Abstract: Proposed one kind of simple effective current limiting protection circuit, elaborated this protection circuit to apply in the wide scope input is stirring up the converter and the wide scope input instead stirred up when the converter the working condition difference, and gave one to be suitable for the wide scope input instead stirs up converter’s compensating circuit. The final experimental result has confirmed the current limiting protection circuit and compensating circuit’s principle of work and the validity.
Key word: Overflow protection; Just violent; Counter-violent
Introduction
The overflow protection circuit is in the power source product an essential constituent, may divide into the shutdown way and the current limiting way approximately according to its control method. Because the current limiting way it has the electric current sagging characteristic, after the breakdown relieves, the switching power supply can the self-recovery work, therefore, obtains the quite widespread application.
The current limiting protection circuit must first have an electric current sample link, at present, generally makes the law is connects a small resistance or is obtains the electric current signal with the Hall part. When the sample electric current is quite small, these two method of sampling is may take. But works as the sample electric current to be quite big, the resistance sample can have the big loss, cut converter’s efficiency, but the Hall part takes a sample its volume to be quite big, and the price is expensive, to the entire power source’s cost is also a question.
Based on the above consideration, this article proposed that one kind of simple effective current limiting protection circuit, overcame the above two ways to take a sample time the big electric current shortcoming. It is suitable in is stirring up, instead stirs up and so on each kind of converter, moreover the cost is also quite low.
1 current limiting protection circuit principle of work
Figure outside 1 dashed line frame’s electric circuit is the ordinary summit current way PWM control circuit, the use current transformer sample summit current. In the chart shows the PWM chip is L5991[1] which ST Corporation produces. In the dashed line frame is the current limiting protection circuit which this article proposed. It uses in the summit current control the electric current signal to take the input signal, the peak value which is composed of D1, R1, C1 maintains through one electric circuit and by the PI link which transports puts is composed obtains an error signal, in converter’s output current surpasses the limitation value time, this error signal will control the PWM chip the dutyfactor, will thus cause the output current maintains at the limitation value. As a result of the D2 existence, when the output current is lower than the current limiting value, this part of electric circuits the control do not have an effect to the dutyfactor.
Below take is stirring up the converter as the example, the elaboration current limiting protection circuit’s principle of work.
Is stirring up converter as shown in Figure 2. Supposes Figure 1 A voltage is a va, B voltage is a vb, C voltage is vc, Figure 2 the average switching valve’s electric current is, the inductance electric current is iL, the output current is io. The electric current sample transformer primary side electric current, namely winds through switching valve’s electric current is. And makes the following hypothesis:
1) the diode D1 breakover pressure drop is VD1 and maintains invariable;
2) R1 is composes the RC absorption network absorption peak in the actual electric circuit’s function with C1, here supposes is zero;
3) is stirring up the converter inductance L inductance quantity to be big, the electric circuit work is small in the CCM pattern and the inductance current fluctuation.
Is stirring up converter current limiting protection circuit’s theory work profile as shown in Figure 3. Its switching period may divide into 3 working stages.
Stage 1 (t0-t1) t0 time vg>0, switching valve S and diode DR1 breakover, iL linearity rise, therefore, the primary side electric current is also linearity rise, va also along with it rise, this time compartment va-vb<VD1, diode D1 is at the shutdown condition, vb through the R3 electric discharge, assumes the declining trend.
The stage 2 (t1-t2) t1 time va-vb>VD1, diode D1 starts the breakover, vb along with the va linearity rise.
The stage 3 (t2-t3) t2 time vg=0, S shuts off, is=0, then va=0, diode D1 shuts off, vb through the R3 electric discharge, until next cyclical arrival.
May see from Figure 3 vb is a fluctuation voltage, but in actual electric circuit, because Figure 1 time-constant R3C1 obtains is quite big, the vb fluctuation is very small, may be approximate is a DC voltage.
According to supposes 3), the inductance electric current’s fluctuation is small, namely the va slope is quite small, moreover VD1 is small (is because winds through diode’s electric current to be very small, in experiment uses the 1N5819 actual value is about 200mV), then the vb value is equal to vaD approximately (va in DT time mean value). May see from Figure 3 VaD and output current io is proportional, is also vb is approximate with output current io is proportional, supposes vb=Kio, K is the constant.
We knew, when the current limiting protection circuit works and achieves the steady state, vb=vc=vref=Kio, this time output current io namely for current limiting protection value. Therefore, then changes the current limiting protection value through change reference voltage Vref.
2 current limiting protection compensating circuit
Is certain in the output voltage, when input voltage for wide scope, will change as a result of the dutyfactor along with input voltage’s change, applies in the different analysis situs, the current limiting protection circuit’s working condition will differ from, below take will be stirring up with instead stirs up-like the converter to carry on the theoretical analysis as the example.
Before analysis makes a hypothesis first: Analyzed by front already knew vb value approximately equal vaD, in it is hereby ordered vb=vaD, and the DC voltage appeared in the following oscillogram.
2.1 are stirring up the converter
And above supposes according to the current limiting protection circuit’s principle of work, then has
vb=vaD=isDn2R=ilon2R/n1=ion2R/n1 (1)
io=n1vb/n2R (2)
In the formula: isD is in the DT time mean value; n1 is the transformer original vice-side number of windings ratio;
n2 is the current transformer original vice-side number of windings ratio;
iLo is in inductance electric current one cycle mean value.
When the current limiting protection circuit works and achieves the steady state, vb=vc=Vref, io namely for current limiting protection value iomax. Then
iomax=n1Vref/n2R (3)
From the type (3) may see that n1, n2, R is a constant, under Vref certain condition, iomax is a constant, does not change along with input voltage’s change.
2.2 instead stir up the converter
Instead stirs up converter as shown in Figure 4, similarly has
vb=vaD=isDn2R=iLon2R= (iDD’n2R) /n1=(ion2R)/n1D’ (4)
io= (D’n1vb) /n2R=[(1-D)n1vb]/n2R (5)
In the formula: iLo is in inductance electric current one cycle mean value (instead stirs up converter’s inductance is transformer primary side excitation inductance);
iDD ‘ to wind through vice-side diode D electric current iD in the (1-D)T time mean value.
Also has Vout=VinD/n1(1-D) (6)
Promotes D=n1Vout/(n1Vout Vin) (7)
-like (7) substitution type (5)
io=[1-(n1Vout/n1Vout Vin)n1vb]/n2R (8)
When the current limiting protection circuit works and achieves the steady state, vb=vc=Vref, io namely for current limiting protection value iomax. Then
iomax=[1-(n1Vout/n1Vout Vin)n1Vref]/n2R (9)
From the type (9) may see that n1, n2, R is a constant, in Vout and under Vref certain condition, iomax increases along with Vin increases.
(4) may discover compared with-like (1) and the type: When vb is certain (i.e. current limiting protection circuit works and achieves when steady state reference voltage Vref certainly), no matter is stirring up the converter instead to stir up the converter, inductance average current iLo does not change along with input voltage’s change. Causes both difference the key to lie in: Is stirring up converter’s output current is continual, but instead stirs up converter’s output current is off and on. Regarding was stirring up converter io=iLo, but regarding instead stirred up converter io=n1 (1-D) iLo. As a result of when the output voltage is certain, dutyfactor D will change along with the input voltage change, therefore, will instead stir up converter’s current limiting value to change along with the input voltage change.
Figure 5 and Figure 6 gave separately has supposed when io was invariable, the different input voltage is stirring up the converter and instead stirs up the converter current limiting protection circuit’s theory profile, in the chart input voltage Vin2>Vin1.
May know according to the above analysis, when the reference voltage is constant, is stirring up the converter current limiting value is also constant, has not related with the input voltage. What here need pointed out: The above theoretical analysis is based on the vb=vaD hypothesis, when input voltage change, the vb=vaD approximate degree will be also different, therefore, in fact will be stirring up the converter current limiting value
Also will change along with the input voltage change, only will be the fluctuation is very small, this may see in the experimental result.
Instead stirs up the converter current limiting value to have the sweeping change along with the input voltage change, therefore, needs to use certain measure to carry on the compensation, causes the current limiting value the change in the acceptable scope. (9) may know the current limiting value from the type to increase along with the input voltage increases, is also the hypothesis current limiting value invariable words, vb increases along with the input voltage reduces. Therefore, needs to make certain compensation to vb, the balancing waveform should increase along with the input voltage increases, thus counter-balances vb the change. Comes with the input voltage as the compensation signal is the method which one kind may select. The input voltage receives Figure through a resistance 1 C spot, like Figure 4 the dashed line shows, this time the current limiting protection circuit works when and achieves the steady state, vc no longer is equal to vb, but is
vc=vb [(vin-vb)R2]/(R2 R4)
vc first part of vb increases along with Vin reduces, but the second part increases along with Vin increases, thus achieves the counter-balance the goal. The R4 value may act according to the biggest input voltage theoretically and when the smallest input voltage vc equal obtains (the R2 value already in situation which decides), carries on the trimming again in the concrete experiment, obtains the smallest current limiting value range of variation in order.
3 experimental results
The current limiting protection circuit which has this article to propose is stirring up the converter, had the current limiting protection circuit and the compensating circuit with one instead stirs up the converter to confirm the above theory result, its circuit value like table 1 arranged in order.
Table 1 circuit value
|
Converter |
Forward |
Flyback |
|
Input voltage /V |
9~15 |
9~15 |
|
Output voltage /V |
24 |
5 |
|
Output /W |
240 |
35 |
|
Operating frequency /kHz |
100 |
100 |
Figure 7 what gives is input voltage 12V, time the electric circuit full load work current limiting protection circuit work profile, from the chart may see that its actual electric circuit profile with the theory profile is consistent.
Figure 8 and Figure 9 respectively has given the input voltage respectively is 9V,12V,15V, when the electric circuit full load work is stirring up the converter and instead stirs up converter current limiting protection circuit va the profile, with Figure 5 and Figure 6 theory profile is also consistent.
Figure 10 the piece has given just violently, after before instead stirs up the compensation, and instead stirs up the compensation, the actual current limiting value along with the input voltage change curve. Is stirring up the converter current limiting value basically along with the input voltage change invariable, but instead stirs up the converter current limiting value to have the big fluctuation before the compensation along with the input voltage change. But, after adding the compensating circuit instead stirred up the converter current limiting value the stability to have the distinct improvement, has proven this compensating circuit’s validity.
4 conclusions
This article proposed the current limiting protection circuit has the simple effective characteristic, overcame the electric circuit operating current was quite big when the resistance sample consumed power to be big and the Hall part sample volume was big, cost high shortcoming.
This article analyzed this current limiting protection circuit to apply in is stirring up with instead stirs up time converter’s working condition, and proposed applied instead stirs up time a converter’s simple effective compensating circuit in the wide scope. Needs to attach the compensating circuit regarding other analysis situs, the reader may be continuously or off and on voluntarily analyzes according to the output current.
