DI-89
DI-138
protectsQ2fromshortcircuitconditionsbylimitingthecollector
current, while R5 maintains regulation even if the -5Voutput is
unloaded. Locating transistors Q1 and Q2 physically close to
one another provides tracking of VBE drops, minimizing output
voltage variation with temperature.
and EMI requirements. For single 100/115 VAC
applications, the voltage of C1 and C2 can be reduced
to 200 V.
• For correct operation of Q1, select the value of R5 to
give 1 mA at no-load.
• Limit the maximum value of R4 such that a minimum
This arrangement works well in this application, where the
load range is limited and the impact on efficiency of linear
regulation is minimized.
VCE voltage of 1 V appears on Q2. This ensures that
Q2 remains in linear operation when the 5 V output is
at full load.
80
70
60
50
40
30
20
10
0
Key Design Points
QP
AV
• Diode D2 must be an ultra-fast type. The MURS160
selected has a tRR of 25 ns; a slower ultra-fast diode
(≤50 ns) may be used, but efficiency may be reduced.
• A Zener with a low test current should be selected for
VR1. The initial tolerance directly affects the output
tolerance, a 2% part gives an overall variation including
line and load regulation of ±5%.
• The temperature coefficient of VR1 is –0.8 mV/°C, giving
a further ±0.4% variation over a temperature of 0 to 50 °C.
• R1 and R2 should be 1% parts for better accuracy of 12 V
output.
-10
• Pre-loads R5 and R6 are only necessary if regulation at
no-load is required.
-20
0.15
1.0
10.0
100.0
MHz
• For single 230 VAC applications, the value of C1 and C2
can be reduced to 2.2 μF, depending on differential surge
Figure 3. Conducted EMI Scan to EN55022B Limits Measured at
230 VAC Input.
4
3
2
1
0
-1
-2
-3
-4
7 V output at 265 VAC (5 V at min load)
-5 V output at 265 VAC (12 V at no load)
7 V output at 185 VAC (5 V at full load)
-5 V output at 185 VAC (12 V at full load)
0
20
40
60
80
100
% Full Load
Figure 2. Worst Case Load and Line Regulation Results.
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