# Output in PI expert explained

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Hello,
I want to use the innoswitch3 family for my news designs. I never worked before with InnoSwitch family and PI expert either. I usually use a mathcad sheets to develop my calculations. Now I am designing a sheet for the Innoswitch3 family and comparing results with the PI expert. Then I realized that I don't understand some outputs or maybe they are wrong. Hopefully you could clarify my doubts.
I attached a screen capture with some results.
WORST CASE ELECTRICAL PARAMETERS
Fswitching_max:95kHz
DutyCycle:0.418
T_on:6.15us primary switch on-time
T_off:6.12us primary switch off-time

In my understanding
Fswitching_max=1/(T_on+T_off) however 1/(T_on+T_off)=81.5kHz (different of 95kHz)
DutyCycle:T_Ton/(T_on+T_off)=0.51 (different of 0.418)

Could you explain to me what I am doing wrong?

Thanks

AttachmentSize
PI expert screen capture100.39 KB

Hi JGE,

your understanding of the formulas is correct and these formulas are used inside of PI Expert. I just need to clarify what PI Expert means by "Worst case".

Once a physical converter is built based on some design calculations, its operating frequency will not exactly match the calculated value, but will vary depending on actual values of its parts, which vary within some tolerances.

The tolerances of primary inductance and current limit of the InnoSwitch3 part have a large impact on the switching frequency. PI Expert computes the highest possible operating frequency over the range of possible values and reports it in the "Worst Case Electrical Parameters".

The highest frequency occurs at the lowest inductance and current limit, but the worst case (highest) TON and worst case (lowest) TOFF happen at different tolerance combinations. The worst case values, from the perspective of violating their datasheet limits, are all listed in one table, but might not be consistent with each other, as you have noted.

At the bottom of the spreadsheet, you should see the "TOLERANCE ANALYSIS" section. That section lists the electrical parameters for a given tolerance and all values should be consistent.