Tektronix DM 5010, Инструкция по эксплуатации

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Theory of Operation—DM 5010
Initially, as the power module is turned on, the PWR MOL
line is held low for a short time. This keeps Q1105 and
Q1104 turned off, holding the output of U1000 high. This
keeps Q1101 and the series-pass transistor in the power
module turned off.
Then the PWR MDL line goes high and Q1105 is turned
on. This forward biases Q1104, allowing current to flow
through Zener diode VR1001. The inverting input of U1000
is now held at
+5 volts and its output goes negative, turn­
ing on Q1101. As Q1101 comes on, base current begins to
flow in the series-pass transistor to turn it on.
primary winding connected to the power module’s +25 V
supply, the ends of the primary coil are alternately switched
to ground at an approximate 27.78 kHz rate. This rate is
synchronized
to the A/D conversion process and minimizes
any converter error caused by power supply ripple in the
Isolated Section.
Flip-flop U1325B and inverter U1520B convert the
unsymmetrical 27,78 kHz timing signal from the Timing logic
to a pair of symmetrical and complementary signals to drive
the transformer switching amplifiers. Figure 4-11 illustrates
the timing relationships of the symmetry conversion.
As its emitter is pulled toward the +8 volt supply, current
flows
in the voltage-sense network, R1005 and R1033.
When the voltage at the noninverting output r5
volts, bias current to Q1001, and thus the series-pass tran­
sistor, is reduced. The supply stabilizes at this point and the
output closely approximates +5 volts.
The complementary squarewaves at the outputs of
U1325B drive two identical current-switching amplifiers to
control current flow in isolation transformer T1311. A pair of
transistors in each amplifier are driven in a push-pull con­
figuration to provide the large base currents and fast switch­
ing
times required for efficient power transfer.
Zener diode VR1216, resistor R1217, and the associated
power-module transistor provide an overvoltage protection
network on the +5 V supply line. If the +5 V line exceeds
¾5.7 V, the transistor becomes forward biased and begins
to shunt excess current to ground. This protects the bulk of
the circuitry in the Grounded Section of the instrument
should a component in the +5 V regulator fail.
TRANSFORMER DRIVE <4>
The Transformer Drive circuitry switches current from the
TM 5000-Series power module through transformer T1311
to drive the Isolated Supplies. With the center tap of the
ISOLATED POWER SUPPLIES
<4>
The Isolated Supplies provide rectification and initial fil­
tering for the power transferred from the power module to
the Isolated Section via the Isolation transformer. Capcitor
network C1301, C1401, C1403, and C1404 provide noise
cancellation by summing out-of-phase currents until cancel­
lation occurs.
CONTROL LOGIC
<5>
The Control Logic generates the control signals that con­
trol the A/D conversion process. It uses time-related signals
0
1 2 3 4 5 6 7 7 8 9 IB 11 12 13 14 15 15
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QC J — ™ -------------- ! — !________s____________
I-------
4
-------------- U _________ ! _________ F"
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SiK
n _______ n---------------i_________ n-------------- 1__
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0
I------------------ 1________ I-------------------1________ I
PROPACATJON
■» — DELAY
THROUGH
COUNTER
299419
Fig. 4-11. Transformer drive timing.
4-16
ADD JAN 1982