Roger Stockton wrote:
>
> Victor Tikhonov [mailto:[EMAIL PROTECTED]] wrote:
>
> > This is exactly how expensive Brusa chargers are built. An I
> > suspect, its HF isolation transformer is the reason why it
> > cannot put quite as
> > many Amps in the pack at low AC input voltage (120VAC) as PFC can.
>
> I suspect the isolation transformer is not the limiting factor. The
> more likely limitation is the current handling capability of the
> input/PFC boost stage.
>
> A typical wide-range PFC topology (and I haven't been inside a Brusa to
> confirm that it is similar) is for the PFC input stage to boost the
> rectified AC line voltage to something like a 400VDC bus; this bus is
> then fed through an isolation transformer by a DC/DC stage that switches
> at high frequency. Regardless of the AC line voltage, the isolation
> transformer is always fed from about 400VDC, so it is not the limiting
> factor. However, for the same output power (or power at the 400VDC
> internal bus), the input stage must switch higher currents and run at
> higher duty cycles when the AC line voltage is low; this is the likely
> limitation.
>
> The isolation transformer limitation is that the output voltage range of
> the charger must be restricted to a narrower band since the voltage is
> determined by the duty cycle of the DC/DC stage, and this duty cycle
> cannot vary over a very wide range if good efficiency is to be achieved.
>
> Cheers,
>
> Roger.
Correct
Our PFCXX are a PFC front end that we use a voltage sense circuit to
control it's output.
Doing a Iso means a Buck stage that has the same power ratings as the
input boost stage.
That means another inductor two switches, and a high speed diode
rectifier, just for the iso stage. That's exactly double the power
devices of the single stage.
We have looked at this. And could put it all in a box the same size as
the PFC20. 2x the volume.
We have shetches and drawings.
Cheaper by about $1000 is a 5Kw Medical grade torroidal 60 Hz
transformer, and a stout fan.
a 5 Kw ferro resonat surplus might be as low as $100 bucks.
I got a 15 Kw Cutler Hammer for the power bench for $550, new and it
weighs 216 lbs.
Also using other topololgies on the PFC front end would get us iso but
NOT the huge output voltage ranges.
I most defiantley understand the ISO nightmare that the Magnachargers
created. They used so many conversion stages and cute magnetic tricks
that thier losses were on the range of a Bad boy chargers. But they did
it on about 4 or 6 stages.
The simplicity of Joe's design, and the execution of it on my part, I
think makes for a unique design. We targted the over 120 volt world, got
that all the way up to 450 volts, then found that we could also Buck
down to 12 volts, without blowing breakers, then found we could Buck
with major power down to the lower voltages. It's been a long road.
There's more to come.
--
Rich Rudman
Manzanita Micro
www.manzanitamicro.com
1-360-297-7383,Cell 1-360-620-6266
