Application note AN-47
Hello,
I am doing a design with a TOP266 with the application note AN-47. Before doing nothing, I have reviewed the application note. How do you make the calculation of the primary inductance (LP)? I'm using the formula LP = ((Vmin-VDS)/(KP*Ip))*Ton=((74-10)/(0.5*1.16))*5.65uS=623uH. The result of the AN-47 Lp=1435uH. The result is so different!
Thanks.
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I have reviewed the AN-32 and the result is the same. The AN-32
use the next formula for CCM
1000000 * Po Z * (1-ƞ) + ƞ
Lp = --------------------------------- * -----------------
Ip^2 * KP * (1-Kp/2) * fs min ƞ
If you introduce the AN-47 inputs and outputs
1000000 * 35W 0.5 * (1-0.8) + 0.8
Lp = --------------------------------------- * --------------------- =
1.16^2 * 0.5 * (1-0.5/2) * 119000 Hz 0.8
Lp = 582,87*1.125=655,72µH --> The same
Please reavise if I have introduced some input and output wrong in the formula.
Thanks.
Now you can see better the formula
Lp = ((1000000 * Po)/(Ip^2 * KP * (1-Kp/2) * fs min))*((Z * (1-ƞ) + ƞ)/ƞ)
If you introduce the AN-47 inputs and outputs
Lp = ((1000000 * 35W)/(1.16^2 * 0.5 * (1-0.5/2) * 119000 Hz))
*((0.5 *(1-0.8))/0.8))=
Lp = 582,87*1.125=655,72µH --> The same
Please reavise if I have introduced some input and output wrong in the formula.
Thanks.
Hi
Your calculations are correct. The AN-47 "Lp" value is based on Half frequency (Fsmin=59.4KHZ). Even though app note says "F" for full frequency it never got registered in calculations or may be just a typo by the author.
If you use Half frequency and add 10% to the calculated value you get 1445uH.
Good catch and thanks for pointing this out.
rgds
Hi,
The calculations are based on formula used in AN-32. Please take a look at the AN-32 http://www.powerint.com/sites/default/files/product-docs/an32.pdf. In your formula I don't see output power for calculating the Lp.
rgds