EBNCP1200/D How to Use the Spreadsheet NCP1200 Discont.xls" By Hector NG ON Semiconductor Singapore Application Engineer Email: [email protected] http://onsemi.com ENGINEERING BULLETIN This short note describes the necessary steps to efficiently use the NCP1200 design aid file: “NCP1200 Discont.xls”. Symbol Cell in Spreadsheet Explanation Remarks Vmax B4 Maximum AC Input Voltage in volt Vmin B5 Minimum AC Input Voltage in volt Fline B6 AC Line Frequency in Hz Vo B11 Output Voltage in volt Io B12 Maximum Output Current in ampere h B13 Efficiency in % Vbd B14 Power MOSFET breakdown voltage in volt Vd B16 Diode voltage drop in volt Assume 80% if unknown Assume 1.0 V if an ultra–fast diode is used or 0.8 V if a Schottky diode is used Step 1: Input System Parameters Step 4: Enter Current Density Allowed for the Transformer The first step in power supply design is to understand the system requirements. The following parameters have to be entered into the spreadsheet. The wire size of a transformer has to be chosen suitably to avoid excessive copper loss and heat dissipation. The current density of the wire selected should be in the range of 3 ~ 5 A/mm2 for natural cooling system and current density can be increased to 4 ~ 7 A/mm2 for fan cooled system. This value should be entered into cell B55. Step 2: Enter Capacitance of Input Filter Capacitor Cin Enter the capacitance value of the input filter capacitor into cell B39 in mF. A recommended capacitance value is shown in cell B38. Step 5: Determine Maximum Wire Size Select the maximum wire size for the transformer in the list box in cell G53. The program will limit itself in choosing which wire size for primary and secondary winding with this information. Step 3: Determine Primary Inductance A list box is shown in the spreadsheet which labelled as “Selected Device” – Cell G12. Choose a device which you plan to use from the list. NCP1200 is offered in 40 KHz, 60 KHz and 100 KHz versions. Then the user can set the primary inductance by inputting a value in cell B32. A recommended Lp is shown in cell B31 for the user reference. Depending on the application, the choice of Lp can have a significant noise impact during NCP1200 skip cycle operation. Low Lp implies high peak currents (with possible noise problems in standby) while a high Lp implies low peak current (less noise problems) but possibly a higher leakage inductance. A trade–off has thus to be found between all the design requirements. Semiconductor Components Industries, LLC, 2001 January, 2001 – Rev. 0 Step 6: Enter Flux Density Safety Factor Flux density safety factor determines the magnetizing level of the transformer core, it should in the range from 0.3 to 0.5 Tesla. Enter this value in cell B58. Step 7: Enter Bobbin Usage Factor In a transformer bobbin, not all cross sectional area is available to accommodate the windings. A bobbin usage factor is introduced to account for area occupied by margin, insulation tape and waste space between wires. It should be in the range from 0.3 to 0.5. Enter this value in cell B59. 1 Publication Order Number: EBNCP1200/D EBNCP1200/D Step 8: Enter Magnetic Core and Bobbin Data without excessive hysteresis loss, e.g. N67 from Epcos (Siemens) and PC40 from TDK. The worksheet allows user to input properties of 5 material simultaneously. From the data book of the magnetics, locate the following date and enter into cell B61 to F64. Before we can proceed further, we must have the information of different magnetic cores and bobbins ready. Recommended core types are EE, EI, EF and ETD made of material that can work in the selected switching frequency Symbol Cell in Spreadsheet Core Name B61–F61 Name of the magnetic core Optional data, not for calculation For identification purpose only Ae B62–F62 Effective area of the magnetic core in mm2 Property of the magnetic core Bsat B63–F63 Saturation magnetic flux density at 25oC in Tesla Property of the magnetic material Aw B64–F64 Explanation Bobbin window area in Remarks mm2 Property of the bobbin, some vendors provide several bobbins for one magnetic core Step 9: Enter RDS(on) of the Power MOSFET room for transient response. Ideally Dmax should be kept at 40% to 60% so that there is reasonable balance on primary and secondary ripple current. Decrease Lp if Dmax is too high. b. Maximum voltage across power switch circuit, Vpwr_sw(max): Make sure that this value (cell B23) does not exceed power MOSFET breakdown voltage. Decrease Lp if Vpwr_sw(max) is too high. In fact, we must have headroom to cater for voltage spike generated by the leakage inductance of the transformer. c. Magnetic flux density during start–up, Binit: To avoid magnetic saturation during start–up, Binit (cell B88) should be kept below 70% of Bsat. If Binit is too high, first attempt to reduce Binit should be by increasing the value of the sensing resistor Rsense. If Rsense is already very close to its allowable maximum value, try lowering the value of flux density safety factor (cell B58). This may force you to change to a bigger magnetic core. The spreadsheet provides additional information on maximum conduction loss of the power MOSFET. Enter maximum RDS(on) (usually @Tj = 100°C) of the selected power MOSFET in cell B34, maximum conduction loss is shown in cell B35. Step 10: Determine the Sensing Resistor It is normal for a transformer to have 10% tolerance in its primary inductance. Enter the percentage tolerance in cell B82. The spreadsheet uses the lowest primary inductance and lowest switching frequency to compute worst case primary peak current. Maximum allowable sensing resistance is calculated based on this information and it is shown in cell B86. Select a sensing resistor with value lower than B86 and enter into B87. Please pick a value within the E24 series for easier selection: 0.56 Ω, 0.68 Ω, 0.82 Ω, 1.0 Ω, 1.2 Ω, 1.5 Ω, 1.8 Ω, 2.2 Ω, 2.7 Ω, 3.3 Ω, 3.9 Ω, 4.7 Ω. Step 11: Final Review Step 12: Reading Results Before finalizing on the design, one has to review the calculation results. a. Maximum turn on duty, Dmax: Dmax (cell B25) should be kept below the maximum turn on duty of NCP1200. Referring to NCP1200 data sheet, typical D(max) is at 80%. However, it is not realistic to push Dmax to the limit of the control IC because the secondary peak current will be very high and there is no Results are summarized in the Results page. Select magnetic core/bobbin set to use by list box in cell G6. User should take note whether cell C11 is showing “OK” or “not OK”. Cell showing “OK” implies that the corresponding magnetic core/bobbin set is big enough to accommodate all windings. The information is for reference only, consult your transformer vendor for a conclusive answer. http://onsemi.com 2 EBNCP1200/D Notes http://onsemi.com 3 EBNCP1200/D ON Semiconductor and are trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. 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