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current in LlOO is continuous; therefore, the 30 kHz component is minimal in the load current
presented to the +23 V unregulated supply. R126 is used to lower the Q of LlOO and Cl00 to
less than one. This ensures that there will be no ringing at their resonant frequency. LlOO is a
toroidal inductor to minimize radiated magnetic field.
The synchronous pumping circuit consists of UlOO, U103B, UlOlA, and QlOO and associated
passive components. UlOO is a pulse width modulated switching regulator IC. It has a ramp
generator and several comparators that can terminate the output pulse that starts at the
beginning of the ramp. R118 and Cl17 determine the slope of the ramp.
the high voltage transformer primary. When the voltage passes through zero volts from high
to low, the output of U103B goes high. This signal is connected to the EXT SYNC input of
ensuring that the comparator will trigger a ramp at start up (when there is no ac signal in the
tank circuit).
The output pulse, OUT COLLECTOR, starts at the beginning of the ramp and is terminated
by one of several events. The output is an open collector that is pulled to +15 V by R121.
This signal is inverted and buffered by UlOlA. UlOlA is able to provide large peak currents to
drive the gate capacitance (approximately 1200 pF maximum) of QlOO. R122 limits the peak
current at the gate and slows the switching time slightly. This protects UlOlA and helps to
reduce EMI.
When QlOO is turned on, current begins to flow in LlOl. LlOl is a toroidal inductor to
minimize radiated magnetic field. This current builds up during the duration of the output
pulse from UlOO. When the output from UlOO is turned off, QlOO is turned off also. Then the
current in LlOl continues to flow, forward biasing CR103. The current begins to flow into
Cl22 and through R120 and back into LlOl. R120 dissipates most of the stored energy from
The turn off time of QlOO also slows the rise of the voltage into CR103, minimizing EMI.
This shows the ability to sense the downward zero crossing of the sine wave voltage of the
tank circuit and pull it down in phase so that energy may be added to the tank. UlOO has
the ability to control the maximum possible output pulse length. This is done by RllO and
is essential to prevent a gain reversal where longer pulses would begin to subtract energy from
the tank once the voltage began to go positive.
The current in LlOl is sensed by R124 and is fed back to the CURRENT LIMITING input of
happen in a fault situation and is used primarily for damage control. R124 also provides a
test point to observe the currents in LlOl.
The cathode voltage output from the high voltage power module is sampled by AlA3R3 and
is fed to this board as the HV FEEDBACK signal. The cathode voltage is approximately
-2400 V dc and AlA3R3 is 30 Ma. HV FEEDBACK comes into a pot R103 and then to
a resistor, R102 to +15 V dc. CR100 and CR102 protect the input of U105A in case of
arcing in the high voltage section (AlA3) that might cause any excessive voltage on the HV
FEEDBACK input. El is a spark gap that is designed in as part of the PC board traces. It
should break down at about 1500 V in dry air (less in humidity). Normally, there is only a
few volts at El and no leakage should occur.
4 Al A6/Al A7
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