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Pulse start circuit

source:Industry News Popular:adapter release time:2021-06-18 09:25:38 Article author:sznbone

  The resistors R1 and R2 are usually the discharge resistors of the energy storage capacitors C1 and C2. After they are switched on, they cause current to flow into the capacitor C3, and the capacitor C4 of the auxiliary charger will discharge at this time.

  The voltage of C3 will increase until both ends of the diac reach the trigger voltage. The bidirectional two-pole thyristor will trigger and transfer part of the charge from C3 to C4, and the size of the transferred current is limited by the resistance R3.

Pulse start circuit(图1)

  Choose the appropriate capacitors C3, C4 and the voltage value of the bipolar thyristor, so that the required auxiliary charger voltage is gradually formed at both ends of C, and the converter starts to work through a normal soft start.

  Again, through D5 and additional winding positive feedback, the main transformer provides an auxiliary charger.

  As C4 is further charged and the voltage rises, the bipolar thyristor turns off. Because of the clamping action of ZD1 on C3, the voltage across the bipolar thyristor no longer reaches the trigger value.

  This circuit has such advantages:

  During the turn-on transient, a large current is provided, and there will be no excessive loss on the resistors R1 and R2. In the case that the first pulse of the converter fails to start (this situation rarely occurs), once C4 is discharged and C3 is recharged to the turn-on voltage of the bipolar thyristor, the start action will be repeated.

  The choice of bipolar thyristor is very important. It must be able to conduct the required conduction current, and the conduction voltage should be less than (V1-Vstart) and greater than (V1-V2) (where V1 is the clamping voltage of ZD1, Vstart is the starting voltage of the control circuit, and V2 is the converter C4 voltage during operation); otherwise, the lock will occur after the first pulse. It is feasible to replace the bidirectional triode with a small SCR and an appropriate gate drive circuit.


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