Abstract: Introduced one kind of active clamp Flyback converter ZVS realizes the method, and to its soft switch parameter redesign. When this plan can realize main auxiliary switching valve’s ZVS, the limit output rectification diode shuts off di/dt, reduces the rectification diode’s switching loss, simultaneously also effectively reduced switching valve’s voltage stress.
Key word: Zero potential switch; Current reversal; Active clamp
Introduction
Because the Flyback converter its electric circuit is simple, is used generally in the low power situation. But, as a result of the transformer leakage inductance’s existence, causes on the switching valve the high voltage stress. Ordinary RCD inlays the position Flyback converter its leakage inductance energy consumption to inlay on position resistance R, on the switching valve voltage stress’s size is decided in the consumption in inlays in the position resistance the energy size. The consumption in inlays in the position resistance the energy to be more, switching valve’s voltage stress is lower, but has also affected the entire converter’s efficiency, therefore, ordinary RCD inlays the position Flyback converter always to have between the switching valve voltage stress and the entire converter efficiency contradiction.
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. 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.
This article introduced one kind active inlays the position Flyback soft switching circuit, not can only realize ZVS, moreover also solved fore-mentioned ordinary RCD to inlay the question which in the position Flyback converter existed.
1 principle of work
Electric circuit as shown in Figure 1, its two switch S1 and the S2 supplementary breakover, middle has certain dead area to prevent altogether condition the breakover. Transformer initiation inductance Lm designs in a big way, causes the electric circuit work in the electric current continual pattern (CCM), like Figure 2 iLm profile shows. But inductance Lr designs slightly (Lr 頛 m), causes to wind through Lr the electric current to be possible to be reverse in one cycle, like Figure 2 iLr profile shows. Considered switch’s junction capacitance as well as the dead time, one cycle may divide into 8 stages, each stage’s equivalent circuit as shown in Figure 3. Its principle of work is as follows.
1) the stage 1(t0, t1) this stage S1 breakover, Lm and the Lr series withstanding input voltage, winds through Lm and the Lr electric current linearity rise.
V2=Vin(Lin/Lm Lr) (1)
As a result of Lr 頛 m, therefore the type (1) may simplify as
V2≈Vin (2)
2) stage 2(t1, t2)t1 time S1 shuts off, on Lm and the Lr electric current for the S1 output junction capacitance Cr1 charge, simultaneously causes S2 the output junction capacitance Cr2 electric discharge. t2 time S2 leaks the source voltage to drop zero, this stage ended.
Figure 2
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. Lr and the Lm series with inlays the position electric capacity Cclamp resonance, on Cclamp the voltage vc slow rise, on v2 the voltage also slowly rises.
v2=(Lm/Lm Lr)vc (3)
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. This time the compartment still maintains Lr and the Lm series with inlays the position electric capacity Cclamp resonance, the v2 slow rise.
5) stage 5(t4, t5)t4 time v2 rises to certain voltage causes the vice-side diode D breakover, v2 to inlay the position in - NVo. Lr and Cclamp resonance. In guaranteed that t5 the time Lr current reversal in the situation, its resonant cycle should satisfy
In the formula: toff primarily in switching valve S1 one cycle turn-off time.
Figure 3
t5 time S2 shuts off, this stage ended.
6) stage 6(t5, t6)t5 on the time Lr direction of current for negative, this electric current part causes S1 the output junction capacitance Cr1 electric discharge, another part to the S2 output junction capacitance Cr2 charge. 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 on the breakover, S1 leaks the source voltage clip in the zero potential condition, was also the S1 zero potential breakover has created the condition. This time, on Lr withstanding voltage v1 is
v1=Vin NVo (5)
On Lr the electric current rises fast. Winds through vice-side rectification diode D electric current iD fast to drop.
diD/dt=-N[Vin NVo]/Lr NVo/Lm) (6)
Considered Lr 頛 m, the type (6) may simplify as
diD/dt=-N(Vin NVo)/Lr (7)
stage 8(t7, t8)t7 the time S1 gate becomes the high level extremely, the S1 zero potential clear, winds through the parasitic diode’s electric current to flow through S1. t8 the time vice-side rectification diode D electric current drops zero, the D nature shuts off, the electric circuit starts to enter the next cycle.
May see that under this kind of plan, two switch S1 and S2 has realized the zero potential clear, the diode D nature shutdown.
2 soft switch’s parameter designs
Supposes the electric circuit work in the CCM condition. Because the S2 soft switch realizes is Lr and Lm jointly is sufficient to Cr1 and Cr2? Electricity, but the S1 soft switch realizes is independent Lr to Cr1 and the Cr2 charging and discharging. Therefore, the S2 soft switch realizes is quite easy, but the S1 soft switch realizes must be much more difficult on the other hand. Therefore, in the parameter design, the key is must consider S1 the soft switch condition.
The electric current continual pattern active inlays position Flyback the converter ZVS design procedure to state as follows.
2.1 transformer initiation inductance Lm hypothesis
As a result of the Lr 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 as a result of t5~t8 time iLr ascending velocity unusual quick, therefore may think Deff=D approximately. Thus, according to Flyback electric circuit work in the CCM condition, then
In the formula: Eta is the converter efficiency;
fs is the turn-on frequency;
PoCCM is converter’s output.
In actual design, for guarantee circuit in underloading time can also work in the electric current continual pattern, Lm takes is generally
2.2 inductance Lr hypothesis
In order to realize the S1 ZVS, t5 time to store up in Lr the energy is discharged sufficiently S1 output junction capacitance Cr1 zero, simultaneously causes S2 output junction capacitance Cr2 to charge in a big way. Namely
In the formula: vds=vds1=vds2≈Vin NVo;
Cr=Cr1 Cr2.
(4) determines the appropriate resonant cycle according to the type to be possible to make
2.3 electric capacity Cclamp hypothesis
(4) has according to the type
In satisfies-like (15) under the premise, determines appropriate Cclamp to make iLrmax=iLrmin.
2.4 dead time determination
In order to realize S1 ZVS, must guarantee that in t6 to the t7 time, S1 starts the breakover. Otherwise on Lr the current reversal, to the Cr1 charge, such S1 ZVS condition will lose again. Therefore, after S2 shuts off, before, the S1 clear’s dead time hypothesis realizes very important to S1 ZVS. Appropriate dead time for inductance Lr and S1 and S2 output junction capacitance resonance cycle 1/4, namely
Strictly speaking, the switching valve output junction capacitance 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 switching valve S1 dutyfactor D.
Deff=D-ΔD (17)
[(Vin NVo)/Lr] δdt≈2 (P/DVin) (18)
δD≈2PLrfs/DVin (Vin NVo) (19)
Substitution type (17)
Deff=D-2PLrfs/(DVin(Vin NV0) (20)
2.6 switching valve voltage stress computation
Vs1, s2≈Vin NVo (2PLrfs/DVin(1-D) (21)
The type (21) the third item is on the other hand small, therefore switching valve’s voltage stress approaches in Vin NVo.
3 experimental results
In order to confirm above ZVS to realize the method, has designed an experiment electric circuit, its specification and the main parameter are as follows:
Input voltage Vin48V;
Output voltage Vo12V;
Output current Io0~5A;
Operating frequency f100kHz;
Main switch S1 and S2IRF640;
Transformer initiation inductance Lm144μH;
The transformer original vice-side number of windings compare n=N8/3;
Inductance Lr10μH;
Electric capacity Cclamp2μF.
Figure 4 what gives is time the load current Io=2A experiment profile. From Figure 4 (e) and Figure 4 (f) may see that S1 and S2 have realized ZVS. Figure 5 has given two kind of Flyback electric circuit’s efficiency curve, may see that active inlaid the position Flyback soft switching circuit to promote converter’s efficiency effectively.
4 conclusions
Active inlays the position Flyback soft switching circuit while to realize the main switch and auxiliary switch ZVS, also has realized the output rectification diode’s nature shutdown, therefore, reduced the switching loss effectively, raised the converter efficiency. Moreover, it also greatly reduced switching valve’s voltage stress, this may look quite from the experiment profile clearly.
