Abstract: Analyzed one kind very suitably to work under the ultra-high frequency multi-resonant DC/DC converter. This converter’s all switching valve work under the ZVS condition, all rectification diode work under the ZCS condition. This converter structure is simple, entire converter only needs a magnetic cell. And multianalysis this converter’s ultra-high frequency compatibility. A 135V input, the 54V/3A output, the turn-on frequency was higher than 1MHz the prototype to confirm its principle of work and the ultra-high frequency compatibility. This prototype the efficiency achieves 88.7% under the rated conditions.
Key word: Multi-resonance; Soft switch; Converter
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. At present applied the soft switch technology, the converter turn-on frequency already might very with ease surpass 100kHz. The soft switching circuit may divide into the cushion and the control two kinds. The cushion soft switch analysis situs often attached the extra line, increased the cost, reduced the reliability, lets the user accept with difficulty. The control soft switch does not increase main circuit’s primary device, realizes the soft switch through the reasonable design control circuit, compared with easy to let the user use. At present, the mature control soft switching circuit are not many, typical has phase shifting entire bridge [1], asymmetrical half bridge [2] and so on. These are the PWM converters, realizes the soft switch through the edge resonance, can reduce the switching loss, but basic does not increase the voltage or the electric current effective value. However, this kind of electric circuit is very difficult achieves all semiconductor device truly (including switching valve and diode) the soft switch. For example, the phase shifting entire bridge and the asymmetrical half bridge’s rectification diode is the hard shutdown, has seriously restores the question reverse. Therefore, these electric circuit being unable works in the higher turn-on frequency. Therefore, when the turn-on frequency must further enhance, quite suitable to use the resonance converter.
Below proposed more than one kind of resonance’s DC/DC converter, the turn-on frequency has surpassed 1MHz. This converter’s all semiconductor device has realized the soft switch, is an ultra-high frequency converter’s very good choice.
1 principle of work
Shown in Figure 1 is half bridge structure LLC connects the multi-resonant converter: Two main switch S1 and S2 constitute -and-a-half a bridge structure, its driving signal is the dutyfactor fixed 50% supplementary signal, realizes output voltage constant through the change turn-on frequency. Therefore, this kind of resonance converter may also classify in the control soft switching circuit. Inductance Ls, electric capacity Cs and transformer’s excitation inductance Lm constitutes a LLC resonance network. This resonance network connections between half bridge’s center point and place, therefore, resonant electric capacity Cs also plays separates the straight electric capacity’s role. In output side, rectification diode D1 and D2 constitution center tap’s leveling circuit, on rectification diode direct connection output capacity Co.
Figure 1
The LC intrinsic resonance frequency defines as
This article states LLC connects the multi-resonant converter’s turn-on frequency scope is fm<f<fs.
In the following analysis, Co was considered that is the infinity, but replaces by constant pressure source Vo, the main switch has the reverse parallel diode. This converter’s switching period may divide into 6 working stages, its equivalent circuit as shown in Figure 2. Corresponding work profile as shown in Figure 3. 6 working stage’s principle of work is as follows.
Figure 2 various stages equivalent circuit
1) the stage 1(t0~t1) shuts off in t0 time S2, resonance current ir to the S1 output capacity electric discharge, S1 leaks - source voltage vds1 to start to drop, when vds1 drops zero, S1 body diode breakover. The input voltage adds in the LLC series connected return route. In vice-side, transformer winding’s polarity for on under negative, the D1 breakover, the Lm voltage by the output voltage Vo clamp, the resonance is in fact had between Ls and Cs, on Lm electric current im linearity rise.
2) the stage 2(t1~t2) clears in t1 time S1 under the zero potential condition. im continues the linear rise, ir flows through S1 and rises gradually by the sine wave form. Winds through D1 the output current difference of for the resonance current and the exciting current. The switching period is bigger than Ls and the Cs resonant cycle, therefore, after ir passes through half resonant cycle, S1 was still at the clear condition. When ir drops to is equal with im, the D1 electric current because of the zero crossing shuts off. This working stage ended.
Figure 3 prime task profile
Because adds on Lm the voltage is nVo, im may express is
In the formula: Im is the exciting current maximum value;
Vo is the output voltage;
n is the transformer primary side to the vice-side ratio-turn.
3) the stage 3(t2~t3) shuts off under the t2 time D1ling electric current condition. The output side and the resonant tank is separated from completely. The Lm voltage no longer receives the Vo limit, Lm and the Ls series participation resonance. Circuit design Lm>>Ls usually, therefore, the resonant cycle lengthens obviously. ir maintains invariable basically, may think
ir(t)=im(t)=Im (5)
In this stage, ir continues to the Cs charge, the Cs voltage to continue to rise, yizhi dao the t3 time, S1 shuts off, starts the second half action cycle.
Working stage 4, 5, 6 and working stage 1, 2, 3 similar. What is different is the resonant initial energy provides by resonant electric capacity Cs. Work profile and stage 1, 2, 3 full symmetries.
4) the stage 4(t3~t4) shuts off in t3 time S1, ir to the S2 output capacity electric discharge, S2 leaks - source voltage vds2 to start to drop, when vds2 drops zero, S2 body diode breakover. In vice-side, transformer winding’s polarity for on negative under positive, the D2 breakover, the Lm voltage by the Vo clamp, the resonance is in fact had between Ls and Cs, on the Lm electric current im linearity drops.
5) the stage 5(t4~t5) clears in t4 time S2 under the zero potential condition. im continues the linearity to drop, ir flows through S2 and by the sine wave form negative growth. Winds through D2 the output current difference of for the resonance current and the exciting current. In this operating frequency scope, the switching period is bigger than Ls and the Cs resonant cycle. Therefore, passed through a half cyclical in ir the resonance, S2 was still at the clear condition. When ir drops to is equal with im, the D2 electric current because of zero has shut off. This working stage ended.
6) the stage 6(t5~t6) shuts off under the t5 time D2ling electric current condition. The output side and the resonant tank is separated from completely. The Lm voltage no longer receives the Vo limit, Lm and the Ls series participation resonance. ir maintains invariable basically, continues to the resonant electric capacity Cs electric discharge, the Cs voltage to continue to drop, yizhi dao the t6 time, S2 shuts off, new action cycle start.
Supposes ir to t6 maintains invariable in t2 to t3 as well as t5, and indicated by Im, then output voltage Vo may express is
In the formula: Vin is the input voltage;
T is the switching period;
Ts is Cs and the Ls resonant cycle, Ts=1/fs=
(6) may see from the type, the output voltage increases along with the switching period increases.
2 high frequency compatible analyses
Above analyzes the LLC multi-resonance converter is suitable to use in the turn-on frequency very high situation, its reason is as follows.
1) all switching valves work under the ZVS condition, the switching loss is zero nearly. Switching valve’s zero potential is realizes by the initiation inductance in excitation circuit to switching valve’s junction capacitance charging and discharging. Therefore, regarding load current’s change, its zero potential clear’s condition basic will not change, this point must surpass the phase shifting entire bridge and so on other control soft PWM electric circuit. Moreover, the LLC multi-resonance converter’s initiation inductance is an achievement resonance inductance, uses for to adjust the input output voltage the relations, itself will design quite slightly. Looking from passes condition the loss, this point is disadvantageous, but, realizes the condition from the soft switch to look that is actually very advantageous, therefore, has the superiority in ultra-high frequency situation this electric circuit. The ZVS boundary condition like type (7) shows (boundary condition meaning is supposition dead time may be willfully big, can realize the ZVS critical condition).
In the formula: Coss1 and Coss2 respectively are two switching valve’s output capacities.
-like (4) the substitution type (7), might result in ZVS again the boundary condition further expression for the type (8).
In fact, in the LLC multi-resonance converter, the type (8) is very easy to satisfy, but the dead time will not be big, therefore, may think approximately in the initiation inductance’s electric current will maintain invariable in the dead time, will be a constant current in carries on the charging and discharging to switching valve’s junction capacitance. The ZVS condition in this case is called the ample condition, the expression for the type (9).
In the formula: tdead is the dead time.
-like (4) the substitution type (9), might result in ZVS again the ample condition further expression for type (10).
2) all vice-side diodes work under the ZCS condition, restores reverse the influence is very small. But the ordinary control soft PWM electric circuit only has realized switching valve’s soft switch, but does not have to solve the diode to restore the question reverse well, therefore, (e.g. above 1MHz) uses in the turn-on frequency very high situation has the difficulty. The vice-side diode’s current waveform approximate is a sine, passed condition the loss regarding the reduction is the shortcoming, but applied in the ultra-high frequency situation, the switching loss must compared to pass condition the loss difficultly to process a lot, therefore, this electric circuit application also had a superiority in the ultra-high frequency situation.
3) the ordinary control soft PWM electric circuit works above 1MHz other reason is with difficulty, the transformer leakage inductance is very difficult under high frequency to process. Specially considered original vice-side insulation strength time, the transformer leakage inductance is very difficult to do is small, but under the ultra-high frequency, the leakage inductance influence is also very obvious. The LLC multi-resonance converter’s leakage inductance is an achievement resonance inductance perhaps a resonant inductance part, itself hoped that can design the leakage inductance big somewhat. Usually with difficulty designs the leakage inductance which in the low frequency situation needs, but wants the sur- resonant inductance, but is quite easy in the high frequency situation the leakage inductance which designs needs. Therefore, this is also this electric circuit suitable to use in the ultra-high frequency situation the reason.
3 experimental results
Turn-on frequency 1MHz the above DC/DC converter has confirmed this multi-resonant converter principle of work and the high frequency compatibility.
This converter’s specification and the main parameter are as follows:
Input voltage Vin 135V;
Output voltage Vo 54V;
Output current Io 0~3A;
Lowest operating frequency f 1MHz;
Main switch S1 and S2 IRFP250;
Rectification diode D1 and D2 30CPQ150;
Transformer T n=13∶ (7 7), Lm=15μH, Ls=6μH;
Resonant electric capacity Cs 4.4nF (the Cs actual capacity must be smaller than under high frequency this value).
Figure 4 has given this converter’s under different load conversion efficiency. Its peak efficiency achieved 89.5%, the full load efficiency has achieved 88.7%.
Figure 5 is inputs when 135V the main experiment profile. Figure 5 (a) is the full load (3A) when S2 vds and the vgs profile, may see that S2 slaving voltage vgs is in the vds voltage drops after the zero only then starts to rise, therefore, is the zero potential clear. S1 vds and the vgs profile is also similar, here 11 did not give. Figure 5 (b) is the primary side resonance voltage and the current waveform, every a half cycle has two resonant processes, respectively is Cs and the Ls resonance, Cs and (Ls Lm) resonance. Figure 5 (c) is on the rectification diode D1 voltage and the current waveform. May see that the electric current is arrives at zero by the sine shape resonance, but presents certain reverse recovery current. This is because the turn-on frequency is 1MHz, although is the sine current waveform, but its di/dt is quite big. If changes into the general PWM electric circuit under the similar frequency, restores the question to be able reverse to be more serious. Therefore, uses the ordinary Short base or restores the diode quickly, PWM electric circuit also being unable generally works under the 1MHz frequency. Here diode voltage also because will restore reverse, but overswing, but, its overswing voltage has not surpassed 2 time of output voltages, therefore, here may use 150V the Short base diode, this will be unable in the general PWM electric circuit to achieve.
Figure 6 has given this converter the work profile which (120kHz) corresponds under low frequency, uses for to take the contrast. May see presented certain reverse recovery current besides diode D1, under 1MHz turn-on frequency working condition and under 120kHz turn-on frequency working condition basic same, from this explained that this electric circuit’s ultra-high frequency compatibility is good.
4 conclusions
This article proposed the LLC multi-resonance converter’s switching valve realized ZVS, the rectification diode has realized ZCS, and happen to made the resonant inductance using the leakage inductance, therefore, very suitable to work under the ultra-high frequency switch (above 1MHz). The entire converter is also simple, only needs magnetic elements, therefore, this converter is suitable for the superelevation power density situation.
