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Primary Side Regulation (PSR) in flyback

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VSef

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Hello,

I am about to use the UCC28910 from TI to design a multi-output converter.
My outputs are 24Vdc and 5Vdc.

To understand how this IC functions, I was reading its datasheet found on this link: Datasheet.

The IC uses primary side regulation where the aux terminal is used to sense the output voltage in order to regulate the output voltage. As can be seen in figure 21 of the datasheet the sensed value is captured right after the diode current is settled to 0 A. This voltage is equivalent to (Vout + Vf)*(Nps/Npa) where Vout is the voltage on the output of the secondary, Vf is the diode forward drop, Nps is the primary to secondary turns ratio and Npa is the primary to auxilary turns ratio.

The thing that I am not able to understand is what if I have multiple outputs with different windings (Nps1 and Nps2 instead of just Nps) whereI have variable loads on my outputs? How will the voltage regulation still be possible?? Section 10.2.3 have mentions something about multiple outputs but without mentioning this issue.

Thank you!
 
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Hi,

you simply need the correct winding ratio.

let´s say your 24V output has 100 turns.
You calculate with 24V +0.6V diode drop = 24.6V winding output

For 5V you need to calculate with 5V + 0.6V = 5.6V winding output.

Then the 5V output needs 100 * 5.6V / 24.6V = 22.76 = 23 turns.

Mind the voltage drop in the windings... it may influence output voltage accuracy.

Klaus
 
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    VSef

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Thank you KlausST for your reply.

I have another question regarding the calculations done for the design.
I don't understand how Nps(max) is being calculated in section 10.2.1.2.3. How is this formula derived?

Thank you
 

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