Scion Instruments 436-GC Service Manual page 92

436-GC/456-GC
Bead power supply.
The bead power supply has four sections: a regulated bias voltage source, a linear current regulator, a
DAC to set the current, and a switching pre-regulator for the current regulator. R26, R27, and AR4 (pins 8-
10) provide a stable, low impedance source of -4V to bias the bead. C25 keeps switching supply noise out
of AR4, and R28 prevents C25 from destabilizing AR4.
DAC U6, together with AR4, generates a voltage which varies between 0 and -10V with 12-bit resolution.
C29 eliminates DAC output noise due to digital sources. The Thevenin equivalent of the DAC output
(through R21 and R15), combined with the -10V reference (through R16), is a generator which varies
between -4V and -10V, with a source resistance of 6K. This equivalent generator drives the inverting input
of AR3 and resistor R14. Ignoring R28 for the moment, the other end of R14 is held at -4V, irrespective of
the bead current. AR3 will adjust its output, which controls the bead current through Q4, to bring its
noninverting input to the same voltage as the inverting input. The result is that R17 must have the same
voltage drop across it as does R14. As the DAC setting is varied from 0 to 4095, this voltage drop goes
from 0 to 572mV, and the current through R17 varies from 0 to 3.8A. Since the bead current switching
supply is floating, all of the current through R17 also flows through the bead (connected across J2-2&3),
and the DAC controls the bead current. C19 suppresses digital noise.
To keep the power dissipation in Q4 low, a switching pre-regulator holds the voltage across Q4 at 390mV.
U7 is operated in the flyback mode, alternately connecting T1-4 to ground and then opening the
connection. When T1-4 is grounded, CR3 is reverse-biased, and the current in the primary of T1 rises
linearly. When the output switch in U7 opens, the voltages at T1-4 and T1-9 rise rapidly, turning on CR3,
and transfering the energy stored in the primary into C22 and C23. CR4 and VR1 clamp the voltage spike
across the primary, absorbing the energy stored in the leakage inductance of T1. C30 supplies the high
current pulses required when U7 turns on.
The voltage across Q4 is amplified by AR4 (pins 12-14), which is configured as a differential amplifier by
R22-R25. U7 adjusts its duty cycle to maintain 1.24V at its feedback pin (U7-2), which occurs when the
voltage across Q4 is 390mV. C32 provides a high-frequency feedback path to preserve loop stability.
When the bead is unplugged, the feedback loop is opened, and the regulator would continue to increase its
output voltage indefinitely. This is prevented by CR5, which closes the loop and holds the output voltage
steady when it rises to 1.9V. (Normal output voltages are always below ground, since the bead resistance
is between 0.5 and 0.75Ohmm.) CR6 prevents U7 from being latched up by driving pin 2 negative during a
transient or fault condition, and R31 limits the current from AR4 when either CR6 or CR5 is conducting.
C33 and R32 provide loop frequency compensation, and CR7 allows the supply to be turned off by setting
U3-5 low.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 91