DER-49
-12V/3W, Non-Isolated Buck-Boost Converter
April 20, 2005
3 Schematic
R3
12k 1%
+
C4
10uF/50V
U1
LNK305P
R4
2.05k 1%
D1
1N4007
J1
J3
L1
D3
RF1
1
1
-12V @
250mA
C5
0.1uF
LINE
2.2mH
47/3W
MUR160
R2
3.3k
+
+
+
108 - 265VAC
50/60Hz
C1
10uF/400V
L2
1500uH
C3
100uF/25V
C2
10uF/400V
D2
1N4005GP
J2
J4
RF2
1
1
NEUTRAL
47/3W
* RF2 not used
Figure 2 –Schematic.
4 Circuit Description
The circuit shown in Figure 2 is a non-isolated buck-boost (inverting) topology. The input
voltage range is 108 to 265VAC 50/60Hz and provides a regulated –12V at 250mA. The
buck-boost topology is essentially the non-isolated version of the Flyback Converter, in that
the transformer is replaced with a single low cost inductor (L2).
RF1 is a fusible link resistor. The input AC is half-wave rectified and filtered by D1 and C1.
C1, L1 and C2 form a pi-filter network to reduce common-mode emissions imposed to the
input line, this in conjunction with the built-in frequency jitter of the LinkSwitch-TN (U1)
ensure sufficient conducted EMI margins. U1, D2 and L2 form the buck-boost switching cell,
which converts the rectified bulk positive DC voltage on C2 into a negative voltage on C3
(w/r/t Neutral/GND). D2 samples the output voltage onto C4 as a positive voltage with
respect to the source of U1. The EN pin of U1 is internally set to 1.65V (w/r/t pins 1,2, 7 and
8) this in conjunction with resistors R3 and R4 form a simple voltage divider to precisely set
the output voltage to the desired level. C5 is a bypass capacitor that serves as high
frequency decoupling and energy storage. This capacitor provides power to the IC as well as
controls the auto-restart mechanism in the LinkSwitch-TN. Resistor R2 serves to reduce
peak charging effects on C3 which tend to increase the output voltage, its static power
dissipation is limited to less than 30mW. Without this additional resistor the standby power
consumption would be less than 100mW.
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