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Offline Flyback: input EMC filter downstream of the diode bridge.

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cupoftea

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Hi,
Will we have EMC problems with our 7W offline flyback if we put all of the mains input filter (common mode and diff mode) downstream of the mains diode bridge?
(only two Y caps will be upstream of the diode bridge)
Doing this, enables us to have an inrush resistor shorted out by an NFET rather than a relay.
Also, there is sometimes inrush into the Diff mode caps of the EMC filter, so the way it Is shown here means that the inrush into the EMC filter is also limited.
..as the attached LTspice sim and PDF schem show.
 

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  • Offline flyback _filter downstream of diode bridgee.pdf
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You are essentially moving the control micro to its own PSU upstream of the switcher. As long as the micro itself doesn't cause emissions it should work but consider that any interference 'trapped' by the common mode choke is superimposed on the base of Q5. How switching signals fed back to the compensation input is anyone's guess but it might work. Consider what might happen if incoming mains borne interference is blocked by the choke though. Maybe an optocoupler as well as or maybe instead of Q5 could be used to break the voltaic connection.

Brian.
 
Thanks, Q5 does need to be reached by a digi isolator or do it as an opto as you say.
 

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