Re: [funwithtransistors] Care and feeding of a large transistor


Aug 8, 2016

 


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#2071 Aug 8, 2016

Hi all!



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

uplink.



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

on.



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.



Norm n3ykf



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#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.



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