DomainKey-Signature: a=rsa-sha1; c=nofws;
        d=gmail.com; s=gamma;
        h=message-id:from:to:references:in-reply-to:subject:date:mime-version
         :content-type:content-transfer-encoding:x-priority:x-msmail-priority
         :importance:x-mailer:x-mimeole;
        b=DAyen9fBt6gkGtCRITENFf0hRtZ5X9+qwqzsyFVPTRGOGrvZNGUjSnRT3q718aidgB
         CHVedZcv0XqfUPvfPsbZ/V57COcUtu/SUQbLUNEPjHvmZNkZ/7jN26F2P2eoPtlG99kl
         TLUimdXm94M1vGx5OKzgm/sIaPxroEfA6PNS0=
Message-ID: <D9D1BE4F862E4056B791BDB6A4C686F5@PcMinto>
From: Minto Witteveen <minto.witteveen@gmail.com>
To: <rsgb_lf_group@blacksheep.org>
References: <003801cbb33e$8b59e620$0401a8c0@xphd97xgq27nyf> <4929CA3383DD45FCBC44A80A2733B139@PcMinto> <004801cbb34a$5b8ab580$0401a8c0@xphd97xgq27nyf> <1294948612.1430.60.camel@gerhard-desktop>
In-Reply-To: <1294948612.1430.60.camel@gerhard-desktop>
Date: Thu, 13 Jan 2011 21:15:24 +0100
MIME-Version: 1.0
Importance: Normal
DomainKey-Status: good (testing) 
Subject: Re: LF: FET RDS
Content-Type: text/plain;
	format=flowed;
	charset="UTF-8";
	reply-type=original
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Gerhard,

It is my experience that using a separate resistor for each gate (abt 10 
ohms) greatly reduces a tendency for parasitic oscillations.
Main cause might be my particular setup but without the 10 Ohms per fet I 
had trouble keeping the amp quit when mismatched, with the R's it is 
unconditionally stable..

Regards,

Minto pa3bca


--------------------------------------------------
From: "Gerhard Hickl" <gerhard.hickl@aon.at>
Sent: Thursday, January 13, 2011 20:56
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: FET RDS

> Hello Mal and group!
>
> ....."a few in parallel"....
>
> My PA is using a single IRFP260N and if I want to put another one "in
> parallel" is it necessary to "decouple" the gates by two resistors (a
> few Ohms each) or can the gates be paralleled directly?
>
> I would prefer the decoupling with separate gate resistors but is it
> essential?
>
> 73 es tnx
>
> OE3GHB
> Gerhard
>
>
>
>
> Am Donnerstag, den 13.01.2011, 17:50 +0000 schrieb mal hamilton:
>> 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
>> >
>> >
>> >
>>
>>
>
>
>