CN1032719C - Logarithmic curve soft start circuit for switching power supply - Google Patents

Abstract

The present invention relates to an exponential curve type soft start circuit for a switching power supply of a pulse width modulation type, which is realized by connecting an electronic circuit of which the voltage rises according to an exponential curve rule after a voltage is supplied at the inphase input end of an error amplifier of a pulse width modulation (PWM) integrated device. The exponential curve type soft start circuit has the advantages of simple circuit design and less elements, and solves the problem of heavy current impact when the switching power supply is started. The exponential curve type soft start circuit is suitable for both an SG3525A chip and other switching power supplies of a pulse width modulation type.

Description

The logarithmic curve soft start circuit of Switching Power Supply

The present invention relates to a kind of soft starting circuit of Switching Power Supply, exactly, relate to a kind of logarithmic curve soft start circuit that is used for the PWM-type high frequency switch power, belong to the electronic circuit category in the power technique fields.

The modernization power supply has all been realized HF switchization, and this power supply is because can save Industrial Frequency Transformer and huge heating radiator, and volume and weight all greatly reduces; And because switching tube is in the saturation region interrupter duty, its merit hair is much smaller than linear power supply, so efficient is also high, obtains application widely in various electronic equipments, wherein has more technical advantage with the pulse-width modulation type Switching Power Supply especially.The LC filtering circuit is all adopted in the voltage stabilizing of this power supply and filtering usually, because the output voltage of power supply all is taken from the electric capacity two ends of filtering circuit, this will cause the heavy current impact state that powers up capacitance short-circuit when starting.The heavy current pulse that this duration is very short can make and bear the very poor semiconductor devices puncture inefficacy of momentary overload capacity.So people study the heavy current impact problem when overcoming start as possible." the electric capacity----constant current source that extensively adopts in now general in the world width modulation (PWM) the special IC device " soft starting mode addresses this problem.

But in the present PWM integrated circuit, soft start (claiming again: the start slowly) time can only reach several seconds zero point at most, and this just is difficult to adapt to the requirement of high power load.In fact, the heavy current impact problem solution still not yet in effect during the starting up.For example, the peripheral hardware soft start capacitor maximum of width modulation (PWM) integrated circuit (IC) chip SG3525A (its circuit block diagram is referring to Fig. 2) only allows to use to 10 μ F, like this, its slow start-up time can only be deferred to Tss=10 μ F/50 μ A=0.2S at most, for the load of time constant, just heavy current impact can't have been solved greater than 0.2 second.And, above-mentioned data or no-load condition, in case be with load, soft start function also will weaken.At this moment shown in Figure 1A and Figure 1B, the time shaft after abscissa is represented to start shooting among the figure, ordinate represent to flow through in the machine strength of current (figure A) and the output voltage (figure B) of power switch VMOS pipe respectively respectively for starting current, output voltage characteristic.Can be found out that by Figure 1A starting current is very big, be a kind of strong current pulse that is similar to short circuit, though the time is very short, destructiveness is quite big.This also is the big defective in the existing switch power technology.

The purpose of this invention is to provide a kind of logarithmic curve soft start circuit that is used for the pulse-width modulation type Switching Power Supply, make and start moment, the voltage climbing speed is by 0 volt of beginning, and quickening gradually, heavy current impact problem when powering up startup to overcome existing in prior technology.

The present invention is achieved in that it includes width modulation (PWM) integrated circuit (IC) chip and peripheral circuit element thereof, is characterized in that: insert one at the in-phase input end of the error amplifier of PWM integrated device and powering up its voltage of back rule electronic circuit of rising and changing exponentially.

The novel uniqueness of circuit design of the present invention, ingenious practicality, circuit structure is simple, and element is few, and principle of work is clear.Index curve formula of the present invention starts rule and reaches tens of seconds more than soft-start time, is that existing various " submissive " startup method institute is irrealizable.Outside the SG3525A chip that it not only is adapted to the present invention and is exemplified, also can be widely used in all universal pulsewidth modulated intergrated circuit chips and pulse-width modulation type Switching Power Supply, and can both reach desirable startup protection effect.The present invention provides a kind of fruitful technical measures for the startup safety of protection power supply self.

Below in conjunction with accompanying drawing, the present invention is specifically addressed.

Fig. 1 is the starting characteristic curve map of existing Switching Power Supply.(figure A is that current characteristics, figure B are voltage characteristics).

Fig. 2 is a PWM integrated device SG3525A chip block diagram.

Fig. 3 is an electrical schematic diagram of the present invention.

Fig. 4 is a soft-start feature curve map of the present invention.(figure A is a current characteristics, and figure B is a voltage characteristic).

Fig. 2 is the inner structure block diagram of PWM integrated device SG3525A chip, and its frame numeral of circle in addition is the Pin locations numbering: 1. anti-phase input, 2. homophase input, 3. synchronous, 5.CT, 6.RT, 7. discharge, 8. submissive startup, 9. compensation, 10. close 11. output A, 12. ground, 13.Vc, 14. output B, 15.Wi, 16.VREF (DC source of stable pressure output terminal).Can show by figure: between 15 pin Vi and 12 pin, add that supply voltage is (after 8～40V), as long as 5,6 pin are connected resistance, the electric capacity of decision frequency, connect dead band (place of safety) resistance between 7 pin and 5 pin, just can obtain two pulse-width signals of phase phasic difference semiperiod at output terminal 11,14 pin, can remove the driving power switching tube, the total output voltage of forming by SG3525 of power supply, after dividing potential drop, feed back to " 1 " pin, work as feedback signal for width modulation, automatically regulate output voltage, to reach the purpose of voltage stabilizing.Simultaneously, also can connect a little electrolytic condenser on 8 pin, output voltage is slowly set up, reach submissive starting, the phenomenon of the heavy current impact that reduces to start shooting, this also is the fairly simple application process of existing PWM formula Switching Power Supply.

According to inventor's experimental verification, for the Switching Power Supply below thousands of watts, no matter its load is that pure resistive also right and wrong are resistive, when slow start-up time is taken at 2～3 seconds, just can effectively prevent the generation of heavy current impact phenomenon; And in the initial moment that powers up, the speed that rises with electric current, voltage is good more more slowly.The soft start measure that the present invention takes the index curve formula to power up just in view of the above, feasible start is added in the output filter capacitor two ends when powering up voltage is by initial 0 volt and slowly increase, rather than increase suddenly, so can adapt to the characteristics that the response time is grown when exporting big capacitive accumulation electric charge, its voltage is set up at leisure, can be to not producing dash current in the machine.Then the output voltage climbing speed is accelerated rapidly, does not cause to drag start-up time oversizely.

Fig. 3 is an electrical schematic diagram of the present invention, and the IC1 here is typical PWM device SG3525A.Principal character of the present invention is to send into a reference voltage that changes by index law at the error amplifier in-phase input end (2 pin) of PWM device when start, output terminal (11, the 14 pin) output pulse width of device is 0 when making start, and then rule output pulse broadens gradually exponentially, the output voltage Changing Pattern that makes general supply is " rise slowly earlier, then rise soon ", can obviously improve the carrying load ability of whole power supply.

This logarithmic curve soft start circuit is made of resistance R 1～R6, capacitor C and transistor V.As shown in Figure 3: three resistance R 1, R2, R3 be serially connected with in proper order power supply Vi (15 pin---SG3525A) and between the ground, the two ends shunt capacitance C1 of R2 wherein; The base stage of transistor V is connected between resistance R 2 and the R3, grounded emitter, its collector be connected to VREF (16 pin----SG3525A) with ground between mutually between two resistance R 4 and R5 of serial connection, (2 pin----are SG3525A) to be connected to the error amplifier in-phase input end of PWM chip by resistance R 6 again.Start shooting moment, capacitor C is with the R2 short circuit, resistance R 1 provides a critical saturated electric current to transistor V, make V1 saturated, then 2 pin of the error amplifier in the integrated device are positioned at 0 volt, subsequently, " R1-C1-V is in parallel with R3 " forms a branch road to capacitor C 1 charging, and it is fixed that its charging rate was selected by the output rise time.Like this, the output voltage of transistor V just rises along with the charging of C1, in the time of setting for example 4～10 seconds, after output voltage reaches R4, the R5 series connection dividing potential drop numerical value to source of stable pressure (VREF, 16 pin), the variation of transistor V just no longer influences circuit, circuit enters the stable region, thereby has finished soft start-up process.For the output of general supply, then be when beginning output voltage rise very slowly, accelerate gradually afterwards, and output voltage reaches design value in the time of setting, this voltage rising rule is exactly the index curve formula.Two figure of Fig. 4 are respectively the current curve and the voltage curve rising synoptic diagram of soft-start feature of the present invention.The selection of R3 should be when the C1 charging termination, and the pressure drop of the dividing potential drop result of R1～R3 on R3 ends transistor V.

The present invention test of many times implements, and has obtained satisfied technique effect.After adopting the present invention, soft-start time was more than 30 seconds, was not activated dash current, and the PWM waveform dynamic change situation after the start is that the beginning pulsewidth is 0, after be widened to Nominal Width gradually.And conventional soft-start time is＜＜0.1 second, it is very big to start dash current, the PWM waveform dynamic change after the start is that the beginning pulsewidth is the wideest, after narrow to Nominal Width.Through test of many times relatively, the present invention obtains following two groups of typical datas: first group is R1=39K, R2=100K, and R3=3.3K, C=100 μ F, slow start-up time is 5～7.5 seconds, is applicable to that output power is 0～1 kilowatt a Switching Power Supply; Second group is R1=75K, R2=200K, and R3=7.5K, C=100 μ F, slow start-up time is 15～20 seconds, is applicable to that output power is 1～2 kilowatt a Switching Power Supply.

Claims (2)

1, a kind of logarithmic curve soft start circuit that is used for the pulse-width modulation type Switching Power Supply, include pulsewidth modulated intergrated circuit chip and peripheral circuit element thereof, it is characterized in that: insert one at the in-phase input end of the error amplifier of width modulation integrated device and powering up its voltage of back rule electronic circuit of rising and changing exponentially; This electronic circuit is made of resistance R 1-R6, capacitor C and transistor V: three resistance R 1, R2, R3 are serially connected with between the Vi and ground of above-mentioned pulsewidth modulated intergrated circuit in proper order, the two ends shunt capacitance C1 of R2 wherein, the collector of transistor V is connected between two resistance R 4 and the R5, the 16th pin of the other end of resistance R 4 and integrated circuit (IC) chip joins, the other end of R5 is ground connection then, is connected to the in-phase input end of the error amplifier of said integrated circuit chip again by resistance R 6.

2, logarithmic curve soft start circuit as claimed in claim 1 is characterized in that: the resistance of described resistance R 3 is selected, and should make when capacitor C 1 charging termination, and behind resistance R 1-R3 dividing potential drop result, the pressure drop on R3 ends transistor V.

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