connection flyback regulator transformer with LM2587

Hello,

Thankyou for including the exact part number of the transformer you are using from Coilcraft.

Two points I wish to clarify first:

1) In the definition of a flyback regulator; when the power switch is conducting, only current in the primary winding is flowing; all secondary windings are configured such that their respective catch diodes are reversed biased.

2) It is common in transformer manufacturing to place a dot marking to illustrate the starting terminal of a particular winding and secondarily to illustrate the phase relationship between the windings.

With that said, it is most common to connect the primary dot marked end to Vin (AC grounded through CIN) and the positive output voltage dot marked secondary terminals to ground.  (+12VOUT and +5VOUT)  This means the primary non-dot marked end ties to the switch terminal of the power switch(IC) as well as the snubbing network across the primary. On the secondary side, for the positive output voltages, the non-dot marked terminals connect to the rectifier diode anodes. The Cathode of the diodes go to the + terminals of the output capacitors. Of course the minus terminals of the output capacitors go to ground. I assume you are trying to create +/- 12V and 5V for the circuit application. This leaves the question of how to configure the secondary for the negative output winding. There are two possibilites. The preferred configuration is that the dotted winding terminal connect directly to minus VOUT and the negative terminal of that output capacitor and the non-dot marked end connect to the associated rectifier diode anode. The cathode of the diode for the negative output ties to ground along with the + terminal of the negative rail COUT.

Also, It is wise to place series R-C snubbers across each of the three output rectifier diodes to tame ringing and reduce EMI.

If you need a schematic from us to clarify all of this let us know.

Alan Martin