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At present I am designing the transformer for this power supply. Requirement is 12W output at 12V. Input can vary from 19Vdc to 273Vdc. (ie lower end of 24V battery to peak rectified 264Vac). I am considering a flyback convertor and possibly discontinuous mode of operation. At 19Vdc I understand that the power supply will be switching at maximum frequency (135KHz) and 50% duty cycle (50% helps the control loop). My question is that due to the large input voltage range, if there is full load at 350Vdc (264ac x 1.414) input, the frequency will fall below the audible frequency and the peak current reduces to about 30% of the maximum peak (according to the data sheet) . (1) If that is the case then, does this controller maintain that 30% of the peak current (to keep mechanical resonance low) and increase the frequency to above audible range to deliver full power? (2) What happens at light load and 370Vdc input? (3) If I have to consider higher switching frequency then what is the maximum frequency I can consider with this ic? (4) Do you expect any problem using this ic for the above design? (5) Do you have more technical application notes for this ic? (6) Is there any other ic more suited for this application? (NCP1380?.....)

Thank you for considering ON Semiconductor. Yes, at the lowest input voltage, the controller will force the maximum switching frequency, 135 kHz in your case. At high line and lighter load, the frequency will reduce as you properly pointed out. Let's see your points now: (1) If that is the case then, does this controller maintain that 30% of the peak current (to keep mechanical resonance low) and increase the frequency to above audible range to deliver full power? --; the controller freezes the peak current at 30% of the maximum but the frequency can down to zero Hz if necessary (2) What happens at light load and 370Vdc input? --; the converter reduces its frequency and enters skip cycle (3) If I have to consider higher switching frequency then what is the maximum frequency I can consider with this ic? --; I believe that 150 kHz is a limit, but the power dissipation is another constraint. The average driving current is Fsw x QG, so depending on the selected MOSFET and the switching frequency, you must ensure the power dissipation is within the SO8 capabilities. (4) Do you expect any problem using this ic for the above design? --; No, the 1351 works well, make sure the PCB routing is compact, wire the 1351 components very close to the IC. (5) Do you have more technical application notes for this ic? --; No. (6) Is there any other ic more suited for this application? (NCP1380?.....) --; We have a lot of controllers. The NCP1250 might work well in your circuit. This is a fixed-switching frequency featuring frequency fold back. Answered by:
ON Semiconductor
2011-07-21 17:58:10.0