The full-bridge PWM converter where the phase shift controls is one of the most frequently used circuit topology forms in middle high-power DC/DC inverter. The ones that utilize junction capacitance and high-frequency voltage transformer of the gas switching tube in phase shift PWM control mode leak the inductor as the syntony component, make the gas switching tube reach the zero voltage to open and shut off. Thus has reduced switching loss and switching noise of the circuit effectively and reduced the electromagnetic disturbance that is produced in the switching process of the device, have offered the good condition that the converter raised switching frequency, raised the efficiency, reduced the size and weight. The intersection of hold circuit and the intersection of topological structure and succinct, the intersection of control mode and simple, the intersection of switching frequency and invariant, voltage and the intersection of electric current and the intersection of stress and a series of advantage such as being little of components and parts at the same time.

Converter, PWM of full-bridge, that phase shift control have one main shortcoming that lag behind the intersection of arm and gas switching tube lay, realize the zero voltage switch while being difficult in light load, make it not fit for the load range to change the great occasion [1 ]. When the circuit can’t realize the zero voltage switch, will produce the following several consequence:

1Because of the existence of the switching loss, need to increase the volume of the heat sink;

2The gas switching tube has very big di/dt when being opened, will cause big EMI;

3Because of the backward recovery of the secondary diode, the electric current on the high-frequency voltage transformer secondary leakage inductance turns into function in wink, produce the voltage overshoot and shake on the diode, so, must join R-C to absorb on the secondary diode in real application.

To the above-mentioned problem, the common solution is in a saturation inductance Ls of voltage transformer primary side tandem connection, expand the switch range of the zero voltage of the converter [2 ] [3 ]. However, after adopting this method, the circuit still can’t reach the zero voltage switch in the whole operating range. Moreover, because the saturation inductance can not have ideal saturation properties in real application, this will cause:

1Increase the circuit circulation, thus increase the feed-through of the converter to loss;

2Having aggravated the duty ratio of the secondary voltage to lose, thus increase the electric current of primary side and stress of diode voltage of secondary;

3The saturation inductance is switched over among plus-minus saturation value with very high frequency, the loss of the magnetic core will be very large, it is serious to generate heat.

The improved full-bridge phase-shift ZVS-PWMDC/DC converter is a kind of circuit topology [4 ] put forward according to above-mentioned shortcoming [5 ] [6 ]. It, through increasing the auxiliary branch in the circuit, enables gas switching tube to reach the zero voltage switch within the range of all load, it is low-powered ‘ <3kW??????????????????????PWM????,???ZVS?????????,????????????????,???????????????????????????ZVS????

A improved full-bridge phase-shift ZVS-PWM DC/DC converter

1.1 Circuit topology

It is a kind of improved full-bridge phase-shift ZVS-PWM DC/DC converter that Fig. 1 shows, compared with fundamental full-bridge phase-shift PWM converter, it is only lagging behind the arm is increasing an auxiliary branch composed of inductor Lrx and electric capacity Crx two components.

Circuit topology of Fig. 1In the auxiliary syntony branch composed of Lrx and Crx, the electric capacity Crx is enough large, voltage VCrx should be satisfied on it

VCrxVin 1

It was a voltage of rectangular wave of symmetric exchange of plus-minus half cycle whose duty ratio is 50% that was then got on the inductance Lrx, its amplitude is Vin/2. Electric current crest value ILrx max on the inductance For

ILrxmax = 2

In the type: Vin, in order to input the direct-current volts;

Ts is switch cycle.

Circuit control mode of lasting phase shift, the it one main circuit operating principle too and full-bridges fundamental the PWM is getting the more same totally in converter. And the existence of auxiliary branch, can guarantee to lag behind the arm gas switching tube is opened and shut off in all load range inner zero voltage.

1.2 Procedure analysis of running of the circuit

The full-bridge PWM circuit controlled because of the phase shift has detailed discussions on much literature, so this text does not analyze its working phase concretely, the switching process of the arm gas switching tube that only discuss to lag behind and reaching the condition of the zero voltage switch. In order to analyze, suppose:

–All power gas switching tube and diode are ideal devices;

–All inductor and electric capacity are ideal elements;

–Consider the nonlinearity of the junction capacitance of the gas switching tube output of the power, there is C1 =C2 =C3 =C4 = 4/3 Coss, and write C3 C4 =C;

–Consider the leakage inductance Llk of the voltage transformer;

–Because inductance Lrx and electric capacity Crx are enough large, can be thought to maitain unchanged in the inert zone td in inductance Lrx upper electric current iLrx.

1Before t0 moment

Before t0 moment, as shown in Fig. 2, voltage transformer primary side diode D1, gas switching tube S3, the voltage transformer secondary diode D5 is in turning on state, the primary side electric current ip of the voltage transformer circulates through diode D1 and gas switching tube S3, and the linear decline under the function of the output voltage nVo, the circuit is in circulation state, the actual current direction is contrary to electric current reference direction. At the point of t0, the primary side electric current ipt0 of the voltage transformer For

ipt0 ==-I1 3

In the type: I1 is that the secondary outputs the current value that the filtering inductance Lf electric current minimum reflects to the primary side, obviously, the magnitude of I1 depends on the condition of loading.

In the picture suffix ‘ : iptd When less than or equal to I1, : ipt =When I1 t is less than or equal to tdMain waveform Dead Time is enlarged of the circuit of Fig. 2

At this moment, the inductance Lrx upper electric current ILrxt0 of auxiliary access road For

iLrxt0 =ILrxmax 4

2T0- t1 time quantum

At the point of t0, the zero voltage is shut off under the gas switching tube S3 function in the electric capacity C3 and C4. Since t0 moment, the circuit has begun LC syntony to taken place, makes C3 charge, C4 discharges, the equivalent electrical circuit has been shown as in Fig. 3 this stage, among them C is C3 and parallel connection of C4, voltage and electric current of primary side of the voltage transformer are vp and ip, voltage and electric current on the electric capacity C are vc and ic. This time quantum respectively

Fig. 3 t0- t1 time quantum circuit equivalent is topologicalvp =Llk 5

ic =C 6

vp vc =Vin 7

ip-ic =ILrxmax 8

The initial condition is

ipt0 =-I1, vct0 =Vin

Solve the equation, and insert the initial condition and can be had

ip =-ILrxmax I1 Cos t ILrx max 9