Re: LF: FET RDS

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

Mal,

Yes, putting several in parallel means that the total capacitance that the 
driver stage sees (sum of gate capacitances and the reverse transfer 
capacitances) increases so more power is needed to hard-switch the fets. I 
use a TIP31/32 totem-pole to accomplish this.
And the capacitance becomes quit big, Ciss = 4200 pF,  Crss= 135 pF 
(translates roughly to approx 135 * 20 = 2700 pF) so total capacitance the 
driver sees is somewhere near 14 nF!!
Cx @ 500 KHz = 22.7 Ohms.
The IRFP360 is even worse.
Now image 4 of these in parallel.


Regards,
Minto pa3bca.



--------------------------------------------------
From: "mal hamilton" <g3kevmal@talktalk.net>
Sent: Thursday, January 13, 2011 18:50
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: FET RDS

> 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