Design Idea DI-80

Utility Meter Power Supply

Design Highlights

• Universal AC input voltage range

• Meets EN62053-21:2003 (<1 W real and <5 VAapparent

input power at 265 VAC)

• 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

Operation

Using the buck topology, the circuit shown in

Figure 1 generates a non-isolated 12V, 50 mAoutput.Typical

uses include utility meter and industrial and motor control

applications.

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.

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.

LinkSwitch-TN

useson/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.

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.

The value of L1 is selected according to the LinkSwitch-TN

Design Guide (AN-37) or the PIXIs design spreadsheet (part of

power supply design software). Both are available

at www.powerint.com.

www.powerint.com

Figure 1. LinkSwitch-TN 0.6 W, 12 V, Universal Input Power Supply Using a Non-Isolated Buck Topology.

DI-80

®

September 2004

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

RTN

12 V,

50 mA

85-265

VAC

PI-3918-081104

FB

BP

S

D

LinkSwitch-TN

C1

4.7

µF

400 V

C3

100 nF

D2

1N4007

D1

1N4007

D3

UF4005

LNK302

D4

1N4005GP

C5

47

µF

16 V

RF1

8.2

Ω

2 W

R4

11.0 k

Ω

1%

R1

220

Ω

0.5 W

R2

220

Ω

0.5 W

R3

1.82 k

Ω

1%

L1

1.5 mH

130 mA

C2

4.7

µF

400 V

C4

1

µF

16 V

R5

3.3 k

Ω