Re: LF: FET RDS

[Stefan Schäfer <schaefer@iup.uni-heidelberg.de>]

Hi Gerhard,

I know many people using a 1 Ohm to 4.7 Ohm resistor in series when 
paralleling the FETs. This is to prevent e.g. "oscillations" and is used 
in SMPS applications, switching at some 10 kHz. In my PAs i never used a 
gate resistor (they just increase the switching time in my opinion) and 
never lost a FET by that. I would rather say its important to make the 
coupling impedance between the driver output and the different gate legs 
as low-impedant as possible. In my 160m PA is was necessary to use a 
copper plate the connects the driver and all the 4 gates. But on 137 kHz 
this is not that problem.
A IRFP260N can handle at least 1 kW in a class E PA at 137 kHz, so what 
do you plan? :-)
73, Stefan

Am 13.01.2011 20:56, schrieb Gerhard Hickl:
> 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
> > >
> > >
> > >
> > >        
> >      
>