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MAX258 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX258 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 12 page Component Selection Transformer Selection Transformer selection for the MAX258 can be simplified by the use of the ET product. The ET product relates the maximum allowable magnetic flux density in a trans- former core to the voltage across a winding and switch- ing period. Inductor magnetizing current in the primary winding changes linearly with time during the switching period of the device. Each transformer has a minimum ET product, though not always stated on the transformer data sheet. Ensure that the transformer selected for use with the device has an ET product of at least ET = VDD/(2 x fSW) for each half of the primary winding, where fSW is the minimum switching frequency of the T1 and T2 ouputs. Select a transformer with sufficient ET product for each half of the primary winding to ensure that the transformer does not saturate during operation. Saturation of the mag- netic core results in significantly reduced inductance of the primary, and therefore in a large increase in current flow. For example, when HICLK is low, the required trans- former ET product to the center tap for an application with VDD (max) = 5.5V, is 13.1V-µs. An application with VDD (max) = 3.3V has a transformer ET product to the center tap requirement of 7.9V-µs. In addition to the constraint on ET product, choose a trans- former with low leakage inductance and low DC-winding resistance. Power dissipation of the transformer due to the copper loss is approximated as: PD_TX = ILOAD2 x (RPRI/N2 + RSEC) where RPRI is the DC-winding resistance of the primary, and RSEC is the DC-winding resistance of the second- ary. In most cases, an optimum is reached when RSEC = RPRI/N2. For this condition, the power dissipation is equal for the primary and secondary windings. As with all power-supply designs, it is important to opti- mize efficiency. In designs incorporating small trans- formers, the possibility of thermal runaway makes low transformer efficiencies problematic. Transformer losses produce a temperature rise that reduces the efficiency of the transformer. The lower efficiency, in turn, produces an even larger temperature rise. To ensure that the transformer meets these requirements under all operating conditions, the design should focus on the worst-case conditions. The most stringent demands on ET product arise for maximum input voltage, minimum switching frequency, and maximum temperature and load current. Additionally, the worst-case values for transform- er and rectifier losses should be considered. The primary must be center-tapped; however the second- ary winding may or may not be center-tapped, depending on the rectifier topology used. The phasing between pri- mary and secondary windings is not critical. The transformer turns ratio must be set to provide the minimum required output voltage at the maximum antici- pated load with the minimum expected input voltage. In addition, include in the calculations an allowance for the worst-case losses in the rectifiers. Since the turns ratio determined in this manner ordinarily produces a much higher voltage at the secondary under conditions of high input voltage and/or light loading, take care to prevent an overvoltage condition from occurring. Transformers for use with the IC are typically wound on a high-permeability magnetic core. To minimize radiated electromagnetic emissions, select a toroid, pot core, E/I/U core, or equivalent. Diode Selection The high switching speed capability of the MAX258 necessitates high-speed rectifiers. Ordinary silicon signal diodes such as the 1N914 or 1N4148 can be used for low- output current levels (less than 50mA), but at high-output current levels, their reverse recovery times might degrade efficiency. At higher output currents, select low forward- voltage Schottky diodes to improve efficiency. Ensure that the average forward current rating for the rectifier diodes exceeds the maximum load current of the circuit. For surface-mount applications, Schottky diodes such as the B230A, MBRS230, and MBRS320 are recommended. Suggested External Component Manufacturers Table 1. Suggested External Component Manufacturers MANUFACTURER COMPONENT WEBSITE Halo Electronics Transformers www.haloelectronics.com Diodes Inc. Diodes www.diodes.com Murata Americas Capacitors www.murataamericas.com MAX258 500mA, Push-Pull Transformer Driver for Isolated Power Supplies www.maximintegrated.com Maxim Integrated │ 10 |
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