Transformer winding question

[pigman]

Hey guys,
I have a transformer that I use its a centre tap 11.25:1 transformer switched by two FETs at 12v putting out ~240v square wave. The secondary is 0.25mm enamel and the Primary is 14 x 0.2mm centre taped. My question is I am currently running a 50W load on the transformer and I have done some tests running 100W load on the transformer, while it works well and the coil runs at about 70deg C I am concerned about the load on the little 0.25mm enamel wire and that maybe this temp is too high? Would winding it with say 0.4mm enamel coated wire instead increase the load capacity of the device and decrease the running temp?

I am fairly new to transformers and whatnot and understand increasing the thickness of the secondary might be an issue?

Cheers

 

[Grossel]

Generally, thicker wires cause less wattage loss for resistance. But you need to make room for it.
If you already have a transformer in hand, it's not always possible to "upgrade" it because there isn't enough free space.

 

[pigman]

It is a ETD29 bobbin so I'll just wind a new one . So you think that 0.4mm will lower resistance, improve performance and output and then reduce temp?

 

[ronsimpson]

Bigger wire will help.
Wire loss is not the only thing. There is core loss. At some point you will need a bigger core.

 

[crutschow]

Bigger wire will help.
Wire loss is not the only thing. There is core loss. At some point you will need a bigger core.

The main parameter determining the required core size is the size of the wire and the number of turns. If you can't get the number of turns you need on the bobbin, then you will need a larger core.

 

[awt]

increasing the thickness of the wind reduces copper loss hence reduces the temperature. but this may not necessarily increase the capacity of the transformer as capacity depends on many transformer parameters

 

[ronsimpson]

increasing the thickness of the wind reduces copper loss hence reduces the temperature. but this may not necessarily increase the capacity of the transformer as capacity depends on many transformer parameters

I agree. That is why I said somethng about the core. It can only handle so much power. If the core saturats then you can not get more power.

 

[awt]

yeah! ronsimpson.

I agree. That is why I said somethng about the core. It can only handle so much power. If the core saturats then you can not get more power.

 

[crutschow]

I agree. That is why I said somethng about the core. It can only handle so much power. If the core saturats then you can not get more power.

The core does not saturate from power, its saturation is proportional to the applied voltage and inversely to the frequency. The power it can handle is determined mainly by the amount of heat it can tolerate as generated by I²R heating of the winding resistances.

 

[awt]

The core does not saturate from power, its saturation is proportional to the applied voltage and inversely to the frequency. The power it can handle is determined mainly by the amount of heat it can tolerate as generated by I²R heating of the winding resistances.

consider this tansformer's equation,

**broken link removed**

W = Window
a = Cross sectional area
B = flux density
you can see clearly those parameters that determine the power of a transformer.

 

[pigman]

Thanks guys, I did some calcs and found I could wind the right amount of turns of 0.4mm wire on. Doesn't seem to have reduced core temp much but at least I feel a bit better about it it is working well. Frequency is around 40kHz and core temp maxes at about 70 deg C so I guess at the load and frequency that is just what it runs at, which is still well within the limits of the core being used.