Re: LF: Re: FET RDS

[Andy Talbot <andy.g4jnt@gmail.com>]

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