Design Idea DI-80
®
LinkSwitch-TN 0.6 W Non-Isolated
Utility Meter Power Supply
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Utility Meter
LNK302
0.6 W
85-265 VAC, 47-63 Hz
12 V, 50 mA
Buck
In meter applications, capacitors C1 and C2 are sized for the
required holdup time to write the meter reading to non-volatile
memory after a power outage.
Design Highlights
•
•
Universal AC input voltage range
Meets EN62053-21:2003 (<1Wreal and <5VAapparent
input power at 265 VAC)
LinkSwitch-TNuseson/offcontroltoregulatetheoutputvoltage.
When the current into the FEEDBACK (FB) pin exceeds
49 µA, MOSFET switching is disabled for the next switching
cycle. The 49 µA threshold is specified at an FB pin voltage of
1.65 V (±7% over the entire operating temperature range),
allowing it to be used as a reference.
•
•
•
•
•
Long holdup time (140/580 ms at 115/230 VAC)
High efficiency (>60 % at 265 VAC)
Very low no-load consumption (0.15 W at 265 VAC)
EMI compliant (EN55022B, CISPR-22B)
Extended line surge withstand with integrated 700 V
MOSFET
•
•
Small size and light weight
ScalableoutputpowerusinglargerLinkSwitch-TNfamily
members
The power processing stage is formed by the LinkSwitch-TN,
freewheelingdiodeD3,outputchokeL1,andtheoutputcapacitor
C5. To a first order, the forward voltage drops of D3 and D4
are equal. Therefore, the voltage across C4 tracks the output
voltage. The voltage across C4 is sensed and regulated via the
resistor divider R4 and R3 connected to the LNK302 FB pin.
The values of R3 and R4 are selected such that, at the desired
output voltage, the voltage at the FB pin is 1.65 V. Resistor R5
establishes a small 3.6 mA pre-load and is only necessary for
applications where regulation to zero load is required.
Operation
Using the buck topology, the circuit shown in
Figure1generatesanon-isolated12V, 50mAoutput.Typical
uses include utility meter and industrial and motor control
applications.
The value of L1 is selected according to the LinkSwitch-TN
Design Guide (AN-37) or the PIXIs design spreadsheet (part of
PI Expert™ power supply design software). Both are available
at www.powerint.com.
TheAC input is rectified and filtered by D1, D2, C1, C2, RF1,
R1, and R2. Two rectifiers improve both line surge withstand
(2 kV) and conducted EMI. Resistor RF1 should be a fusible
flameproof type, whereas R1 and R2 can be flameproof only.
C3
R4
100 nF
C4
R3
1.82 kΩ
1%
11.0 kΩ
RF1
8.2 Ω
2 W
R1
220 Ω
0.5 W
1 µF
D4
1N4005GP
1%
12 V,
50 mA
16 V
FB
BP
S
D
L1
D1
1.5 mH
130 mA
LinkSwitch-TN
1N4007
C5
C1
4.7 µF
400 V
C2
4.7 µF
400 V
85-265
VAC
R5
3.3 kΩ
LNK302
47 µF
D3
UF4005
16 V
D2
1N4007
RTN
R2
220 Ω
0.5 W
PI-3918-081104
Figure 1. LinkSwitch-TN 0.6 W, 12 V, Universal Input Power Supply Using a Non-Isolated Buck Topology.
September 2004
DI-80
www.powerint.com
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