Rerurn to Romy the Cat's Site
In the Forum: Analog Playback
In the Thread: The last phonocorrector: “End of Life" Phonostage
Post Subject: Is SiC sick?Posted by N-set on: 11/28/2011
fiogf49gjkf0d
Ok, I think I've finally vivisected my ham--it's connected to the rectifier switching (at least some part is).
Below is the waveform of the loaded HT secondary (red trace) and the signal from a search coil
catching the leakage flux close to the transformer core (with LT everything very much the same):
Well, I remember seeing much nicer switching....of voltages
well above 1kV and with a bridge of huge MV tubes...SiC's, loaded with my big input chokes, hold the voltage for a considerable
period at switching!! A rough estimate from the above trace is 0.8ms! (when unloaded it's much shorter
0.2ms perhaps, and the spikes are proportionaly smaller) and is the same for the LT too--perhaps this a property of a choke-loaded SiC?
Interesting if this has ever been examined as 99% of commercial supplies are either switching or cap-input.
One should have in mind that all those marvelous features of SiC's we've been fed with by manufacturers'
propaganda, have been measured in labs, under precise "test conditions" etc
and here we have a real life situation.
What happens is that the core integrates the voltage (the resulting flux is an integral of the applied voltage), so in the death zones
the flux starts to rise linearly with time, producing the spikes. A closer examination indeed shows that the
blue spikes above start exactly at the beginning of the death zone and peak exactly at the right end of it.
Integral of a constant is a linear function in time.
Now, this flux happily leaks out of the transformers due to a one inevitable compromise.
I've specified my PS transformers to be isolating type with the secondary and primary beeing on different legs
of the C-core. This minimizes capacitive coupling to the line but the price to pay is of course a high
leakage flux. I've put Cu leakage bands around all the transformers and chokes; transformers in the shooting direction
are blocked with a Cu plate (very efficient indeed) not to shoot on the input chokes but still the flux leaks
out in other directiuons.
BTW, one sees a gentle core saturation as a result of me being lazy to persuade the winders to overwind the primary
some 20%. But this saturation is harmless compared to the death zones.
I believe this is this HT flux which somehow couples to my signal circuit. Below is a trace of my output ham with
the search coil signal from the HT transformer:
I think the correlation is not entirely by chance (interestingly the ham is anticorrelated with LT flux).
Reducing the ham with box orienation seems to confirm it.
The ways out of spikes:
1) DIY-style grotesque way:change SiC's to tubes, esp. TV dampers...and burn 2x the power of the rest of
the circuit i their heaters.
2) try to close trannies in steel coffins, thick steel, 1mm at least; this would be my choice; the efficiency is unknown
and can be low at the 50Hz fundamental of the spikes but should increase greatly with harmonics;
The ugly choke ringing observed earlier I ignore at this moment as at the first caps behind the chokes
I have an almost perfect 100Hz sine; if I have a harch fatiguing sound then I try snubbers.
Cheers,
Ham-set
PS Those flux peaks are also most probably (co)responsible for the mechanical noise of the transformers.
Rerurn to Romy the Cat's Site