tube Q1, and an other input of comparator 106 is connected and fixed power supply threshold value VCS, for example VCS=0.5V.
The signal FB of compole feedback circuit 403 outputs of transformer T1, it includes the current over-zero time information of diode D2, after demagnetization testing circuit 103 receives signal FB, demagnetization testing circuit 103 can detect the current over-zero moment of diode D2, be exactly the moment that demagnetization finishes, from the current over-zero of diode D2 constantly, index maker 104 can ramp signal Vramp who rises with index in time of output, wherein
, a wherein, b, c are the constant greater than 0, and when transformer T1 demagnetization process began, ramp signal Vramp was resumed and is initial value c.After ramp signal Vramp is input in comparator 105, compare with the signal of error amplifier 102 outputs, if ramp signal Vramp is greater than the signal of error amplifier 102 outputs, obtain an energizing signal, be sent to trigger 107, by driver 108, switching tube Q1 is opened, switching tube Q1 can be a triode again, and it is also MOS transistor.After switching tube Q1 opened, the electric current that flows through transformer T1 former limit winding Np also rose with certain slope, so, voltage CS on resistance R cs is along with rising, when voltage CS reached fixing voltage threshold VCS, comparator 106 upsets were closed switching tube Q1 by trigger 107 and driver 108.After Q1 closed, transformer T1 began again a new demagnetization process, when demagnetization finishes, ramp signal Vramp compares with the signal that error amplifier 102 is exported again, the energizing signal that obtains can be opened switching tube Q1 again, and above process moves in circles, and system keeps output voltage by these actions.
As seen, above-mentioned switching power converters circuit, the output loading of its system is big or small, has directly affected the length of demagnetization time, has also determined the time length of switching tube Q1 remain off.When the output loading of system was larger, it is shorter that the electric current that flows through diode D2 drops to time of zero from peak value, causes the transformer T1 demagnetization time shorter, makes the time of switching tube Q1 remain off also shorter.Otherwise, when the output loading of system hour, it is longer that the electric current that flows through diode D2 drops to time of zero from peak value, causes the transformer T1 demagnetization time longer, makes the time of switching tube Q1 remain off also longer.
Therefore, above-mentioned switching power converters circuit, when the output loading of system when load variations is heavy duty on the lenient side suddenly, during due to underload, the time of switching tube Q1 remain off is longer, when System Sudden changes, the demagnetization testing circuit does not also have enough time to detect the current over-zero moment of diode D2, when system is not able to do in time in time to open switching tube Q1, the output voltage of system has produced decline by a relatively large margin, cause system's output voltage unstable, system is poor for the dynamic responding speed of load variations.Again due to system when underload and the heavy duty, the energy that the transformer secondary output coil receives is the same, therefore, when load changing was to heavy duty on the lenient side, the output voltage fluctuation was larger when system, system's output voltage ripple is also larger at least.In addition, system is when underload, and because the time of switching tube Q1 remain off is longer, switch periods is lower, and such switching frequency can enter people's ear induction range, makes people's ear can hear the noise that transformer sends.
Summary of the invention
For this reason, technical problem to be solved by this invention is: dual threshold control system and method that a kind of Switching Power Supply is provided, make system when being suddenlyd change to heavy duty by underload, the dynamic responding speed of raising system, reduce the output voltage ripple of system, allow people's ear can't hear the transformer noise that produces because of load changing.
so, the invention provides a kind of dual threshold control system of Switching Power Supply, comprise rectification circuit, the translation circuit that is connected with rectification circuit and the control circuit that is used for the control change circuit output voltage that is connected with translation circuit, described translation circuit comprises transformer T1, the former limit circuit of transformer T1, secondary output circuit and compole feedback circuit, compole feedback circuit output comprises the control circuit that feeds back signal to of current over-zero time information in T1 time grade of output circuit of transformer, control circuit detects described zero passage after the moment, the signal that control circuit feeds back to according to the compole feedback circuit is adjusted the output voltage of T1 level output circuit of transformer, wherein, one dual threshold selector is set in control circuit, this dual threshold selector receives two fixed thresholds of different sizes, and select corresponding threshold value and export to the former limit circuit of transformer T1 according to the pulse frequency modulated PFM signal of input, when system's output loading is heavy, pulse frequency modulated PFM signal notice dual threshold selector is selected described larger threshold value, when system's output loading is light, pulse frequency modulated PFM signal notice dual threshold selector is selected described less threshold value.
Wherein, the secondary output circuit of described transformer T1 comprises: the secondary winding Ns of the transformer T1 that is one another in series, diode D2 and capacitor C 2, and the resistance R 2 in parallel with capacitor C 2.
The former limit circuit of described transformer T1 comprises: former limit winding Np, the diode D1 of transformer T1, capacitor C 1, resistance R 1, switching tube Q1, resistance R cs, former limit winding Np and diode D1, capacitor C 1 are connected mutually, capacitor C 1 and resistance R 1 parallel connection, the collector electrode of switching tube Q1 is connected with the positive pole of diode D1, base stage is connected with control circuit, emitter is by resistance R cs ground connection, and emitter also is connected with control circuit simultaneously.
The compole feedback circuit of described transformer T1 comprises: compole winding Na, diode D3 and the capacitor C 3 of the transformer T1 that is one another in series, and the resistance R 3 that is one another in series and resistance R 4, in resistance R 3 after series connection and the two ends of resistance R 4, one end of resistance R 3 is connected with the positive pole of diode D3, one end ground connection of resistance R 4, resistance R 3 and the middle tie point f that takes out of resistance R 4 from series connection as signal FB, input to control circuit.
described control circuit comprises: sample circuit, error amplifier, for detection of current over-zero demagnetization testing circuit constantly in T1 level output circuit of transformer, be used for from the demagnetization testing circuit detect T1 time grade of output circuit current over-zero of transformer constantly, the index maker of the ramp signal that output is risen with index in time, comparator 205, comparator 206, trigger, driver and dual threshold selector, the feedback signal of exporting in described compole feedback circuit inputs to sample circuit and demagnetization testing circuit simultaneously, the output of sample circuit is connected with an input of error amplifier, the output of demagnetization testing circuit is connected with the input of index maker, the output of index maker is connected with another input of comparator 205, the output of comparator 205 is connected with trigger one input, another input of the output of comparator 206 and trigger is connected, the output of trigger is connected with driver input end, driver output end is connected with the base stage of switching tube Q1, one input of comparator 206 is connected with the emitter of switching tube Q1, another input connects the output of dual threshold selector.
described control circuit comprises: sample circuit, error amplifier, for detection of current over-zero demagnetization testing circuit constantly in T1 level output circuit of transformer, be used for from the demagnetization testing circuit detect T1 time grade of output circuit current over-zero of transformer constantly, the index maker of the ramp signal that output is risen with index in time, comparator 205, comparator 206, trigger, driver and dual threshold selector, the feedback signal of exporting in described compole feedback circuit inputs to sample circuit and demagnetization testing circuit simultaneously, the output of sample circuit is connected with an input of error amplifier, the output of demagnetization testing circuit is connected with the input of index maker, the output of index maker is connected with another input of comparator 205, the output of comparator 205 is connected with trigger one input, another input of the output of comparator 206 and trigger is connected, the output of trigger is connected with driver input end, driver output end is connected on the former limit circuit of transformer T1, one input of comparator 206 is connected with the former limit circuit of transformer T1, another input connects the output of dual threshold selector.
Described pulse frequency modulated PFM signal, switching signal frequency dependence connection in its frequency and varying circuit, the demagnetization time correlation connection of the duty ratio of pulse frequency modulated PFM signal and system, switching frequency in varying circuit uprises along with the increasing of load, and the demagnetization time of system diminishes along with the increasing of load.
The present invention also provides a kind of dual threshold control method of Switching Power Supply, it is characterized in that, comprising:
The dual threshold selector receives two fixed thresholds that vary in size, and selects corresponding threshold value and output according to the pulse frequency modulated PFM signal of input;
When system's output loading was heavy, pulse frequency modulated PFM signal notice dual threshold selector was selected described larger threshold value, and when system's output loading was light, pulse frequency modulated PFM signal notice dual threshold selector was selected described less threshold value.
Wherein, described threshold value is determined according to Switching Power Supply application scenario and actual loading situation.
Dual threshold control system and the method for Switching Power Supply of the present invention, by set up the dual threshold selector in the control circuit of Switching Power Supply, make this selector to select different threshold value output according to system's output loading weight, this selector only receives two fixed thresholds of different sizes, when system's output loading is heavy, select larger threshold value, when system's output loading is lighter, select less threshold value.Like this, when system sports heavy duty by underload, the selector of control circuit can in time be adjusted the threshold value of output, and then in time adjusted the time that switching tube Q1 opens, improved the dynamic responding speed of system, reduce the output voltage ripple of system, allow people's ear can't hear the transformer noise that produces because of load changing.
Description of drawings
Fig. 1 is PFM inverse-excitation type AC-DC switching power converters circuit diagram in prior art;
Fig. 2 is the dual threshold control system circuit diagram of the described Switching Power Supply of the embodiment of the present invention.
Embodiment
Below, describe the present invention by reference to the accompanying drawings.
As shown in Figure 2, the present embodiment provides a kind of dual threshold control system of Switching Power Supply, the translation circuit 400 that comprise rectification circuit 300, is connected with rectification circuit and the control circuit 200 that is used for control change circuit 400 output voltage V o that is connected with translation circuit 400.
Wherein, translation circuit 400 comprises: former limit circuit 401, secondary output circuit 402 and the compole feedback circuit 403 of transformer T1, transformer T1, what compole feedback circuit 403 output packets contained the diode D2 current over-zero time information of flowing through in transformer T1 time level output circuit 402 feeds back signal to control circuit 200, the flow through electric current of diode D2, the electric current in T1 the level output circuit of transformer of flowing through exactly.control circuit 200 detects described zero passage after the moment, the current over-zero that is closed to diode D2 from switching tube Q1 is exactly the demagnetization time of transformer T1 constantly, the current value of secondary output circuit is less, the demagnetization time of transformer T1 is longer, correspondingly, control circuit is by a ramp signal, opens switching tube Q1 after waiting for a longer time again, the current value of secondary output circuit is less in other words, and the switching frequency of switching tube Q1 is lower, simultaneously, one dual threshold selector 209 is set in control circuit 200, this dual threshold selector receives two fixed threshold VR1 and the VR2 of different sizes, suppose that VR1 is less than VR2, dual threshold selector 209 is selected corresponding threshold value according to the pulse frequency modulated PFM signal of its input and is exported to the former limit circuit 401 of transformer T1, when system's output loading is heavy, pulse frequency modulated PFM signal notice dual threshold selector 209 is selected described larger threshold value VR2, when system's output loading is light, pulse frequency modulated PFM signal notice dual threshold selector is selected described less threshold value VR1.The output loading of system shown in the present embodiment refers to the output loading of T1 level output circuit of transformer.
Here, of particular note, in the present embodiment, dual threshold selector 209 only can receive and carry out the selection of two fixed thresholds, it selects to have Duoed an alternative condition than the only fixing threshold value in background technology, make this selector to select different threshold value output according to system's output loading weight, when system's output loading was heavy, dual threshold selector 209 was selected larger threshold value VR2, when system's output loading was lighter, the dual threshold selector was selected less threshold value VR1.
Concrete, the secondary output circuit 402 of transformer T1 comprises: the secondary winding Ns of the transformer T1 that is one another in series, diode D2 and capacitor C 2, and the resistance R 2 in parallel with capacitor C 2.
The former limit circuit 401 of transformer T1 comprises: former limit winding Np, the diode D1 of transformer T1, capacitor C 1, resistance R 1, switching tube Q1, resistance R cs, former limit winding Np and diode D1, capacitor C 1 are connected mutually, capacitor C 1 and resistance R 1 parallel connection, the collector electrode of switching tube Q1 is connected with the positive pole of diode D1, base stage is connected with control circuit, emitter is by resistance R cs ground connection, and emitter also is connected with control circuit simultaneously.
The compole feedback circuit 403 of transformer T1 comprises: compole winding Na, diode D3 and the capacitor C 3 of the transformer T1 that is one another in series, and the resistance R 3 that is one another in series and resistance R 4, in resistance R 3 after series connection and the two ends of resistance R 4, one end of resistance R 3 is connected with the positive pole of diode D3, one end ground connection of resistance R 4, resistance R 3 and the middle tie point f that takes out of resistance R 4 from series connection as signal FB, input to control circuit.
The specific implementation principle is: pulse frequency modulated PFM signal, the switching signal frequency dependence connection of its frequency and switching tube Q1, the demagnetization time correlation connection of the duty ratio of pulse frequency modulated PFM signal and system, the switching frequency of switching tube Q1 uprises along with the increasing of load, and the demagnetization time of system diminishes along with the increasing of load.When the output loading of system was heavier, pulse frequency modulated PFM signal made the output VCS of dual threshold selector select the larger threshold value VR2 signal of magnitude of voltage; When the output loading of system was lighter, pulse frequency modulated PFM signal made the output VCS of dual threshold selector select the less threshold value VR1 signal of magnitude of voltage.
The current sensing signal CS of an input connection transformer T1 former limit circuit of comparator 206, another input connects the output of dual threshold selector 209, this output output VCS signal, when switching tube Q1 opens, current sensing signal CS signal rises with certain slope, the larger current sensing signal CS of the electric current level that flows through switching tube Q1 is higher, when current sensing signal CS reaches VCS signal predetermined level value, and the upset of the output level of comparator 206.When current sensing signal CS signal reached VCS predetermined level value, the upset of the output level of comparator 206 was overturn the output of trigger assembly 207 and driver 208, thereby reduces to flow through the electric current of switching tube Q1, even closing switch pipe Q1.
In embodiment of the present invention, according to formula
We can obtain as drawing a conclusion:
1. work as
Hour, less threshold level, the peak current when reducing switching tube Q1 conducting are selected by the dual threshold selector by system
Thereby the switching frequency of raising power switch pipe Q1 makes Switching Power Supply when load is lighter, and switching frequency can not enter people's ear scope, that is: people's ear be can't hear the transformer noise that produces because of load changing;
2. when underload was switched to heavy duty, the switching frequency raising due to switching tube Q1 waited until the time of demagnetizing reduces next time, makes the response speed of system be improved;
3. because the VCS level is lower when the underloading, the peak current during switch conduction
Less, so the energy of transmission of per cycle
, the voltage variety on output capacitance C2 is also just reduced, and has reduced the ripple of output voltage;
As seen, the described system of the present embodiment, when system sports heavy duty by underload, dual threshold selector 209 can in time be adjusted the threshold value of output, and then in time adjusted the time that switching tube Q1 opens, not only improved the dynamic responding speed of system, reduced the output voltage ripple of system, and allowed people's ear can't hear the transformer noise that produces because of load changing.The described dual threshold of the present embodiment is controlled and is compared with other similar techniques means, and its method for designing is succinct, the good stability of system's loop.
The present embodiment also provides a kind of dual threshold control method of Switching Power Supply, comprising:
The dual threshold selector receives two fixed thresholds that vary in size, and selects corresponding threshold value and output according to the pulse frequency modulated PFM signal of input;
When system's output loading was heavy, pulse frequency modulated PFM signal notice dual threshold selector was selected described larger threshold value, and when system's output loading was light, pulse frequency modulated PFM signal notice dual threshold selector was selected described less threshold value.
Wherein, described threshold value is determined according to Switching Power Supply application scenario and actual loading situation.
In sum, dual threshold control system and the method for the described Switching Power Supply of the present embodiment, by set up the dual threshold selector in the control circuit of Switching Power Supply, make this selector to select different threshold value output according to system's output loading weight, this selector only receives two fixed thresholds of different sizes, when system's output loading is heavy, selects larger threshold value, when system's output loading is lighter, select less threshold value.Like this, when system sports heavy duty by underload, the selector of control circuit can in time be adjusted the threshold value of output, and then in time adjusted the time that switching tube Q1 opens, improved the dynamic responding speed of system, reduce the output voltage ripple of system, allow people's ear can't hear the transformer noise that produces because of load changing.
The above is only preferred embodiment of the present invention, and is in order to limit the present invention, within the spirit and principles in the present invention not all, any modification of doing, is equal to replacement, improvement etc., within all should being included in protection scope of the present invention.
Claims (9)
1. the dual threshold control system of a Switching Power Supply, comprise rectification circuit, the translation circuit that is connected with rectification circuit and the control circuit that is used for the control change circuit output voltage that is connected with translation circuit, described translation circuit comprises transformer T1, the former limit circuit of transformer T1, secondary output circuit and compole feedback circuit, compole feedback circuit output comprises the control circuit that feeds back signal to of current over-zero time information in T1 time grade of output circuit of transformer, control circuit detects described zero passage after the moment, the signal that control circuit feeds back to according to the compole feedback circuit is adjusted the output voltage of T1 level output circuit of transformer, it is characterized in that, one dual threshold selector is set in control circuit, this dual threshold selector receives two fixed thresholds of different sizes, and select corresponding threshold value and export to the former limit circuit of transformer T1 according to the pulse frequency modulated PFM signal of input, when system's output loading is heavy, pulse frequency modulated PFM signal notice dual threshold selector is selected described larger threshold value, when system's output loading is light, pulse frequency modulated PFM signal notice dual threshold selector is selected described less threshold value.
2. the dual threshold control system of Switching Power Supply according to claim 1, it is characterized in that, the secondary output circuit of described transformer T1 comprises: the secondary winding Ns of the transformer T1 that is one another in series, diode D2 and capacitor C 2, and the resistance R 2 in parallel with capacitor C 2.
3. the dual threshold control system of Switching Power Supply according to claim 2, it is characterized in that, the former limit circuit of described transformer T1 comprises: former limit winding Np, the diode D1 of transformer T1, capacitor C 1, resistance R 1, switching tube Q1, resistance R cs, former limit winding Np and diode D1, capacitor C 1 are connected mutually, capacitor C 1 and resistance R 1 parallel connection, the collector electrode of switching tube Q1 is connected with the positive pole of diode D1, base stage is connected with control circuit, emitter is by resistance R cs ground connection, and emitter also is connected with control circuit simultaneously.
4. the dual threshold control system of Switching Power Supply according to claim 3, it is characterized in that, the compole feedback circuit of described transformer T1 comprises: compole winding Na, diode D3 and the capacitor C 3 of the transformer T1 that is one another in series, and the resistance R 3 that is one another in series and resistance R 4, in resistance R 3 after series connection and the two ends of resistance R 4, one end of resistance R 3 is connected with the positive pole of diode D3, one end ground connection of resistance R 4, resistance R 3 and the middle tie point f that takes out of resistance R 4 from series connection, as signal FB, input to control circuit.
5. the dual threshold control system of Switching Power Supply according to claim 4, it is characterized in that, described control circuit comprises: sample circuit, error amplifier, for detection of current over-zero demagnetization testing circuit constantly in T1 level output circuit of transformer, be used for from the demagnetization testing circuit detect T1 time grade of output circuit current over-zero of transformer constantly, the index maker of the ramp signal that output is risen with index in time, comparator 205, comparator 206, trigger, driver and dual threshold selector, the feedback signal of exporting in described compole feedback circuit inputs to sample circuit and demagnetization testing circuit simultaneously, the output of sample circuit is connected with an input of error amplifier, the output of demagnetization testing circuit is connected with the input of index maker, the output of index maker is connected with another input of comparator 205, the output of comparator 205 is connected with trigger one input, another input of the output of comparator 206 and trigger is connected, the output of trigger is connected with driver input end, driver output end is connected with the base stage of switching tube Q1, one input of comparator 206 is connected with the emitter of switching tube Q1, another input connects the output of dual threshold selector.
6. the dual threshold control system of Switching Power Supply according to claim 1, it is characterized in that, described control circuit comprises: sample circuit, error amplifier, for detection of current over-zero demagnetization testing circuit constantly in T1 level output circuit of transformer, be used for from the demagnetization testing circuit detect T1 time grade of output circuit current over-zero of transformer constantly, the index maker of the ramp signal that output is risen with index in time, comparator 205, comparator 206, trigger, driver and dual threshold selector, the feedback signal of exporting in described compole feedback circuit inputs to sample circuit and demagnetization testing circuit simultaneously, the output of sample circuit is connected with an input of error amplifier, the output of demagnetization testing circuit is connected with the input of index maker, the output of index maker is connected with another input of comparator 205, the output of comparator 205 is connected with trigger one input, another input of the output of comparator 206 and trigger is connected, the output of trigger is connected with driver input end, driver output end is connected on the former limit circuit of transformer T1, one input of comparator 206 is connected with the former limit circuit of transformer T1, another input connects the output of dual threshold selector.
7. the dual threshold control system of the described Switching Power Supply of according to claim 1 to 6 any one, it is characterized in that, described pulse frequency modulated PFM signal, switching signal frequency dependence connection in its frequency and varying circuit, the demagnetization time correlation connection of the duty ratio of pulse frequency modulated PFM signal and system, switching frequency in varying circuit uprises along with the increasing of load, and the demagnetization time of system diminishes along with the increasing of load.
8. the dual threshold control method of a Switching Power Supply, is characterized in that, comprising:
The dual threshold selector receives two fixed thresholds that vary in size, and selects corresponding threshold value and output according to the pulse frequency modulated PFM signal of input;
When system's output loading was heavy, pulse frequency modulated PFM signal notice dual threshold selector was selected described larger threshold value, and when system's output loading was light, pulse frequency modulated PFM signal notice dual threshold selector was selected described less threshold value.
9. method according to claim 8, is characterized in that, described threshold value is determined according to Switching Power Supply application scenario and actual loading situation.
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