Keithley 194 Instruction Manual page 174

PRINCIPLES OF OPERATION
6.8.3 Digital Supplies
The digital supplies include a +5V supply that powers the
bulk of the digital circuitry within the instrument, as well
as separate +15V and -15V supplies that power
miscellaneous circuits. The +5V supply uses a switching
regulator to maximize efficiency, as well as to keep power
supply transformer bulk to a minimum.
DC Rectification
AC power is supplied to the digital supply section by one
secondary winding of TlOl. Coupled voltage is rectified by
CRllO, which is a bridge rectifier. Input filtering is done
by C109. The nominal +25V developed across Cl09 is used
by the +5V, +15V, and -15V supplies, as well as the cool-
ing fan.
Switching Regulator
Key components associated with the switching regulator
include QlOl, CRlOl, T102, LlDl, U119, Q103, Q104, as well
as numerous miscellaneous resistors and capacitors. QlOl,
a power MOSFET, is the switching transistor for the sup-
ply, while U119 is the pulse width modulator. 4104 drives
the primary of TlO2 which couples switching pulses to the
gate of QlOl tu control that device. LlOl provides current
storage between input pulses, while CR101 provides a
flyback path when QlOl is off.
LJ119 uses an internally generated 50kHz clock to govern
the modulation process. This operating frequency is set
by R121 and C145. Through use of internal comparators,
LJ119 monitors the amplitude of the +5V supply output
and changes the duty cycle of the pulses at the Cl and
C2 outputs accordingly. The duty cycle of the pulses that
switch QlOl is changed by a corresponding amount in
order to regulate the supply output.
For example, assume that the supply voltage begins to rise
slightly. Circuitry within LJ119 then acts to reduce the duty
cycle of 0101, thus lowering the output voltage. Converse-
ly, the duty cycle will increase if the supply output drops,
maintaining voltage within regulator limits.
Since a switching supply operates in a pulsating fashion,
more stringent filtering than normal is required. Cl39,
L102, and Cl43 make up B pi filter to attenuate any switch-
ing noise or ripple at the supply output.
Supply Protection
Three forms of protection are included with the switching
regulator to protect the supply and external circuits from
over voltage or over current conditions: the fuse, a crowbar
circuit, and a current limiting circuit.
FlD2 is a 6A fuse that protects the power supply irtrm
catastrophic failures. The fuse also works in conjunction
with the crowbar circuit made up of Vlill~l, 1~115, C141, and
Q102. This circuit monitors the supply output voltage to
determine if a safe level has been cxccedcd. If the volt,lge
exceeds GV, SCR 4102 fires, causing fuse FIO2 to blo\v.
The current limiting circuitry includes Qlll3 ,uld .lssori,ltcd
components. If an overcurrent condition too fat
hr the
fuse react is detected, Q103 shuts down L!Il'), cffcctivt*l>
turning off QlOl. With QlUl shut down. the supply outs
put is turned off.
+l5V and -15V Supplies
The nominal +25V generated by CIUIO and 009 is tur-
thcr filtered by Cl07 and regulated to + IS\' bv VRIOI. .I
standard IC regulator (78M~I5). Additional filtc$ 11i this
+15V supply is performed by ClOh.
The -15V supply originating in this section is generated
somewhat differently. Pulses from the switching transistor
are coupled through Kl08 to a voltage doubler made up
of C122, CRlo4, and CRlOs. 'The doubled output is
regulated to a stable -15V v&c by VR102, which is .I
negative 15V regulator (7915).
6.8.4 Analog Supplies
Separate, isolated analog supplies arc available for each
input channel in the Model 194. Each supply group is fed
from a separate transformcr secondary winding and ill-
eludes +5V, -5V, +VV, and -15V supplies. Since the two
groups are identical, only one is covered in the following
discussion.
Rectification for the +15V and -15V supplies is performed
by CRlll. Filtering for the +15V supply is performed b!
C123, while regulation is done by VR104, an IC regulator
(78M15). In a similar manner, Cl25 filters the -15V supply,
while VR105, a -15V regulator IC (7Y15), provides the regula
tion. Capacitors across the t15V and -15V outputs add x-
ditional filtering and provide stability.
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