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LM3447 LM magnetizing inductance ?

phuong hoang
Prodigy 10 points
Other Parts Discussed in Thread: LM3447-A19-230VEVM

I'm designing LED driver for LED bulb 9W , Vout = 30V , I =275mA , f = 75Khz , n(efficiency) = 0.82

Input Voltage range : 190-265V , Pin =10W

I based on reference design LM3447-A19-230VEVM .

I bought Transformer (750815047) WE-Midcom,Np:Ns =5.6  , inductance 3.2 mH - I think this is magnetizing inductance(LM), it's right ?

When I calculate LM follow datasheet , i got LM  < 2.6mH .

1)This is my problem , I don't understand Why LM3447-A19-230VEVM choose Transformer LM=3.2mH ?

2) In the formula  OUTPUT BULK CAPACITOR , what is the fL (line frequency) ? Can you examles with ILED(rip) = 100mA  ?

I hopefully looking forward to hearing from you, Thank you very much , Ti employee  !

  • 0
    TI__Expert 5130 points

    Hi,

    1)Your calculation is correct and it is base on the formulas in data sheet. However the transformer was designed by WE base on the electronic spec that TI gave them. Base on those spec, WE designed the transform with turn ratio 5.6 and inductance 3.2mH which works fine with the EVM and has the electrical performance as listed in the EVM user guide. As you may noticed, the equation 9 in the data sheet pg 15 is for the equation 8 to be valid and also for PFC. If you want to design the transformer by yourself, you can definitely follow the design process in the data sheet.

    2)Yes, fL is the line frequency. For your case, the line frequency should be 50Hz(+/-5Hz). So Cbulk should be larger than 108uF.

    Best regards,

    Victor