TNY278 design with PI expert, Bp is correct?

3 posts / 0 new


I have tried to design a Flyback converter with a TinySwitch-III and the PI Expert Suite (Ver 10.1).
The basic parameters are:

Input: 85~265VAC 60Hz
Output: 12VDC, 1A (12W Simple output)

In the design results the primary number of turns (Np) is 77.
When I tried to calculate this parameter with the formula of application note AN-23 (Page 7, Step 18) the result for the peak flux density (Bp) is about 3190 gauss.
But, in the same page, "Calculate primary and secondary number of turns for peak flux density (Bp) not exceed for 3000 gauss" is written.

My question is:
1) There is a problem with the PI Expert or the formula in application note?
2) Bp > 3000 its not important?

I atttached the design (.uds file) and the apllication note scan.
PD. Excuse me for my ugly english
Raul Diez.

첨부 파일Size
12V_1A_60Hz_EE19.uds1001.5 KB
AN-23_Page_7_191115.pdf231.35 KB

Hello Raul,

Thank you for using Power Integrations in your design.

I believe you are correct in the use of the formula, where you used the maximum value of Ipeak to get the highest Flux Density. Im guessing that in the PI Expert version that you are using, the Ipeak used is the nominal value, 0.55A,which will lead to ~2900 Gauss Bpeak.

Have you tried using PI Expert Online? The version there contains the latest updates on the formuals and equations used so it might be worth trying.

Using 3190 Gauss peak flux density is ok as long as the saturation of the core is not reached. Today, most cores can operate up to 3500 to 3800 Gauss so your 3190 should be ok. The only concern that you might want to consider is Audible Noise. As noted in the same App Note (AN-23), if you encounter audible noise, you might need to consider putting the Bpeak below 2500 Gauss.


Hello PI-Bombadil

Thank you very much for your help.

I will try the online version.