5_V_to_isolated_5_V_at_20_mA

Published:2009/7/24 9:08:00 Author:Jessie | From:SeekIC

In this circuit, a negative-output dc/dc converter generates a -5-V output at point A. To generate this -5 V at A, the transformer primary must flyback to a diode drop that is more negative than -5 V. If the transformer has a tightly-coupled 1:1 turns ratio, 5 V (plus a diode drop) is across the secondary. The 1N5817 rectifies this secondary voltage to generate an isolated 5-V output (which is not fully regulated because the -5 V at point A is sensed by the MAX635). With careful transformer selection, the 5-V output will be within 10%. Bifilar winding of the transformer provides better load regulation of the isolated 5-V output, but this does reduce the isolation by increasing the capacitance between the primary and secondary. The isolation voltage breakdown is determined by the characteristics of the transformer, not the MAX635.



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