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top switch understanding

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Bjtpower

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Hi,
i would to like understand the Importance of auxillary winding..

Does it really needs..

It can drive from the Direct DC Bus Voltage (Bulk capacitor) then why it cant..??

Also the device is rated for the 725 voltage..
i obseved the Peak voltage at the drain is almost 635V..
But the Bulk capacitor i used having capacity of voltage of 400V.

doesnt it cause high voltage spikes of drain and get burst..??

Can anyone guid me on the same.
'
Attached is the circuit..
topswitch.jpg
 
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By 'bulk' capacitor I presume you mean the one across the bridge rectifier. It is charged by the incoming AC so will be at approximately 1.4 times AC RMS voltage. The drain pin has that voltage AND the voltage from the current switching in the transformer on it so it is normal for the peaks to be much higher.

In theory you do not need the auxiliary winding, it can take feedback from the main low voltage output and still work perfectly well, however if you do that there would be a direct connection between the input side and output side so the +/- DC out would both be 'live' and unprotected from the AC line going in. It would present a very serious safety hazard, especially as there is no fuse in the circuit.

Brian.
 
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