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Re: [TCML] understanding DRSSTC
In 14/02/2013 07:18, Udo Lenz wrote:
I was commiting an error in my previous frequency analysis, now
corrected. Really, there are frequencies where the
input impedance is resistive close to the two pole frequencies, but not
exactly at them.
But this system has two resonances. Looking at the frequency response
of the system, what I see is:
At the poles the input impedance is never purely resistive, and is
purely reactive in the unloaded case,
inductive above and capacitive below.
At the central frequency the input impedance is purely resistive,
with any resitive load at the output.
Greater load widens the frequency range where the input impedance is
I agree. I've carelessly equated the terms "pole frequency" and "ZCS
As you have shown, they are not identical.. You should be able to run
with ZCS (i.e.the input being purely resistive) in the vicinity of the
The center frequency at light loads and the central frequency at large
have different properties. The first one will be more inductive below
it and more
capacitive above. The latter one is the other way around, behaving
like a pole.
So the frequency with the first behaviour does not exist anymore at
A PLL, which corrects the frequency according to the phase between
primary current and
voltage can only lock onto one of these frequencies. It will be driven
away from the other one.
This is interesting, and something that I was not observing due to my
error. A PLL would really have
problems. But in my time-domain simulations of the system, I used a
simple comparator forcing
the input voltage to have the same polarity of the input current, turned
on after a few cycles. This
control doesn't show instability with any load. It fails just if the
current reverses with energy
returning from the load, as happens if a burst is continued after a
complete beat of the input
current, in a lightly loaded case. But a PLL control controlled by the
input current would also fail.
operate at a pole frequency, because both pole frequencies are
identical, at the central frequency
In the case of a heavily loaded system with only a single ZCS
(with different Qs).
If you are using a PLL, compare the frequency where it operates at
the end of a long burst
with arc load with the two pole frequencies of the unloaded system.
It's true that arc load adds
capacitance to the secondary and changes the tuning of the system, so
I would expect the final
frequency to be somewhere below the central frequency. Or you can
operate the system with
small arcs, so the unloaded tuning is preserved.
frequency will be close to the primary resonant frequency. The secondary
resonance will be wide at this point, as you have stated above, so
that the value
of that frequency will not matter as much as in the lightly loaded case.
I have updated my document, with some frequency response curves:
Frequency response doesn't say everything in these systems, because at
the start of a burst there is a
long transient involving the two natural oscillation frequencies of the
system. They can be used to predict
the behavior only for a heavily loaded system or at the end of a long
burst. And this supposing that
everything is approximately linear.
Antonio Carlos M. de Queiroz
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