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MAX256ASA Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX256ASA Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 14 page Component Selection Transformer Selection Transformer selection for the MAX256 can be simplified by the use of a design metric, the ET product. The ET product relates the maximum allowable magnetic flux density in a transformer core to the voltage across a winding and switching period. Inductor current in the primary linearly increases with time in the operating region of the MAX256. Transformer manufacturers specify a minimum ET product for each transformer. For the MAX256, the requirement on ET product is calculat- ed as: By choosing a transformer with sufficient ET product in the primary winding, it is ensured that the transformer will not saturate during operation. Saturation of the magnetic core results in significantly reduced induc- tance of the primary, and therefore a large increase in current flow. Excessive transformer current results in a temperature rise and possible damage to the trans- former and/or the MAX256. When CK_RS is unconnected, the internal oscillator is programmed for the minimum frequency. The default required ET product for the MAX256 is 42.3Vµs, (assum- ing +5.5V maximum VCC), or 27.7Vµs for +3.3V opera- tion (assuming +3.6V maximum VCC). Both of these ET products assume the minimum oscillator frequency of 65kHz. See the Typical Operating Characteristics plot, RS vs. Required ET Product to determine the required ET product for a given value of RS. In addition to the constraint on ET product, choose a transformer with a low DC-winding resistance. Power dissipation of the transformer due to the copper loss is approximated as: where RPRI is the DC-winding resistance of the primary, and RSEC is the DC-winding resistance of the sec- ondary. In most cases, an optimum is reached when: For this condition, the power dissipation is equal for the primary and secondary windings. RN R SEC PRI = 2 PI N R R D TX LOAD PRI SEC _ =× + ⎛ ⎝ ⎞ ⎠ 2 2 ET V f CC SW =× × 1 2 3W Primary-Side Transformer H-Bridge Driver for Isolated Supplies 10 ______________________________________________________________________________________ GND MAX256 MODE CK_RS +5V 47kΩ VCC ST2 ST1 7815 10µF 0.1µF 0.1µF -15V COMMON 4.7µF 470nF 7815 10µF 0.1µF MBRS140 TGM-280NS TGM-280NS MBRS140 MBRS140 MBRS140 0.1µF +15V Figure 5. +5V to Isolated Regulated ±15V |
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