[Amps] LDMOS amp off battery?

[Manfred Mornhinweg]

Steve,

> When I travel the world on my yacht I shall come and see
> you!  Maybe we could take a trip at sea and work DX and satellites /MM
> all day?

Or maybe we can meet at sea, when I'm sailing on the yacht Capricornio.

If you can tell me how to get an HF antenna to perform well on a 
wooden-hulled yacht, please let me know. I have tried working a litle DX 
from it, both using the boat's IC-M700 and my TS-450, and the results 
are lousy, no matter what I try. If you have something lenghty about 
this, it would probably best to go by direct mail, to keep the amps 
forum on-topic.

> Yes, the house inverter is the easy way - just plug the amp in!  My
> first paragraph lets the cat out of the bag - and space is a little tight!

If the inverter on board is large enough, then I would indeed do that. 
You know, batteries have a very limited life. Instead of having separate 
batteries for different things, and having to replace all of them every 
few years, it's simply better economy to have one set of batteries, 
that's large enough for everything.

> Yes, or switch in a couple of fat diodes in series to knock off a volt
> or two.

Not such a good idea. A typical power diode has 0.4V drop at very low 
current, and 1.2 to 1.5V drop at high current. So a string of diodes 
significantly worsens the stability of the voltage, and that impacts 
signal quality.

> The real question is, do I even need to do this?  Is 58V on the amp a
> deal breaker?

With some luck, caution, well tuned antennas, a device like the BLF188XR 
should survive that. But if you really want to go with 58V, I would 
suggest using the new MRF1K80H, which can comfortably and safely handle 58V.

> So maybe the combination will survive..  Obviously drive levels will
> vary..  Maybe I will have to add input ALC so it limits at 800 watts -
> to avoid overdrive during high volts, and to avoid distortion at low volts.

Peaks will still reach over 100V at the drains.

A word of caution regarding direct battery feed for an LDMOSFET amp: 
Batteries don't have a usable current limit. If anything happens that 
makes the amp suddenly draw 60A or whatever, such as an arc at the 
output, or a self-oscillation at an odd frequency, the amps will happily 
supply that current, and you can count on the FET burning out faster 
than any fuse you might have there. So it would be a good idea to add an 
overcurrent protection circuit: Simply a big high current MOSFET, driven 
by a Schmitt trigger controlled by some sort of current sensor. If the 
current exceeds a certain limit, say 38A, for more than 200 
microseconds, the big FET turns off and shuts down the amp.

With switching power supplies that have an internal fold-back current 
limiting circuit, you don't need such additional overcurrent protection, 
of course.

Manfred



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