HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Mr.ArchVile on October 22, 2016, 0325 UTC
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The MOSFET is getting hot when I connect direct DC to modulation PIN of the LuLu Board.
But when I used a Cap in series with the Mod pin, it stay cool.
I think though this is a trade off in power.
I included a drawing.
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Ok we have resolved this problem off board.
An antenna of the correct impedance must be connected to any transmitter before power is applied. That or a 50 ohm dummy load.
There is no need to connect a capacitor as shown as that will simply block DC and only pass current when modulation is applied. To be honest I have no idea how that would turn out but I suspect not well!
However, Thanks for posting this as it's clear that I need to provide more instructions with my kits.
I will be making a video in the near future!
Hope that this will explain everything!
Hopefully!
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Stretchy should the mosfet give off any heat at all?
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Yes, of course but it's reasonably efficient as it's class E. Worst case is 85% efficiency so then 15% loss, so at 10W o/p that's 1W5 of heat and 4 times that on mod peaks. It does NOT need a Massive heatsink, just mount in a metal case and bolt to that with the supplied insulating kit.
The IRF510 (o/p FET) is easy to drive but has a fairly pants on resistance of 0R5. This is what causes the loss, as if it was 0R it would be the perfect switch.
Class E tries to be the perfect switch as it's either on or off, so S/C or O/C (Short/Open Circuit). As long as you switch quick and have a low on resistance and you have a efficient amp.
An amplifier is just a switch, that's all!
Sooper simple once you get your head round a few basic principles!
Wierd I know but this is RF at its most basic and efficient of modes, you simply cannot beat class E, it's the Biz. It truly is!