Output current problem

Godyreal

New member
Another thing that had been bothering me is whether the primary wire diameter determine the secondary output current? In a smps transformer
 

Silvio

Active member
The assumption is not right. You have to consider power input versus power output and adding around 20% more on the input to cover losses.
If for example I have an off line smps which works with an input voltage of 300vdc and the output is say 30v. If say the current in the output is 10A then the power absorbed is 300w. Turning back to the input power then at 300v the power in the input is 300w + 20% = 360w so the current in the primary is not 1A but 1.2A.
The wire in the primary then must be able to handle this current. On the other hand it is the same rule for the secondary winding. The wire must be thick enough to carry 10A.
I hope it is more clear for you now.
 

Godyreal

New member
thanks so much Sir though to be frank i still cannot relate it to how i use a 0.5mm diameter of wire for primary and 1.4mm for the secondary and i got 150volts at secondary output though i regulate it to 60volts but when i used it on my 200watts amplifier and tested the current the meter only read 1ampper and if i increase volume the smps dim the standby led and the sound become low and distorted, i uses uc3844 with 9n60 mosfet and the transformer is EE42 30turns for the primary and 4-0-4 turns for the secondary. The question is why am i having a small current when i used 1.4mm wire diameter which supposed ti have atleast 4amppers?
 

Silvio

Active member
thanks so much Sir though to be frank i still cannot relate it to how i use a 0.5mm diameter of wire for primary and 1.4mm for the secondary and i got 150volts at secondary output though i regulate it to 60volts but when i used it on my 200watts amplifier and tested the current the meter only read 1ampper and if i increase volume the smps dim the standby led and the sound become low and distorted, i uses uc3844 with 9n60 mosfet and the transformer is EE42 30turns for the primary and 4-0-4 turns for the secondary. The question is why am i having a small current when i used 1.4mm wire diameter which supposed ti have atleast 4amppers?
You relate power in the input and power in the output according to the voltage in each case.
Volts per turn Pri = 320v / 30 = 10.7v per turn.
Sec voltage then = 10.7 x 4 turns = 42.8v.
If you need a regulated voltage of 40v then you can clearly see that 2.8v is not enough. Your headroom voltage to allow the chip to regulate at load must be x 1.4 to 1.5 more.
So 40v x1.5 = 60v. You can see here that each secondary winding must supply 60v when the chip is at full pulse width. Remember that voltage tend to drop with load so when this happens the chip will widen the pulse width to allow more voltage at the secondary thus keeping the output voltage stabilised via the feedback loop.
Using the same reasoning then turns needed at the secondary are 60v / 10.7 = 5.5 turn. Due to that we cannont wind half a turn then we either wind 5 or 6 turns for each secondary winding.
Ampere turns.
Sec current is 10A so 80v x 10A = 800w. You can see here that an EE42 will never give you this power in flyback topology but it will in half or full bridge. 300 or 350w is a more reasonable power output for flyback.
350w / 80v = 4.4A
Using a current density of 8 for audio purpose then using 0.5mm copper wire.
This is found using the cross sectional area.
So 0.25 x 0.25 x 3.142= 0.196mm² x 8A per mm² = 1.57A per wire.
We now find how many wires needed for 4.4A.
So 4.4 A/ 1.57 = 2.8 wires (3 wires of 0.5mm for secondary).
For primary using the same rule then 350w / 320v = 1.09A. So 1 wire x 0.5mm is adequate for primary winding.
 
Last edited:
Top