From: Winfield Hill
Subject: Re: low loss transformer availability?
Date: 5 Jan 2003 16:57:11 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> I might be able to go down to 100 kilohertz or as high as 5MHz.
> I need 12 V AC RMS in..... and (about) 120 V AC RMS out.
> Even when I chose a high mu toroid and wound with small wire, I could
> not get the impedance of the primary up high enough to prevent a lot
> of input current from being drawn by the primary when there was very
> little current being drawn by the load on the secondary.
Keep in mind that the load seen driving the primary is N^2 times the
secondary load, in parallel with the primary magnetizing impedance.
Naturally you seek a low primary magnetizing current, but this means
more turns, which means 10x more turns on the secondary, driving up
the secondary winding's self capacitance. Say that's Cs + Cload = 3pF,
including all wiring and the load itself. While this is a pretty small
capacitance, it looks like 300pF to the primary, thanks to the 1:10
step-up transformer. At 5MHz that's a reactance of j106 ohms, which
causes a drain of 95mA on 10V. :>(
As has been suggested, it's a good idea to resonate that capacitance.
This means using a tuned inductor for the secondary, generally on a
ferrite core complete with air gap. You can drive this inductor with
a transformer (the inductor resonates the transformer's capacitance as
well), with a "primary" winding, or you can simply tap the inductor 10%
of the way up (i.e. one turn for a 10-turn inductor) and drive that.
Here's an example. Let's pick a nice small pot core ... [more later]
You can also reduce your drive frequency, but you'll still find that
for fixed frequencies a resonate system can save you _lots_ of power.
How about telling us about your load, and what you're trying to do.