Systems peripheral functions often require separate 3.3V supplies. Thi
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High Efficiency DC-DC Isolated Converter
Systems peripheral functions often require separate 3.3V supplies. This function require isolated power supply, logic supply cannot sink source enough current. Isolated DC-DC converters readily available market place, however there some spare inverters available hardware design then cost solution configured. circuit Figure replacement MAX253 driver circuit. uses same transformer recommended Maxim application circuit (part number 76253/55 5V-5V isolated conversion, 76253/35 3.3V-5V isolated conversion).
inverters produce generated complimentary square wave output which turn used drive (FMMT617 SuperSOT transistors). These power `off shelf' transformers specified above. operation inverters transformer imitates operation MAX253 provides oscillator frequency 240KHz. centre primary winding transformer connected power supply (3.3 de-coupled ground
150pF
0u1F FMMT617
47uH Vout ZHSC750 15uF 15uF 15uF
7404
7404
7404
0u1F FMMT617 7404 ZHSC750
Figure
schottky rectifier diodes Zetex ZHCS750) then employed reproduce supply. network acts filter ensure steady output signal. Maximum efficiency transformer approx. ideally design needs close this possible. This achieved careful selection driving frequency. FMMT617 (surface mount) ZTX618 (through hole) transistors have been chosen. These medium power devices require little base drive their high gain. Also they exhibit very VCE(sat) characteristics making them ideal this switching application. ZHCS750 Schottky diodes have been selected. They have very (forward voltage drop) rating fast switching speed. Consequently they consume very little power enhancing overall efficiency circuit. Setting 22K, 150pF circuit will oscillate 240KHz. This optimum frequency gives highest efficiency result. Figure shows waveform transformer drives into Trace upper waveform (Trace1) shows output from circuit driving into 500mA load. seen frequency operation 240kHz, resultant output waveform ripple free.
Figure
circuit efficiency excess achieved. Also circuit able drive load maximum power rating transformer /55, 500mA /35). Typical applications include RS-485 RS-232 data interface (transceiver) applications. Both transformers manufactured Newport Components,Tel:+44(0)1908 615232.
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