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MAX15004 Datasheet(PDF) 18 Page  Maxim Integrated Products 

MAX15004 Datasheet(HTML) 18 Page  Maxim Integrated Products 
18 / 27 page 4.5V to 40V Input Automotive Flyback/Boost/SEPIC PowerSupply Controllers 18 ______________________________________________________________________________________ Selecting VCC Resistor (RVCC) The VCC external supply series resistor should be sized to provide enough average current from VOUT to drive the external MOSFET (IDRIVE) and ISUPPLY. The VCC is clamped internally to 10.4V and capable of sinking 30mA current. The VCC resistor must be high enough to limit the VCC sink current below 30mA at the highest output voltage. Maintain the VCC voltage to 8V while feeding the power from VOUT to VCC. For a regulated output voltage of VOUT, calculate the RVCC using the following equation: See Figure 5 and the Power Dissipation section for the values of ISUPPLY and IDRIVE. Flyback Converter The choice of the conversion topology is the first stage in powersupply design. The topology selection criteria include input voltage range, output voltage, peak cur rents in the primary and secondary circuits, efficiency, form factor, and cost. For an output power of less than 50W and a 1:2 input voltage range with small form factor requirements, the flyback topology is the best choice. It uses a minimum of components, thereby reducing cost and form factor. The flyback converter can be designed to operate either in continuous or discontinuous mode of opera tion. In discontinuous mode of operation, the trans former core completes its energy transfer during the offcycle, while in continuous mode of operation, the next cycle begins before the energy transfer is com plete. The discontinuous mode of operation is chosen for the present example for the following reasons: • It maximizes the energy storage in the magnetic component, thereby reducing size. • Simplifies the dynamic stability compensation design (no righthalf plane zero). • Higher unitygain bandwidth. A major disadvantage of discontinuous mode operation is the higher peaktoaverage current ratio in the primary and secondary circuits. Higher peaktoaverage current means higher RMS current, and therefore, higher loss and lower efficiency. For lowpower converters, the advantages of using discontinuous mode easily surpass the possible disadvantages. Moreover, the drive capabil ity of the MAX15004/MAX15005 is good enough to drive a large switching MOSFET. With the presently available MOSFETs, power output of up to 50W is easily achiev able with a discontinuous mode flyback topology using the MAX15004/MAX15005 in automotive applications. Transformer Design Stepbystep transformer specification design for a dis continuous flyback example is explained below. Follow the steps below for the discontinuous mode transformer: Step 1) Calculate the secondary winding inductance for guaranteed core discharge within a mini mum offtime. Step 2) Calculate primary winding inductance for suffi cient energy to support the maximum load. Step 3) Calculate the secondary and bias winding turns ratios. Step 4) Calculate the RMS current in the primary and estimate the secondary RMS current. Step 5) Consider proper sequencing of windings and transformer construction for low leakage. Step 1) As discussed earlier, the core must be dis charged during the offcycle for discontinuous mode operation. The secondary inductance determines the time required to discharge the core. Use the following equations to calculate the secondary inductance: where: DOFFMIN = minimum DOFF. VD = secondary diode forward voltage drop. IOUT = maximum output rated current. Step 2) The rising current in the primary builds the energy stored in the core during ontime, which is then released to deliver the output power during the offtime. Primary inductance is then calculated to store enough energy during the ontime to support the maximum out put power. DMAX = Maximum D. L VD Pf D t t P INMIN MAX OUT OUT MAX ON ON = ×× ×× = + 22 2 η () tt OFF L VV D If D S OUT D OFFMIN OUT OUT MAX OFF ≤ + ()× () ×× 2 2 () == + t tt OFF ON OFF R V II VCC OUT SUPPLY DRIVE = + − () () 8 
