7M0680 Ver la hoja de datos (PDF) - Fairchild Semiconductor

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Fabricante

7M0680

Fairchild Power Switch

Fairchild Semiconductor 

FS7M0880

Block Diagram

It can be divided into several large, functional sections:

under voltage lockout circuitry (UVLO); reference voltage;

oscillator (OSC); pulse width modulation (PWM) block;

protection circuits; and gate driving circuits.

described in Fig 2. Although VCC only needs to be set above

9V during operation, it should be set such that OVP does not

execute during an overload condition. For a full load, about

18~20V is appropriate for the VCC voltage and for no load,

about 13~14V.

Start Up

Input voltage range: 85 ~ 265 V (AC)

When Vac is minimum and it is started by the DC Link bulk

capacitor, the starting resistance is calculated as follows:

Rstart = 8----5---8---0--2--µ---–-A---1---5-- = 1.3MΩ

When Vac is maximum and it is started by the DC Link Bulk

capacitor, the power loss is calculated as follows:

Ploss

=

(---2---6----5-------2-----–-----1---5----)--2-

1.3MΩ

=

0.1(W)

Protection

The FPS has several self-protection circuits, which can

operate without additional external components, thereby

acquiring reliability without increase in cost. After a

protection circuit comes on, it can completely stop the SMPS

until the cutoff AC power is reconnected (Latch Mode

Protection) or it can make the SMPS operate above the

UVLO by unlatching the control voltage below the ULVO

(Auto Restart Mode Protection).

Va

265V

85V

UVLO

When it is started by the one-phase of the AC-Lines and Vac

is minimum, the starting resistance is calculated as follows:

RStart

=

-2----•-----8---5--------2-----–----1----5---π--

2π

÷

(80µA)

= 38MΩ

When it is started bythe one-phase of the AC_Line and Vac

is maximum, the power loss is calculated as follows:

∫ Va(rms) =

2--1--π--

π

o

(Vp

sin

t

–

15

)2dt

= 177V(Vp = 265 2)

Ploss

=

V-----a----(--r--m------s---)--2--

Rstart

=

(---1---7----7---)---2-

38M

= 82(mW)

The starting current across the starting resistor charges the

SPS VCC capacitor. When the VCC becomes greater than the

starting voltage, the SPS starts to switch the built-in

MOSFET. Once it starts, the current in the SPS control IC

abruptly is increased to 7mA, makes it difficult to operate

with the current through the starting resistor. Therefore, after

it starts, the auxiliary winding of the transformer supplies

most of the power to SPS. It is best to use an appropriate size

VCC power capacitor, generally about 33µF, because if it is

too large, the starting time can be delayed. This operation is

Figure 1. Detail of the undervoltage lockout (UVLO)

circuitry in a Fairchild Power Switch. The gate

operating circuit holds in a low state during UVL

thereby maintaining the SenseFET at turnoff.

These two operations are user-command operations, so the

user can select the

operation from the IC or by carefully controlling circuit

constants. The operation and applications for each protection

are described below.

Icc

[ m A]

20

7

Power On Reset

R a nge

0.1

6V

9V

15V

Vcc

Vz [V]

Fig 2 < Start-up Waveform >

Figure 2. Start-up Waveform

Over Load Protection

In abnormal status of SMPS over load is distinguished from

load short. This happens when a load exceeds a pre-set load

during normal operation. That is, the FPS overload

protection circuit stops the FPS if an instantaneous load

increases and becomes greater than 50W during normal

operation, when the maximum SMPS output had been

pre-set to 30W. In this type of protection, the protection

5

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