From: Gerhard Hickl <gerhard.hickl@aon.at>
To: rsgb_lf_group@blacksheep.org
In-Reply-To: <D9D1BE4F862E4056B791BDB6A4C686F5@PcMinto>
References: <003801cbb33e$8b59e620$0401a8c0@xphd97xgq27nyf>
	 <4929CA3383DD45FCBC44A80A2733B139@PcMinto>
	 <004801cbb34a$5b8ab580$0401a8c0@xphd97xgq27nyf>
	 <1294948612.1430.60.camel@gerhard-desktop>
	 <D9D1BE4F862E4056B791BDB6A4C686F5@PcMinto>
Date: Thu, 13 Jan 2011 21:59:30 +0100
Message-ID: <1294952370.1430.64.camel@gerhard-desktop>
Mime-Version: 1.0
Subject: Re: LF: FET RDS
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


Hello Minto!

Thank you for the reply. That confirms my thoughts.

I will try to put another transistor in parallel soon.

73 es tnx

OE3GHB
Gerhard




Am Donnerstag, den 13.01.2011, 21:15 +0100 schrieb Minto Witteveen:
> 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
> >> >
> >> >
> >> >
> >>
> >>
> >
> >
> > 
>