Re: LF: FET RDS

[Minto Witteveen <minto.witteveen@gmail.com>]

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
> > >
> > >
> > >