Design Idea DI-138
LinkSwitch-TN
®
1.2 W, Non-isolated Dual Output Supply for White Goods
Application
White Goods
Device
LNK304DN
Power Output
1.2 W
Input Voltage
85-265 VAC
Output Voltage
+7 V, -5 V
Topology
Buck Derived
Design Highlights
• Universal input
• Excellent regulation – 12 V ±8% and 5 V ±5%
(line, load, and tolerance)
• Temperature compensated
• Low stand-by input power consumption:
<300 mW at 230 VAC with 50 mW output load
• Meets CISPR-22/EN55022B limits for conducted
EMI with >10 dB margin
Resistor RF1 is a fusible
fl
ame-proof type that acts as a fuse
in the event of a catastrophic failure. Diode D1 provides AC
input rectification while C1 and C2 provide smoothing and,
together with L1, act as a
π
fi
lter. This
fi
ltering, together with
LinkSwitch-TN’s
(U1) integrated switching frequency jitter,
provides a generous EMI margin (see Figure 3).
During each enabled switching cycle, U1’s internal MOSFET
is enabled, causing a linear ramp in current through L2 and
C5. Once the internal current limit is reached, the MOSFET
turns off, and the inductor current can freewheel via D2,
C5, and C6.
Regulation is maintained by adjusting the ratio of enabled to
disabled cycles. A cycle is skipped once the current into the
FEEDBACK (FB) pin exceeds 49
μA.
As this is specified
at a voltage of 1.65 V, it allows this pin to can be used as a
reference. With the values of R1 and R2 as shown, this sets
the voltage across C5 plus C6 to 12 V.
The –5 V output is regulated by Q2 using VR1 as a reference.
Zener VR1 is biased with a
fi
xed current of approximately 2
mA, set by R3, to reduce the voltage variation of VR1 and
therefore of the –5 V output over load changes. Resistor R4
Operation
The circuit shown below provides two outputs, –5 V and
+7 V. Referencing the load to the –5 V output creates a 5 V
and 12 V output, the 5 V driving the control electronics and
the 12 V output the mechanical relays.
This is ideal in applications where a negative gate drive is
required for controlling a triac. As the triac gate is referenced
to the RTN/N line when the control electronics are referenced
to the –5 V output (V
EE
), the gate can be driven with a 0 V to
–5 V signal (w.r.t. RTN/N).
R2
12.4 k
1%
RF1
8.2
2.5 W
D1
DL4007
L1
1000 H
C3
100 nF
50 V
FB
D
BP
S
L
R1
2k
1%
C4
10 F
50 V
L2
1500 H
D3
DL4007
+7 V, 80 mA
C5
100 F
16 V
Q1
MMBT3904
R3
3.3 k
R4
20
Q2
MMBT3904
R6
3.3 k
RTN
C1
4.7 F
400 V
LinkSwitch-TN
U1
C2
4.7 F LNK304DN
400 V
D2
MURS160T3G
N
C6
100 F
16 V
VR1
MAZS0510M
-5 V, 50 mA
PI-4633-040507
R5
5.1 k
Figure 1. LinkSwitch-TN 1.2 W Dual Output Supply.
DI-138
April 2007
POWERINT [ POWER INTEGRATIONS, INC. ]
