Design Idea DI-124

LinkSwitch-TN

Ultra-wide Input Range (57-580 VAC) Flyback Power Supply

Figure 1. Schematic Diagram of 3 W Bias Supply using LinkSwitch-TN in StackFET Configuration.

DI-124

September 2006

Application

Device

Power Output Input Voltage

Output Voltage

Topology

Metering / Industrial

LNK304

3 W

57-580 VAC

12 V, 250 mA

StackFET Flyback

Design Highlights

extremely wide input voltage range

common-mode EMI (>10 dBµV margin)

• 66 kHz switching frequency with jitter reduces

conducted EMI

• Simple ON/OFF controller – no feedback compensation

required

• Auto-restart function for automatic and self-resetting

open-loop, overload and short circuit protection

• Built-in hysteretic thermal shutdown at 135 ºC

Operation

The AC input is rectified and filtered and the resultant DC

applied to one end of the transformer primary winding. The

450 V input capacitors are stacked with parallel balancing

resistors to meet the required voltage rating. Resistors R1 to

R4 provide fusing in case of a catastrophic failure. Inductor

L1, C1 and transformer E-Shield windings allow the design

to meet EN55022 B conducted limits with good margin.

A600 V MOSFET, Q1, and U1 are arranged in the StackFET

configuration (cascode). The drain of U1 drives the source

of Q1 while the drain of Q1 drives the transformer primary.

The drain voltage of U1 is limited to 450 V by VR1-3. This

extends the maximum peak composite drain voltage of U1

and Q1 to 1050 V. The resistor chain R6-R8 provides startup

charge for the gate of Q1 and R9 dampens high-frequency

ringing. Once the converter is operating, the gate is largely

driven by the charge stored in the capacitance of VR1-3.

Zener VR4 limits the gate to source voltage of Q1. Leakage

inductanceenergyisclampedbyVR5andD9withR10added

to reduce ringing and thereby, EMI.

The operation of U1 is unaffected by the StackFET

configuration. WhentheinternalMOSFETturnson,Q1isalso

turned on, applying the input voltage across the transformer

primary. Oncetheprimarycurrentreachestheinternalcurrent

limit of U1, the MOSFET is turned off and the energy stored

is delivered to the output. Regulation is maintained using

ON/OFFcontrol. Switchingcyclesareenabled/disabledbased

on current into the FEEDBACK pin of U1. This is ideal as

it results in a lowering of the effective switching frequency

®

PI-4487-081506

D

S

FB

BP

R5

1 k

L1

1 mH

R13

475 k

Ω

1%

0.5 W

R14

475 k

Ω

1%

0.5 W

R6

680 k

Ω

0.5 W

R7

680 k

Ω

0.5 W

R10

200

Ω

1%

R12

1 k

Ω

R11

330

Ω

R8

680 k

Ω

0.5 W

R9

10

Ω

R16

475 k

Ω

1%

0.5 W

R15

475 k

Ω

1%

0.5 W

R1

10

Ω 1 W

R2

10

Ω 1 W

R3

10

Ω 1 W

R4

10

Ω 1 W

C7

15

µF

450 V

C5

15

µF

450 V

C4

100 nF

50 V

U1

LNK304P

U2B

PC817A

U2A

PC817A

C6

15

µF

450 V

C8

15

µF

450 V

C1

2.2 nF

250 VAC

L2

Ferrite Bead

C9

5.6 nF

1 kV

C2

470

µF

16 V

T1

EEL16

C3

100

µF

16 V

D10

UF4004

NC

4

1

5

7

9

10

Q1

IRFBC20

D9

UF4007

D1

1N4007

D2

1N4007

D3

1N4007

D4

1N4007

D5

1N4007

D6

1N4007

D7

1N4007

D8

1N4007

VR1

P6KE150A

VR5

P6KE150A

VR4

1N5245B

15 V

VR6

BZX79-C11

11 V

VR2

P6KE150A

VR3

P6KE150A

J1

J5

J6

J2

J3

J4