HP 5300A - Page 33

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conduct, pulling its anode down to about zero volts.
This turns on A2Q8, which turns on A2Q9. A1Q3
base current is then shunted through A2Q9 to ground,
such that A1Q3 receives no drive voltage. This shuts
the power supply off. A2Q6 remains on, however, as
long as the input voltage is present. The circuit is
reset by removing the ac input for about 5 seconds,
which allows A1C2 to discharge through A2R15 and
A2R16.
4-61. Dc-to-Dc Converter
4-62. This is basically a blocking oscillator converter
using a single switch transistor, A1Q3, with the dc in-
put voltage available across A1C2. Resisitor A1R11
supplies initial start current into the base of A1Q3 to
start oscillations. Diode A1CR5 allows base current to
flow to A1Q3 during normal operation with capacitor
A1C4 as an ac bypass. Printed circuit wiring is such
that no drive voltage is applied to A1Q3 if A2 Regu-
lator Assembly is removed from its socket. During
normal operation A1Q3 alternately switches on into
saturation and then off. With A1Q3 “on”, an increas-
ing current flows through A1T1 and the primary of
A1T2.
4-63. The polarity of the rectifiers on the secondary
of A1T2 is such that when A1Q3 is turned on, they do
not conduct. Thus, the dc input voltage sees only the
primary inductances of A1T1 and A1lT2. AI1T1 is a
small current transformer and drops very little volt-
age across primary pins 1 and 3. A2Q1 collector cur-
rent builds up linearly when it is turned or. The
impedance of AIT1 is such that about 1/15th the
A1Q3 collector current flows into the base of A1Q3.
This is sufficient to keep it in saturation. After a
period of time, designated time Tl and controlled by
the rest of the circuit (Figures 4-11 and 4-12), A1Q3
switches off. The magnetic energy stored in the core
of A1T2 transfers into the secondaries and current
flows through each of the rectifiers, A1CR7, 8, 9, 10,
and 13, until the magnetic flux in the core of A1T2
is zero. This defines the end of time T2. Time T1 is
the time A1Q3 is turned on and time T2 is the time
A1Q3 is turned off. During time T1, energy builds up
in the core of transformer A1T2. Time T2 is deter-
mined by the amount of time it takes the flux in trans-
former AIT2 to reach zero and is a function of the
transformer and load only.
4-64. Time T1 is varied by the regulating circuit to
provide the proper amount of enery storage so that
secondary voltages are regulated at their proper
value. The secondary voltages are all held in fixed
ratios with respect to one another and are deter-
mined by the turns ratios of the secondary windings.
The +5 V is compared to a reference voltage on regu-
lator board A2 and regulated to +5 V +.1 volts. This
regulates the other voltages to their correct values.
Capacitors A1C12, 9, 11, 13, and 14 filter the
secondary voltages. The -15 V bias for the MOS cir-
cuits is provided by resistor AIR17 and zener diode
A1CRI11. A1CR12 is across the -17 volt supply to pre-
vent the output voltage from overshooting a large
amount when the supply is initally turned on; it does
not conduct during normal operation.
Model! 5300A
Theory of Operation
4-65.
The
regulation
circuit
must
generate
time
T1
to
properly
regulate
output
voltages,
and
it
must
sense
the
end
of
time
T2
so
that
a
new
cycle
may
be
initiated.
4-66.
A2CRI1
generates
the
reference
voltage
which
is
compared
with
the
+5
V
supply.
The
comparison
takes
place
in
the
differential
current
source
A2Q2
and
A2Q3.
Resistors
A2R12,
A2R5,
A2R3,
and
capaci-
tor
A2C2
provide
a
frequency-selective
compensation
network
to
ensure
fast
regulator
response
and
pre-
vent
oscillation
of
the
feedback
loop.
Resistor
A2R1
biases
zener
diode
A2CRI1
from
the
+17
V
supply,
and
A2R2
supplies
a
relatively
constant
current
to
the
dif-
ferential
pair,
A2Q2
and
A2Q3.
Resistor
A2R9
helps
keep
the
output
voltages
constant
as
the
input
voltage
varies
over
a
wide
range.
A2C6
and
A2R11
provide
instantaneous
voltage
compensation
to
minimize
120
Hz
ripple
on
the
regulated
output
voltages.
4-67. The collector of A2Q2 supplies a current to uni-
junction transistor A2Q5 and capacitor A2C4. This
current varies depending on the difference between
the regulated +5 V and the reference voltage from
A2CRI1.
4-68. A2Q1 is a series-gating transistor for uni-
junction transistor A2Q5. Its base is driven through
resistor A2R4 which goes to the secondary of trans-
former A1T2. Diode A2CR2 protects the base-
emitter junction of A2Q1 from excessive reverse bias.
The phasing of the signal from transformer A1LT2 to
A2Q1 is such that UJT A2Q5 has a voltage from B1 to
B2 during the time A1Q3 is turned on (Time T1).
Figure 4-12. Power Supply Waveforms
~50vV
Q3
COLLECTOR
VOLTAGE
Q3
COLLECTOR
CURRENT