SMPS Method easy but hard

Thread Starter


Joined Oct 27, 2015
Hello Guys, Researchers Tinkerers,
I come to you, because my knowledge is limited and would like to learn and develop.

for this I would like to use the paper AND8128.
It shows a method to calculate smps. The question is: How do I find the right transformer can anybody help me with that?
especially if I have the following values:

+20V bei Iout=400mA, Ippk=0,157A, Fmin=100KHz,Fmax=146KHz,4mH. what would be your method in selecting the Transformer?

If I have a double secundar output with a ground in the center can I than create a Bipolar power supply?, do I need to double the overall transformer wattage? if the Load is equal of course.

would be great If somebody can nudge me in the right direction.

again thanks for reading this mail, time effort and dedication.

sincerely yours



Joined Aug 1, 2013
There are a lot of circuit variables that affect the transformer design, and without a schematic of your circuit we know none of them. Please post your schematic.

Depending on which controller chip you use, there might be an off-the-shelf transformer available. Some transformer manufacturers have product lines based on popular control chips.


Thread Starter


Joined Oct 27, 2015
Hello AnalogKid,Dodgydave,

thanks for the fast reply, thank you very much:) As Dodgydave suggested connecting between the pins 6-10 based on the design of the AND8128 see attached file. I can use the termination pin for the +15 and -15V.
The load calculation I can keep based on the AND8128 if the load is symmetrical from the +15 and -15V. or are there some secrets of the trade I should know if I would like to implement this transformer Dodgydave suggested?, especially on EMI?

I hope I can resort to another question on a simular subject within this thread, The transformator given in the simulation file from MAXIM, shows me that I need to build a custom design Transformer... with 2 primary, and 1 secundary.
and the simulation spits me out a complex portrayal of a transformer which is understandable, but a bottleneck less the same. how can I convert these values to an understandable dimensioned transformer?

  1. I cannot use the same transformer due to the high voltages primary,and bias. which results in the second question
  2. what kind of transformer do I have to look for?
thanks already upfront.:):oops: