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How numbers of Primary & Secondary turns affect the output Power of SMPS?

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Sep 2, 2019
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how the numbers of primary & secondary turns of flyback smps affect output power. i have a smps battery charger
with transformer winding with N0.0 = 20T(swg 26x3 ), Nsec = 6T(swg 26x6), Nvcc = 6T(swg 26 x 1) & N0.1 = 18T(swg 26x3). the output
power of smps is Vout = 14.0Vdc & Idc = 6amp, Pout = 84watt. how to increase it upto Iout = 10amp? how the numbers of primary & secondary turns and wire
gauge affect the output current if Vout is kept constant?

more pri & sec copper would be needed to keep losses the same for the increased sec side ( & pri side ) rms current

note losses are proportional to I^2R, so going from 6A to 10A is 100/36 = 177% higher, so the R needs to decrease by the same amount, for both pri & sec

i.e 36/100 or 0.36 of previous

...Yes , also, as you know the power throughput you can get depends on if your core is big enough not to saturate at the peak current....
Here is a Flyback sim in the free LTspice for you to play with if you want.

With offline flybacks you usually operate them in DCM, and arrange so that peak secondary current is not too the primary on time ends up often being quite also have to watch out for referred voltage, as if you make NS/NP too small, then you throw back a lot of off-state volts to your fet, and can blow it up if not wary.
The power integrations do a full online design for you....using their gives you all txfmr design etc etc.


  • Flyback offline _48V 1A
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there are many equations regarding the input avg-primary current that's define which wire gauge & how many turns is to be used
with other factor such as Bmax, Fosc, core Ae, permeability to achieve more power rating. i saw a formula to calculate the input current
primary-avg current that is " Ipri = Pout/(Vindc-min * 0.9) " let say Pout = 140watt, Vin-dc(min) = 141volt & efficiency = 0.9
then Iprim-avg = 1.10amp is this calculation Iprim-avg correct?

The attached is the design doc for a flyback ...using standard equations of magnetism.
Also shows the transformer design doc.
There is the LTspice sim there too.

Just using standard equations can lead to problems......its best to work it through...doesnt take long...the attached is about half hour of work.

The maths needed is in here


  • 60VIN 15vOUT
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