Gerhard,

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
>
>
>