The actual circuit is composed of non-ideal components, and many unexpected situations may be encountered in the design.When debugging the normal full-bridge power supply as shown in figure 1, the output is not a stable waveform expected, but intermittent oscillation occurs from time to time, and a "squeak" sound is emitted, sometimes even burning the switch tube.After analyzing the circuit, no factors that may cause instability in the structure were found, so the voltage ratio of output sampling was changed, and the output was set at the half-voltage 24V, and the input dc voltage of 90V was used for debugging under the condition of ensuring the safety of the power tube.After the circuit works normally, it slowly increases the input dc voltage. After several tests, it is found that when Ui is 180 ~ 250V, it may cause oscillation. Finally, it is determined that the distributed capacitance between the windings of the driving transformer is making trouble.
The capacitance distribution of two switching tubes, where C2 is the distributed capacitance between the lower end M of winding NA and the upper end P of NB.When drive transformer windings is NA output pulse NB negative pulse output, TA tube by the deadline to saturation conduction, and the source of the TA pipe is M point potential rapid rise, and through the capacitance C2 NB winding the upper P increased potential, rise in value and the distribution of two winding capacitance C1, C2, C3, and point P to the high frequency impedance and the M point potential rising speed.If the value of lifting is greater than the negative pulse amplitude of the NB winding itself, the instantaneous conduction of TB tube will be triggered, resulting in the intermittent oscillation described above.Similar conditions will occur when other tubes are switched on.
There are generally three ways to solve electromagnetic interference. One is to reduce the intensity of the interference source, the other is to enhance the anti-interference ability of the driven MOS tube, and the third is to block the interference path.In this case, the source of the interference is the pulse that the transformer is transmitting, which cannot be reduced.Adding negative voltage to the driver can greatly enhance the anti-interference ability of MOS tube, which is adopted by many power sources.The third method is adopted in this example, which is to wrap a shield between the windings of the driving transformer. The structure is shown in figure 3, with 5 windings and 5 shielding layers.The whole transformer includes the shield layer wound layer by layer from left to right, N1 is connected to the ground of the control loop;The two lower tube drive windings, due to little potential change, are connected to N2 at the same time, which is actually connected to the power ground.N3 and N4 wrap the upper tube winding NA and connect it with the different end of NA.N5 isolates the winding ND from NA.So that each winding is at the same potential as its shield, and there is no capacitive current between them.When the TA conduction of the upper tube and the potential jump of the upper tube winding NA, the potential jump of the shielding layer N3 and N4 should also be the same. Due to the distributed capacitance between N2 and N3, this jump will generate current between the two shielding layers, but it has no effect on the drive of the tube, only a little main power will be consumed.After the electromagnetic shielding driving transformer is used in the actual circuit, the problem is completely solved.
For the misguide of cut-off tube caused by distributed capacitance, two methods can be adopted: setting negative pressure drive and shielding isolation.The design of driving transformer is complicated by adding shielding layer to transformer, but it is still a practical and effective method without modifying the circuit.