[Prev][Next][Index][Thread]
Re: Solid-state TC - transformer design
Roderick wrote:
>I'm going to use a 10 ohm 225w power resistor placed before the
>filter capacitors in the circuit to limit the rate of charge to the
>capacitors. In previous circuts I blown the bridge rectifier even though
>it was a high current, high voltage type!
I use much less capacitance (2000uf ), a 25A bridge and an
"inrush limiter" this little gizmo has a resistance of 10ohms
when cold and 0.1 ohm when hot (from memory). I found these
in the RS catalogue over here. It got rid of the switch on bang.
Malcolm wrote:
>Leakage inductance is modelled as an inductor completely separate
>from and uncoupled to the transformer. This inductor can be seen
>in both primary and secondary and has a value reflecting the turns
>ratio when looking into the appropriate winding. If one models it
>that way, it is then a component capable of storing 0.5LI^2 which
>is basically a rogue lump of energy. If you short one of the windings
>then measure the inductance of the other winding, you are measuring
>the leakage inductance (uncoupled flux). A perfect transformer will
>theoretically give zero inductance in this situation. I think I got
>that right but stand to be corrected.
Jim wrote:
>You've got it right on. The portion of the primary's flux that
>does not cut through the secondary, is the "leakage inductance" and
>acts just like you put an inductor in series with your transformer (of
>reduced primary inductance). Yes, it does both: resist the current
>rise and increase the back EMF. Like I said, you got it right on;)
>Harri wrote:
>A good model for most of coiling transformer might be like this:
> ideal T
>-----L(leak)----+-+-----)(-------+-----
> | | )( |
> Lm Rp )( = C
> | | )( |
>-----Rw---------+-+-----)(-------+-----
>The reason for need of very low capasitance (vied on the secundary
>side) is that it will be reflected to the primary side as square
>of turns ratio! A nice example from literature was a 2000-turn
>transformer with high transformer ratio. Capasitance on the secundary
>side was only 13pF. However, it was reflected to the primary side as
>something like 70nF in parallel with the ideal T. Wow! That will
>most certainly mean problems..
thanks Harri, Jim and Malcolm, "rogue lump of energy" sounds about right -
well it gives the snubbers something to do :).
>The reason for need of very low capasitance (vied on the secundary
>side) is that it will be reflected to the primary side as square
>of turns ratio! A nice example from literature was a 2000-turn
>transformer with high transformer ratio. Capasitance on the secundary
>side was only 13pF. However, it was reflected to the primary side as
>something like 70nF in parallel with the ideal T. Wow! That will
>most certainly mean problems.
Yes and the bigger the transformer, the bigger the potential problems -
sorrowfully he shelves his plan for a 100kW driver using 200 irf740's -
much to the relief of radio listeners everywhere.