[Amps] BLF188xr amp
chris.scott at wku.edu
Thu Oct 15 09:33:45 EDT 2015
I should clarify my question; my intent is to do everything practical to protect the FETs - overtemp, overcurrent, and excessive reflected power. The usual location for the reflectometer is after the low pass filter - this provides an accurate indication of the load match. But if the wrong band LPF is selected, this does not protect the fets.
Logically one could move the reflectometer to between the fet output transformer and the input of the low pass filter. This would protect the fets, but would always show about a 2:1 SWR due to the high 3rd harmonic (-10 to -12) reflecting back from the filter. Perhaps this is the best solution - and maybe up the reverse power threshold to normalize it.
Or, modify the LPFs with a "diplexer" arm that shunts 3rd harmonic to a small 50 ohm load. This would have the effect of reducing the reflection to the fets as well ...
But my question relates to swr mismatch protection. My project is a 2 x Blf188XR 1200 watt amp, 80/40/20, for ssb service. I've ordered a 4z4rb board, and got my two Fets for 129 euros each from www.pcs-electronics.com<http://www.pcs-electronics.com>. I spent $190 on a 0.75" x 6" x 12" copper heat spreader, and it will be mated to a large aluminum forced air heat sink. As much as I hate to do it, it seems that soldering the fets in is best to keep case temps reasonable. That makes one doubly worried about blowing the fets.
They brag about 65:1 swr survivability, but this is pulsed. Clearly a post LPF reflectometer ref sample is necessary to shut it down with a poor match, say above 2:1. But I see another pitfall; the switched LPFs being set to the wrong band.
So, is the best solution adding another reflectometer after the output transformer and prior to the filter to protect against that? With an expected third harmonic being 10-12 dB down at this point, the threshold might need to be high, like 5:1 ...
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