AM 320-01-808
83
In the PS 21 we are working with the UC 3842, a very
modern integrated circuit. In the overall circuit diagram,
the internal structure of the IC is indicated schematically.
We can best understand the function if we "run through" a
working cycle.
The oscillator runs at a frequency of about 100 kHz. The
positive wave shape sets the flip-flop and hence the DRIVE
output; Q 1 switches on (make phase). The current I
s
now
flows in the primary winding, Q 1 and R 18 (Fig. 2). The
voltage developed across R 18 (which is proportional to the
current I
s
) is applied to the positive input of the comparator.
(The filter R 34, C 28, R 45, R 33, C 27 and R 44 keeps out
interfering high-frequency oscillations). Once this linearly
increasing voltage has reached the level at the negative
input, the comparator switches, resetting the flip-flop; the
drive output goes to 0, Q 1 opens and we are now in the
break phase.
The cycle starts over again with the next positive edge
of the oscillator. Let us again consider that the current
Is (and hence the voltage at the positive input) rises as
a linear function of time and let us change the voltage at
the negative input.
If we make this voltage more positive, it will take longer
for the linearly increasing voltage to be reached at the other
input; the make phase is longer. Accordingly, the make
phase becomes shorter if we reduce the voltage. The length
of the make phase and hence the make-to-break ratio
therefore depends on the voltage at the negative input.
This voltage is provided by the (inverting) control ampli-
fier, which is also integrated. Its positive input is connected
to the internal +2.5 V reference voltage; its negative input
is connected via the voltage divider R 42, R 43 to the 5 V
output of the power supply. If the voltage at the output
decreases (because of the increased load), the amplifier
output, that is, the comparator threshold, becomes more
positive and the make phase becomes longer. Thus the
output voltage rises again to 5 V and the control process is
concluded. The entire system also naturally functions in the
other direction.
The network R 30, C 24, D 11 generates a positive "ramp"
during the make phase. Via R 31, this ramp is added to the
"current ramp" and thus guarantees stable
no-load operation, however, it also limits the maximum
make-to-break ratio (via comparator IC 3a and Q 3) to
approximate 0.6.
Another feature of the control IC must be mentioned. It
exhibits Schmidt trigger behavior relative to its supply
voltage (pin 7). These thresholds are about 16 V and
10 V. This means that over 16 V, it begins functioning and
below 10 V it "switches itself off."
In Practical Terms This Means That:
After the AC voltage is switched on, we have about +50 V
at the charging electrolytic capacitor C 13. At electrolytic
capacitor C 20, the voltage rises relatively slowly (R 17 =
10k); the IC draws a current of about 1 mA. When the 16 V
threshold is reached, the IC and hence the power supply,
starts. The output voltages build up, and a +15 V output
then starts to supply the IC via R 9 and D 9 (the IC's
consumption is now 6.8 mA). If this takeover does not
occur for some reason, the elevated consumption at R 17
causes an increased voltage drop, so the voltage at C 20
drops rapidly below 10 V; the IC switches off again, the
consumption drops to 1 mA, and the process repeats itself
cyclically.
The comparators IC 3b, c, d are responsible for current
limitation on the secondary side. The output currents are
sensed via R 13, R 14, R 15. The respective comparators
are correspondingly biased. When the threshold is reached
(5V about 10A, +1-iSV about 1.5 A), the comparators
switch (NPN open collector outputs) and bring about a
lower make-to-break ratio via Q 3; the corresponding
output becomes the constant voltage source.
If the load increases (to a short circuit), the output voltages
decrease. At 10 V at the 1 5 V output, the control IC
switches off and the power source goes into the above-
described cyclic "start test" mode. C 25, D 12, R 39 provide
a "soft start".
Th 1 and Q 2 form the "crowbar" circuit, which protects the
5 V output from over-voltage by producing a short circuit
at about 5.5 V.
IC 2 is supplied by R 16 and generates the system reset.
IC 1 is not exactly a part of the power supply unit. It is
supplied from the outside (BATT) and acts - in com-
bination with the on/off switch on the control panel CB 43
and the relay on RS 1 - like an electronic switch.
Technical Data PS 21:
Design: Single-ended flow transducer
Operating frequency: about 100 kHz
Input: 50 to 55V
Output: +5 V (+/-2%) max. 10
A
+15 V (+1-10%) max. 1.5
A
-15V (+5/-10%) max. 1.5
A
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