Message-ID: <1082666681.40882eb931593@webmail2.kuleuven.be>
Date: Thu, 22 Apr 2004 22:44:41 +0200
From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.ac.be>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Questions about (G0MRF) 136kHz transmitter
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Hi Dick,

> First of all the FET's used.
> The original design uses 2 times 34NB20 FET's Vds200V/Id 34A, Rds(on)
> <0,075 Ohm
> > I don't have them, but I can replace them by IRFP260N or IRFP360
> The figures for a IRF360 are: 400V/23A Rds 0.2Ohm
> > I'll plan to run the stuff on 50Volt.
> The IRF360 has an higher Vds but also a litle higher Rds, I wonder if
> the Rds has an effect on the output power.

Assume you run the FETs at 15A (and 50% duty cycle) then the "switched 
state" dissipation of the 34NB20 will be less than 8 Watt while the 
IRF360 will dissipate 23 Watt. 
But much more important is the "dynamical" dissipation : when the 
FET's are switched you have both rather high current and rather high 
voltage, thus rather high dissipation. The faster you can switch the 
FET's the lower the losses, both the ON/OFF times and the gate-source 
capasitance will affect this.

> Second and more important question is about the output transformer.
> I don't have the original 3C90 core, so I'm planning to use the core
> of an old computer monitor for this.
I'm using a "double-U' core from an old TV set, it doesn't get hot at 
500 W output.

> Did any of you do this before on the G0MRF's design, and if so I
> would like to know if there was any difference in number of turns 
> used.
> I guess the amount of power greatly depends on this output
> transformer so that makes it to one of the important parts of the
> project........
If you're running a few 100 W from a 50V power supply the output 
impedance of the PA is very low (few Ohm), so you will need only a few 
turns on the primary (FET) side.
I'm using a variation of the G3YXM design, I believe it is pretty much 
the same as the G0MRF design. Primary winding is 2 x 3 windings, 
secondary (to LPF) has 25 windings with a lot of taps. By selecting 
the tap I can easily vary the output power from 50 W to 500 W.

> Maybe the number of turns might also be dependend in some kind of way
> to the type of FET's used.
As far as I know : no

73, Rik  ON7YD