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A prospective s.s.t.c. input-synchronizer
Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>
In my s.s.t.c. project, I need a) a system-signal running all the
time to keep the IGBT-drivers' power supplies charged-up and b) a
means for seamlessly substituting, for that signal, the feedback
signal from a current-transformer when the primary becomes
excited. The screen-captured schematic & waveforms at
<http://hot-streamer/temp/tch-synctest.jpg>http://hot-streamer/temp/tch-synctest.jpg
show what I've come up with -- in simulation, that is. It might be
useful for others, for deriving a logic-level signal, even if not
needing a) above.
I'd initially designed it without D3, D4 & D5. But then I realized
that the current transformer would be required to provide a dc
current component -- not possible -- since the voltage excursions at
Q1's gate were not equal. To correct that, a series capacitor would
ordinarily be required. But in this application such capacitor might
need to pass an appreciable peak current. So then I thought to add
D3, D4 & D5, eliminating the capacitor.
For plus-going I1 current, D4 turns on Q1 & D3, with D1 clamping the
collector voltage to ~7V. For negative-going I1 current, D5 turns on
Q1 via its emitter and D2. In that case, Q1's collector can only go
to ~-1.4V because of D1. In both cases, R2 protects U1 from the
slight over-voltage. Notice from the simulation that the mean I1
current is a minimal 2.4 mA out of 10A.
When I1 is providing current, C1's influence is negligible since
either Q1's collector current or I1's current via Q1 will rapidly
change its state of charge at signal excursions. Absent an I1 input,
Q1 remains off and the C1/U1/R3 oscillator functions normally. And
Q1's gate:emitter or gate:source voltage is kept within bounds: it cannot go
One could just as well use a MOSFET instead of the IGBT. Better,
maybe: I checked it out in simulation & found a mean current of only
1 mA instead of 2.4. So I'll try the MOSFET.
N.B: R1 is required in the simulation; in the real world I'll use a
pair of 1Ks at the right sides of D4 & D5 -- to ensure Q1 stays off
with no I1 input.
KCH