Asymmetric semi-bridge DC/DC converter of zero voltage switch
0 foreword The asymmetric semi-bridge DC/ DC converter has advantage such as being simple in control small in quantity of working, device of a soft switch, so, widely used in the middle low power inverter less than 1000W. However, in traditional asymmetry semi-bridge circuit topology, realize the zero voltage switch when sending oxen electric capacity of voltage transformer leakage inductance and main switch produce syntony, so, in order to realize the soft switch, syntony inductance (namely voltage transformer leakage inductance) Worth must enough large. And serious syntony will take place in current conversion in the syntony inductance and parasitic capacitance of outputting the kenotron, produce voltage surge, even puncture the output diode, and great leakage inductance will cause the great duty ratio to lose. In order to prevent the output diode from working and damaging by mistake, the over voltage that must limit from the voltage transformer leakage inductance and the parasitic parametric resonance of diode produces. Usually, can limit this voltage while adding clamping and absorbing circuit at both ends of the diode, for example, the method to often use is to add the resistance - electric capacity - absorbing circuit of diode (RCD circuit) at both ends of the diode To restrain the voltage. But the shortcoming of the maximum of this circuit is that energy is nearly all consumed on the resistance, this will obviously reduce the efficiency of this converter. In addition, fluctuation of voltage will sustain with lower frequency appearing, and very much difficult to dispel.
1 catching diode of function A very good solution is to add the catching diode on the primary side of the voltage transformer Tr, : The purpose to add catching diode is on the premise of not changing the operating characteristic of the converter, over voltage produced with external inductance syntony while dispelling and outputting the current conversion of the rectifier tube, clamp the voltage transformer Tr primary side voltage in the terminal voltage Vc1 and Vc2 of the electric capacity C3 and C4 through these two diodes. Its course is: Unless last duty ratio to be D if switch S1, then S, duty ratio for 1- D,turn on to S1, clamp being Vc1 voltage transformer the voltages of the primary sides diode Dg1, when S2 turns on, it is - Vc2 that the voltage of the primary side of the voltage transformer is clamped by Dg2, the electric potential of the secondary has been clamped accordingly, export the kenotron (Dr1 and Dr2) Having will not present obvious voltage surge either. At this moment, the energy of importing to voltage source and electric capacity and absorbing and outputting the rectifier tube through the catching diode and the external inductance syntony produces, and the electric current through the catching diode is very small, and they act only while exporting current conversion of rectifier tube, so, their impact on working phase of the whole converter is very small. Clamp and decrease voltage transformer leakage inductance through primary side of the voltage transformer, it is possible to totally remove and export and commutate RCD absorption circuit of the pipe end. However, the leakage inductance of the voltage transformer can not be totally dispelled, can't be restrained and outputted the over voltage which commutate the pipe end of the dipole sometimes only through the clamping of primary side, must also be outputting and commutating the pipe end of the dipole and connecting RCD absorbing circuit in parallel, RCD absorbing circuit only plays an auxiliary role at this moment, its every parameter value is different while too and only using a piece of CD absorbing circuit, usually fetch minor electric capacity and relatively large resistance. The original side band clamp circuit method of this kind of voltage transformer is only suitable for the discontinuous working condition (DCM) of inductive current of syntony . Now make a concrete analysis of the course of the switch of zero voltage of this circuit implementation.
2 working procedure analysis In order to simplify analyzing, we assume as follows: 1)The gas switching tube S1 and S2 regards and parasitic capacitance (C1, C2) as the ideal switch respectively , backward diode (D1, D2) Connect in parallel, does not consider MOSFET tube reverse leakage current; 2)Simplify and unite the initiation inductance (Lm) for the ideal voltage transformer Zhyuan with voltage transformer , series connection leakage inductance (L1k) Model; 3)The electric capacity C3 and C4 is regarded as the invariable voltage source; 4)Export and regard as the invariable current source, its value is Io; 5)Consider the current conversion effect of the diode D1, D2, Dc1, Dr2; 6)Other devices are ideal devices, the circuit enters the steady state; Knowing by Fig. 1, when S1 turns on, the voltage that A clicks is DE, and the average voltage on a cycle internal inductance Lm, L1k and Lr is 0, so, the voltage on the electric capacity C2 is DE, and the voltage on the electric capacity C1 is (1 - D) E. The feed-through time to export the kenotron Dr1, Dr2 is unequal, forward current and reverse current of the primary side of the voltage transformer are unequal, it is zero that the inductance Lm can absorb the average current by guaranteeing to flow through electric capacity C3 and C4 of its difference. Sting and regard dividing as 12 working stages in one switch cycle of this converter, its working waveform, among them vGS1 and vGS2 are S1 and drive wave form of S2 respectively. Can see the working waveform is symmetric in the first half a piece of cycle sum latter half and a cycle, the working phase is similar, so, only analyze half a piece of 6 periodic working stages as follows, 6 pieces of equivalent electrical circuit parting.
1)S1 turns on at the point of t0 the stage 1 [t0 t1 ], the electric current of primary side flows through S1, the direction, the magnitude is the specified load current In and sum Io +iLM of the exciting current iLM. The voltage between A, B is (1 - D) E, the inductance Lm of initiation is endergonic, the electric capacity C3 discharges. This stage is that power conveys stage, this end of procedure when t1 moment S1 shuts off. 2)2 stage S1 shut off electric current begin circulation among C1, C2, the intersection of electric capacity and C1 charge linearly, the intersection of electric capacity and C2 discharge linearly, so, S1 shuts off for the zero voltage. The voltage begins the linear decline too among A, B, VAB voltage is zero hour at the point of t2, this stage is over. 3)VAB voltage of the stage 3 [t2 t3 ] is zero, exports the kenotron and shorts out (Dr1, Dr2 current conversion) ,The Ausgang absorbs the exciting current, inductance Lr, L1k, electric capacity C1, C2 syntony, in order to realize the transformation of the working condition. 4)4 stage the intersection of inductance and Lr, the intersection of L1k and residual energy present, return power through the intersection of diode and D2, as iLr being zero hour, S2 turns on, this stage is over, S2 opens for the zero voltage. 5)S2 turn on at the point of t4 the stage 5 [t4 t5 ], the resonance current iLr veers, the voltage on Lr, L1k is DE, iLr increases to Io +iLM reversely linearly, this stage is over. 6)At the end of stage 5, exporting the kenotron Dt2 is shut off at the stage 6 [t5 t6 ], the voltage of the original brink of voltage transformer rises rapidly. Because the inductance Lr and clamping and syntony of the parasitic capacitance of kenotron, the voltage of the original brink of voltage transformer will be higher than stable value DE, at this moment, the diode Dg2 clamps the electric potential of the point C, the electric capacity of syntony passes inductor Lr release energy. Make half periodic 6 working stage and the above-mentioned resembling while being subsequent, no longer detail. Its waveform sees Fig. 2. Half periodic 6 working stage can be learnt under the analogize from half periodic 6 working stage on being already analytic: S1, S2 work in the on-off state of the zero voltage. The each switch one voltage is in stress E, it clamp catching diode Dg1, Dg2 voltage transformer Tr primary side voltage UCB until - DE and (1 - D) Between E, then the voltage of Tr secondary is clamped too, exporting will not present obvious voltage surge either on the kenotron Dr1, Dr2.
3 output characteristic analysis Knowing by the analysis of the working phase above, the leading-in of the catching diode Dg1 and Dg2, have not obviously changed the operating characteristic of the converter. It for two reasons: First the electric current through this catching diode is very small, second they act only while exporting current conversion of kenotron, recovery time is very short, so, introduce the operating characteristic that the catching diode has not changed the converter. The zero-frequency gain q of this converter is
In the type: Vo ' Io ' whether output voltage, electric current convert value to reach the primary side separately. By formula (1) It is obvious, the zero-frequency gain of this converter is average voltage drop on inductance Lr of syntony V( =4LrfsIo' ) And the function of the duty ratio D. The output characteristic can be shown by Fig. 4.
4 experimental results In order to verify the above-mentioned analysis, produce a direct-flow input 300~450V, output the asymmetry semi-bridge experiment prototype machine of 54V/ 6A, its specification and specification are: Input the voltage 300~450V; Voltage 50V of the output; Export the electric current 0~6A; Working frequency 100kHz; Main switch S1 and S2 IRF840; Catching diode Dg1 and Dg2 MUR860; Kenotron Dr1 and Dr2 30CP0150; Syntony inductance Lr 40μ H; Parameter n of the voltage transformer =50: 20: 20,Lm =1.2mH, Ls =162μ H. Fig. 5(a) It is S1 vGS1 and vDS1 waveform of normal working hour, S2 vGS2 and vDS2 waveform and Fig. 5(a) of normal working hour Similar, they open when the voltage is zero. Fig. 5(b) It is the oscillograms of the voltage born in S1 one cycle and electric current flowing through, Fig. 5(c) It is the oscillograms of the voltage born in S2 one cycle and electric current flowing through. Can see S1 and S2 current conversion of all to happen during zero crossing of the voltage by these two pictures. There is more transient state course of S2, to open the course is a bit more complicated too, so Fig. 5(c) China more shake.
In order to verify the operating characteristic of this circuit topology, and Dr1 Dr2 add the intersection of RCD and the intersection of experiment and prototype machine of absorbing circuit compare with output kenotron only by another the intersection of experiment and prototype machine this. The parameter of RCD absorbing circuit is chosen as: The resistance is 330kΩ / 3W, the electric capacity is 4.7nF/ 1kV, the diode adopts FRl07. Fig. 6(a) Output the voltage of kenotron Dr1 both ends while only adopting RCD absorbing circuit, Fig. 6(b) Output the voltage of kenotron Dr1 both ends while adopting the stated clamp circuit of this text. Easy to find out, Fig. 6(a) The voltage overshoot of China Dr1 end has reached above 250V, and adopt the clamp circuit and can obviously reduce and output the voltage overshoot on the rectifier tube, help to prevent this rectifier tube from being punctured.
7 Fig. converter this different to support efficiency curve under when the input voltage is 350V. Efficiency can be more than 94% when this circuit is fully loaded with, but the primary side of the voltage transformer does not adopt the clamp circuit, add RCD absorbing circuit only in the output diode, efficiency is at most 93.1%.
5 concludes the speech This text has introduced the asymmetry semi-bridge DC converter with clamp circuit of primary side of a kind of voltage transformer, have carried on detailed analysis to the switching process of its main switch, has produced the prototype machine of an experiment, to circuit this with lead RCD prototype machine of absorbing circuit to compare only. The characteristic of this circuit is as follows: 1)The main switch S1 and S2 can realize the soft switch within the range of full load; 2)Export the voltage overshoot of the rectifier tube to obviously reduce. Help to prevent this rectifier tube from being punctured, can choose the rectifier tube slightly low in withstand voltage at the same time, have expanded the range of choice; 3)This circuit is higher than to only adopt the efficiency of RCD absorbing circuit; 4)The leading-in clamping the dipole and managing Dg1, Dg2 and syntony inductance Lr, has not changed the operating characteristic of the converter. Because this circuit topology has such great advantages in only adopting RCD absorbing circuit, so, this kind is topological can further generalize to other DC converters.