source:Industry News Popular:adapter release time:2021-05-25 11:43:08 Article author:sznbone
The drive current loop established from P2 through R3, Q1 base-emitter junction, R1, and back through D2 is like a forward diode. The larger drive current value makes the transistor fully saturated during the entire turn-on period.
When Q1 is turned on, the collector current will increase with time, and its size is determined by the primary inductance, the converted secondary inductance L and the load.
As the current increases, the voltage across R1 and the base voltage of Q1 also increase until the Zener diode turns on.
Choose the value of R1 reasonably so that the drive current is cut off when the collector current reaches the maximum control value. At this time, the magnetic core just enters the saturation point, that is, the point S1' in the figure. Since H cannot be further increased when the clamping current is reached, the voltage across all windings must drop to zero. In the end, only a very small flyback is required to turn off Q1 and turn on Q2 and Q2 to continue to complete the work cycle. The collector limit current no longer depends on the transistor gain.
When the converted load current is lower than the magnetizing current, the diodes D5 and D6 connected to the collector provide a path for the reverse current. These two diodes work in a cross-connected manner. For example, when Q1 is turned off, the magnetizing current will quickly make the collector of Q1 positive. At the same time, the collector of Q2 quickly becomes negative until D6 is turned on. The dislocation effect provided by D6 makes the collector voltage of Q1 twice of Vcc. In order to reduce the shadow of leakage inductance, P1 and P3 should be wound with double strands.
In order to reduce radio frequency interference and secondary side breakdown stress, a buffer circuit is required in addition to D5 and D6.
In order to ensure a good switching effect, sufficient regenerative feedback is required, and P2 in the circuit shown should provide a driving voltage of at least 4 volts.
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