CN102468742A

CN102468742A - Current limit signal generation method applied to flyback power converter

Info

Publication number: CN102468742A
Application number: CN2010105411971A
Authority: CN
Inventors: 吴信义; 黄志丰
Original assignee: Richpower Microelectronics Corp
Current assignee: Richpower Microelectronics Corp
Priority date: 2010-11-11
Filing date: 2010-11-11
Publication date: 2012-05-23

Abstract

The invention relates to a current limit signal generation method applied to a power converter, which can be carried out without detecting the input voltage, and comprises the steps of: detecting the current which passes through a power switch and obtaining a sensing signal; calculating the time needed by the sensing signal which rises from a first reference position to a second reference position; finally, deciding a current limit signal according to the time, and limiting the maximum value of the current; and therefore, the maximum output power of the power converter is ensured to be stable.

Description

Be applied in the current limiting signal production method of flyback power converter

Technical field

The present invention is relevant a kind of power supply changeover device, particularly is applied in the current limiting signal production method that flyback power converter need not to detect input voltage about a kind of.

Background technology

Fig. 1 shows traditional power supply changeover device, and it comprises that transformer 12 has first siding ring Lp and second siding ring Ls; Power switch Q1 connects first siding ring Lp; Sensing resistor Rs connects with power switch Q1; Controller 10 output control signal PWM power controlling switch Q1 are so that transformer 12 transfers input voltage vin to output voltage V o.Controller 10 obtains and the relevant sensing signal Vcs of electric current I p that passes through power switch Q1 by the cross-pressure that detects sensing resistor Rs; Under perfect condition, when sensing signal Vcs reaches preset current limiting signal Vlimit, controller 10 will cut out the maximum of (turn off) power switch Q1 with restriction electric current I p; And then make load current Io stable; In other words, the ON time of power switch Q1 equals the operating time (on-time) of control signal PWM, therefore passes through the electric current of power switch Q1:

Ip=(Vin/Lp) * Ton formula 1

Wherein, Ton is the operating time (on-time) of control signal PWM.Can try to achieve the peak power output of power supply changeover device according to formula 1:

Po=0.5 * (Lp/Ts) * Ip ²=Vin ²* Ton ²/ (2 * Lp * Ts), formula 2

Wherein, Ts is the switching cycle of power switch Q1.Shown in formula 2, the peak power output Po that the maximum of Control current Ip can the stabilized power supply transducer.Yet; In fact, reach current limiting signal Vlimit from sensing signal Vcs and close to power switch Q1 and have fixing transmission delay (propagation delay) time T d, as shown in Figure 2; Therefore the actual ON time of power switch Q1 equals Ton+Td, formula 2 thereby can be rewritten as

Po＝0.5×(Lp/Ts)×Ip ²＝Vin ²×(Ton+Td) ²/(2×Lp×Ts)。Formula 3

With reference to Fig. 2, because the influence of transmission delay time T d, the true peak of sensing signal Vcs will be greater than Vlimit, and under different input voltage vin, the slope of sensing signal Vcs is different, and its peak value is also different.Shown in the waveform 14 of Fig. 2 when input voltage vin is high pressure; The slope of sensing signal Vcs is bigger; Opposite, when input voltage vin was low pressure, the slope of sensing signal Vcs was less shown in the waveform 16 of Fig. 2; Because transmission delay time T d fixes, so the peak value of the sensing signal Vcs of the peak value of the sensing signal Vcs of input voltage vin when being high pressure when being low pressure greater than input voltage vin.Because the peak value of sensing signal Vcs changes along with the variation of input voltage vin; So the peak value of electric current I p also will be followed the variation of input voltage vin and change; Can know by formula 3 that again the change of the peak value of electric current I p will make the peak power output Po of power supply changeover device unstable.For fear of the problems referred to above; The input IN of controller 10 connects the power input of power supply changeover device through resistance R i; In order to detect input voltage vin; Controller 10 will be adjusted current limiting signal Vlimit according to detected input voltage vin, so that the peak value of electric current I p does not change with the variation of input voltage vin.

Yet, detecting in the method for input voltage at this, resistance R i will cause power loss, be under the situation of high pressure in input voltage vin especially.In order to reduce power loss; At present existing many methods can obtain the information of input voltage vin with adjustment current limiting signal Vlimit under the situation that does not detect input voltage vin, for example United States Patent (USP) the 6th, 674; No. 656 and the 7th; Utilize for 643, No. 313 operating time of control signal PWM to judge the size of input voltage vin, adjust current limiting signal Vlimit according to this.U.S. Patent number the 7th, 616, No. 461 then is the slope of judging sensing signal Vcs by the difference that obtains sensing signal Vcs on two preset time points, and then the size of judgement input voltage vin is with adjustment current limiting signal Vlimit.

Summary of the invention

The object of the invention is to propose a kind of current limiting signal production method that power supply changeover device need not to detect input voltage that is applied in.

A kind ofly be applied in current limiting signal production method that power supply changeover device need not to detect input voltage and comprise that the electric current of detection through power switch is to obtain sensing signal; Obtain this sensing signal is risen to the second accurate position by the first accurate position time; In order to limiting this maximum, and then make the peak power output of this power supply changeover device stable according to this decision current limiting signal through the electric current of power switch time.

Description of drawings

Fig. 1 shows traditional power supply changeover device;

Fig. 2 is presented at the sensing signal waveform under the different input voltages;

Fig. 3 shows current limiting signal production method of the present invention;

Fig. 4 shows the power supply changeover device of using the inventive method;

The sensing signal Vcs that Fig. 5 is presented under the different input voltages rises to the certainly time of position Vref2 by accurate position Vrefl;

Fig. 6 shows the curve of time Δ t to current limiting signal Vlimit; And

The waveform of sensing signal Vcs under different input voltage vin of Fig. 7 displayed map 4.

Drawing reference numeral:

10 controllers

12 transformers

The waveform of 14 sensing signal Vcs

The waveform of 16 sensing signal Vcs

30 load limiters

32 comparators

34 flip-flops

The waveform of 40 sensing signal Vcs

The waveform of 42 sensing signal Vcs

Embodiment

Fig. 3 shows current limiting signal production method of the present invention.Fig. 4 shows the power supply changeover device of using the inventive method, and the circuit of itself and Fig. 1 comprises controller 10, transformer 12, power switch Q1 and sensing resistor Rs equally, but controller 10 does not detect input voltage vin.In this embodiment, controller 10 comprises that load limiter 30 is according to sensing signal Vcs decision current limiting signal Vlimit; Comparator 32 is current limiting signal Vlimit and sensing signal Vcs generation comparison signal S1 relatively; And flip-flop 34 is according to clock pulse CLK and comparison signal S1 decision control signal PWM.With reference to Fig. 3 and Fig. 4, shown in the step S20 of Fig. 3, controller 10 is by detecting voltage on the sensing resistor Rs detecting and electric current I p through power switch Q1, and then obtains the sensing signal Vcs relevant with electric current I p.Then carry out step S22, load limiter 30 is obtained sensing signal Vcs and is risen to the time Δ t of accurate position Vref2 by accurate position Vref1, and the slope of the sensing signal Vcs under different input voltage vin as shown in Figure 5 is different; When high input voltage Vin1, the slope of the sensing signal Vcs shown in waveform 40 is bigger, so the time Δ t1 that rises to accurate position Vref2 by accurate position Vref1 is shorter; Opposite, when low input Vin2, the slope of the sensing signal Vcs shown in waveform 42 is less; So the time Δ t2 by accurate position Vref1 rises to accurate position Vref2 is longer, in other words, load limiter 30 can be judged the slope of sensing signal Vcs by the time Δ t that is obtained; And then the value of differentiation input voltage vin; Therefore, method of the present invention need not to detect input voltage vin, promptly knows the variation of input voltage vin.

Next carry out step S24, load limiter 30 is multiplied by preset parameter K with resulting time Δ t and produces adjusted value K * Δ t.Carry out step S26 at last; Load limiter 30 produces current limiting signal Vlimit=Vc+K * Δ t with adjusted value K * Δ t with preset critical value Vc addition; Because parameter K and critical value Vc are definite value, therefore can obtain time Δ t as shown in Figure 6 curve to current limiting signal Vlimit.Can know that by Fig. 6 when time Δ t increased, current limiting signal Vlimit also and then increased; That is to say that when low input Vin2, load limiter 30 will provide higher current limiting signal Vlimit shown in the waveform 42 of Fig. 7; Opposite; When high input voltage Vin1, load limiter 30 will provide lower current limiting signal Vlimit shown in the waveform 40 of Fig. 7, therefore under different input voltage vin; The peak value of sensing signal Vcs is all identical, and then makes the peak power output Po of power supply changeover device stable.

Above-mentioned parameter K can get by derivation; With reference to Fig. 7; Because the peak value of sensing signal Vcs under different input voltage vin is all identical, and the peak value of sensing signal Vcs equals current limiting signal Vlimit and adds the variable quantity that is caused because of transmission delay time T d, therefore can push away:

(Vc+K×Δt1)+[(Vin1/Lp)×Rs×Td]

=(Vc+K * Δ t2)+[(Vin2/Lp) * and Rs * Td], formula 4

Can push away at last parameter

K＝[(Vin2-Vin1)/(Δt1-Δt2)]×Rs×Td/Lp。Formula 5

More than the narration done for preferred embodiment of the present invention be purpose for illustrating; And be not intended to limit the present invention accurately is the form that is disclosed; Based on above instruction or to make an amendment or change from embodiments of the invention study be possible; Embodiment is for explaining orally principle of the present invention and let those skilled in the art utilize the present invention in practical application, to select with various embodiment and narrate, and technological thought attempt of the present invention is decided by claim and equalization thereof.

Claims

1. one kind is applied in the current limiting signal production method that flyback power converter need not to detect input voltage, it is characterized in that, described current limiting signal production method comprises the following steps:

(A) detect electric current through power switch in the said flyback power converter to obtain sensing signal;

(B) obtain said sensing signal and rise to second accurate time by the first accurate position, the said time is relevant with input voltage; And

(C) according to said time decision current limiting signal, in order to the maximum of the electric current of the said power switch of restricted passage.

2. current limiting signal production method as claimed in claim 1 is characterized in that, said step (C) comprising:

With said time and the preset parameter generation adjusted value that multiplies each other; And

A said adjusted value and a preset critical value addition are produced current limiting signal.