Tcd Electronics Circuit Description - Scion Instruments 436-GC Service Manual

436-GC/456-GC

TCD Electronics Circuit Description

Power supplies and grounds
There are four grounds serving the TCD electronics circuitry. Ground 1 is the return for the +5V digital
supply. It is connected directly to Ground 4, the unregulated +24V return, at the card edge connector on
the Mother board. The analog return is Ground 2, which provides a low-noise return for the ±15V and
+5.25V supplies. Ground 3 is the reference for analog signal distribution, which carries almost no DC
current. All of the grounds must be tied together externally for the board to function properly.
Three RC filters remove noise from the analog power supplies. These consist of R39 with C28, R40 with
C29, and R41 with C30.
Digital circuits
U8 decodes bus address information to access latch U6, buffer U9, and DACs U3 and U4. The latch holds
all of the digital control signals for the board, while the buffer transmits the board identification number and
filament temperature limit status to the bus. R42 allows the inputs of U9 to be pulled high for test, while
remaining low in normal operation.
Power Converter
The TCD requires a variable, stable, floating excitation source, which is derived from the unregulated +24V
instrument supply. U1 is a current-mode switching regulator controller, operating as a flyback regulator at
40kHz. Its internal pass transistor is connected between pin 4 (VSW) and pin 3 (ground). When the switch
turns on, the primary current of T1 begins to increase at about 0.8A/µs. Current flow stops abruptly when
the switch turns off, causing the voltage at U1-4 to rise rapidly. CR8 and VR3 clamp the voltage at 25V
above the input supply voltage, conducting until the energy stored in the leakage inductance of T1 is gone.
VR2 does not normally conduct, but will protect U1 if a transient raises the +24V supply excessively.
When the primary current is interrupted, and the voltage on primary pin 5 of the transformer rises, the
voltages on secondary pins 7 and 10 also go positive. Diodes CR2 and CR9 are then forward biased,
allowing current to flow in both secondaries. Because the secondaries are bifilar wound, the voltages
across the secondary windings remain equal to each other throughout the cycle. The main output from
CR9 is smoothed by C7, and the high-frequency ripple and noise are further attenuated by L2, C3, and C5.
The rectified voltage across C4 is equal to the main output voltage, which is floating. This provides a
ground-referenced feedback signal, allowing the output voltage to be controlled without any direct
connection to the main secondary circuit. Leakage inductance in T1 degrades the tracking accuracy
between the two secondary voltages. This effect is reduced by loading the sense output with R2, and by
choosing CR2 to have a longer recovery time than CR9.
The sense voltage at C4 is combined through R4, R5, and R26 with the output of the bridge voltage control
DAC at AR4-1, which varies between 0 and -10V. U1 adjusts its conduction duty cycle to try to hold its
feedback pin (pin 2) at 1.24V. The output voltage across C7 follows the DAC setting according to the
following equation:
CR3 prevents the output voltage from falling below about 10V at low DAC settings, so that the circuitry
powered by the floating supply remains active. CR4 keeps U1-2 from going negative if the DAC is set to a
large (negative) output while the converter is disabled, which would otherwise cause U I to lock up. High-
frequency noise between ground 4 and ground 2 is filtered off of the control signal by C11, to avoid
instability in the switching converter. C1 and R3 provide feedback loop compensation for U1, and CR5
holds the control pin of U1 (pin 1) low when U6-19 is low, thus turning off the converter.
SCION Instruments
V = -3.14 * V + 4.14.
out DAC
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 92