Harris DX 25U, Техническое руководство

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the drain current and decrease the effective source-to-drain “re-
sistance.” An input voltage of less than +10 Volts will effectively
“saturate” the MOSFETs in this circuit and result in minimum
source-to-drain resistance.
E.4.4
Regulator Section Circuit Description
The regulator section includes an input amplifier stage Q2 and
series regulator pass transistors for the two regulated outputs.
Parallel pass transistors Q3 , Q4, and Q7 are used for the Section
D8A output voltage, and Q5, Q6, and Q8 are used for the Section
D8B output voltage.
Each series pass transistor section can also be thought of as a
source follower circuit, with D8 as the load. The regulator output
voltage will be 2 to 10 Volts less positive than the gate voltage.
The gate voltage of Q3-Q4-Q7 will be determined by Q2 drain
voltage and the voltage divider; the gate voltage of Q5-Q6-Q8
will be determined by Q2 drain voltage and the voltage offset
circuit.
E.4.5
DC Amplifier Stage (Q2)
The reference voltage from U2-1 is fed through R16 to a current
summing network at the gate of Q2. The inputs to the summing
network include R19, Q2 stage negative feedback, R41, negative
feedback from regulator section A output, and R42, negative
feedback from regulator section B output. The output of the
summing network is offset 1.4 Volts by diodes CR5 and CR6.
The sum of the four currents creates a voltage drop across R20
which is the gate voltage for Q2. Capacitor C8 bypasses AC
components around the voltage offset diodes to speed up regu-
lator response time.
N-channel power MOSFET Q2 is used as a DC amplifier, with
its drain connected to the +230 VDC supply through load resis-
tors R25 and R26 on Fuse Board A24. The MOSFET begins
conducting when its gate voltage goes above a +2 to +4 Volt
threshold. Once the threshold is exceeded, the MOSFET’s drain
current will increase as the gate goes more positive and the drain
voltage will decrease because of the voltage drop across load
resistors R25 and R26 on Fuse Board A24. The output of the Q2
amplifier stage is coupled to Q3-Q4-Q7 gates through a resistive
voltage divider, and to Q5-Q6-Q8 gates through zener diodes
CR7, CR9, and CR10.
E.4.6
Series Pass Transistors Q3, Q4, and Q7 (For
Section D8A Supply Voltage)
The regulated output voltage to driver section D8A is controlled
by series pass transistors Q3, Q4, and Q7. When their gate
voltage is zero, they are cut off and the section D8A output
voltage is zero. Q3, Q4, and Q7 begin conducting (turning on)
when their gate voltage is a few Volts positive (+2 to +4 Volts
relative to their source). As the gate voltage becomes more
positive, they conduct more heavily, and the section D8A output
voltage increases.
E.4.6.1
Voltage Divider
The voltage drop across resistor R25 is the gate voltage for Q3,
Q4, and Q7; R25 is part of a voltage divider between Q2 drain
and Q3-Q4-Q7 source. The voltage DIFFERENCE between
Q2’s drain voltage and regulator output voltage “A” is divided
by the voltage divider consisting of R22, R24, and R25. As Q2
drain becomes more positive, voltage across R25 increases,
Q3-Q4-Q7 conduct more, and the section D8A output voltage
increases.
Capacitor C9 and R23-C10 provide a low impedance path around
R22 and R24 for AC components. Their effect is to speed up
response to sudden variations in output; this action will also
reduce AC ripple in the regulated output voltage.
Resistors R58, R59, and R64 in the source circuit are current
equalizing resistors which compensate for variations in charac-
teristics of the paralleled MOSFETs. Ten Volt zener diodes CR8,
CR12, and CR16 protect the MOSFETs against excessive
source-to-gate voltage.
Diode CR13 at the regulator output protects the circuit against
negative transient voltages.
Resistor R41 provides negative feedback for the regulator section.
E.4.7
Series Pass Transistors Q5, Q6, and Q8
(For Section D8B Supply Voltage)
The regulated output voltage to driver section D8B is controlled
by series pass transistors Q5, Q6, and Q8. Except for the zener
voltage offset diodes, this section operates in the same way as
pass transistors Q3-Q4-Q7.
E.4.7.1
Voltage Offset
The voltage across resistor R34 is the gate voltage for Q5, Q6,
and Q8. Resistors R34, R33, and zener diodes CR7, CR9, and
CR10 are all in series, between Q5-Q6-Q8 source and Q2 drain.
The zener diodes will not conduct until the voltage at the drain
of Q2 exceeds the 133 Volt sum of the zener voltages. Until the
zener diodes conduct there will be no voltage drop across R34
and Q5, Q6, and Q8 remain cut off.
When the voltage at the drain of Q2 is high enough to overcome
the zener voltage, Q5, Q6, and Q8 begin to turn on. At this point,
Q3, Q4, and Q7 are conducting heavily so that output voltage
“A” is nearly at the +115 VDC input. As the voltage at the drain
of Q2 becomes still more positive, Q5, Q6, and Q8 turn on more,
causing output voltage “B” to increase while output voltage “A”
remains at maximum.
Transistor Q1 is used to ensure that Q5, Q6, and Q8 can turn on
fully, so that the supply voltage to driver Section D8B can
approach the +115 VDC input when required. The base-emitter
voltage for Q1 is the voltage drop across R22, which is part of
the voltage divider that controls Q3, Q4, and Q7. When Q3, Q4,
and Q7 are nearly saturated, Q1 will begin turning on so that the
voltage across Q1 and CR7 will be less than 39 Volts and the
zener offset will be less than 133 Volts.
Capacitor C11, and R32-C12 provide low impedance paths
around the zener diodes for AC components. Their effect is to
speed up response to sudden variations in output; they also
reduce AC ripple in the regulated output voltage.
Resistors R60, R61, and R65 are current equalizing resistors.
Resistors R35, R36, and R63, in the gate circuits, are parasitic
suppressors. Ten Volt zener diodes CR11, CR14, and CR17
DX-25U
E-2 888-2297-002
WARNING: Disconnect primary power prior to servicing.