[Amps] Direct rectification of AC mains to drive the amp, VDD Supply

[Manfred Mornhinweg]

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.
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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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