Abstract: Proposed one kind of Boost electric circuit soft switch realizes the method, namely the synchronized rectification adds on the inductance current reversal. Realized the soft switch’s condition according to two switching valves to be different, proposed the strong tube and the weak tube’s concept, has given the full weak feet switch condition design method. A 24V input, the 40V/2.5A output, the turn-on frequency was the 200kHz synchronized Boost converter prototype has further confirmed the above method accuracy, its full load efficiency has achieved 96.9%
Key word: Boosted circuit; Soft switch; Synchronized rectification
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 component’s switching loss, therefore the soft switch technology arises at the historic moment. Generally, must realize the quite ideal soft switch effect, needs one or one above auxiliary switch creates the soft switch’s condition for the focus switch, simultaneously hoped that the auxiliary switch itself can also realize the soft switch.
The Boost electric circuit takes one kind of most basic DC/DC analysis situs, but widely applies in each kind of power source product. Because the Boost electric circuit only contains a switch, therefore, must realize the soft switch often to attach active or the passive extra electric circuit, increased converter’s cost, reduced converter’s reliability.
The Boost electric circuit besides has a switching valve also to have a diode. Is comparing the low pressure output the situation, itself hoped that replaces the diode with MOSFET (synchronized rectification), thus obtains the quite high efficiency. If can use this synchronizer switch to take main switch’s auxiliary tube, creates the soft switch condition, simultaneously itself can realize the soft switch, that will be a quite good plan.
This article proposed one kind of Boost electric circuit realizes the soft switch’s method. This plan is suitable for the output voltage low situation.
1 principle of work
What shown in Figure 1 has two switching valve’s synchronized Boost electric circuits. Its two switch supplementary breakover, middle has certain dead area to prevent altogether condition the breakover, as shown in Figure 2. Usually in the design in the inductance electric current is a direction, like Figure the 2 5th profile shows. Considered switch’s junction capacitance as well as the dead time, one cycle may divide into 5 stages, each stage’s equivalent circuit as shown in Figure 3. Below described the inductance electric current not to change the direction simply the synchronized Boost electric circuit’s principle of work. Under this kind of design, S2 may realize the soft switch, but S1 can only work in the hard on-off state.
1) the stage 1(t0~t1) this stage, the S1 breakover, on L withstands the input voltage, on the L electric current linearity increases. In the t1 time, S1 shuts off, this stage ended.
after 2) stage 2(t1~t2)S1 shutdown, the inductance electric current carries on the charge to the S1 junction capacitance, causes S2 the junction capacitance to carry on the electric discharge, S2 leaks the source voltage to be possible to think the linearity to drop approximately, until drops 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 clip in the zero potential condition, was also is the S2 zero potential breakover has created the condition.
4) the stage 4(t3~t4)S2 gate becomes the high level extremely, the S2 zero potential clear. On the inductance L electric current winds through S2. On L withstands difference of the output voltage and the input voltage, the electric current linearity reduces, shuts off until S2, this of stage ended.
5) on stage 5(t4~t5) this time inductance L direction of current still for positive, therefore this electric current can only shift to the S2 parasitic diode, but is unable to the S1 junction capacitance to carry on the electric discharge. Therefore, S1 is the work in the hard on-off state.
Then the S1 breakover, enters the next cycle. From the above analysis may see that S2 has realized the soft switch, but S1 has not realized the soft switch. After its reason is the S2 shutdown, in the inductance direction of current is, is unable to cause S1 the junction capacitance to carry on the electric discharge. But, if designs enough L is small, lets the inductance electric current when the S2 shutdown for negative, as shown in Figure 4, might carry on the electric discharge to the S1 junction capacitance to realize the S1 soft switch.
In this case, one cycle may divide into 6 stages, each stage’s equivalent circuit as shown in Figure 5. Its principle of work description is as follows.
1) the stage 1(t0~t1) this stage, the S1 breakover, on L withstands the input voltage, on the L electric current is increasing to the linearity, becomes from the negative value just when. In the t1 time, S1 shuts off, this stage ended.
after 2) stage 2(t1~t2)S1 shutdown, inductance electric current for positive, carries on the charge to the S1 junction capacitance, causes S2 the junction capacitance electric discharge, S2 to leak the source voltage to be possible to think approximately the linearity drops. Leaks the source voltage until S2 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 clip in the zero potential condition, was also is the S2 zero potential breakover has created the condition.
4) the stage 4(t3~t4)S2 gate becomes the high level extremely, the S2 zero potential clear. On the inductance L electric current winds through S2. On L withstands difference of the output voltage and the input voltage, the electric current linearity? Is small, until becomes the negative value, then S2 shuts off, this stage ended.
5) on the stage 5(t4~t5) this time inductance L direction of current for negative, happen to may cause S1 the junction capacitance to carry on the electric discharge, carries on the charge to the S2 junction capacitance. S1 leaks the source voltage to be possible to think approximately the linearity drops. Leaks the source voltage until S1 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 on the breakover, S1 leaks the source voltage clip in the zero potential condition, was also is the S1 zero potential breakover has created the condition.
Then S1 under the zero potential condition the breakover, enters the next cycle. May see that under this kind of plan, two switch S1 and S2 may realize the soft switch.
2 soft switch’s parameter designs
The above the means realizes the Boost electric circuit’s soft switch with the synchronized rectification coil loading current reversal, two switches realize the soft switch’s difficulty degree not to be the same. The inductance electric current’s Feng Feng value may express is
δi= (VinDT)/L (1)
In the formula: D is a dutyfactor;
T is the switching period.
Therefore, in the inductance the electric current maximum value and the minimum value may express are
Imax=ΔI/2 Io (2)
Imin=ΔI/2-Io (3)
In the formula: Io is the output current.
-like (1) the substitution type (2) and the type (3) may result in
Imax=(VinDT)/2L Io (4)
Imin=(VinDT)/2L-Io (5)
May see from the above principle analysis S1 the soft switch condition is by Imin to the S2 junction capacitance charge, causes S1 the junction capacitance electric discharge to realize; But the S2 soft switch condition is by Imax to the S1 junction capacitance charge, causes S2 the junction capacitance electric discharge to realize. Moreover, usually in full load situation|Imax| 韡 Imin|. Therefore, S1 and the S2 soft switch realizes the difficulty degree to be also different, S1 must be much more difficult than S2. Here is called S1 the weak tube, S2 is called the strong tube.
Strong manages S2 the soft switch boundary condition is L and S1 junction capacitance C1 and the S2 junction capacitance C2 resonance, can let on C2 the series resonance to zero condition, may express for the type (6).
-like (4) the substitution type (6) might result in
In fact, the type (7) very easy to satisfy, but the dead time is impossible to be big, therefore, may think approximately the electric current maintains invariable on dead time internal inductance L, is a constant current to the S2 junction capacitance charge, causes S1 the junction capacitance electric discharge. The ZVS condition in this case is called the ample condition, the expression for the type (8).
(C2 C1)Vo≤ (VinDT/2L Io) tdead2 (8)
In the formula: before tdead2 is the S2 clear dead time.
Likewise, weakly manages S1 the soft switch ample condition is
(C1 C2)Vo≤(VinDT/2L-Io)tdead1 (9)
In the formula: before tdead1 is the S1 clear dead time.
In the actual electric circuit’s design, the strong tube’s soft switch condition is easy to realize, therefore, the key designs the weak tube’s soft switch condition. First determined may withstand the biggest dead time, then according to type (9) calculated the inductance measures L. Because, in can realize under the soft switch’s premise, L is not suitable is too small, in order to avoid creates on the switching valve the oversized electric current effective value, thus causes switch’s breakover loss to be oversized.
3 experimental results
A turn-on frequency was 200kHz, the power is the 100W inductance current reversal the synchronized Boost converter further confirms the above soft switch to realize the method accuracy.
This converter’s specification and the main parameter are as follows:
Input voltage Vin24V
Output voltage Vo40V
Output current Io0~2.5A
Operating frequency f200kHz
Main switch S1 and S2IRFZ44
Inductance L4.5μH
Figure 6(a), Figure 6 (b) and Figure 6(c) is the full load (2.5A) the time experiment profile. May see from Figure 6(a) on inductance L the electric current can be reverse in DT or in the (1-D)T time interval, also has created the S1 soft switch’s condition. From Figure 6(b) and Figure 6(c) may see that has realized ZVS to two switch S1 and S2. But from the voltage vds drop slope looked that S1 must miss compared to the S2 ZVS condition, this is the strong tube and the weak tube’s difference.
Figure 7 has given this converter’s under different load current transfer efficiency. The peak efficiency has achieved 97.1%, the full load efficiency is 96.9%.
4 conclusions
This article proposed one kind of Boost electric circuit soft switch realizes the strategy: Synchronized rectification coil loading current reversal. Under this plan, two switching valves act according to the soft switch condition the difference, divides into the strong tube and the weak tube. In the design must act according to the weak tube’s critical soft switch condition to decide inductance L the size. Because has realized the soft switch, the turn-on frequency may design quite high. The inductance quantity may design very much slightly, needs the inductance volume may also quite small (usually be possible to use I magnetic core). Therefore, this kind of plan is suitable for the high power density, the low output voltage situation.
