Tektronix 2213 - Page 51

T heory of O p
eratio n -2213 Service
Q917 will be biased into its
active regio
n
and t h e res u lting
collector c urr e n t will cause
α voltage drop across R 916 . T h is
voltage dro p will b ias
on
Q915, and t h e positive
feed b ac k
t h roug h R 914 will r ei n fo r
ce t
h
e turn-on of Q917 . T hu s Q915
and 0917 will drive
eac
h
ot h er into saturation very quic k ly .
O n ce Q915 is on, U 920 will b egi n to f u cn tio n .
Pu lse-wi d t h modulator IC U 920 controls t h e out p ut
voltage of t h e Pr ereg u lator b y reg u lati n g t h e du ty cycle of
t h e p u lse app lied to t h e gate of Q933 . It utilizes an oscillato r
w h ose fre q uency is determined b y R 920 a n d C920
(a p proximately 40 kH z) and w h ose o u t pu t at p in 5 is α
sawtoot h voltage . An i n ternal com p arator com p ares t h is
sawtoot h voltage wit h t h e outp u t voltage pro d uced by t h e
two error amplifie r s . Wh e n ever t h e sawtoot h voltage is
greater t han t h ee r ro r -am p lifie r output voltage, Q933 is
b iased on to su pply c u rre n t to b ot h C934 a nd t h e rest of t h e
circ u itry . T h e two e rr or am p lifiers a r e used to mai n tai n α
constant output voltage an d to mo n ito r t h e outp u t cu rre n t of
t h
e
Pr
eregulato
r .
One
i npu
t of eac
h
am p
lifie
r is conn ected
t h
roug
h
α
divider
netwo
rk
to t
h
e
i n
te
rn
al +5 V
r
efe r e n ce .
T h e outp u t voltage of t h
e
P
reregulator is monitored b y t h e
voltage d ivi d er at p in 2 . T h e voltage drop across R
933,
p
roduced by
t h
e
P
reregulator output current, is app lie d to
t h
e
current-limit
am
p lifier via
R 929 .
Wh en t h e i n str u ment is first turned on, t h e current limit
am p lifier co n trols t he conductio n time of 0933 . Wh ile Q933
is co nd ucting, t h e output curre n t i n creases until α
sufficiently large voltage drop is developed across R 933 to
invo k e t h e c ur re n t limit mode . T h e curre n t-limit am p lifie r
h olds t h eo u t p ut curre n t to t he current-limit t h res h old at
a pp roximately 1 am pere . Wh en t h e voltage across C934
reac h es app roximately 43 V , t h e voltage am p lifier starts
controlli n g t h e d uty cycle of Q933 and t h e P reregulator will
not limit current unless t h e r e is excessive current deman d .
W it h
Q933
off,
C933
charges to t h e outp u t voltage of t h e
P
ower
I npu t circuit . Wh
en Q933
turns
on, current t
h
ro u g h
t h e FE T will come from t h e win d i n g connected to pi n s 1
a
n
d
2 of Τ 933 an d from C933 .
Cu
r rent to C933 is s upp lie d
by
t h e win ding co n nected to pins 4 and 5 of Τ 933 . Wh en
U 920
s h uts off Q933, t h e colla p sing magnetic field will raise t h e
voltage at
t h
e anode of
C R 933
.
T h
is
diode t h
e
n
b ecomes
forward
biased and p asses current sup p lied by t
h
e
win
d ing
connected to pi n s 1 a n
d
2 of Τ 933 . T h is p
rocess
will
continue for eac h p e r io d of t h e oscillator, an d t h e du ty cycle
will be alte r ed as necessa r y to mai n tain 43 V across C934 .
To
s hu t off Q933 du ring eac h oscillator perio d ,
Q931
is u
sed
to disc h arge t h e gate-drain capacitance . P in 10 of U 920
goes L O, reverse biasing C R931 and
turning
on Q931 to
effectively
s
h
ort toget
h
er t
h
e gate and source,
t h
us s
h utting
off t h e FE
T .
Once t
h
e su pp ly is running, power to U 920 will be
su p plie d from t h e win d ing co nn ected to p ins 6 and 7 of
Τ 933 . Diode C R 913 h alf-wave rectifies t h
e voltage across
pins 6 and 7 to keep filte r
cap acitor C913 c h a r ge d a nd to
maintain V
cc voltage to U 920 t hr oug h Q915 .
3- 2 4
I n strument protection from
excessive o u tp u t voltage is
supp lied by silico n -controlle d rectifie r
0935 . S
h
o u
l
d t h e
P reregulator outpu t voltage excee d
51V, ze
n
er d io d e
VR 935 will co n duct, causi n g Q935
to also con d u ct . T h e
Pr e r egulator o u t pu t current will t h
e
n
b e s hu nted t h roug h
Q935, a n d t h e outpu t voltage will
very
q
uic
k ly
go to zero .
W it h t h e V cc voltage of U 920 no lo n ge r being s upp lie d
by
t h e wi n di n g conn ecte d to pins 6 a n d 7 of
T933, the
P re r egulator will s h ut down a n d Q935 will be r eset . T h e
supp ly will t hen attempt to power up, but will agai n
s
hu t
d own o n ce t h e overvoltage con d itio n is reac h e d . T h is
sequence co n tin u es until t h e overvoltage conditio n is
corrected .
Inverte
r
T h
e Inverter circuit c h a n ges t he d
o voltage f r om t h e
P re r
egulator to ac for use by t he
sup plies t h at a r e conn ect-
ed to t h
e seconda r ies of Τ 940 .
T h e outp u t of t h
e
P
reregulator circuit is ap plied
to t h e
center ta p of Τ 940 . P
ower-switc h ing transistors
Q940 a
n
d
0942 alternate co n
ducti n g cu r re n t t hr oug h R941
from t h e
primary ci r cuit common
to t h e Pr ereg u lato r outp
u t line . T h e
transistor switc h ing action
is controlled by Τ 942,
α sat u rat-
i n g b ase-drive tra n sfo r mer .
Wh en t h e instrume n t is first
tu rn ed on, one of t h e switc h -
i n g
transistors will start to co n
duct a n d t h e collector voltage
will d r o p
toward t h ecommon voltage level .
T h is will in du ce α
p ositive
voltage from t h e lea d of Τ 942 w h
ic h is co nn ecte d to
t h
e b ase of t h e cond u cting tra n sisto r
to
r
einforce cond u c-
tion . E
vent u ally Τ 942 will saturate,
an d as t h e voltage
across Τ
942 (a n d Τ 940) b egins to
reverse, t h e co ndu cting
transistor cuts off b ecause of t h e
drop i n b ase drive . T h e
ot h er tra n sistor will n
ot start cond u ctio n un til
t
h
e voltage on
t h e lea ds of Τ 942
reve
r
se e n o u g h to b ias it
on .
T h
is p rocess
will conti n
ue, a n d t h e saturation time of Τ 942 p l
u s t h e tran-
sisto r -switc h i n g time will
d ete r mine t h e fre q uency
of Inverter
operatio n (ty p ically 20
kHz) . After t he initial Inverter
start up,
t h e switc h i n g transistors
do n ot saturate ; t h ey
remain i n t h e
active regio n during switc h ing .
In instrume n ts h aving t h e C u rre n t L imit
b oard, dio d es
C R 940 and C R 942 serve as α negative-pea k
detector to
ge n erate α voltage co n t r olli n g t h e o u t p
uts of both t h e
P rereg u lator a n d t h e error amplifier .
In instruments h avi n g
t h eP rereg u lator board, dio d
es C
R
940 and C R 942 serve as
α negative- p ea k detecto r to
generate α voltage fo r co n trol-
ling t h e outpu t of t h e error amplifier .
Ca p acito r C951 will
c h arge to t h e pea k am p litude of t h
e collector voltage of
0940 and 0942 . T h is voltage level
is a pp lie d to t h e d ivi d er
com p osed of
R
945,
R
946, a n dR 947 . T h e e rr or amplifie r ,
composed of Q948 and Q954, is α differential amplifie r t h at
com p ares t h e reference voltage of VR
951 wit h t h e voltage
on t h e wiper of pote n tiometer R
946 . T h e cu rr e n t t h ro u g h
Q954 will set t h e base drive of Q956 and t h e r eby co n trol t h e
REV N O V 1982