Scion Instruments 436-GC Service Manual page 99

436-GC/456-GC 610 Hardware description
coupling between the halves of the winding. VR1, C1, and C9 provide a stable, low-impedance source of
+15V to the switching supply.
The control signal which turns on the igniter supply (U7-9) also enables the high voltage supply. When this
signal is low, the dead-time control input of U1 (pin 4) is pulled low through R13. This sets the dead time,
when neither switching transistor is turned on, to its minimum value of 3%. When the control signal is high,
the DT pin (U 1-4) is raised above 3.5V through CR8, setting the dead time to 100% and shutting down the
supply. C5 pulls U1-4 up when the supply is turned on, creating a soft start, as the dead-time control
voltage is slowly pulled down through R13.
Transformer T1 has a turns ratio of about 29:1 from the secondary to one half of the primary. The peak
secondary voltage of about 275V maximum is quadrupled by the network consisting of CR3-6 and C6-8
and C11. R5 and C10 attenuate the 20kHz ripple and introduce a maximum drop of 75V under normal
loading.
Since pin 2 of U1 is at a virtual ground, R12 and R7 divide the high voltage output by a factor of about
2700. This creates the HV_ SENSE signal, which is routed through analog switch U8 (pins 2-3) to the ADC
for diagnostics. AR4 (pins 12-14) is a unity-gain buffer which isolates the supply from the glitches created
by switching of the system ADC input multiplexer.
Page: 98
436-GC/456-GC Service Manual Revision B February 2019
SCION Instruments