Rev. C 03/16

4

LNK64x4-64x8

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Applications Example

Circuit Description

This circuit shown in Figure 4 is configured as a primary-side

regulated flyback power supply utilizing the LNK6448K. With an

average efficiency of 78% and <30 mW no-load input power this

design easily exceeds the most stringent current energy efficiency

requirements.

Input Filter

AC input power is rectified by bridge BR1. The rectified DC is filtered

by the bulk storage capacitors C1 and C2. Inductors L2 and L3,

together with C1 and C2 form a pi (π) filter, which attenuates

conducted differential-mode EMI noise. This configuration along with

Power Integrations transformer E-Shield™ technology allows this

design to meet EMI standard EN55022 class B with good margin

without requiring a Y capacitor, even with the output connected to

safety earth ground. A ferrite bead for L3 is sufficient especially

when the output of the supply is floating. Fuse F1 provides protection

against catastrophic failure. NTC (Negative Thermal Coefficient)

thermistor RT1 is used to limit the rush current to below the peak

specification of BR1 during start-up especially at high-line input

voltage. High-line results in the highest current into C1 and C2. F1

and RT1 can be replaced by a single fusible resistor. If the reduction

in efficiency is acceptable, a bridge with a higher I

FSM rating may also

allow removal of RT1. If a fusible resistor is selected, use a

flameproof type. It should be suitably rated (typically a wire wound

type) to withstand the instantaneous dissipation while the input

capacitors charge when first connected to the AC line.

LNK6448K Primary

The LNK6448K device (U1) incorporates the power switching device,

oscillator, CC/CV control engine, start-up, and protection functions.

The integrated 725 V MOSFET provides a large drain voltage margin

in universal input AC applications, increasing reliability and also

Figure 4.

Energy Efficient USB Charger Power Supply (78% Average Efficiency, <30 mW No-load Input Power).

reducing the output diode voltage stress by allowing a greater

transformer turns ratio. The device is completely self-powered from

the BYPASS pin and decoupling capacitor C7. For the LNK64xx

devices, there are 4 options for different amount of cable drop

compensation determined by the third digit in the device part

number. Table 2 shows the amount of compensation for each device.

The LNK644x devices do not provide cable drop compensation.

The optional bias supply formed by D3 and C8 provides the operating

current for U1 via resistor R8. This reduces the no-load consumption

from ~200 mW to <30 mW and also increases light load efficiency.

The rectified and filtered input voltage is applied to one side of the

primary winding of T1. The other side of the transformer’s primary

winding is driven by the integrated MOSFET in U1. The leakage

inductance drain voltage spike is limited by an RCD-R clamp

consisting of D2, R3, R11, and C6.

Output Rectification

The secondary of the transformer is rectified by D1, a 10 A, 45 V

Schottky barrier type for higher efficiency, and filtered by C3, L1 and

C4. If lower efficiency is acceptable then this can be replaced with a

5 A PN junction diode for lower cost. In this application C3 and C4

are sized to meet the required output voltage ripple specification with

a ferrite bead L1, which eliminates the high switching noise on the

output. A pre-load resistor R2 is used to meet the regulation

specification. If the battery self-discharge is required, the pre-load

resistor can be replaced with a series resistor and Zener network.

Output Regulation

The LNK64xx family of devices regulates the output using ON/OFF

control in the constant voltage (CV) regulation region of the output

characteristic and frequency control for constant current (CC)

regulation. The feedback resistors (R6 and R7) were selected using

standard 1% resistor values to center both the nominal output

voltage and constant current regulation thresholds.

PI-7209-020515

D

S

FB

BP

R11

200 kΩ

R2

2.55 kΩ

1%

R3

150 Ω

R6

44.2 kΩ

1%

R5

2.4 Ω

D3

RS1ML

R8

2.37 kΩ

1%

R1

10 Ω

R10

4.7 kΩ

R7

10 kΩ

1%

D2

S1ML

D1

SBR1045SP5-13

T1

EPC17

5

FL1

FL2

4

3

6

C4

680 µF

10 V

C5

1 nF

50 V

C3

680 µF

10 V

C1

10 µF

400 V

C2

15 µF

400 V

C7

1 µF

50 V

C8

10 µF

25 V

L2

1 mH

L1

Ferrite Bead

(3.5 × 7.6 mm)

L3

Ferrite Bead

(3.5 × 7.6 mm)

C6

470 pF

250 V

U1

LNK6448K

LinkSwitch-3

BR1

B10S-G

1000 V

RT1

10 Ω

F1

1 A

tO

90 - 265

VAC

L

TP1

N

TP2

5 V, 2 A

J1-1

RTN

J1-4

6.0

3.0

4.0

5.0

1.0

2.0

0.0

0

0.5

1

1.5

2

2.5

3

Output Current (A)

115 VAC

230 VAC