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at 30V, CR12 must be on and CR11 must be off
(point
© = 46.5V).
Thus, the 60V raw-dc source
is supplying the load.
Notice, again, that in the
high voltage range,
up to 1 amp. supplied
by the 60V source so that the voltage drop across
R5 is still not great enough to bias VR2 past its
breakdown point.
VR2, then, still has no affect on
the operation of the series regulator.
The switch¬
ing point between the two ranges occurs when the
voltage at point
© equals that at point ©
(30V); this occurs when the output is approximately
2.1V (51V for the 6105A and 6115A).
4-42 Figure 4-5 illustrates the in-range operation
of the series regulator from a power dissipation
point of view.
When the output voltage is in the
low range (between zero and 21 volts for the 6104A
and 6114A or 0-51 volts for the 6105A and 6115A),
transistors Q2 and Q3 are controlling the output.
In this region, the power dissipation of Q1 is very
low (approximately 1W), since the only current
going through it is that flowing through R8.
When
the output voltage is in the high range (between
21 and 40 volts or 51 and 100) most of the regula¬
tor dissipation occurs in transistor Q1 with.tran¬
sistors Q2 and Q3 clamped to a low voltage (and
dissipating
approximately 7. 5W).
SERIES
REGULATOR
POWER
DISSIPATION
(APPROXIMATE)
Figure 4-5.
Series Regulator Power Dissipation
4-43
The obvious advantage in this type of regu¬
lator circuit is the saving in power dissipation.
Consider the case discussed in paragraph 4-40,
where the supply output is 5 volts at 2 amps, and
the regulator is dissipating 48 watts.
If the supply
used a conventional single-stage regulator, the
raw-dc source would have to be approximately 48V
(20% higher than the maximum rated output voltage
of 40V).
The power dissipated by the regulator
would therefore be the voltage drop across it (43
volts) times the current conducted (2A), or 86 watts.
The use of the power sharing regulator thus repre¬
sents a power dissipation saving (in this particular
case) of more than 170%.
4-44 Refering back to paragraph 4-41, it was noted
that since the output current is held (by the con¬
stant current comparator) to 1A when the output vol¬
tage is in the high range, zener diode VR2 had no
effect on circuit operation since the voltage across
it does not exceed its breakdown voltage.
How¬
ever, assuming that the CURRENT control is set to.
2A and the VOLTAGE control is still set at 40V, if
the load resistance decreases and attempts to draw
more than approximately 1A through Q1 (and R5),
the reverse voltage across VR2 exceeds the break¬
down voltage of the zener diode.
At breakdown,
the diode becomes a constant current source and
limits the current through the regulator to approxi¬
mately 1.3A (at nominal line voltage).
If load re¬
sistance decreases further, output voltage is re¬
duced (since the zener is a constant current source)
until the crossover point between the raw-dc sources
is reached (point © is at approximately 21V). At
this point, CR12 is cut off and CRH is forward
biased and the regulator reverts to the low voltage
range of operation described in paragraph 4-40.
The supply, then, will output the specified current
and operate as a constant current source, respond¬
ing to the error signal developed by the constant
current comparator.
The voltage setting, then, is
overriden and output voltage will be adjusted be¬
tween 0 and 20V to maintain the output current at
the required value.
Resistor R7 allows the gross
current limit region to be calibrated (R7 is set at
the factory at low line voltage to allow gross cur¬
rent limit at approximately 1.2A for the 6104A and
6114A supplies or at 0. 48A for the 6105A and 6115A
supplies).
4-45 REFERENCE SUPPLY
4-46 The reference supply includes a feedback reg¬
ulator, similar to the main supply, that provides
stable reference voltages (referenced to
l+Sj
) that
are used throughout the unit.
The regulated + 16V
reference voltage is derived from dc obtained from
full wave rectifier A1CR1-CR2 and filter capacitor
A1C1.
In addition, zener diode A1VR3 provides
-14. 7V reference voltage that is also used through¬
out the unit.
Also, zener diode VR4 (with R26) pro¬
vides a regulated -6. 2V for use in the overvoltage
crowbar circuit.
The -14.7V and -6.2V are de¬
rived from dc obtained from full wave rectifier
A1CR3-CR4 and filter capacitor A1C2.
The + 16V
and -14. 7V/-6.2V reference.'rectifiers are returned
to common point (+T3J
.
Notice, too, that the
raw-dc input to the'% 16V regulator is 28V (nominal)
which is used, unregulated, in the CURRENT MODE
indicator and overvoltage crowbar circuits.
4-7
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