#2071 Aug 8, 2016
Getting ready to build a solid state VHF RF power amp. Input on the
order of 0-10 dBm. Output 30 watts or so. Mitsubishi part built on the
correct circuit. Do have filtering for the output and a sequencer.
Understand that power needs to be "on" and time needs to pass for
stabilzation before RF can be produced.
Plan in running this is in class A, for SSB, CW and FM. Keeping it
simple. Power out follows power in. I do understand that if there is
no input, the chip sinks ALL of the power as heat.
This is battery powered. Planned on regulating the B+ to keep the
operating point of the chip constant. Will be used for satellite
Battery stack is D cells rated at 11Ah(!).
Lots of heatsinking and fans. LM 196 is the regulator chip. Output
load (planned) is 6A at 13.5VDC. Cobbled a test load from power
resistors. Planned of doing a 100% duty cycle power supply test early
What quasi preventable disasters lurk on the path to a completed project?
Is a UHF implementation of this design the same (or similar?)?
Have spent many quality hours with the datasheets, schematics and docs
for the build.
#2072 Aug 9, 2016
If this is a single transistor design, then I do not remember any real
gotchas at these power levels. If it uses a power amplifier module,
then it is even easier since all of the input and output matching and
bias control is already done.
Just quickly off the top of my head here are some problems I have seen
or planned for:
1. In a discrete transistor design, thermal runaway is possible do to
excess or uncontrolled bias current. After the bias has been
adjusted, run a test by monitoring the supply current and see how it
changes as the temperature rises to make sure it does not increase to
a dangerous level.
2. RF transistors and modules can be damaged instantly by supply
voltages which exceed their maximum ratings. Consider including an
SCR crowbar circuit or big transient voltage suppressor to blow the
fuse in the event of regulator or supply failure. Also check and tune
the regulator's transient response.
3. Do not skimp on the heat sink. If active cooling (a fan) is used,
then also include a thermal shutoff. Or include a thermal shutoff
anyway for when someone buries the passive heat sink in pillows.