LNK362低功耗单火线开关芯片：优化系统成本与高效能解决方案

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LNK362-364

12/05

Applications Example

A 2 W CV Adapter

The schematic shown in Figure 5 is a typical implementation of

a universal input, 6.2 V ±7%, 322 mA adapter using LNK362.

This circuit makes use of the Clampless technique to eliminate the

primary clamp components and reduce the cost and complexity

of the circuit.

The EcoSmart features built into the LinkSwitch-XT family

allow this design to easily meet all current and proposed

energy efﬁ ciency standards, including the mandatory California

Energy Commission (CEC) requirement for average operating

efﬁ ciency.

The AC input is rectiﬁ ed by D1 to D4 and ﬁ ltered by the bulk

storage capacitors C1 and C2. Resistor RF1 is a ﬂ ameproof,

fusible, wire wound type and functions as a fuse, inrush current

limiter and, together with the π ﬁ lter formed by C1, C2, L1

and L2, differential mode noise attenuator. Resistor R1 damps

ringing caused by L1 and L2.

This simple input stage, together with the frequency jittering of

LinkSwitch-XT, a low value Y1 capacitor and PIʼs E-Shield™

windings within T1, allow the design to meet both conducted

and radiated EMI limits with >10 dBµV margin. The low value

of CY1 is important to meet the requirement for a very low

touch current (the line frequency current that ﬂ ows through

CY1) often speciﬁ ed for adapters, in this case <10 µA.

The rectiﬁ ed and ﬁ ltered input voltage is applied to the primary

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

integrated MOSFET in U1. No primary clamp is required as the

low value and tight tolerance of the LNK362 internal current

limit allows the transformer primary winding capacitance to

provide adequate clamping of the leakage inductance drain

voltage spike.

The secondary of the ﬂ yback transformer T1 is rectiﬁ ed by D5,

a low cost, fast recovery diode, and ﬁ ltered by C4, a low ESR

capacitor. The combined voltage drop across VR1, R2 and the

LED of U2 determines the output voltage. When the output

voltage exceeds this level, current will ﬂ ow through the LED

of U2. As the LED current increases, the current fed into the

FEEDBACK pin of U1 increases until the turnoff threshold

current (~49 µA) is reached, disabling further switching cycles

of U1. At full load, almost all switching cycles will be enabled,

and at very light loads, almost all the switching cycles will be

disabled, giving a low effective frequency and providing high

light load efﬁ ciency and low no-load consumption.

Resistor R3 provides 1 mA through VR1 to bias the Zener

closer to its test current. Resistor R2 allows the output voltage

to be adjusted to compensate for designs where the value of the

Zener may not be ideal, as they are only available in discrete

voltage ratings. For higher output accuracy, the Zener may be

replaced with a reference IC such as the TL431.

Figure 5. 2 W Universal Input CV Adapter Using LNK362.

1N4005

1N4934

PI-4162-110205

1N4005

RF1

8.2 Ω

2.5 W

3.9 k

1/8 W

1 k

1/8 W

390 Ω

1/8 W

6.2 V,

322 mA

85-265

VRMS

1 mH

3.3 µF

400 V

3.3 µF

400 V

CY1

100 pF

250 VAC

330 µF

16 V

VR1

BZX79-

B5V1

5.1 V, 2%

EE16

NC NC

100 nF

50 V

PC817A

LNK362P

LinkSwitch-XT

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