# TNY277 design with PI expert, VMIN is correct?

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Hi

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

Input: 85~265VAC 50Hz
Output: 12VDC, 1.2A (14.4W Simple output)

In the "Design Results" report, in the VMIN (Minimum DC Input Voltage) there is 83.4V.

But, when i tried to calculate this parameter with the formula of application note AN-23 (Page 6, step 5) or AN-16 (Page 13, step 3) the results is about 120V (diferent from 83.4).

My question is:

1) There is a problem with the PI Expert or the formula in application note?
2) Can you show me the correct formula for VMIN?

I atttached the design (.uds file) and the apllication note scan.

Thanks

PD. Excuse me for my ugly english

Raul Diez.

Attachmentサイズ
12V_1.2A_Tiny.uds702 KB
AN-23_Page6_190509.pdf282.3 KB

Hi Raul,

Thank you for using PI Expert for your designs and thank you for providing the details of your design, so I can better understand the issue.

When I compute the hand-written formula for Vmin that you provided at the bottom of AN-23 sqrt(2 * 85^2 - 2 * 14.4 * (1 / 100 - 0.003)/(0.767 * 47e-6)) , I get 94V. This is closer to 83.9V, but there is still a big difference.

I am looking into why there is a difference and I will get back to you as soon as I fully understand it.

Raul,

PI Expert uses the same formula as the application note AN-23. The confusion arose because AN-23 and PI Expert select different parameters for the design.

In particular:
1) When I load your attached design into PI Expert, I can see from the schematic page that it has the Input Capacitor in a PI Filter arrangement, with two 18uF capacitors for a total of 36uF. It selected this arrangement for EMI performance. If you would like to change that, you can right click on one of the capacitors and select "Functional Dialog". There, you can change the arrangement and the actual capacitors being selected.

2) The application note recommends 3 ms as the estimated conduction time. PI Expert computes the conduction time based on the input voltage and the selected capacitor. For your design, it was 2.69 ms.

3) The estimated efficiency you use in the Vmin formula and the one used by PI Expert are different, 76.7% vs 78%. The PI Expert value is an educated guess based on typical designs of this type. If you would like to use 76.7% for the efficiency estimate, you can specify it by either editing it in the Navigation Tree Menu on the left (in Design->Key Parameters->Efficiency Estimate) or in the Efficiency Estimate field of the Design Results Report tab. Once a better estimate of efficiency is available from actual design prototypes, updating this field improves the accuracy of the calculations in PI Expert.