Scion Instruments 436-GC Service Manual page 85

436-GC/456-GC
VR1 pre-regulates the power supplied to the flyback converter, thereby stabilizing the current which is
supplied by the converter to the zener diode. It also powers U2 and U3, which sets the drive voltage for the
gates of Q4-Q6 at 15V.
U4, a TL494, is a controller for pulse-width-modulated switching power supplies. In this application,
however, it is used only to provide fixed, 45% duty cycle pulses. The internal error amplifiers are set to their
maximum duty cycle condition by tying their noninverting inputs (pins 1 and 16) to ground, and connecting
their inverting inputs (pins 2 and 15) to the 5-volt VREF (pin 14). Maximum duty cycle (45%) is achieved by
turning off analog switch U1 (pins 6-7), so that R14 pulls the dead time input (pin 4) down to ground. The
converter is turned off by closing the switch, pulling up the dead time pin to 5 volts and setting 100% dead
time (0% duty cycle). R15 and C19 set the operating frequency to 50kHz.
The outputs of U4 are uncommitted transistors, which conduct alternately without overlap. Their emitters
are tied to ground (pins 9 and 10), and their collectors are pulled up to 15 volts through R12 and R13. U3
(pins 1-3) inverts the negative-going pulses from one of the outputs, and two sections of U2 provide
provide a high-current drive to switch the large gate capacitance of Q4 quickly. Q4 is on for about 10µs for
each pulse, placing 15 volts across 1.2mH inductor L1. The current in Ll ramps up to about 100mA while
Q6 is conducting. When Q6 turns off, the voltage on the drain of Q4 (pin 3) rises rapidly, turning on CR2.
The energy which was stored in the inductor is transferred into C14. The voltage across C14 stabilizes at
the voltage which lets the energy flow out of it at the same rate as it flows in. This occurs at about 210V,
with about 500µA flowing through R17 and VR2. R18 and R19 limit the current from the supply to a safe
value in case of operator contact, while C15 and C18 reduce noise pickup on this high-impedance line.
Resistors R20 and R21 divide the high voltage by 202 for measurement by the system ADC. The current
drawn by this divider drops the nominal voltage at J2-1 to 191VDC. If this point is measured with a typical
meter having a 10M input resistance, the nominal reading falls to 173V.
Igniter power supply.
The igniter power converter supplies 6.4Vp-p (unregulated) square waves for the F1D flame igniter. It is
driven from both outputs of U4, so the high voltage must be turned on in order to enable the igniter supply.
The igniter supply is then turned on by turning on U1 (pins 9-11). U3 and U2 invert and buffer the push-pull
outputs to drive transistors Q5 and Q6, as in the high voltage supply. If U1 (pins 9-11) is turned off, R11
pulls U3-6 and U3-9 high, holding both transistors off by grounding their gates. Transformer T1 steps down
the 24V square waves at pins 1 and 3 to 6.4Vp-p across pins 4 and 6.
Stray capacitance from the primary of T1 to its secondary circuit couples 50kHz square wave currents into
Ground 3. C20 provides a direct path to return these currents to Ground 4.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 84