Can secondary side wires of SMPS transformer be interchangeable?

Thread Starter

Younes Thabet

Joined Jan 9, 2019
60
Hello All,

I have an SMPS that i want to change its transformer (image below) in the original transformer, pin 7 is connected to ground and pin 9 to a rectifier diode.
Screenshot 2021-10-07 092702.png
with another transformer (image below)

Screenshot-2021-10-07-092719-ConvertImage.png
Now, the primary of both transformers are similar but the secondary of original transformer (first image) and secondary of second transformer (S1 i.e pin 7 and 9) is reversed in polarity!! The second transformer secondary pin 9 was connected to ground and pin 7 to rectifier diode (in another SMPS)..
My question is, can i use the second transformer but connect its pin 7 to ground and pin 9 to rectifier diode!?

Thanks,
 

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Ian0

Joined Aug 7, 2020
3,754
You don’t say the type of circuit that it goes in, but I assume from the transformer specification that it might be a flyback.
If so, the phase is important, so you cannot invert the phase of the secondary.
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
60
You don’t say the type of circuit that it goes in, but I assume from the transformer specification that it might be a flyback.
If so, the phase is important, so you cannot invert the phase of the secondary.
Thanks for the reply,
Yes, the SMPS is a flyback. Can you please elaborate more on your answer!!
I very much appreciate your help.
 

Dodgydave

Joined Jun 22, 2012
10,087
In a Flyback design, the output is designed to peak on collapsing of the primary pulse , so the phase of the secondary is crucial for rectification of positive pulses, so if you reversed the output wires the pulse would be suppressed.

So if you need to reverse the secondary output you may have to reverse the primary too.
 

crutschow

Joined Mar 14, 2008
28,170
Assume an NPN transistor or N-MOSFET switching to ground, connected the primary side with the dot, and the non-dot to V+.
That generates a large positive pulse on the primary (dot), due to the primary inductance, when it switches off.
This will then generate a positive pulse on the dot secondary winding with respect to the non-dot output.

So you can see that, depending upon the desired output voltage polarity, you can't arbitrarily change the polarity between the primary and secondary.
 

Ian0

Joined Aug 7, 2020
3,754
If you reverse the phase of the secondary, it becomes a forward converter, where no energy is stored in the transformer, and the secondary output power at the same time as the MOSFET is ON. It needs an output choke as that is the place that the energy is stored.
 
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