September, 2006 − Rev. 2

1 Publication Order Number:

AND8257/D

Implementing a Medium

Power AC−DC Converter

with the NCP1395

Prepared by: Roman Stuler

ON Semiconductor

INTRODUCTION

This document describes all the necessary design steps

that need to be evaluated when designing the NCP1395

controller in an LLC resonant converter topology. A 240 W

AC−DC converter has been selected for the typical

application.

The design requirements for our 240 W AC−DC

converter example are as follows:

Requirement Min Max Unit

Input Voltage 90 265 Vac

Output Voltage − 24 Vdc

Output Power 0 240 W

Operating Frequency 65 125 kHz

Efficiency Under Full Load 90 − %

No Load Consumption − 1000 mW

LLC series resonant converter topology has been selected

to meet efficiency requirements. The NCP1395 resonant

mode controller is a very attractive solution for such designs

because it offers the following features.

Brownout Protection Input

The divided down input voltage of the converter is

permanently monitored by the Brownout pin (pin name). If

the voltage on the bulk capacitor falls outside of the desired

operating range, the controller drive output will be shut off.

This feature is necessary for an LLC topology because it is

usually optimized to operate over a narrow range of bulk

voltages.

Immediately Fault Input

This input can be used as a shutdown input in some

applications (LCD television SMPS, etc.). It can also be

used to induce skip mode operation of the LLC converter,

during the light load conditions. Standby consumption of

whole supply can thus be significantly decreased.

Timer Based Fault Protection

The converter stops operation after a programmed delay

when this input is activated. This protection can be

implemented as a cumulative or integrating characteristic.

Thus under transient load conditions the converter output

will not be turned off, unless the extreme load condition

exceeds the timeout.

Internal Transconductance Amplifier

This internal transconductance amplifier can be used to

create effective overload protection. As the result the power

supply can be operated in either CV or CC mode. This

feature is very useful for the battery chargers applications.

Common Collector Optocoupler Connection

The open collector output allows multiple inputs to the

feedback pin, for example overcurrent sensing circuit,

overtemperature sensor , etc. The additional input can pull up

the feedback voltage level and take over the voltage

feedback loop.

Please refer to the NCP1395A/B data sheet for a detailed

description of all the functions.

Demo Board Connection Description

The schematic for the 240 W demo board is shown in

Figure 1. The demo board contains three blocks: a PFC front

stage (which is necessary for the required power level and

to restrict the bulk voltage operating range of the

downstream resonant converter), an LLC converter, and an

auxiliary buck converter which provides bias power for PFC

and LLC controllers.

The NCP1653A PFC controller is used to control the PFC

front stage. Capacitors C

−C

together with BALUNs L

, L

and varistor VDR

forms the EMI filter which suppresses

noise conducted to the mains.

APPLICATION NOTE

http://onsemi.com

AND8257/D

http://onsemi.com

Figure 1. Schematic of the NCP1395 Demo Board

Fmax

timer

AGnd

NINV

OUT

S.F.

F.F.

PGnd

IC2

R22 3k3

R21 150 k

C18 100 n

C16 4u7

C17 NU

C15 1 u

R20 330 k

R19 680 k

R18 100 k

R17 NU

C19 100 n

C20 1 n

E12 NU

R30 56 k

R23

C21 10 n

R25220 k

R27 5k6

R24

1M2

R29 NU

R26 33 k

BC846BLT1

7V5

D16

R47 2k7

R28

100 R

R33 2k2

D12 NU

NCP1395

off

DRV

Gnd

IC1NCP1653

1 nC9

39 nC12

1 nC13

100 nC14

470 k

C10 4n7

R12

56 k

C11

100 n

R10

4R7

11 k

R8 3k3

2M2

0.1R

R11

10 k

C7 330 n

C6 330 n

750 k

470 k

R16

470 k

R15

470 k

R14

300 k

KBU810B1

3m3

C2 NU

AC INPUT

90−265 VAC

470 n

T3, 15 A

RTH1

L1L2

470 n

2n2/Y1

CV275K10B1

VDR1

MURA160

1N5408

2u2/450 V

650 mH

SPA20N60C3

CSD0660A

150u/450 V

15 V

IC4

PC817

1 n

Gnd

MURA160

220 m/25 V

Drain

MURA160

47 m

1m5

R13

1 k

E11

220 m/63 V

2.2 mH

TR1

MBRF20100CT MBRF20100CT

D14A D14B D15A D15B

E10

1000 m/35 V

R40 1 k

R44

1 k

DRV_HI

DRV_LO

GND

G_HI S_HI G_LO S_LO

10 k 10 k

DRIVER MODULE

R45 NU

R46

18 k

C27

C290

1 n

C26 22 n

R41

2k7

C28

TL431

IC6

R42

3k3

R43

5k6

R48 4M7

C25

2n2/Y1

IC5

PC817C

D13

R38

C24

R39

C23

33 n

TR2

CST1−100LB−

COILCRAFT

R36

1k2

NUR37

220 pC22

R34

R35

180

D10

D11

M3 M2

STP12NM50FP

100 mH

R32R31

STP12N

M50FP

IC3

NCP1012AP065