Figure 1. LinkSwitch-TN 1.2 W Dual Output Supply.

DI-138

April 2007

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

White Goods

LNK304DN

1.2 W

85-265 VAC

+7 V, -5 V

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

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

–5 V signal (w.r.t. RTN/N).

Resistor RF1 is a fusible flame-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 filter. This filtering, 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 fixed 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

®

Design Idea DI-138

LinkSwitch-TN

1.2 W, Non-isolated Dual Output Supply for White Goods

PI-4633-040507

D

FB

BP

S

R5

5.1 k

R6

3.3 k

R4

20

R3

3.3 k

RF1

8.2

2.5 W

R1

2 k

1%

R2

12.4 k

1%

L1

1000 H

U1

LNK304DN

L2

1500 H

C5

100 F

16 V

C6

100 F

16 V

C1

4.7 F

400 V

C2

4.7 F

400 V

C3

100 nF

50 V

C4

10 F

50 V

D1

DL4007

D3

DL4007

VR1

MAZS0510M

Q1

MMBT3904

Q2

MMBT3904

D2

MURS160T3G

LinkSwitch-TN

+7 V, 80 mA

RTN

-5 V, 50 mA

L

N