I can in principle concur with your 3 salient points here Manfred.
However, as I said in a recent post an amateur / ham could consider doing
such an abominable thing in a one-off homebrew project; but directly
rectifying the AC power mains would never pass regulatory approval for
commercial equipment sale in most jurisdictions as the rules have tightened
up in recent years, particularly in respect of a DC and harmonic components
going back into the mains, and power factor requirements mandated by CISPR,
CENELEC, EN standards, IEC, etc. For these reasons we're unlikely to see
such a dubious direct AC mains rectification scheme deployed in the QRO amp
commercial marketplace.
I would also not wish sacrifice the immense superiority of TLT xfmrs for
conventional RF xfmrs to provide the necessary robust electrical isolation
from the potentially lethal AC mains supply. Facilitating robust AC mains
electrical isolation in a traditional flux coupled RF transformer covering
the entire 1.8 to 30 MHz HF range is nontrivial and somewhat problematic.
Achieving the requisite isolation with coupling / blocking capacitors is a
risk as they have a propensity to fail...and not in a failsafe manner :-(
In summary such schemes whilst seemingly attractive and tempting to do on
the surface of things have many ugly hairs growing on them that make them
unattractive.
BTW Manfred, I really like your succinct piece titled "Output architectures
of conventional class AB push-pull amplifiers" at:
http://ludens.cl/Electron/mosfetamps/amps.html
I concur with what you say here about how circuit design misinformation is
perpetuated.
73
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Tuesday, 17 September 2013 5:06 AM
To: amps@contesting.com
Subject: Re: [Amps] Direct rectification of AC mains to derive the amp VDD,
> supply
Leigh,
> I would not consider schemes that directly rectify the mains to
> provide the DC supply rail to a QRO MOSFET RF amplifier because it's
> a daft idea from the concern of safety issues relating to electrical
> isolation.
If you don't feel comfortable working in equipment with dangerous
voltages in them, surely you shouldn't use direct mains feeding. But
then even more you should never even consider building a tube type
amplifier, with its several kilovolts inside!
Personally I feel perfectly comfortable with powering an amplifier
directly off-line. There are just three things to be aware of:
1- Simplicity
2- Safety when using it
3- Safety when working on it.
In regard to simplicity, MOSFET HF power amplifiers couldn't be simpler:
Anyway there are always transformers at the input and output. These can
be easily designed to provide the required insulation. The bias is only
a voltage, with no signiifcant current, so it can be derived from the
main supply. That means that you need no separate bias input. All
measuring (power, SWR) is done at the output, ground-referenced. Only
the amplifier module proper is floating at line level. It's really VERY
simple! The only limitation is that it forces the designer to use
conventional transformers, instead of transmission line ones. The other
option, using transmission lines and make the insulation by coupling
capacitors, that have low impedance at RF but high at line frequencies,
can be used, but is less advisable.
Regarding safety in use: Safety regulations used in many countries say
that any electrical device needs to fulfill either one of these: Having
all user-accessible metal parts securely grounded, or having double
insulation between the line and any such metal parts. A ham amplifier
will usually be grounded, and so the insulation in the RF transformers,
plus this grounding, is already safe enough to meet the law. You can add
a built-in ground fault interruptor to exceed the legal safety requirements.
If you prefer, double insulation can also be implemented, by using
additional RF transformers in series with input and output of the
amplifier module, or even by using two separate layers of insulation on
the transformer wire!
I have used simple insulation, winding with thin stranded wire bundles
inside teflon tubing, and using safe grounding, along with the ground
fault interruptors existing in my home.
Regarding safety when working on the amp: It can be done using nothing
more than caution. If you want more, an insulation transformer is great,
but expensive, big and heavy. Buying one just for that one amp sort of
defeats the purpose of using a transformerless supply! For a commercial
amp using direct off-the-line power, an interlock at the cover can be
used, so that the amplifier can't be powered up when open, unless
tampered with.
But any household machine has live line voltage inside, and that's
considered safe as long as it carries writing in the lines of "no user
serviceable parts inside - do not open", and the like.
Manfred
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http://ludens.cl
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