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Guest

Sun May 03, 2020 9:45 am   



Hi again,

Tim Williams <tiwill_at_seventransistorlabs.com> wrote:
Quote:
Alternately, use MOSFETs with the gates driven from opposite drains, no need
for a separate drive winding. Usually a series diode and pull-up resistor
is used to limit gate on voltage.

Example:
https://www.seventransistorlabs.com/Images/HVPower1.png
This was a custom transformer so the voltage isn't incredible, but the
circuit overall is actually a bit relevant...

I have a few questions about this circuit, if you don't mind:


1) I don't have such a high Hfe BJT as the 2SD1273, I think a sziklai pair
in place of it should be fine, since it seems to me it will never be saturated
anyway, unless I'm missing something?

2) the 1/2 358 opamp has no DC negative feedback, that's well, unusual to
me, I would like to understand better that part.
Thanks

Frank

Tim Williams
Guest

Sun May 03, 2020 10:45 am   



<frank_at_invalid.org> wrote in message news:r8ltgm$dq9$1_at_dont-email.me...
Quote:
I have a few questions about this circuit, if you don't mind:

1) I don't have such a high Hfe BJT as the 2SD1273, I think a sziklai pair
in place of it should be fine, since it seems to me it will never be
saturated
anyway, unless I'm missing something?

2) the 1/2 358 opamp has no DC negative feedback, that's well, unusual to
me, I would like to understand better that part.


This is what we call an error amplifier. Its output can indeed be saturated
+/- if the load is taking its sweet time to catch up, or if the input is
sudden and large. The 10k in, and 0.01 + 100k across it, sets the response
time and gain (compensation).

2SD1273 can of course be replaced with anything of suitable rating, Sziklai,
Darlington, MOSFET if you don't mind the lost Vgs(on); or even better, a
buck converter -- the purpose is simply to make voltage at the primary CT.
Or current at the CT more specifically, since the oscillator is current
mode. Which means a current-mode buck, and usually a relatively large
inductor so the current ripple is small, is ideal.

Eventually the voltage output responds to the error amp's output (changing
the oscillator's supply voltage), and the voltage divider closes the DC
feedback loop.

The feedback is also buffered, so there's an accurate x1000 output sense (I
don't have HV probes, as it happens).

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/


Guest

Sun May 03, 2020 11:45 am   



Tim,
first of all thanks for all the information!

Tim Williams <tiwill_at_seventransistorlabs.com> wrote:
Quote:

This is what we call an error amplifier. Its output can indeed be saturated
+/- if the load is taking its sweet time to catch up, or if the input is
sudden and large. The 10k in, and 0.01 + 100k across it, sets the response
time and gain (compensation).


well yes I figured out it would be closing the loop with that time constant
of 1ms, it was just the lack of DC path that was looking odd to me, but it's
just me Smile
Nothing bad happens if the output saturates. New things are always welcome
for learning.


Quote:

2SD1273 can of course be replaced with anything of suitable rating, Sziklai,
Darlington, MOSFET if you don't mind the lost Vgs(on); or even better, a
buck converter -- the purpose is simply to make voltage at the primary CT.
Or current at the CT more specifically, since the oscillator is current
mode. Which means a current-mode buck, and usually a relatively large
inductor so the current ripple is small, is ideal.


ok I understood, I think I'll just stick with a sziklai pair since that
will allow me to have a prototype sooner.
I'm still a long way from the needed 11 kV.

Quote:

Eventually the voltage output responds to the error amp's output (changing
the oscillator's supply voltage), and the voltage divider closes the DC
feedback loop.

The feedback is also buffered, so there's an accurate x1000 output sense (I
don't have HV probes, as it happens).


yes I've noticed that, I won't be needing it since it's a bad idea to
wire a long string of resistors down from the CRT anode. I'll be using
a low voltage G2/G4 supply for feedback, if I can make it to work in the
first place that is.

Thanks
Frank

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