Hi Chris,
Am 02.12.2018 18:01, schrieb Chris Wilson:
Any chance of a few photos of the H Bridge amp construction please?
OK, here are some impressions ;-)
Outside:
http://www.iup.uni-heidelberg.de/schaefer_vlf/LF/pa/20181204_151804.jpg
Filters:
http://www.iup.uni-heidelberg.de/schaefer_vlf/LF/pa/20181204_151945.jpg
FETs mounted to heat sink:
http://www.iup.uni-heidelberg.de/schaefer_vlf/LF/pa/20181204_152015.jpg
Gate transformer:
http://www.iup.uni-heidelberg.de/schaefer_vlf/LF/pa/20181204_152050.jpg
Some PTT watchdog stuff:
http://www.iup.uni-heidelberg.de/schaefer_vlf/LF/pa/20181204_152158.jpg
I take it the BF862's in the optocoupler feeds act as current limiters?
Yes. I had the idea just to reduce the dependency of the duty cycle from
the supply voltage level and shape of the driver a bit.
Any reason you didn't just use resistors instead? Or is it for
galvanic isolation?
Yes, galvanic isolation. One aspect is the safety (oh yes, i like safety
but only when it is useful to consider), the other aspect is the
prevention of EMC issues in the shack. This way, the PC or other
sensible signal generators is carefully decoupled from the power side.
The IXDD414CI appears obsolete, the IXDD614CI is
still current from none Chinese suppliers, should I use a 614?
Yes, there seems to be no difference except the name.
I got them from https://www.digikey.com/products/en?keywords=ixdd614ci
Any
thoughts on more modern FET's as the IRFP460A is also off the current
device listings from named suppliers :( Thanks!
Oh! At least here they are available:
https://www.reichelt.de/mosfet-n-ch-500v-20a-280w-to-247-irfp-460a-p118998.html?&trstct=pos_0
Thanks for the note. I should order a bunch of them as long as they are
available.
For LF, the normal IRFP460 worked reasonable too but at MF you already
notice a significant difference in upwarming due to higher switching losses!
Try to get them from somewhere. It should be possible. Or search another
FET with similar parameters.
RS allows a good parametric search, e.g.
https://de.rs-online.com/web/c/halbleiter/diskrete-bauteile/mosfet/?sra=p&applied-dimensions=4294424079,4294424036,4294341069,4294374353,4294393595,4294386882,4294421462,4291196625,4294392327,4294407012,4293040733,4294389435,4294337998,4293043627,4291658865,4294421649,4292771966,4294416407,4294215551,4294324063,4294136743,4294325511,4292899339,4294220644,4294204641,4294326243,4294095386,4294303277,4294136519,4293253351,4294324990,4294124426,4294222002,4294136239,4294124482,4294325988,4294222053,4294324870,4294133462,4294137170,4294325552,4294325005,4294324039,4291653797,4294325177,4294325265,4294325021,4294325269,4294325291,4294325157,4294099290,4294136648,4293272063,4294136333,4291453942,4294098433,4294324788,4291658391,4294136487,4294330306,4294324584,4294325413,4294323450,4294136639,4294119632,4294132386,4291653453,4294325018,4294086943,4293245047,4294221031,4293048710,4294116300,4294126751,4291653865,4291453937,4294324581,4292940441,4293063774,4294200481,4294131612,4291653452,4294116869,4294332397,4294086951,4294126698,4292801591,4294136028,4294325168,4294226681,4294095381,4294325172,4294136903,4294323509,4294086948,4291653818,4294087757,4291658390,4294193644,4291653837,4294325565,4291653538,4294116716,4294136800,4294421967,4294422427,4294424078,4294137854,4294413678,4294136280,4294421650,4294127968,4294132407,4294420404,4294409034,4294419000,4294421459,4294421386,4294421369,4294410955,4294128048,4294410089,4294418957,4294421504,4293247874,4294419079,4294403642,4294419075,4294409642,4294416416,4294421582,4294397063,4294422422,4293250149,4294398715,4294421360
Can you upload screen shots of the
gate and drain waveforms you are now seeing for comparison, they are
kind of hard to envisage in words ;)
Unfortunately i did not document this very well in 2012. My camera and
also the scope didn't really make impressive photos during that time ;-)
I remember the curves were quite smooth and there was a transient swing
on the positive edge, maybe up to 140 % and then back to 100%. This
transient swing is smoothed with the gate resistor, on the cost of a
slightly lower rise time.
Regarding the 2 kW statement: You know this always depends on the
temperature of your heat sink and thus, the strength of the fan and the
ambient temperature in the shack. I never run it at 2 kW but something
near 1.5, during winter.
My fan is stepless regulated in speed (not like the annoying
on/off/on/off switched fans in most commercial power supplies), so the
speed gives a feedback about the heat sinkl temperature. In winter, at
high power, it was running quite relaxed mostly, even at WSPR-30! :-)
Hope this helps...
73, Stefan
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