PI Expert giving wrong value resistor

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I have just done a topswitch (TOP264EG) design on PI Expert and it has given me a resistor value which is not feasible.

Do you know why this has happened?

This regards the resistor in series with the opto-diode (the resistor that sets a gain in the feedback loop).

The spec is...

Flyback

85-265VAC

Fsw = 132khz

TL431 regulation

 

Vout = 25V

Iout = 1.3A

 

The resistor in series with the opto-diode came out as 15800 ohms…….but as you know, this resistor should never be more than 4700 ohms..otherwise insufficient current will be able to flow into the control pin of the topswitch during light load.

The CTR of the suggested opto is 300-600 (PS2501-1-K-A) , but as you know, we must derate this by 40% to take into account CTR degradation with time. Even without CTR degradation, the series resistor would still be limited to 6800 ohms. Therefore, why has PI Expert given such a far out value of 15800 ohms?

I am not knocking PI Expert, ..topswitch and PI expert are the best in the world for flyback design with monolithic chip, but I am just wondering if there is some reason.?

 

 

 

 

Hello,

By the way, for some reason i cannot now use PI Expert software at all, when i put in the spec, it just says "loading, please wait", and just hangs like that for ages, and does nothing. Do you know what's wrong?

The higher value of the gain resistor (15800 ohms) is due to the fact that the crossover frequency is very low (1kHz). If you increase the crossover frequency in the feedback dialog box to 2kHz, the value of the DC gain resistor will change accordingly.

Let me know if you have further questions.