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PI expert software miscalculating the required number of turns

Posted by: steven.aherne@… on

Hi,

PI expert is giving me the wrong number of turns on one of the outputs of an offline flyback design.

the outputs are +5v, +5v,+18v

The turns are too low on the +18 volt out put to give 18 volts out . This is an offline design with 114 turns on the primary so at 85 volts rms (=120 volts pk) input the primary winding gives about 1 volt per turn. Both the 5 volt outputs have 10 turns each (at 50% duty cycle this is 5 volts) so why does the+18 volt output only have 24 turns? It would make more sense if it had 36 turns.

 Is this a mistake in the software?

Please find attached the design and the jpg summary.

Thank you for looking at this.

Best Regards,

Steven  

Files

Attachment Size
BIQ2Design_EE16.uds 1022 KB
BIQ2Design_5v.pdf 544.21 KB

Comments

Submitted by PI-Kenobi on 12/06/2016

Steven,

Per your schematic, the 18V, 0.15A and 5V, 0.15A outputs are AC stacked. In such a case, the total turns for the 18V output is 24T + 10T, which provides you a total of 34T.

Let me know if this helps clarify your concern.

 

Submitted by steven.aherne@… on 12/06/2016

In reply to by PI-Kenobi

Thank you PI-Kenobi. This makes complete sense. Thank you.

Apologies but I have 2 more questions

Is it critical that the feedback winding is wound quadfilar? Is there a magnetic coupling reason for this or is it just to fill a complete layer on the transformer?

Can the feedback winding have a dual purpose ie: be used to power a chip on the secondary as well as provide primary side feedback? 

Once again thank you for clarifying,

Best Regards,

Steven

Submitted by PI-ODO on 12/12/2016

Dear Steven,

 

The feedback winding is wound quadfilar for two reasons.  First, for manufacturability, it is easier to wind thinner-gauge wire (e.g. AWG25).  Second, thicker wire can have increased losses due to proximity and skin effects.

 

The feedback winding can indeed be used for a dual purpose.  For example, if you refer to Figure 8 in the LinkSwitch-CV datasheet, you can find a case where an external bias circuit is added to supply BP to reduce no load input power consumption.  In such cases, it is particularly important to pay attention to good layout practices to avoid noise coupling, etc.

 

-PI-ODO

Submitted by steven.aherne@… on 12/13/2016

Hi PI-ODO ,

Thank you for answering my questions on the bias winding,

Best Regards,

ASteven