CN101207333A - Switch power supply and control method with awaiting orders mode thereof - Google Patents

Abstract

The invention discloses a standby-mode controlling method of a switching power source; when the outputted voltage Vct1 of a feedback circuit is detected to be decreased from more than V3 to be less than V1, a pulse controlling signal is stopped outputting; when Vct1 is detected to be increased from less than V1 to be more than V2, a signal PG1 is outputted and the time-counting function is started; when Vct1 is detected to be increased again from less than V2 to be V2, the time-counting function is stopped; if the counted time is less than T1, a signal PG2 is outputted and the time-counting function is again started; the duty ratio of the signal PG2 is more than that of the PG1; if the counted time is more than T1, a signal PG3 is outputted and the time-counting function is again started; the duty ratio of the signal PG3 is less than that of the PG1; the voltage V3 is more than V2 that is more than V1. The invention also discloses the switching power source. The invention can reduce the power loss of the switching power source under the condition of the light load, and can control the noise under the standby mode within a given range.

Description

A kind of Switching Power Supply and control method with awaiting orders mode thereof

Technical field

The present invention relates to switch power technology, particularly a kind of Switching Power Supply and control method with awaiting orders mode thereof.

Background technology

Switching Power Supply can change into one or more direct current output to a power supply expeditiously, and the voltage of these out-put supplies can be greater than or less than former supply voltage.Switching Power Supply generally is used for the power supply of electronic equipment, particularly battery supply set, for example, and mobile phone, notebook computer or the like.These equipment have normal mode of operation and the mode of operation of awaiting orders, and need more energy at these equipment of normal mode of operation work, and under the mode of operation of awaiting orders, need less relatively energy.

Under the mode of operation of awaiting orders, though some electronic equipment does not need power supply that a lot of energy is provided, need the noise of supply voltage can be controlled at known among a small circle in, introduce noise with the exempt from customs examination electronic equipment.When electronic device works during at standby, traditional Switching Power Supply is by reducing output voltage or utilizing the accessory power supply power supply to reach the purpose that reduces power consumption.Though this Switching Power Supply is not introduced noise under the mode of operation of awaiting orders, increased the cost of Switching Power Supply.

Summary of the invention

The objective of the invention is defective at prior art, a kind of Switching Power Supply and control method with awaiting orders mode thereof are provided, this Switching Power Supply can enter the mode of operation of awaiting orders automatically according to the situation of load, and can keep output voltage on certain level, the noise that produces within the specific limits.

Technical scheme of the present invention is: a kind of control method with awaiting orders mode of Switching Power Supply, comprise step: the output voltage V ct1 to the feedback circuit of Switching Power Supply samples, detect when voltage Vct1 when dropping to greater than the 3rd reference voltage V3 less than the first reference voltage V1, stop to export the pulse control signal that is used for the control switch tube grid; Detect when voltage Vct1 when rising to less than the first reference voltage V1 greater than the second reference voltage V2, export the first pulse control signal PG1, and the startup clocking capability; Detect when voltage Vct1 once more when rising to the second reference voltage V2 less than the second reference voltage V2, stop clocking capability, if the comparison timing gained time exports the second pulse control signal PG2, and restarts clocking capability less than first with reference to T1 time of delay; The duty ratio of the described second pulse control signal PG2 is greater than the duty ratio of PG1; If the timing gained time exports the 3rd pulse control signal PG3, and restarts clocking capability greater than first with reference to T1 time of delay; The duty ratio of described the 3rd pulse control signal PG3 is less than the duty ratio of PG1; And, voltage V3＞V2＞V1.

Concrete, described pulse control signal PG1, PG2 and PG3 are the pulse signals with fixed pulse number.

The inventive method can further comprise step: detect when voltage Vct1 rises to greater than the 3rd reference voltage V3, according to the size output consecutive pulses control signal of described voltage Vct1.

Further, when circuit reached dynamic equilibrium, the time interval between the pulse control signal of fixed number can intersect circulation between T and T '; Predetermined time of delay, Ts was greater than time T, less than time T '.And when circuit reached dynamic equilibrium, the duty ratio size of pulse control signal was by underloaded size decision.

The present invention also provides a kind of Switching Power Supply, comprises transformer, switching tube, feedback circuit and pwm control circuit; The drain electrode of described switching tube links to each other with the different name end of the former limit winding of transformer, its source ground, and its grid links to each other with the output of pwm control circuit; The input of feedback circuit links to each other with the output of transformer secondary circuit, and its output links to each other with the feedback voltage sampling end of pwm control circuit; Described pwm control circuit comprises that standby control circuit and PWM produce circuit; The input of described standby control circuit links to each other with the output of clock signal, first reference voltage, second reference voltage, the 3rd reference voltage and feedback circuit respectively; Described PWM produces in the circuit, and the input of current source links to each other with the former limit winding of transformer, and its output links to each other with the output of feedback circuit and an end of switch by two diodes respectively; The other end of described switch links to each other with second positive input of PWM comparator, and its control end links to each other with the 3rd output of standby control circuit; First positive input of described PWM comparator links to each other with second output of standby control circuit, its reverse input end is by a grounding through resistance, as the PS end of standby control circuit, its output links to each other with the R end of rest-set flip-flop by a not gate simultaneously; The S end of described rest-set flip-flop links to each other with first output of standby control circuit and the first input end of first NAND gate, and its inverse output terminal links to each other with second input of first NOR gate; The output of first NOR gate links to each other with gate driver circuit, and the output of gate driver circuit links to each other with the grid of switching tube as the output that PWM produces circuit.

Described standby control circuit comprises that feedback voltage testing circuit, timing circuit, reference voltage control circuit, decoding circuit, generating circuit from reference voltage, control impuls produce circuit and fixed number of times pulse-generating circuit; The input of described feedback voltage testing circuit links to each other with the output of first reference voltage, second reference voltage, the 3rd reference voltage and feedback circuit respectively, its first output signal and clock signal are after NAND gate, with the output signal of fixed number of times pulse-generating circuit through or behind the door from first output output of standby control circuit, its second output links to each other with the input that control impuls produces circuit, and its 3rd output is as the 3rd output of standby control circuit; First output that control impuls produces circuit links to each other with the fixed number of times pulse-generating circuit, and its second output links to each other with the input of timing circuit, and its 3rd output links to each other with the first input end of reference voltage control circuit; Second output signal warp and second input that outputs to reference voltage control circuit behind the door of the output signal of described timing circuit and feedback voltage testing circuit; Described reference voltage control circuit, decoding circuit and generating circuit from reference voltage are connected in series, and the output of described generating circuit from reference voltage is as second output of standby control circuit.

Described feedback voltage testing circuit comprises first comparator, second comparator, first not gate and first and door; First positive input of first comparator links to each other with the 3rd reference voltage, its second positive input links to each other with first reference voltage, its reverse input end links to each other with the output of feedback circuit, its output is as first output of feedback voltage testing circuit, and is non-behind the door as the 3rd output of feedback voltage testing circuit through first simultaneously; The positive input of second comparator links to each other with the output of feedback circuit, and its reverse input end links to each other with second reference voltage, the output signal of its output signal and described not gate as first with the door input signal; First with the output of door as second output of feedback voltage testing circuit.

Further, circuit of the present invention also can comprise start-up circuit, and its output links to each other with the dc power supply terminal of pwm control circuit, and links to each other with the end of the same name of the former limit winding of transformer by a current-limiting resistance.

Among the present invention, described switching tube can adopt power field effect pipe.

Beneficial effect of the present invention is: can reduce the power loss of Switching Power Supply under light load condition, and can be with Switching Power Supply in the noise under the standby is controlled at certain scope.

Description of drawings

Fig. 1 is the circuit structure schematic diagram of a kind of Switching Power Supply of the present invention.

Fig. 2 is the circuit structure schematic diagram of pwm control circuit among the present invention.

Fig. 3 is the control signal sequential chart of pwm control circuit among the present invention.

Embodiment

The present invention is further elaborated with specific embodiment with reference to the accompanying drawings below.

As shown in Figure 1, Switching Power Supply of the present invention is an isolation type switching power supply, can comprisetransformer 110,switching tube 116,feedback circuit 114,pwm control circuit 115 and start-up circuit.Wherein, theend points 101 that links to each other withtransformer 110 former limits end of the same name for alternating current by the direct current behind the bridge rectifier.What link to each other withend points 101 in other embodiments, also can be direct voltage source.The drain electrode ofswitching tube 116 links to each other with the different name end of the former limit winding oftransformer 110, and its source electrode is by a grounding through resistance, and its grid links to each other with the output of pwm control circuit 115.The input offeedback circuit 114 links to each other with theoutput 111 oftransformer 110 secondary circuits, and its output links to each other with the feedback voltage sampling end ofpwm control circuit 115, is used for output voltage V out is sampled, and produces feedback voltage V ct1.Pwm control circuit 115 is used for according to feedback voltage V ct1 the duty ratio of PWM ripple being modulated, and comes the conducting state of by-pass cock pipe 116, is stabilized on certain level to guarantee output voltage V out.In this preferred embodiment, switchingtube 116 adopts power field effect pipe.

Among the present invention,pwm control circuit 115 has two mode of operations: start-up mode and normal mode of operation.Under start-up mode, the operating current ofpwm control circuit 115 very little (can reduce start-up time, improve the stability of system), and under normal mode of operation, its operating current is bigger.

The condition that enters different mode is described: in a. system starting process,pwm control circuit 115 can detect the voltage (VCC) of itspower ends 104, and as VCC during less than a reference voltage,pwm control circuit 115 can be operated in start-up mode; B. when the voltage of VCC during greater than a reference voltage, withswitching tube 116 closures,pwm control circuit 115 enters normal mode of operation topwm control circuit 115 afterwards by its output (GDV) output control signal.The control method of Switching Power Supply standby of the present invention is carried out afterpwm control circuit 115 enters normal mode of operation.

In order to startpwm control circuit 115, Switching Power Supply of the present invention is equipped with a start-up circuit, comprises theauxiliary winding 112,charging diode 109,charging resistor 108 and the start-up circuitelectric capacity 103 that are wrapped on the transformer 110.One end of start-up circuitelectric capacity 103 links to each other withpwm control circuit 115 and current-limitingresistance 102 as the output of start-up circuit, links to each other its other end ground connection with an end ofresistance 108 simultaneously.The DC input voitage of 101 ends is charged by 102 pairs of start-up circuitelectric capacity 103 of current-limiting resistance.The negative electrode ofdiode 109 links to each other with the other end ofresistance 108, and its anode links to each other with the different name end of auxiliary winding 112.The end ground connection of the same name of auxiliary winding 112.Wherein, start-up circuitelectric capacity 103 can be composed in parallel by two or more electric capacity.Start-up circuit is used topwm control circuit 115 that initial power is provided.In the power initiation process, for the power supply ofpwm control circuit 115 is born by start-up circuit fully.

As shown in Figure 2,pwm control circuit 115 comprises thatstandby control circuit 300 and PWM produce circuit 200.Wherein, the standby control circuit comprises that feedbackvoltage testing circuit 301,timing circuit 302, referencevoltage control circuit 303,decoding circuit 304, generating circuit fromreference voltage 305, control impuls producecircuit 307 and fixed number of times pulse-generating circuit 306.

Feedbackvoltage testing circuit 301 comprises first comparator 308 (adopt stagnate whirl comparator),second comparator 309, first not gate 310 and first and 311.First positive input offirst comparator 308 links to each other with the 3rd reference voltage source V3, its second positive input links to each other with the first reference voltage source V1, its reverse input end links to each other with the output offeedback circuit 114, its output is as an output of feedbackvoltage testing circuit 301, its output signal and clock signal clk after NAND gate with the output signal of fixed number of times pulse-generatingcircuit 306 input one or door, then from first output, 312 outputs of standby control circuit 300.The output offirst comparator 308 links to each other with the input of first not gate 310 simultaneously.The output of first not gate 310 links to each other with an input ofdoor 311 with first as the3rd output 314 ofstandby control circuit 300 simultaneously.The positive input ofsecond comparator 309 links to each other with the output offeedback circuit 114, and its reverse input end links to each other with the second reference voltage source V2, and its output links to each other with another input ofdoor 311 with first.First withdoor 311 output another output as the feedback voltage testing circuit, the input that producescircuit 307 with control impuls links to each other, the output signal of its output signal andtiming circuit 302 through one with an input that outputs to referencevoltage control circuit 303 behind the door.First output that control impuls producescircuit 307 links to each other with the input of fixed number of times pulse-generating circuit 306, its second output links to each other with the input oftiming circuit 302, and its 3rd output links to each other with another input of reference voltage control circuit 303.Referencevoltage control circuit 303,decoding circuit 304 and generating circuit fromreference voltage 305 are connected in series, and the output of generating circuit fromreference voltage 305 is assecond output 312 ofstandby control circuit 300.

PWM producescircuit 200 and mainly comprisesPWM comparator 201, rest-set flip-flop 204 and gate driver circuit 202.Wherein, thepower end 104 ofPWM generation circuit 200 links to each other with the input of current source 205.The output ofcurrent source 205 links to each other with the output offeedback circuit 114 and an end of resistance R 1 by two diodes respectively.The other end of resistance R 1 links to each other with an end of switch S 1 and an end of resistance R 2.The other end ground connection of resistance R 2.The other end of switch S 1 links to each other with second positive input ofPWM comparator 201, and its control end links to each other with the3rd output 314 of standby control circuit 300.First positive input ofPWM comparator 201 links to each other withsecond output 313 ofstandby control circuit 300, and its reverse input end produces the earth terminal (as Ps among the figure) ofcircuit 200 by a grounding through resistance as PWM.The Ps end also links to each other with the source electrode of switching tube 116.The output ofPWM comparator 201 links to each other with the R end of rest-set flip-flop 204 by a not gate.The S end of rest-set flip-flop 204 links to each other withfirst output 312 ofstandby control circuit 300 and the first input end of a NOR gate, and its inverse output terminal links to each other with second input of described NOR gate.The output of described NOR gate links to each other withgate driver circuit 202, and the output ofgate driver circuit 202 links to each other with the grid ofswitching tube 116 as the output that PWM produces circuit.

Among the present invention,standby control circuit 300 is by detecting feedback voltage V ct1, and the state of control switch S1 enters or withdraw from standby thereby control PWM produces circuit 200.Shown in Figure 3 is the sequential chart ofstandby control circuit 300 control signals, and wherein, V1, V2 and V3 are reference voltage source, its voltage V3＞V2＞V1.Vct1 is a feedback voltage, and Vn is the PWM ripple reference voltage in the normal operating conditions, also is the input signal of second positive input ofPWM comparator 201, and this voltage changes along with the variation of feedback voltage V ct1.Vs is the reference voltage source under the operating state of awaiting orders, it is the output signal ofsecond output 313 ofstandby control circuit 300, this voltage has predetermined n step voltage level, and voltage levvl can change along with the comparative result of time of delay (being the output of timing circuit 302) and scheduled delay (promptly first with 311 output).PG is the output signal ofgate driver circuit 202, it is the input signal ofswitching tube 116 grids, in normal operating conditions, PG is a square wave, its duty ratio changes with the variation of feedback voltage V ct1, in the mode of operation of awaiting orders, PG is the square wave of fixed pulse number, and its duty ratio can change according to the variation of voltage Vs.

In embodiment illustrated in fig. 2,301 couples of feedback voltage V ct1 of feedback voltage testing circuit detect, the testing result signal produces the input signal ofcircuit 307 as control impuls, control impuls produces pulse signal CP1, CP2 and the CP3 of three non-overlapping copies ofcircuit 307 corresponding generations, respectively as the triggering signal with reference tovoltage control circuit 303,timing circuit 302 and fixed number of times pulse-generating circuit 306.Scheduled delay and PWM ripple compare time of delay, and the signal of generation is as the control signal of referencevoltage control circuit 303, and decision reference voltage Vs improves a level or reduces a level.The signal decoding thatdecoding circuit 304 produces referencevoltage control circuit 303 becomes the output signal of generating circuit fromreference voltage 305, thereby produces reference voltage Vs.

To sum up, the control method with awaiting orders mode of Switching Power Supply comprises step among the present invention:

S1,standby control circuit 300 detects feedback voltage V ct1 in real time, drop to (T1 stage in as Fig. 3) the V1 process below horizontal from level at feedback voltage V ct1 greater than V3,pwm control circuit 115 work in normal operation, switch S 1 closure, the duty ratio of the pulse control signal PG ofgate driver circuit 202 outputs changes along with the variation of Vct1.

S2, detect, when feedback voltage V ct1 drops to below the V1, the 3rd output output control signal ofstandby control circuit 300, control switch S1 disconnects, and PWM producescircuit 200 and quits work, and no longer is switchingtube 116 output pulse control signals, that is:pwm control circuit 115 withdraws from normal operating conditions and enters cut-off state, as the T2 stage among Fig. 3.Output voltage V out reduces along with the discharge of load, and feedback voltage V ct1 increases.

S3, detect, when Vct1 rises to greater than V2,pwm control circuit 115 enters armed state (as the T3 stage among Fig. 3), control impuls producescircuit 307 output pulse signal CP1, CP2 and CP3, CP2 starts clocking capability, and timing is carried out in the delay of PWM ripple, and CP1 and CP3 signal control finally thatgate driver circuit 202 output is a string to have the pulse control signal of fixed number to switchingtube 116.

Under the effect of the PWM of this fixed number of times ripple, output voltage V out increases thereupon, and feedback voltage V ct1 begins to descend.After above-mentioned pulse control signal (being the PWM ripple) with fixed number stopped, output voltage V out descended along with the discharge of load again, and when output voltage was lower than desired value in thefeedback circuit 114, feedback voltage V ct1 began to rise.

S4 detects, and when Vct1 rises to greater than V2 once more, stops clocking capability, and the timing number that timing is obtained and predetermined time of delay output of door 311 (first with) compare.If the time of timer is less than predetermined time of delay, the output voltage V s of generating circuit fromreference voltage 305 can increase a level, and start fixed number of times pulse-generatingcircuit 306 simultaneously, with the duty ratio of pwm signal output, that have fixed number that increases gate driver circuit 202.If the time of timer is greater than predetermined time of delay, the output voltage V s of generating circuit fromreference voltage 305 can reduce a level, and start fixed number of times pulse-generatingcircuit 306 simultaneously, with the duty ratio of pwm signal output, that have fixed number that reduces gate driver circuit 202.Start clocking capability simultaneously.As the T4 stage among Fig. 3.

Under the effect of the PWM of this fixed number of times ripple, output voltage V out increases thereupon, and feedback voltage V ct1 begins to descend.After above-mentioned pulse control signal (being the PWM ripple) with fixed number stopped, output voltage V out descended along with the discharge of load again, and when output voltage was lower than desired value in thefeedback circuit 114, feedback voltage V ct1 began to rise.

S5, repeating step S4.After several circulations, whole system will reach a dynamic balance.As shown in Figure 3, will circulate the time of delay of PWM ripple at T4 and T5 intersection, and T4＜Ts＜T5.Wherein Ts is predetermined time of delay.

S6, detect, when load increases, feedback voltage V ct1 will increase and exceed the voltage levvl of V3,pwm control circuit 115 withdraws from standby, enter normal operating conditions (as Fig. 3 T7 stage),gate driver circuit 202 output consecutive pulses control signal PG, and its duty ratio changes along with the variation of Vct1.

Among the present invention,feedback circuit 114,timing circuit 302, referencevoltage control circuit 303,decoding circuit 304, generating circuit fromreference voltage 305, control impuls producecircuit 307, fixed number of times pulse-generatingcircuit 306 and gate driver circuit and adopt circuit structure commonly used at present to get final product, so not at this detailed description.

Though exemplary embodiment of the subject disclosure has been done concrete description here, but concerning those skilled in the technology concerned, it is apparent that, without departing from the scope of the invention, can carry out various modifications, increase, various variations such as replacement, therefore, the present invention should not be limited to disclosed specific embodiments, and the present invention should comprise all embodiments that fall into the claim scope.

Claims (10)

1. the control method with awaiting orders mode of a Switching Power Supply is characterized in that, comprises step:

Output voltage V ct1 to the feedback circuit of Switching Power Supply samples, detect when voltage Vct1 when dropping to greater than the 3rd reference voltage V3 less than the first reference voltage V1, stop to export the pulse control signal that is used for the control switch tube grid;

Detect when voltage Vct1 when rising to less than the first reference voltage V1 greater than the second reference voltage V2, export the first pulse control signal PG1, and the startup clocking capability;

Detect when voltage Vct1 once more when rising to the second reference voltage V2 less than the second reference voltage V2, stop clocking capability, if the comparison timing gained time exports the second pulse control signal PG2, and restarts clocking capability less than first with reference to T1 time of delay; The duty ratio of the described second pulse control signal PG2 is greater than the duty ratio of PG1; If the timing gained time exports the 3rd pulse control signal PG3, and restarts clocking capability greater than first with reference to T1 time of delay; The duty ratio of described the 3rd pulse control signal PG3 is less than the duty ratio of PG1; And, voltage V3＞V2＞V1.

2. method according to claim 1 is characterized in that: described pulse control signal PG1, PG2 and PG3 are the pulse signals with fixed pulse number.

3. method according to claim 1 is characterized in that, further comprises step: detect when voltage Vct1 rises to greater than the 3rd reference voltage V3, according to the size output consecutive pulses control signal of described voltage Vct1.

4. method according to claim 1 is characterized in that: when circuit reached dynamic equilibrium, the time interval between the pulse control signal of fixed number intersected circulation between T and T '; Predetermined time of delay, Ts was greater than time T, less than time T '.

5. method according to claim 1 is characterized in that: when circuit reached dynamic equilibrium, the duty ratio size of pulse control signal was by underloaded size decision.

6. a Switching Power Supply comprises transformer, switching tube, feedback circuit and pwm control circuit; The drain electrode of described switching tube links to each other with the different name end of the former limit winding of transformer, its source ground, and its grid links to each other with the output of pwm control circuit; The input of feedback circuit links to each other with the output of transformer secondary circuit, and its output links to each other with the feedback voltage sampling end of pwm control circuit; It is characterized in that:

Described pwm control circuit comprises that standby control circuit and PWM produce circuit; The input of described standby control circuit links to each other with the output of clock signal, first reference voltage, second reference voltage, the 3rd reference voltage and feedback circuit respectively; Described PWM produces in the circuit, and the input of current source links to each other with VCC, and its output links to each other with the output of feedback circuit and an end of switch by two diodes respectively; The other end of described switch links to each other with second positive input of PWM comparator, and its control end links to each other with the 3rd output of standby control circuit; First positive input of described PWM comparator links to each other with second output of standby control circuit, its reverse input end is by a grounding through resistance, as the PS end of standby control circuit, its output links to each other with the R end of rest-set flip-flop by a not gate simultaneously; The S end of described rest-set flip-flop links to each other with first output of standby control circuit and the first input end of first NAND gate, and its inverse output terminal links to each other with second input of first NOR gate; The output of first NOR gate links to each other with gate driver circuit, and the output of gate driver circuit links to each other with the grid of switching tube as the output that PWM produces circuit.

7. according to right 6 described Switching Power Supplies, it is characterized in that: described standby control circuit comprises that feedback voltage testing circuit, timing circuit, reference voltage control circuit, decoding circuit, generating circuit from reference voltage, control impuls produce circuit and fixed number of times pulse-generating circuit; The input of described feedback voltage testing circuit links to each other with the output of first reference voltage, second reference voltage, the 3rd reference voltage and feedback circuit respectively, its first output signal and clock signal are after NAND gate, with the output signal of fixed number of times pulse-generating circuit through or behind the door from first output output of standby control circuit, its second output links to each other with the input that control impuls produces circuit, and its 3rd output is as the 3rd output of standby control circuit; First output that control impuls produces circuit links to each other with the fixed number of times pulse-generating circuit, and its second output links to each other with the input of timing circuit, and its 3rd output links to each other with the first input end of reference voltage control circuit; Second output signal warp and second input that outputs to reference voltage control circuit behind the door of the output signal of described timing circuit and feedback voltage testing circuit; Described reference voltage control circuit, decoding circuit and generating circuit from reference voltage are connected in series, and the output of described generating circuit from reference voltage is as second output of standby control circuit.

8. Switching Power Supply according to claim 7 is characterized in that: described feedback voltage testing circuit comprises first comparator, second comparator, first not gate and first and door; First positive input of first comparator links to each other with the 3rd reference voltage, its second positive input links to each other with first reference voltage, its reverse input end links to each other with the output of feedback circuit, its output is as first output of feedback voltage testing circuit, and is non-behind the door as the 3rd output of feedback voltage testing circuit through first simultaneously; The positive input of second comparator links to each other with the output of feedback circuit, and its reverse input end links to each other with second reference voltage, the output signal of its output signal and described not gate as first with the door input signal; First with the output of door as second output of feedback voltage testing circuit.

9. according to claim 6 or 7 or 8 described Switching Power Supplies, it is characterized in that: also comprise start-up circuit, its output links to each other with the dc power supply terminal of pwm control circuit, and links to each other with the end of the same name of the former limit winding of transformer by a current-limiting resistance.

10. Switching Power Supply according to claim 6 is characterized in that: described switching tube is a power field effect pipe.

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