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" <[email protected]>
> Sent: Thursday, January 13, 2011 20:56
> To: <[email protected]>
> 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" <[email protected]>
> >> To: <[email protected]>
> >> 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
> >> >
> >> >
> >> >
> >>
> >>
> >
> >
> >
>
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