Tektronix 7704A User Manual

Inverter
Α simplifie d schematic of the Inverter, Regulator, and
Inverter Control stages is shown in F ig . 2-30 . Once the
Inverter h as been starte d by the Start Network, the Inverter
is self-oscillating .
F
ee dback re q uired for oscillation is
p rovided by base- d rive transformer Τ 1 . P olarity of the
win d ings causes Q1 and Q2 to conduct alternately . This
generates α s quare-wave voltage at the emitter of Q1 with
an am p litu de approximately equal to the DC voltage at the
in p
ut to this stage .
The
switching action of Q1 and Q2
supplies the drive necessary
to
maintain
α
sine-wave
current
in series resonant
circuit
L 1-C1 . N otice that this sine-wave
current is also common to the primaries of Τ 1, Τ 2, and Τ 3 .
OC I NPU T
VOLTAGE
~-

F ig . 2-30 . Simplified schematic of Inverter, R egulator, and I nverter
Control stages .
Q3 represents the Regulator stage, which is controlle d
by
the Inverter Control stage . These stages are use d to d elay
the
turn-on of
Q1 or Q2 to provide p re-regulation of the
voltages in d uce d in the secondary of Τ 2 . When 03 is on,
the
voltages on
all secondaries of Τ 1 will be zero, and b oth
Q1 and Q2
will be off . Current in t he resonant circuit will
flow through C R1 and CR2 when Q1 and Q2 are off .
Transformer Τ 3 samples the resonant-circuit current to
provi
d
e current-phase information to the P re-Regulator
inputs of the Inverter Control stage to control the
conduction of the Regulator stage .
The normal sequence of operation is as follows : Assume
that the sine-wave current in t he series resonant circuit is
p assing through zero an d is increasing in the d irection to
cause CR1 to conduct . At zero crossing,
Q3 turns
on to
h old Q2 off (CR1 will conduct as long as Q2 is off) . At α
controlle d time after zero crossing, Q3 is turne d off by the
Inverter Control stage . Q2 then turns on, causing CR1 to be
reverse biased . Q2 conducts as the current goes through its
peak an d b ack to
zero
.
At
zero crossing, with c urrent increasing in t he opposite
d irection, Q3 turns
on
to h old Q1 off . During this Inverter
Ο
Circuit Description- 7704A Service
Control time, CR2
conducts
while
Q1 is
off . When
the
Inverter Control stage turns Q3 off, Q1 will b e turned on,
causing CR2 to be reverse biase d . 01 con d ucts as the
current goes through its peak and b ack to zero . The cycle
then repeats with Q3 turning on to h ol d Q2 off .
Now, refer to the complete Inverter/Rectifiers d iagram .
The L C circuit is made up of L 3037-C3037 . After the
circuit h as b een started by the Start N etwork, as d iscussed
previously,
L
3037-C3037 resonates at α f requency of about
25 kilohertz
. The feedback
to
the
base
circuits
of
Q3034-Q3047,
through Τ
3030,
alternately turns Q3034
and Q3047 on . These transistors operate at α forced beta of
four due to the turns ratio of Τ 3030, an d t heir output
current sustains resonance in L3037-C3037 . The 60-turn
center-tapped winding of
Τ
3030 is used to delay the
turn-on of
Q3034
or
Q3047
to p
rovide pre-regulation
of
the voltages produced in the secondary of Τ 3101 . This
operation will b e d iscusse d in more d etail u nd er P re-
Regulator . Dio d es CR3034 and CR3045 provide reverse
conduction paths across Q3034 and Q3047 respectively
when these transistors are h el d
off
for p
re-regulation
p urposes .
Over-Voltage Sto p
Q3048 and Q3052 p rovi d e α means to stop
the Inverter
whenever the voltage across the primary of
Τ 3101 exceeds
α safe level . This stage is activated
whenever the load on
Τ 3101 is remove d or the normal
voltage regulating p ath
t hrough Q3102 and Τ 3030 is inoperative
.
CR3057 charges C3049 to the peak of the voltage across
the primary of Τ 3101 . If this voltage exceeds α safe
level,
VR3059 will con duct and turn on Q3052
. C3049 will
d ischarge through R3052, Q3052, an
d t he b ase-emitter
junction of 03048 .
This current turns Q3048 on to cause
Q3047 to b
e h eld off
an
d stop t he Inverter . The Inverter
cannot restart
until CR3050 h as charged C3048 to about
32 volts (the breakdown
voltage of CR3040) to energize
the
Start
N etwork .
Inverter Control
The Inverter Control stage, made up p rimarily of
U 3105,
p rovides p re-regulation and fault p rotection for
the
low-
voltage p ower supplies .
F
or pre-regulation p urposes, U 3105
provides the Regulator output to the
R egulator
stage
to
vary the Inverter frequency .
F
ault protection
is achieved
through t he Regulator output (as
for p re-regulation) or b y
p rovi d ing the Stop
Trigger
output to
the Inverter Stop stage
to turn t he Inverter off .
U 3105 includes α monostable
multivibrator that is
initially triggere d by
current-phase information fe d back
from the Inverter stage .
The sensing inputs
to
U 3105
d etermine the multivibrator p
ulse wid th . The multivibrator
2- 35

Tektronix 7704A Specifications

General

Brand	Tektronix
Model	7704A
Category	Test Equipment
Language	English