Message-ID: <004801cbb34a$5b8ab580$0401a8c0@xphd97xgq27nyf>
From: "mal hamilton" <g3kevmal@talktalk.net>
To: <rsgb_lf_group@blacksheep.org>
References: <003801cbb33e$8b59e620$0401a8c0@xphd97xgq27nyf> <4929CA3383DD45FCBC44A80A2733B139@PcMinto>
Date: Thu, 13 Jan 2011 17:50:25 -0000
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Subject: Re: LF: FET RDS
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Minto
One approach is to use a few in parallel like they do in plasma tv's but
there must then be other considerations to hinder the application.
mal/g3kev

----- Original Message -----
From: "Minto Witteveen" <minto.witteveen@gmail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, January 13, 2011 5:42 PM
Subject: Re: LF: FET RDS


> Yup that is correct. High(er) voltage fets usually have the higher RDS-on
> values... Tradeoff based on physics...
> I started my 500-600 Watts 500 KHz transverter with two IRFP360's in
> parallel.
> Later I switched to IXFH26N50 (cheaper at EUR 1 a piece and slightly
better
> than the IRFP360).
> The IXFH26N50 has a VDSS of 500 Volts, and a RDS-on of 0.23 Ohms and an Id
> of 25A.
> With two of these in parallel the efficiency is > 90%. DC supply is (max)
54
> Volts.
> Peak voltage on the drains is somewhere around max 250 Volts. So I might
> search for Fets with a somewhat lower RDSon and a lower max voltage, but
> these fets are indestructible in my setup, they survive open and
> short-circuited antennas without a problem for several minutes until heat
> becomes a problem.
>
>
> For more info wrt my setup see www.pa3bca.nl
>
> Regards,
> Minto pa3bca (500 KHz in PA idle at the moment, alas...)
>
>
>
>
> From: mal hamilton
> Sent: Thursday, January 13, 2011 17:25
> To: rsgb
> Subject: LF: FET RDS
>
>
> LF/MF
> It seems to me if you are working with low V high current FETS the RDS
seems
> reasonable 0.02 for example but when a High V low current device is need
the
> RDS of these devices seem to be around 0.4 considerably higher.
> therefore the efficiency of the amplifier will never reach the 90% plus
that
> some claim.
> I stripped a plasma tv recently and found banks of FETS (6 per bank) and
> wondered why the application neederd so many and have come to the
conclusion
> that because of the high RDS lots were required in parallel to reduce the
> losses.
> Maybe there are low RDS fets about that will handle several hundred volts
at
> modest currents ie 10A at 1000 volts
> Room for thought
>
> de mal/g3kev
>
>
>