OK, sure. I will try to explain the background of the problem:
The application is that we own a very good quality (and VERY expensive)
30 KW regulated power supply that was designed to put out anywhere from
~150 VDC to 1500 VDC at up to 20 amps. And it works OK within its
design range, but occasionally we need outputs as low as 50 VDC, and
the supply becomes unstable at those very low settings. (And we can't
afford to just go out and buy another supply.)
So, I thought that I might be able to "trick" the supply into thinking
that it is working into a 150 volt load while actually applying only 50
to the load. (And, by the way: The load is an ionized gas plasma, and
its impedance can vary rapidly over a wide range.)
Duty cycle would be "continuous" i.e. several hours at a time. Accuracy
is not super-important; 5% would be fine, and even 10% might work.
'Transient-response' is an interesting question! The existing power
supply has a lot of very complicated (and proprietary) circuitry built
into it to maintain constant output voltage even while the load
impedance is changing abruptly. I'm not sure, but I think that it also
has circuitry to momentarily turn the output voltage OFF for a few
microseconds if the load impedance goes below some critical threshhold
(i.e. if a localized, intense arc forms in the plasma.) I suspect
that an 'active' voltage dropping device with a finite response-time
might confuse the control circuitry quite a bit, but I'm not sure. At
the present time, we are using resistor banks for the voltage-dropping,
but this is not really satisfactory because the current can vary so
much, depending on the gas pressure & composition.
Hope this helps to clarify the problem.
A long time ago I saw a circuit for a wide range, high power DC supply. It
consisted of a preregulator, which was a phase-fired SCR rectifier, to
provide a raw voltage to the input of a second linear regulator, which would
control the output essentially to zero. The duty cycle of the pre-regulator
was set by the difference between the input and output of the second
regulator. Your application would require a rather hefty linear section,
with 20 amps at 100 V drop, but 2000 watts is not unworkable. This should
work OK as long as the main supply will not create voltage surges high
enough to damage the components of the second regulator. Large capacitors
should be able to sustain rapid load impedance changes, as long as there is
no need to change the supply voltage rapidly.
A more efficient approach might be to use a switching power supply with a
150 VDC output (maybe several in series), and wire it in reverse, to
subtract its voltage from the output. You may need to use large output
capacitors on this supply to handle surges, and probably a big series diode
on the output of your main supply to block reverse current.
A resistive load across the output may also help to stabilize the supply at
lower voltage levels. It is probably a PWM system that gets down to very low
duty cycle when running at its lower limit, and an extra load will increase
the duty cycle to a more stable condition.
Paul E. Schoen
www.pstech-inc.com
