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RF Amp. Heat Problem

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chrreilly

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I've built a RF amp based on a slight modification of the WA2EBY design (**broken link removed**). I'm currently testing it using an 8MHz, 10Vpp signal at the input and a 50ohm dummy load at the output. However, this testing is limited since the IRF510 MOSFETs are becoming very hot very quickly, even with heat sinks attached. I'm slightly puzzled as only 0.5A is being drawn from the +28V power supply.

Anyone have any suggestions? I've attached my schematic for reference, or you can open it here http://imgur.com/3ULJE55.
 

What is your output power by the way!!!
and also Power amplifier requires some special or different PCB designing, there is extra cut out in the board for the Mosfet to protect from over heating!!!
 

There may be a reflection from load and this reflected wave will heat the transistors..
 

It might be poor design in the output balun transformer (also make sure the windings have the right polarity...). Can you show any operating waveforms on the gates or drains? What kind of output power are you delivering?
 

smoothcriminal said:
What is your output power by the way!!!
and also Power amplifier requires some special or different PCB designing, there is extra cut out in the board for the Mosfet to protect from over heating!!!
Click to expand...

I'm getting about 10Vrms across the 50ohm dummy load, so about 2W of power(which is low). Also, the circuit is contructed on a perfboard, so the MOSFETs are through hole with heat sinks screwed on.

BigBoss said:
There may be a reflection from load and this reflected wave will heat the transistors..
Click to expand...

I'll check with an SWR meter to check for reflection. Last time I tried to measure it I didn't see any reflection, but that could be due to the low output power with the meter I'm using.

mtwieg said:
It might be poor design in the output balun transformer (also make sure the windings have the right polarity...). Can you show any operating waveforms on the gates or drains? What kind of output power are you delivering?
Click to expand...

I checked the polariy of the output transformer and it is connected the right way. Any tips for good design/things to watch out for? I tried to make the windings as clean/uniform as possible. Also, the output power is about 2W. I'll try to get some scope waveforms from the gate and drain later on today.
 

There might be some reflection between "the transformer and your transistor", not from load..
 

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