Abstract: The soft switch technology is in a switching power supply’s important research area. According to magnetic elements’ voltage polarity under the different on-off state also the different characteristic, proposed one kind of new ZVS shutdown electric circuit. This kind of shutdown electric circuit attaches the part are very few, but may realize the fine soft shutdown effect. Take CCM the Buck electric circuit as an example, has carried on the detailed theoretical analysis to this ZVS shutdown electric circuit. The final experimental result confirmed the theoretical analysis, indicates this kind of electric circuit’s accuracy and the feasibility.
Key word: Soft switch; Absorbing circuit; Zero potential switch
Introduction
The soft switch (ZVSZCS) the technology may obviously reduce the switching loss, is a switching power supply’s important research area. Until now, realizes the soft switch’s plan to be many, induces may have the following several different forms:
1) directly the resonant soft switch which realizes using the resonance, it may realize the good soft switch effect, but actually needs to use the frequency conversion control policy, makes magnetic elements’ design to optimize with difficulty;
2) uses the active soft switch technology which the auxiliary switch realizes, may realize the PWM control policy, and achieves the very good soft switch effect, but it needs to assist the switching valve and the control driving circuit, does not favor increases switching power supply’s power density, the reliability also has reduces;
3) uses the lossless absorbing circuit to realize the soft switch technology, only needs to attach the few lossless primary devices (electric capacity, inductance and diode), may realize soft switch [1][2][3][4][5][6][7], and may use the PWM control, is advantageous for the design magnetic elements.
This article proposed one kind of new passive lossless ZVS soft switching circuit, it uses the main inductance voltage polarity followed on-off state change the characteristic, comes to control ZVS to shut off electric capacity’s charge and the electric discharge appropriately. This electric circuit has uses the primary device to be few, does not need the extra auxiliary switch, and the soft switch effect is good, the control convenience and so on many merits, the findings proved that this electric circuit is feasible.
1 new magnetic coupling type ZVS shutdown electric circuit
As shown in Figure 1, the new ZVS shutdown electric circuit contains the primary device are very few: Small electric capacity Cs and diode Ds, uses in realizing main switch’s ZVS shutdown; Small inductance LR, diode DR and main inductance coupling inductance Lmc, for Cs creation charge accurate resonance condition. A~E each vertex connects electric circuit’s appropriate position Figure 1, may construct the ZVS shutdown condition for the concrete electric circuit.
Shown in Figure 2 is belt new magnetic coupling type ZVS shuts off electric circuit’s Buck analysis situs, the dashed line surrounds the part for the ZVS shutdown electric circuit which attaches.
Figure 1, 2
This kind of new shutdown electric circuit may promote to all belt single switch’s basic analysis situs, as shown in Figure 3.
Figure 3 belt new ZVS shuts off electric circuit’s basic analysis situs
Because Cuk, Sepic and the Zeta analysis situs includes two main inductance, these two main inductance’s voltage polarity has the change along with on-off state, therefore they may use in constructing the new ZVS shutdown electric circuit.
2 belt new ZVS shuts off electric circuit’s CCM the Buck analysis situs principle of work
Take CCM the Buck electric circuit as an example, explained that new ZVS shuts off electric circuit’s principle of work. For the convenient circuit analysis, makes the following supposition:
1) input voltage Vin is a constant;
2) output capacity Co is big enough, output voltage Vo is constant;
3) all primary devices are ideal, neglects its stray parameter;
4) the coupling inductance may regard as excitation inductance Lm and an ideal transformer composition, exciting current Im is bigger than zero the constant.
The electric circuit altogether has 5 work modalities, as shown in Figure 4, its work profile as shown in Figure 5. The circuit expressed with the heavy line.
Figure 4 electric circuit each work modality chart
Figure 5 each work modality main oscillogram
The modality 1(t0-t1) t0 time, the electric capacity Cs electric discharge finished, Do realized the ZVS clear, the electric circuit enters the Lm discharge condition, its electric current was Im.
Modality 2(t1-t2) t1 time, S clear. Input current iin jumps immediately Im, output diode Do to shut off (S and Do was not immediately soft switch by now, this was not content which this article studied). After S clear, the V1 polarity becomes positively, its induced tension V2 with is also becoming positively; Cs and LR has the series resonance under the V2 function. Supposes the coupling inductance changes the ratio is N2/N1, then has:
This time, iin (t) contains has two components of current, namely
iin(t)=Im iLRc(t) (4)
In the formula: iLRc(t)=(N1/N2)iLR(t)
The modality 3(t2-t3) t2 time, vCs(t) arrives at maximum value Vin, and maintains invariably. iLR(t) the linearity drops under the voltage (Vin-V2) function, namely
Modality 4(t3-t4) iLR(t) drops zero in the t3 time, and maintains invariably, the electric circuit enters the LM charge stage.
Modality 5(t4-t5) t4 time S shuts off, the Cs electric discharge, the vCs(t) linearity drops, realizes main switch’s ZVS shutdown. Therefore on the S voltage is
In t5 (corresponded in front t0) the time Cs electric discharge finishes, started the next action cycle.
3 new shutdown electric circuit ZVS realizes the condition
According to the modality 3, the electric circuit can realize the ZVS condition is in the t2 time, the vCs(t) maximum value must be bigger than Vin. But the vCs(t) maximum value is decided by N2/N1 and difference of the input output voltage, according to the type (1) and the type (3), and supposes m=Vo/Vin, then has
Its curve as shown in Figure 6, satisfies the ZVS condition located at the curve underneath region.
5 may see by the modality, S decided time the shutdown voltage rise speed by Cs and Im, sees type (6). Regarding definite Cs, when Im is bigger (i.e. load current greater time), its slope is also big, behind gives the experimental oscillogram may also explain this point.
4 experimental results
According to the above theoretical analysis, has designed CCM the Buck converter. Its main parameter is as follows: Vin=40V; Vo=13V, LR=10mH, Cs=44nF, N2/N1=1, Lm=74mH; Ds and DR use the super speed to restore the diode, Do are the Short base diodes. The turn-on frequency is 50kHz.
The experimental study electric circuit under the different load current condition, the main circuit switching process’s testing voltage and current waveform distinction like chart 7~ shown in Figure 9. The figure shows, the switching valve has realized ZVS, and switching valve’s voltage stress is equal to the input voltage; The clear process is also approximate in the ZCS clear, the extra resonance current component was opposite in the main electric current is not big, will not have the adverse effect to electric circuit’s work.
Figure 7 and Figure 8
5 conclusions
This article proposed the new ZVS shutdown absorbing circuit, has used under ingeniously the different on-off state the main inductance voltage polar difference, comes to control ZVS to shut off electric capacity’s charge and the electric discharge appropriately, thus has realized the soft switch. May know from the theoretical analysis and the experimental result, this kind of passive lossless absorbing circuit may realize switching valve ZVS to shut off, clear also close ZCS effect. And the switching valve withstands the greatest voltage stress is equal to the input voltage.
This kind of ZVS shutdown absorbing circuit needs sur- primary device quantity are very few, moreover may promote very easily to other each kind of basic topology.
