Jim,
I will take you message as a cue to write another of mine, but it's
addressed to everyone really, and specially to those who are not up to
date with power conversion technology.
Before some people here on the forum think that power factor and its
correction is too big of a problem, I think it would be good to state
that this is NOT so!
To improve the power factor of a rectifier/capacitor group, either a
passive or an active system can be used.
The passive system consists simply of a series inductor. The idea behind
this is the following: As has been said, a rectifier/capacitor group
draw strong current pulses, near the peaks of the voltage waveform. To
be exact, it draws the strong current from some time before the peak
voltage, until just after the peak voltage. After that the current
stops. So we have two problems: The waveform of these pulses is very
different from a half sine, and it is somewhat early in phase.
A properly chosen series inductor will reduce the peak value of these
current pulses, stretch them out, and will also phase shift them. If the
inductance is properly chosen for the power level drawn, the phase of
the current can be made to agree very well with the phase of the
voltage, fixing one side of the power factor problem, and as a
by-product the pulses are softened and broadened, making them assume a
waveform closer to the half sine which would be optimal. The result is
that such a simple inductor in series can improve the power factor very
significantly, but it cannot make it perfect. Also, this correction is
strictly optimized for one fixed power level. At any other power level,
the correction will be less good. Usually the inductor is chosen to
produce best power factor at full load. At partial load the power factor
is then worse, but that's less bad than if it were poor at full power.
The series inductor is simple and reliable, but bulky, heavy, only
partially effective, and somewhat costly.
The active system instead consists of a simple switching boost
regulator, which is controlled in a special way. This boost regulator is
spliced in between the rectifier and the filter capacitor, and consists
basically of one MOSFET, one diode, and one inductor, one small filter
capacitor, plus the control circuit.
The control circuit senses the voltage on the main filter capacitor, and
produces an error signal that has a low frequency response -
considerably lower than the line frequency. This error signal is
multiplied with a sample of the rectified, unfiltered input voltage, and
the resulting product is used to control the current flowing in the
boost converter. As a result, the waveform of the current taken from the
line accurately follows that of the line voltage, is very well
phase-aligned, and the average magnitude of the input current varies to
maintain the output DC voltage roughly constant.
This control system might sound complicated to make, but it comes all
ready made inside an IC that costs less than one dollar! These ICs are
manufactured by the hundreds of millions, and used in every conceivable
electronic equipment.
To build such an active power factor correction circuit, one just needs
the MOSFET, diode, inductor, IC, a capacitor of a few uF, and a very few
small passive components. The hardest part for most people is the
inductor. This uses a ferrite core, can be bought ready-made for the
lower power levels, but for 3kW it probably needs to be wound at home.
One of the largest ferrite cores typically available will be needed, and
it will typically take something like 50 to 200 turns of wire.
Except for that inductor, whose cost is highly variable depending on
where you get it, all the other parts for a power factor correction
circuit for the 3kW level will be about 40 dollars. This will provide
about 0.98 or better power factor, regardless of the actual voltage
waveform in the mains, and across essentially the whole range from
nearly zero to full power!
So, if any law should be passed that requires us to have excellent power
factor even in our homebuilt equipment, this can be done simply and
inexpensively even for a legal limit amplifier. For lower power
equipment, it costs just a few bucks, as little as 5 dollars for small
ham power supplies. So there is no reason to worry.
As long as there is no such law, we have the choice of including such
power factor correction, or skipping it. Without PFC, we save a few
bucks, about two percent loss, and some potential for RFI. Mainly the
latter two are the reasons why I use active power factor correction only
when it is truly required or clearly advantageous.
In high voltage power supplies for tube type amps, active power factor
correction can be included too. In principle it could be done on the
secondary side, after the transformer, at high voltage, but that would
require very expensive MOSFETs and diodes. It would be stupid to use an
expensive, heavy transformer, and then do PFC in an expensive way! So
the obvious solution is to use a switching power supply to make the high
voltage, and build the PFC circuit into it.
This switching supply might not even need regulation - after all, the
PFC circuit itself delivers pretty well regulated DC, and tube
amplifiers usually run from unregulated supplies. So, a very simple and
effective, power factor corrected tube amp power supply could consist of
an active power factor correction circuit followed by a simple,
unregulated upconverter, operating somewhere in the 30 to 100 kHz range,
using a MOSFET bridge with a free-running oscillator/driver chip, a
compact ferrite core transformer, and a high voltage rectifier made with
superfast silicon carbide Schottky rectifiers, followed by a small
filter capacitor (less than 1uF is plenty). This technology is
available and inexpensive. Digikey, for example, has most of the parts
in stock. Only the ferrite cores would have to be ordered elsewhere.
This power supply might weigh 3kg total, would produce much better
voltage stability and less ripple than a conventional supply, have near
to perfect power factor, and might cost around 150 dollars in parts,
which is likely far less than a conventional transformer based supply.
Electronic engineering is the combination of money (for components) and
brains. The more you use of either, the less you need of the other.
Manfred
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http://ludens.cl
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