FN957.10 July 11, 2005 A small heat sink VERSAWATT transistor is used as the series pass element in the fold back current system, Figure 17, si" />
14
FN957.10
July 11, 2005
A small heat sink VERSAWATT transistor is used as the
series pass element in the fold back current system, Figure
17, since dissipation levels will only approach 10W. In this
system, the D2201 diode is used for current sampling.
Foldback is provided by the 3k
and 100k divider network
connected to the base of the current sensing transistor.
Both regulators provide better than 0.02% load regulation.
Because there is constant loop gain at all voltage settings, the
regulation also remains constant. Line regulation is 0.1% per
volt. Hum and noise voltage is less than 200
V as read with a
meter having a 10MHz bandwidth.
Figure 18A shows the turn ON and turn OFF characteristics
of both regulators. The slow turn on rise is due to the slow
rate of rise of the reference voltage. Figure 18B shows the
transient response of the regulator with the switching of a
20
load at 20V output.
FIGURE 16. REGULATED POWER SUPPLY
FIGURE 17. REGULATED POWER SUPPLY WITH “FOLDBACK”
CURRENT LIMITING
5V/Div., 1s/Div.
FIGURE 18A. SUPPLY TURN-ON AND TURNOFF
CHARACTERISTICS
Top Trace: Output Voltage;
200mV/Div., 5
s/Div.
Bottom Trace: Collector of load switching transistor, load = 1A;
5V/Div., 5
s/Div.
FIGURE 18B. TRANSIENT RESPONSE
FIGURE 18. WAVEFORMS OF DYNAMIC CHARACTERISTICS OF POWER SUPPLY CURRENTS SHOWN IN FIGURES 16 AND 17
1
3
75
3k
100
2
1k
1k
D2201
CURRENT
LIMITING
ADJUST
2N6385
POWER DARLINGTON
2
1k
1
3
8
2N2102
1k
+30V
INPUT
4
CA3140
7
1
6
5
100k
2
3
180k
56pF
1k
82k
250
F
+
-
0.01
F
100k
14
10
6
9
8
50k
13
5
F
+
-
12
CA3086
2.2k
3
1
5
4
62k
VOLTAGE
ADJUST
10
F
+
-
2.7k
1k
11
7
2
HUM AND NOISE OUTPUT <200
VRMS
(MEASUREMENT BANDWIDTH
~10MHz)
LINE REGULATION 0.1%/V
LOAD REGULATION
(NO LOAD TO FULL LOAD)
<0.02%
OUTPUT
0.1
24V
AT 1A
1
2
1k
200
D2201
“FOLDBACK” CURRENT
LIMITER
2N5294
3k
8
2N2102
1k
+30V
INPUT
4
CA3140
7
1
6
5
100k
2
3
180k
56pF
1k
82k
250
F
+
-
0.01
F
100k
14
10
6
9
8
50k
13
5
F
+
-
12
CA3086
2.2k
3
1
5
4
62k
VOLTAGE
ADJUST
10
F
+
-
2.7k
1k
11
7
2
HUM AND NOISE OUTPUT <200
VRMS
(MEASUREMENT BANDWIDTH
~10MHz)
LINE REGULATION 0.1%/V
LOAD REGULATION
(NO LOAD TO FULL LOAD)
<0.02%
OUTPUT
0V TO 25V
25V AT 1A
3
100k
“FOLDS BACK”
TO 40mA
100k
CA3140, CA3140A
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