Dear all,
it's funny to see how so many people discuss this matter, with some of
them bringing up good technical reasons in favor or against, and others
just giving their opinion without any good support.
Well, there is an old and wise adage:
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The men who say it can't be done, should abstain from interfering with
the men who are doing it.
------------------
Based on this adage, I will continue using direct mains-connected,
non-isolated power supply schemes, in those situations where it makes
technical and economic sense.
I have build a fair number of such devices, and have never regretted it.
The direct line connected supply I have been designing duirng the last
days for my legal limit amp, is closely based on a smaller (500 watt)
unit I designed and built about two years ago, and which I used among
other things to power several versions of test amplifiers using low cost
MOSFETs. By "low cost" I mean "less than 5 dollars".
This power supply uses just one IC, one power MOSFET, a few diodes, an
inductor, a moderately sized electrolytic cap, and about a dozen small
parts. It delivers tightly regulated output voltage, with adjustable
voltage, current limit and short circuit protection, wide input voltage
range, and features an efficiency of roughly 93% when using cheap parts,
which could be increased to about 96% just by using better, more
expensive parts, and a slightly more complex circuit. The input power
factor at full load is about 0.7 to 0.8, depending on the impedance of
the mains supply. That's not great, but quite usable. An active power
factor correction circuit could of course be added, at the cost of
nearly doubling the complexity and cost of the whole supply, and
reducing the efficiency to slightly under 90%.
The design I'm doing now is for 105VDC output, at 20A continuous duty,
current limit at 23A, taking 180-260V AC input. It has an opto-isolated
enable/disable input too, so the potentially noisy switching regulation
works only during TX. Total cost for the electronic parts, bought in
single quantity at Digikey, is around 70 dollars. In mass production the
whole thing would probably cost less than 30 dollars, but of course we
hams don't do mass production...
I think this is pretty good, compared to any other kind of regulated 2kW
power supply. I'm willing to accept the slight complication of having no
insulation from the mains.
My 500W direct line connected supply, which has basically the same
design but with smaller parts and no enable input, has been working
flawlessly since I built it. And until now I haven't electrocuted myself
nor anyone else, nor have I damaged anything. Having line-connected
electronics is more a matter of inconvenience: One cannot simply connect
any oscilloscope to it! Either one has to use an isolation transformer
during testing, or one has to float the oscilloscope too, which is quite
inconvenient. Some scopes have insulated inputs, and these are great,
but mine isn't like that.
Now a few comments about a linear amplifier powered at about 320VDC from
the rectified and filtered, but unregulated 220-240VAC line, using two
ARF1505 devices:
I looked into this matter when those MOSFETs first appeared. I do see
some problems with it. The main one is some reports about high voltage
MOSFETs having trouble with hotspotting when used in linear modes. The
issue is this: In saturated condition (switching service), a MOSFET's
conductivity has a negative temperature coefficient, which leads to the
current distributing itself evenly over the die, and all is fine. But
the transconductance has a positive temperature coefficient, so that in
linear mode the current will tend to crowd in the hotter spots, further
heating them, and making the devices fail. A certain amount of
resistance distributed inside the devices is effective for avoiding this
effect, up to a specific combination of voltage and current. Below that
voltage, the device operates safely, while above it, it tends to
hotspotting and failing.
Now the question is if these specific MOSFETs can actually work at
300-350VDC in linear class AB, close to their full ratings, without
having this trouble. I have read some papers telling that they cannot,
but I have never tried it muyself - they are too expensive to fry them!
If anybody here has actually used them in linear service at 300V or
more, and high power, for a prolongued time without failures, I would
absolutely love to hear about it! If nobody has done it, the first who
does will be wading uncharted territory.
In switching service (class D, class E, class F, even saturated class C)
there should be no problem, but linear service in class AB or class A
might be expected to cause reliability problems when running at 300V.
And the other, very minor problem: Power factor. To get an acceptable
power factor, at least an inductor should be used in series with the
input to the rectifier. Otherwise the power factor will be really low,
when using a filter capacitance large enough to get a low ripple by
brute force. A pretty good approach would be to use an active power
factor correction circuit instead of the simplistic supply. That would
give 350 or 400VDC, loosely regulated but much better than having no
regulation, and the input power factor would be excellent. But this
power factor correction circuit is just as complex as my direct
off-line, lower voltage supply, which has tight regulation, overcurrent
protection, and reasonable power factor!
OK, back to my design work, or I will never finish it...
Manfred
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Visit my hobby homepage!
http://ludens.cl
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