Abstract: Proposed one kind of novel FLYBACK the converter ZVS soft switch realizes the plan. A small auxiliary transformer and the main transformer series, through enables the auxiliary transformer primary side initiation inductance electric current bidirectional to achieve the main switch tube’s ZVS soft switch condition. This plan has realized the main auxiliary switching valve’s ZVS soft switch, has limited the output rectification diode shuts off when di/dt, and causes the converter in any load situation, can realize the soft switch in the wide input range.
Key word: ZVS soft switch; Auxiliary transformer; Electric current bidirectional
Introduction
In many communications and in the computer system, needs to use the high power density, the high efficiency switching power supply. Raises the turn-on frequency to be possible to reduce part’s and so on inductance, electric capacity volumes, is the present switching power supply enhances the power density one tendency. But, turn-on frequency’s enhancement, the switch component’s loss also increases along with it.
Figure 1
In order to reduce switching power supply’s switching loss, raises its turn-on frequency, the soft switch technology arises at the historic moment. The soft switch technology mainly includes two kinds: Zero potential soft switch (ZVS) and zero electric current soft switch (ZCS). In includes in the MOSFET switch component’s converter analysis situs, the zero potential soft switch must surpass the zero electric current soft switch.
The Flyback converter electric circuit is simple, obtained the widespread application in the low power situation. Soft switch analysis situs also obtained further development [2][3][4] based on Flyback converter’s ZVS. Recently several years, the active clamp ZVS soft switch technology is proposed [5][6][7], but it also has some shortcoming [8][9], for instance, when underloading cannot realize the soft switch.
Figure 2
This article proposed one kind of belt auxiliary transformer’s Flyback zero potential soft switching circuit, compares with the active clamp Flyback zero potential soft switching circuit, it has the following several merits:
1) the electric circuit can realize the soft switch in entire load scope;
2) in any load situation, the electric circuit may realize the soft switch in the wide input range;
3) loses the dutyfactor not to change along with the output load change, favors the circuit value design.
Following has analyzed this electric circuit’s principle of work and the soft switch parameter design, and the experimental result has confirmed this plan validity.
Figure 3
1 principle of work
Figure 1 is the Flyback soft switching circuit which this article proposed, Tr is the auxiliary transformer. Its two switch S1 and the S2 supplementary breakover, middle has certain dead area to prevent altogether condition the breakover. Main transformer T initiation inductance Lm is big, causes the electric circuit work at the electric current continual pattern (CCM), like Figure 2 iLm profile shows. But Tr initiation inductance Lmr designs slightly (Lmr 頛 m), causes to wind through Lmr the electric current to be possible to be reverse in one cycle, like Figure 2 iLmr profile shows. Considered switch’s junction capacitance as well as the dead time, one cycle may divide into 7 stages to carry on the analysis, each stage’s equivalent circuit as shown in Figure 3. Its principle of work description is as follows.
1) the stage 1(t0~t1) this stage, the S1 breakover, Lm and the Lmr series withstanding input voltage, winds through Lm and the Lmr electric current linearity rise. This time compartment
In the formula: Vds2 is S2 leaks the source voltage;
Vo is the converter output voltage;
N1 is the T primary side winding number of windings;
N2 and N3 are the T vice-side two winding number of windings;
n1 and n2 are the Tr original vice-side two winding number of windings.
2) stage 2(t1~t2)t1 time S1 shuts off, on the Lm electric current through the T coupling to vice-side, causes the diode D breakover, the Lm both sides voltage by the clamp at
On the Lm electric current linearity drops.
On a Lmr electric current part to the S1 output junction capacitance Cr1 charge, another part through Tr coupled pair S2 output junction capacitance Cr2 electric discharge. the t2 time, S2 leaks the source voltage to drop zero, this stage ended.
3) stage 3(t2~t3), when S2 leaks the source voltage to drop after the zero, the S2 parasitic diode breakover, S2 leaks the source voltage clamp in the zero potential condition, was also the S2 zero potential breakover has created the condition. Simultaneously Lmr both sides by clamp in
On Lmr the electric current linearity drops. But S1 leaks the source voltage by the clamp in the greatest voltage
4) stage 4(t3~t4)t3 the time S2 gate becomes the high level extremely, the S2 zero potential clear. Winds through the parasitic diode’s electric current to flow through S2. The Lmr both sides still withstanding type (3) shows on voltage V1, Lmr the electric current linearity to drop to zero then increases reverse. the t4 time, S2 shuts off, this stage ended. This time compartment
5) stage 5(t4~t5)t4 time, on Lmr direction of current for negative, this electric current part to the S1 output junction capacitance Cr1 electric discharge, simultaneously, another part through the Tr coupling to vice-side to the S2 output junction capacitance Cr2 charge. To the t5 time, S1 leaks the source voltage to drop zero, this stage ended.
6) stage 6(t5~t6), when S1 leaks the source voltage to drop after the zero, the S1 parasitic diode breakover, S1 leaks the source voltage clamp in the zero potential condition, was the S1 zero potential breakover has created the condition. This time, on the Lmr reverse electrical current flows through the main transformer, for winds through diode D in the electric current iD superimposition electric current
This time in compartment, diode D still breakover, Lmr both sides voltage by clamp in
On Lmr electric current linearity rise. But S2 leaks the source voltage by the clamp in the greatest voltage
7) the stage 7(t6~t7)t6 time, the S1 gate becomes the high level extremely, the S1 zero potential clear. Winds through the parasitic diode’s electric current to flow through S1. Because Lmr both sides withstanding voltage V1 is this time big, the iLmr fast rise, to the t7 time, iLmr=iLm, the main transformer coupling to the vice-side electric current is zero, diode D nature shutdown. This time compartment =
(12)
As a result of Lmr 頛 m, the type (12) may be approximate is
Then Lmr and the Lm series withstanding input voltage, starts the next cycle. May see that under this kind of plan, two switch S1 and S2 zero potential clear, diode D zero electric current shutdown.
2 soft switch’s parameter designs
Supposes the electric circuit work in the CCM condition. Because the S2 soft switch realizes is iLmrmax to Cr1 and the Cr2 charging and discharging, but the S1 soft switch realizes is iLmrmin to Cr1 and the Cr2 charging and discharging, in electric circuit full load situation,|iLmrmax|>>|iLmrmin|, moreover the S2 charging voltage must be bigger than the discharge voltage (to see Figure 2 profile vds2), therefore, the S1 soft switch realizes must be much more difficult than S2. In the parameter design, the key is needs to consider S1 the soft switch condition.
Figure 4, 5, 6, 7
2.1 main transformer initiation inductance Lm hypothesis
As a result of the Lmr existence, converter’s effective dutyfactor Deff (the basis initiation inductance Lm charging and discharging time definition, see Figure 2) to be smaller than S1 dutyfactor D, but, because in the t4~t7 time the iLmr ascending velocity is quick, therefore, but thinks Deff=D approximately. Thus, according to Flyback electric circuit work in CCM condition
In the formula: Eta is the converter efficiency;
fs is the turn-on frequency; For converter output.
In actual design, for guarantee circuit in underloading time can also work in the electric current continual pattern, determines
2.2 main auxiliary transformer original vice-side number of windings compare the hypothesis
According to Lmr 頛 m, and the converter input output relations have
But according to type (8), before to cause to output the filter, electric current io not to be too quick in the t3~t4 time section drop, should better have N3≤N2.
Moreover, to guarantee that t1 time S1 shutdown fashion vice-side diode D electric current iD>0, (7) has had according to the type
2.3 auxiliary transformer initiation inductance Lmr hypothesis
In order to realize the S1 ZVS soft switch, in the (1-D)T time, on initiation inductance Lmr the electric current must be reverse, namely
Moreover, according to Lmr and S1 and S2 output junction capacitance resonance condition
-like (24) substitution type (22) solution
Compared with-like (20) and the type (25), Lmr should according to type (25) establish.
Moreover, (24) may discover by the type, when the input, the output voltage are certain, along with load increase, iLmrmax increases (sees the type (19)), iLmrmin reduces, the soft switch is not easier to realize. Therefore, Lmr must act according to time the full load the soft switch realizes the condition to establish. But works as when input voltage for wide scope, along with input voltage’s reduction, iLmrmax increases (as a result of the electric circuit work in CCM, the full load stylish (19) the second item may neglect), the iLmrmin expression first item reduces, iLmrmin reduces, the soft switch is not easier to realize. , regarding the output load, the input voltage change’s situation, Lmr must act according to the output full load, the input voltage are smallest the soft switch to realize the condition to establish.
Simultaneously needs to point out that in can realize under the soft switch’s premise, Lmr is not suitable is too small, in order to avoid creates on the switching valve the oversized electric current stress and the breakover loss.
2.4 dead time determination
In order to realize the S1 soft switch, must guarantee that in the t5~t6 time, S1 starts the breakover. Otherwise, on Lmr the current reversal, to the Cr1 charge, like this, the S1 ZVS soft switch condition will lose again. Therefore, after S2 shuts off, before, the S1 clear’s dead time hypothesis realizes very important to the switching valve S1 soft switch. Appropriate dead time for inductance Lmr and S1 and S2 output junction capacitance resonance cycle 1/4, namely
Generally speaking, the switching valve output capacity is receives the voltage the function, for convenience, is constant in this supposition Cr1 and Cr2.
2.5 effective dutyfactor Deff computation
Effective dutyfactor Deff is slightly smaller than S1 dutyfactor D, namely
(29) may see from the type, loses the dutyfactor to have nothing to do with the output load. When same electrical specification and circuit value condition, its value probably for active clamp Flyback converter full load loses the dutyfactor 1/2[7].
3 experimental results
In order to confirm the above ZVS soft switch to realize the method, this article has designed an experiment electric circuit, its specification and the main parameter are as follows:
Input voltage Vin40~56V;
Output voltage Vo20V;
Output full load current Io3A;
Operating frequency f100kHz;
S1 and S2IRF640;
Main transformer initiation inductance Lm222μH;
Main transformer original vice-side number of windings N1:N2:N339:15:15;
Auxiliary transformer initiation inductance Lmr10μH;
Auxiliary transformer original vice-side number of windings n1:n213:13.
Figure 4 what gives when is load current Io=2.5A, before outputting the filter, the electric current and winds through the vice-side diode D electric current the experiment profile, its result and the theoretical analysis tally. Figure 5~ Figure 8 has given S1 and S2 separately in time the underloading and the full load slaving voltage, leaks the source voltage and winds through the electric current the experiment profile. May see from the chart, when the slaving voltage is positive, the switching valve leaks the source voltage already was zero, is the zero potential clear. But when the switching valve shuts off, its junction capacitance limits has leaked source voltage the climbing rate, is the zero potential shuts off, from this explained S1 and S2 when underloading and full load has realized ZVS[10]. Leaked the source voltage from the switching valve with to wind through the electric current the comparison also to be possible to see has realized ZVS.
Figure 9 has given the converter efficiency curve. Figure 9 (a) is the input voltage is certain, the load current is different when the conversion efficiency curve, may see, when full load the efficiency is highest, is 91.35%. Figure 9 (b) is the load current is certain, the input voltage is different when the conversion efficiency curve, may see that the efficiency the scope which changes along with the input voltage change is very small.
4 conclusions
This article proposed one kind of Flyback the converter ZVS soft switch analysis situs, has analyzed its principle of work and the soft switch parameter design method. Because the soft switch parameter’s design (key is auxiliary transformer primary side initiation inductance Lmr the design) is according to the full load and time the smallest input voltage working condition design, but along with load reduction with input voltage’s increase, the ZVS soft switch realizes is also easier. Therefore, this soft switch analysis situs may work in the wide input range and any load scope, compares with the active clamp soft switch analysis situs has certain merit, may take applies in the communication, the computer system contour power density situation one kind of choice.
