Rev. E 06/15

8

LinkSwitch-HP

www.power.com

Applications Example

30 W, 12 V Universal Adapter

The circuit shown in Figure 11 is a high efficiency universal input

30 W, 12 V output adapter using the LNK6766E.

The supply uses primary winding coupled sensing for the

following features: output regulation, line undervoltage lockout,

input and output OVP. With primary winding sense there is no

need for an external secondary referenced error amplifier such

as a TL431 and optocoupler. The winding sense of bus voltage

also eliminates the need for direct input voltage sensing which

requires more components and is more dissipative than winding

sense method.

Output regulation is ±5%, active-on efficiency is 86% and

no-load input power is less than 30 mW.

The rectified and filtered input voltage is applied to the primary

winding of T1. The other side of the primary is driven by the

integrated power MOSFET in U1. Diode D1, C3, R2, R3 and

VR1 comprise the clamp circuit, limiting the leakage inductance

turn-off voltage spike to safe value. Zener diode VR1 also helps

to reduce input power consumption during no-load conditions.

Start-up of the power supply is initiated by sensing the forward

negative pulse current from feedback winding through R19 into

Figure 11. Schematic of a Universal Input 30 W, 12 V, 2.5 A Adapter.

the FEEDBACK pin. This sensing is accomplished by periodically

turning on the power MOSFET to sense input voltage condition

with very short low frequency sampling pulses. During the

forward pulse sampling time the FEEDBACK pin is held to zero

volts by an internal clamp. When negative forward pulse current

exceeds 250

mA, LinkSwitch-HP for two consecutive switching

cycles will initiate start-up with a soft-start sequence that reduces

component stress and allows the output to rise in a smooth

monotonic manner. The desired input voltage for start-up is

determined by the turns ratio of primary winding to feedback

winding and the value of R19.

Regulation is accomplished by sampling the feedback winding

during flyback period through the resistor divider R19 and R20

through FEEDBACK pin. This sampled voltage is compared to

an internal error amplifier threshold of 2 V. The value of R19 is

already determined by the line undervoltage function so the

output regulation point is determined by setting the proper

value for R20.

The loop compensation is provided by the network from

COMPENSATION pin to ground. In the case above, a low

frequency to mid frequency gain of 20 dB for the error amplifier

is established by R7 and C7. Capacitor C8 functions essentially

as a noise filter and is typically 100 pF. There is also an internal

16 kHz filter within the device. It is advised to limit R7 to no

greater than 260 k

W to avoid stability and noise sensitivity.

PI-6844-120312

C18

2.2 nF

250 VAC

VR1

BZG03C130

130 V

L4

10 mH

BR2

DF206ST-G

600 V

C2

68

µF

400 V

D2

BAV21WS-

7-F

T1

RM8

D1

DL4937

C6

22

µF

16 V

C8

100 pF

50 V

C23

10 pF

50 V

C20

4.7 nF

50 V

C3

10 nF

630 V

C12

1000

µF

16 V

C13

680 pF

100 V

D8

STPS30100ST

C5

470 nF

50 V

7

FL2

FL1

9

6

8

C14

150 nF

275 VAC

R29

3.3 M

Ω

R30

3.3 M

Ω

F1

2 A

90 - 265

VAC

12 V, 2.5 A

J3

RTN

J4

J1

J2

LinkSwitch-HP

D

S

FB

BP

CP

PD

CONTROL

U1

LNK6766E

C22

10

µF

16 V

L5

100

µH

R19

41.2 k

Ω

1%

R8

23.2 k

Ω

1%

1/8 W

R7

100 k

Ω

1/8 W

R9

4.3 k

Ω

1%

1/8 W

R13

20

Ω

1/8 W

R2

100

Ω

1/2 W

R3

3 k

Ω

R28

27 k

Ω

R20

10.2 k

Ω

1%

C7

100 nF

25 V