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Hello,
I have designed a flyback power supply to work from 20V to 190V using UCC38C45 and my design has completed.
I the data sheet i have seen that it can work only from 0 to 70 degree centigrade.but i need my power supply to work from -30 degree centigrade
So i am planning to replace with UCC28c45 which to make it work from -30 degree. is this right method to do. Please suggest what are things need to be taken care before i proceed.
Thanks
Raju E
Hello Raju,
Sorry for my delayed reply; I was out of the office last week.
Your equation for output voltage is applicable to a forward-mode topology, but you indicated earlier that you are designing a flyback converter.
This equation does not predict Vout for a flyback, so I recommend NOT to change the transformer turns-ratio based on your equation.
In a flyback design, the Ns/Np ratio predicts the reflected voltage at the primary-side winding which adds to the input voltage, and is seen across the drain (or collector) of your primary switch. The main concern, assuming this is a discontinuous-conduction mode design (due to the low power), is that Volt-second balance is preserved. Since the UCC28C45 is a fixed-frequency (user-programmed) controller, the reflected voltage must be high enough to fully demagnetize the transformer during the "off-time" to match the V-s product at the lowest input voltage during the primary "on-time" at peak load.
If this is maintained, the voltage feedback-loop will take care of the regulation, independent of the ratio.
There may or may not be other reasons to fine-tune the turns-ratio during the course of your prototype debug, but don't do it because of that inappropriate equation.
Regards,
Ulrich