Circuit Description—Type 422 AC-DC
Steering Switch. The Steering Switch stage is controlled
by two simultaneous input signals. The positive-going pulse
from the conducting Multivibrator stage transistor provides
a turn-on signal to enable either Q1104 or Q1114 at a fixed
rate as determined by the Blocking Oscillator and Multivi
brator stages. The Steering Switch transistor which is not
turned on is held reverse biased by the negative-going signal
from the other side of the Multivibrator. However, the con
trol signal from the Pulse Width Control stage determines
the actual conduction time of the Steering Switch
transistors. The duration of this signal from the Pulse Width
Control stage is changed to compensate for variations in
input voltage levels. The output voltage from T1201 is
sampled to provide this control (see Pulse Width Control
discussion for details).
To follow the action of the Steering Switch stage,
assume that Q1105 in the Multivibrator stage has just
turned on. This connects a negative-going voltage to the
base of Q1104 to enable it. At the same time, a positive
going voltage is connected to the base of Q1114 to reverse
bias it. The control signal from the Pulse Width Control
stage begins coincidentally with the switching of the Multivi
brator stage (compare waveforms 3 and 7, Fig. 3-15). This
negative-going signal is connected to the center tap of
transformer T1171 to complete the collector current path
for Q1104 through D1104 and one-half of transformer
T1171. Q1104 conducts until the control signal goes posi
tive as determined by the Pulse Width Control stage. This
removes the negative collector voltage necessary for con
duction of Q1104, and both Q1104 and Q1114 remain off
until the Multivibrator stage switches, even though Q1104
is enabled. When the Multivibrator stage switches, condi
tions are reversed and Q1114 conducts as controlled by the
Pulse Width Control stage. Diodes D1104 and D ll 14 pro
tect transistors Q1104 and Q1114 from the high-amplitude
positive spike which is generated by the collapsing field of
T1171 as the transistors are turned o ff at the end of the
control pulse from the Pulse Width Control stage.
Power Control. The Power Control stage is controlled
by the pulses induced into T1171 from the Steering Switch
stage (see previous discussion). The Power Control stage in
turn controls the flow of current from the input voltage
(from AC-DC Power Selector circuit) through the primary
of transformer T1201. The current flow through the pri
mary of T1201 is controlled so as to induce the same volt
age into the secondary of T1201 regardless of the input
voltage.
The conducting transistor in the Steering Switch stage
determines which side of the Power Control stage conducts.
For example, with Q1104 conducting, a voltage is induced
into the secondary of T1171 to forward bias Q1174 and to
hold Q1184 reverse biased. The collector current for Q1174
is supplied through L1182-R1182, L1189-D1189, one-half
of the primary of T1201, and D1176. When either Q1174
or Q1184 in the Power Control stage turn on, a high flow
o f current is demanded. C apacitive netw ork
Cl 170-C1171-C1172 supplies this initial current demand
and inductors L1172 and L1182 prevent the current surges
at turn-on from affecting the AC-DC Power Selector circuit.
Resistors R1172 and R1182 serve as damping resistors for
L1172 and L1182 respectively to prevent oscillation of the
inductors. Capacitors Cl 177 and Cl 187 are in the circuit to
provide protection at turn-off time (this protection is dis
cussed in the next paragraph). However, if these capacitors
were in the circuit at turn-on, they would have to be
charged before the collectors of Q1174 and Q1184 could
drop negative. This would produce nonlinear current in the
primary of T1201. Diodes D1177 and D1187 prevent this
nonlinear current by disconnecting the capacitors when the
associated transistor turns on. For example, when Q1174
turns on, D1177 is reverse biased to allow the collector of
Q1174 to rapidly drop negative without having to discharge
Cl 177 first. Resistor R1177 remains in the circuit to pro
vide a discharge path for Cl 177. Before D1177 becomes
fully reversed biased, it does provide a discharge path for
Cl 177 for a very brief time. Inductor L1189 resists this
sudden current change to prevent nonlinear current in the
primary of T1201. Since L1189 is fairly low in inductance,
it saturates while collector current is still flowing in the
circuit. However, D1189 conducts to prevent the inductive
feedback produced by the collapsing field of L1189 from
affecting the current in the primary of T1201.
When the control pulse from the Steering Switch stage
ends (as determined by the Pulse Width Control stage), the
collapsing field in T1171 reverses the induced voltages to
the bases of Q1174 and Q1184. A negative voltage is
applied to the base of Q1174 and it is reverse biased. The
collector of Q1174 rises positive and capacitor Cl 177 pro
vides protection for the transistor during this time. The
collapsing field in the primary of T1201 attempts to main
tain current through Q1174 even though it is reverse biased
and the simultaneous high current and high positive voltage
would exceed the wattage rating of this transistor. However
as Cl 177 charges, it delays the rise in voltage at the collec
tor of Q1174 until its current has dropped to a fairly low
value. Diode D1176 and zener diode D1174 also provide
protection for Q1174 at this time. Diode D1176 isolates
the collector of Q1174 from the large negative-going tran
sients generated in the primary of T1201 by the collapsing
field, and zener diode D1174 limits the positive collector-
emitter voltage to 75 volts maximum. The collapsing field
in transformer T1171 places a positive-going voltage at the
base of Q1184. Q1184 is forward biased momentarily to
aid the collapsing field in transformer T1201.
The length of time that the Power Control stage con
ducts is controlled by the Pulse Width Control stage so the
same.amount of magnetic flux is built up in T1201 regard
less of the unregulated DC input voltage. Then, a constant
voltage is induced into the secondary ofT1201 to maintain
a regulated output. See the Pulse Width Control discussion
for details on how the regulated output is maintained.
When the Steering Switch stage switches so the opposite
transistor is conducting, the opposite Power Control transis
tor conducts. For example, when conduction of the Steer-
3-27
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