Abstract: Proposed one kind of Flyback the electric circuit ZVS soft switch realizes the method, namely through attaches a winding, causes the initiation inductance current reversal, thus creates Flyback the electric circuit main switch’s ZVS soft switch condition; Has analyzed its principle of work and the circuit value design; The final experimental result has confirmed this electric circuit’s principle of work and the validity.
Key word: Flyback electric circuit; Soft switch; Auxiliary winding
Introduction
Light small is the present power source product pursue goal. But raises the turn-on frequency to be possible to reduce part’s and so on inductance, electric capacity volumes. But, the turn-on frequency enhances the bottleneck is the switch component’s switching loss. Therefore the soft switch technology arises at the historic moment.
This article proposed one kind of belt auxiliary winding’s Flyback zero potential soft switch realizes the method. Through to this electric circuit’s principle of work analysis and experiment’s result, has confirmed this electric circuit’s feasibility.
1 principle of work
Shown in Figure 1 namely soft switching circuit which proposed for this article, auxiliary winding’s number of windings and the output winding are the same. Switching valve S1 and the S2 supplementary breakover, between has certain dead area to prevent altogether condition the breakover, as shown in Figure 2. In the electric circuit the initiation inductance Lm value is small, enables electric current iLm to be possible reverse to achieve main switch S1 the ZVS soft switch condition, like Figure 2 (a) and Figure 2 (b) the iLm profile shows. Because the electric circuit has in time the underloading and the full load working condition slightly differently, as follows concrete study electric circuit underloading time principle of work, time full load principle of work briefing. Considered switch’s junction capacitance as well as dead time, when electric circuit underloading one cycle may divide into 7 stages, its 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 S1 breakover, the Lm withstanding input voltage, excitation circuit iLm is increasing to the linearity, becomes from the negative value just when. Shuts off in t1 time S1, the iLm maximizing, this stage ended.
after 2) stage 2(t1, t2)S1 shutdown, the initiation inductance electric current starts to drop, a part to the S1 output junction capacitance charge, S1 leaks the source voltage linearity rise; Simultaneously another part to vice-side causes S2 through the transformer coupling the output junction capacitance electric discharge, S2 to leak the source voltage to be possible to think the linearity to drop approximately, 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 on the breakover, S2 leaks the source voltage clamp in the zero potential condition, was also is the S2 zero potential breakover has created the condition. Simultaneously diode D also breakover.
4) stage 4(t3, t4)t3 the time S2 gate becomes the high level extremely, the S2 zero potential clear. Initiation inductance Lm withstands reverse voltage nVo (n is transformer original vice-side number of windings ratio), on Lm the electric current linearity drops, the t4 time drops zero, also simultaneously drops zero through switching valve S2 and the diode D electric current, this stage ended.
5) the stage 5(t4, t5) drops through the diode D electric current after the zero, diode D nature shutdown. But S2 continues the breakover, on Lm withstands voltage nVo, winds through Lm the electric current to start the reverse linearity from zero to increase. t5 time S2 shuts off, this stage ended.
6) on the stage 6(t5, t6) this time initiation inductance Lm direction of current for negative, this electric current part causes S1 the output junction capacitance electric discharge, causes S1 to leak the source voltage to be possible to think approximately the linearity drops; Simultaneously another part through the transformer coupling to vice-side to the S2 output junction capacitance charge, causes S2 to leak the source voltage linearity rise. t6 time S1 leaks the source voltage to drop zero, this stage ended.
7) stage 7(t6, t7), when S1 leaks the source voltage to drop after the zero, the S1 parasitic diode breakover, S1 leaks the source voltage clip in the zero potential condition, was also the S1 zero potential breakover has created the condition. t7 time then S1 under the zero potential condition the breakover, enters the next cycle. May see that two switch S1 and S2 have realized the soft switch.
What above analysis is when the electric circuit underloading principle of work, when the electric circuit full load principle of work and the underloading slightly has the difference, namely will not have the diode D electric current to drop the link which will shut off naturally to zero, the diode D electric current in the switching valve S2 shutdown later only then gradually drops zero, like Figure 2 (b) will show.
2 soft switch parameter design
Here soft switch’s parameter design is mainly the transformer initiation inductance design.
The initiation inductance electric current’s Feng Feng value may express is
δILm= (VinDT) /Lm (1)
In the formula: D is a dutyfactor;
T is the switching period.
Then the initiation inductance electric current’s maximum value and the minimum value may express are:
ILmmax=(VinDT)/2Lm Io/n (2)
ILmmin=(VinDT)/2Lm-Io/n (3)
In the formula: Io is the load current.
Figure 3
May see from the above principle analysis S1 the soft switch condition is by|ILmmin| causes S1 the output junction capacitance electric discharge, simultaneously creates through the transformer to the S2 output junction capacitance charge; But the S2 soft switch condition is by|ILmmax| to the S1 output junction capacitance charge, simultaneously causes S2 through the transformer the output junction capacitance electric discharge to create. S1 and the S2 soft switch boundary condition to store up on Lm the energy to S1 and the S2 output junction capacitance charging and discharging, makes sufficiently a junction capacitance discharges zero, but another junction capacitance charges in a big way.
Such S1 boundary condition is
The S2 boundary condition is
In the formula: C1, C2 respectively is S1 and the S2 output junction capacitance.
Because the dead time is quite small in the actual electric circuit, therefore may think approximately the electric current maintains invariable on dead time internal inductance Lm, is a constant current carries on the electric discharge to switching valve’s junction capacitance. The soft switch condition in this case is called the ample condition.
The S1 ample condition is
(C2/n2 C1)(nVo Vin)≤
|ILmmin|tdead1 (6)
The S2 ample condition is
(C2/n2 C1)(nVo Vin)≤
|ILmmax|tdead2 (7)
In the formula: tdead1, tdead2 respectively is S1 and S2 opens
Before passing the dead time.
As a result of the energy by the power source to the load transmission, namely load current IO>0, compared with-like (2) (3) may know with the type|ILmmax|>|ILmmin|, specially when full load,|ILmmax 韡 ILmmin|. Therefore the S2 soft switch realizes S1 must be much easier. Therefore in the concrete experimental design, the key is must design S1 the soft switch condition. First determined may withstand the biggest dead time, then according to type (6) and type (3) calculated the initiation inductance measures Lm. In can realize under the soft switch’s premise, Lm is not suitable is too small, in order to avoid creates on the switching valve the oversized electric current effective value, causes switch’s breakover loss to be oversized.
3 experimental results
Has designed a 48V input, 5V/5The output belt auxiliary winding’s Flyback electric circuit model, has given the experimental result, further confirmed the above soft switch to realize the method accuracy. This converter’s specification and the main parameter are as follows:
Input voltage Vin48V;
Output voltage Vo5V;
Output current Io0~5A;
Operating frequency f100kHz;
Main switch S1, S2IRF730, IRFZ44;
Initiation inductance Lm70μH;
The transformer original vice-side and the auxiliary winding number of windings compare 26∶4∶4.
Figure 4 has given the underloading separately (1A) and the full load (5A) when
The experiment profile, (g) the ~ chart 4 (j) may see from Figure 4 switching valve S1 and S2 when underloading and full load has realized the soft switch.
4 conclusions
This article has analyzed the electric circuit work separately in time the underloading and the full load situation, namely the output rectification diode is at off and on and the continual condition separately, this two conditions have their good and bad points separately, on-off state may realize diode’s zero electric current to shut off, but its electric current stress is high, but the continual condition just is opposite. Therefore, may according to the concrete need, the circuit design in a condition or surmounts two conditions.
