From: Gerhard Hickl <gerhard.hickl@aon.at>
To: rsgb_lf_group@blacksheep.org
In-Reply-To: <004801cbb34a$5b8ab580$0401a8c0@xphd97xgq27nyf>
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	 <4929CA3383DD45FCBC44A80A2733B139@PcMinto>
	 <004801cbb34a$5b8ab580$0401a8c0@xphd97xgq27nyf>
Date: Thu, 13 Jan 2011 20:56:52 +0100
Message-ID: <1294948612.1430.60.camel@gerhard-desktop>
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Subject: Re: LF: FET RDS
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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
> >
> >
> >
> 
>