From: Winfield Hill
Subject: Re: low loss transformer availability?
Date: 5 Jan 2003 19:42:44 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> Smile wrote...
>> 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.
> 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]
To continue... Let's try a quickie 1MHz transformer design suited for
your spec. We'll choose a common small RM-5 core, 10.5 x 15mm, about
0.4 x 0.6 inches. Let's select a Ferroxcube (formerly Philips) 3H3
ferrite material gapping the core at about 60um, so A_L = 400nH/turn^2.
Using N = 50 turns, we get an L = A_L N^2 = 1500uH inductor. At 1MHz
this resonates with 17pF, which will be made up from winding, stray
and load capacitance, plus a tuning capacitor. Our resonance has an
impedance of Q times wL = Q * 9.4k ohms. If the inductor's Q = 100,
the effective resonant impedance is 942k, for a loss current of about
127uA at 120V, or 13mA at 12V rms. Which is P = V^2/QwL = 150mW of
lost power that must be supplied from the primary drive.
You can try to improve on the 150mW loss by increasing L, but you'll
run into trouble with winding capacitance. You can try to improve Q,
but you will already have had difficulty reaching 100 without litz
wire, plus there's core loss to consider as well.