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Date: Thu, 13 Jan 2011 17:50:03 +0000
Message-ID: <AANLkTimK5r8GHioO4N3e4eR-AOqbCrN0=T_dCsb=oCKG@mail.gmail.com>
From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
DomainKey-Status: good (testing) 
Subject: Re: LF: Re: FET RDS
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Yes.  The breadboard is complete.  I've been able to test it up to 60V on
the supply rail and up there can get around 460 Watts.  But the voltage drop
acrosss the FETs is quite significant.    250 Volt devices would be
adequate, but as you say, they're aomehwta rarer, and more expensive.
IRFP450s, on teh otehr hand, cost very little.

100V  PSUs are more diffcult to come by, whereas 50V ones seem quite
pelntiful - old Telephone exchange or oteh rtelecomms facility at a guess -
that's where my 25A rated one came from.

So, basically, it looks as if Class E amps for high powers don't lend
themselves to the cheapest am. construction possible.    My 700W 137kHz
switch mode design is slightly more efficient than this class E one, but
bearing in mind that uses the same FETs, but at a 330V rail in half bridge
not too surprising.

Now this is built, and delivering 400 Watts satisfactorily it can stay as
is.  I now need to revamp the 500kHz loading coil housing, having got rid of
its old plastic bucket back when I thought everything was going to go into
the ATU cabin that burnt out.
Hope to put a half-decent  500kHz WSPR beacon signal out within the week
though.

Andy
www.g4jnt.com



On 13 January 2011 17:31, James Moritz <james.moritz@btopenworld.com> wrote:

> Dear Mal, Andy, LF Group,
>
> There is a trade-off in construction of MOSFETs - basically, for a given
> area of silicon,  higher BVdss requires a thicker active region of the
> MOSFET with higher on resistance. You can reduce Rdson by using a greater
> chip area, but that means higher capacitances, increased cost, etc. So you
> can't have your cake and eat it.
>
> In Andy's breadboard circuit, there is a mismatch between the available
> MOSFET type and the available PSU voltage - the 500V BVdss is a bit too high
> for a 60V DC supply - the peak voltage in an ideal class E is 3.56 x Vdc,
> perhaps you would allow 5 x Vdc for safety. 300V BVdss mosfets seem a bit
> thin on the ground, so more efficient schemes might be to increase Vdc to
> about 100V, or reduce it to about 40V and use lower Rdson 200V mosfets.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>
>

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<div>Yes.=A0 The breadboard is complete.=A0 I&#39;ve been able to test it=
 up to 60V on the supply rail and up there can get around 460 Watts.=A0 Bu=
t the voltage drop acrosss the FETs is quite significant.=A0=A0=A0 250 Vol=
t devices would be adequate, but as you say, they&#39;re aomehwta rarer,=
 and more expensive.=A0=A0 IRFP450s, on teh otehr hand, cost very little.=
=A0=A0 </div>

<div>=A0</div>
<div>100V=A0 PSUs are more diffcult to come by, whereas 50V ones seem quit=
e pelntiful - old Telephone exchange or oteh rtelecomms facility at a gues=
s - that&#39;s where my 25A rated one came from.</div>
<div>=A0</div>
<div>So, basically, it looks as if Class E amps for high powers don&#39;t=
 lend themselves to the cheapest am. construction possible.=A0=A0=A0 My 70=
0W 137kHz switch mode design is slightly more efficient than this class E=
 one, but bearing in mind that uses the same FETs, but at a 330V rail in=
 half bridge not too surprising.</div>

<div>=A0</div>
<div>Now this is built, and delivering 400 Watts satisfactorily=A0it can=
=A0stay as is.=A0 I now need to revamp the 500kHz loading coil housing, ha=
ving got rid of its old plastic bucket back when I thought everything was=
 going to go into the=A0ATU cabin that burnt out.=A0=A0 </div>

<div>Hope to put a=A0half-decent =A0500kHz WSPR beacon signal out within=
 the week though.</div>
<div>=A0</div>
<div>Andy</div>
<div><a href=3D"http://www.g4jnt.com/">www.g4jnt.com</a></div>
<div><br><br>=A0</div>
<div class=3D"gmail_quote">On 13 January 2011 17:31, James Moritz <span di=
r=3D"ltr">&lt;<a href=3D"mailto:james.moritz@btopenworld.com">james.moritz=
@btopenworld.com</a>&gt;</span> wrote:<br>
<blockquote style=3D"BORDER-LEFT: #ccc 1px solid; MARGIN: 0px 0px 0px 0.8e=
x; PADDING-LEFT: 1ex" class=3D"gmail_quote">Dear Mal, Andy, LF Group,<br><=
br>There is a trade-off in construction of MOSFETs - basically, for a give=
n area of silicon, =A0higher BVdss requires a thicker active region of the=
 MOSFET with higher on resistance. You can reduce Rdson by using a greater=
 chip area, but that means higher capacitances, increased cost, etc. So yo=
u can&#39;t have your cake and eat it.<br>
<br>In Andy&#39;s breadboard circuit, there is a mismatch between the avai=
lable MOSFET type and the available PSU voltage - the 500V BVdss is a bit=
 too high for a 60V DC supply - the peak voltage in an ideal class E is 3.=
56 x Vdc, perhaps you would allow 5 x Vdc for safety. 300V BVdss mosfets=
 seem a bit thin on the ground, so more efficient schemes might be to incr=
ease Vdc to about 100V, or reduce it to about 40V and use lower Rdson 200V=
 mosfets.<br>
<br>Cheers, Jim Moritz<br>73 de M0BMU<br><br><br><br></blockquote></div><b=
r>

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