# centre-tap transformer design for SMPS

#### harinikrishna

Joined Apr 3, 2020
33
Hi,
Good day.
I am following the document to calculate the number of turns and the strands for the design of the PSFB converter transformer.
https://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_7.pdf.
The secondary side of the transformer is a centre tap.
my calculated turns are
primary turns = 48,
primary number of strands = 15
secondary turns= 20
secondary number od strands = 49
AWG = #29 ( both primary and secondary)
since it is a centre tap at secondary be 20+ 20 turns or 10+10 turns?

Thank-you
Warm Regards
Harini Krishna

#### ericgibbs

Joined Jan 29, 2010
12,465
hi h,
Is it Fig 7.2 of the datasheet that you are designing.?
Also please post the voltages you expect on each of the windings, primary and secondary.
E

#### harinikrishna

Joined Apr 3, 2020
33
hi h,
Is it Fig 7.2 of the datasheet that you are designing.?
Also please post the voltages you expect on each of the windings, primary and secondary.
E
The diagram that I have posted is the drawing which I have drawn.
at input primary: 240V ( square pulse)
output secondary on each winding is expected to be 103V with a phase shift.
The turns ratio is 2.37

#### Dodgydave

Joined Jun 22, 2012
9,936
48t/ 20t = 2.4 ratio, so 240V input is 100V output per winding approx, less losses ..

Last edited:

#### Lyes

Joined Nov 25, 2019
12
So 20 turn in each secondary!

#### ericgibbs

Joined Jan 29, 2010
12,465
hi h,
I would suggest for a 106V secondary, ideally 21.6 turns on the secondary, say 22 turns allowing for losses.
LTSpice sim.
E

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#### harinikrishna

Joined Apr 3, 2020
33
Thank you for the suggestions.
Since it is a centre tap transformer,
is that 22 turns for both the windings L2 and L3?
is it N(l2)+ N(l3)= 22 or
N(l2)=N(l3)= 22?

#### ericgibbs

Joined Jan 29, 2010
12,465
hi,
Two individual windings of 22 turns,
E
BTW:
If you want a 180deg phase difference on the voltage output of the secondariness, wind on 22 turns , bring out a loop wire and wind the next 22 turns in the same direction as the first winding.
E

Last edited:

#### harinikrishna

Joined Apr 3, 2020
33
hi,
Two individual windings of 22 turns,
E
BTW:
If you want a 180deg phase difference on the voltage output of the secondariness, wind on 22 turns , bring out a loop wire and wind the next 22 turns in the same direction as the first winding.
E
I do follow it. thank-you E.

#### ericgibbs

Joined Jan 29, 2010
12,465
hi h,
Interested to know if the transformer is a EI type lamination or Toroid.?
E
Also VA rating ie: current load and Frequency

#### harinikrishna

Joined Apr 3, 2020
33
I have chosen a PQ core for the application. the operating frequency is 200khz. The load current is 10 A. and according to the calculations the total power I got it to be 1738.

#### harinikrishna

Joined Apr 3, 2020
33
There was actually a miss calculation in the transformer winding and strands calculation.
now with the selected core pq4040, I am getting the following
The primary number of turns: 30
primary strands: 22
Secondary turns: 12
secondary strands: 49
AWG: #29 for both primary and secondary.
With this, I am getting a window utilization factor of 0.37. Initially assumed it to be 0.4 for the calculations.
Last time with the calculated data
primary turns = 48,
primary number of strands = 15
secondary turns= 20
secondary number od strands = 49
AWG = #29 ( both primary and secondary) ,
I ended up with the window utilization factor to be more than 0.47. and during winding did not find space to wind the second winding at the secondary side.