FS7M0680

5

General Application

In general, the FPS consists of several functional sections:

under voltage lockout circuit (UVLO), reference voltage,

oscillator (OSC), pulse width modulation (PWM) block, pro-

tection circuits and gate drive circuit.

Start-Up

The minimum current that FPS requires for the start-up is

80

µA. This current can be provided by the DC link bulk

capacitor (DC start-up) or directly by the AC line (AC start-

up).

DC start-up

Assuming wide range input voltage (85-265V), the maxi-

mum value of Rstart is calculated with the minimum input

voltage as follows:

The maximum power dissipation in Rstart is calculated with

the maximum input voltage as follows:

AC start-up

When the start-up current is provided directly by the AC line

through a single rectifier diode, the maximum value of Rstart

is calculated with the minimum input voltage as follows:

The maximum power dissipation in Rstart is calculated with

the maximum input voltage as follows:

The current provided through the starting resistor charges the

Vcc capacitor. When Vcc becomes higher than the threshold

voltage, the FPS starts the switching operation of the built-in

power MOSFET. Then, the current required by the control

IC is suddenly increased to 7mA, which makes it difficult for

FPS to operate with the current provided through Rstart.

Therefore, after FPS starts, the auxiliary winding of the

transformer should supply most of the power required by the

tor, generally about 33

µF, because the starting time can be

delayed if it is too large. This operation is described in figure

normal operation, it should be set to such an extent that over

voltage protection (OVP) is not activated during an overload

condition. For full load, about 18~20V is appropriate for

Protection

The FPS has not only pulse by pulse current limit circuit, but

also several self-protection circuits. These protection circuits

are fully integrated and do not require external components.

After the protection circuits are activated, the FPS com-

pletely stops the SMPS (Latch Mode Protection) until the

power on reset circuit is activated by removing and restoring

input power, or restarts the SMPS automatically (Auto

Restart Mode Protection).

These two operations are user-selected operations, so the

user can select proper device according to the shutdown

mode. The operations principle and applications for each

protection are described as follows.

Figure 2. Variation of Icc according to Vcc

Rstart

85 2 15

–

80

µA

---------------------------

1.3M

Ω

==

Ploss

265 2 15

–

()

2

1.3M

Ω

--------------------------------------

0.1 W

()

==

265V

85V

Va

RStart

285

•

215

π

–

2

π

----------------------------------------

80

µA

()

÷

=

380k

Ω

=

Va rms

()

1

2

π

------

Vp

t

sin

15

–

()

2

dt

o

π

∫

=

177V Vp

265 2

=

()

=

Ploss

Va rms

()2

Rstart

---------------------------

177

()2

380k

------------------

==

82 mW

()

=

Figure 1. Undervoltage lockout (UVLO) circuit

5V

Vref

Good Logic

Good Logic

Power on

Reset

Latch

Comparator

DC

LINK

Rstart

3

FPS

Vz

15V/9V

6V

UVLO

Vcc

Fig 2 < Start-up Waveform >

Vcc

Vz

15V

9V

20

Icc

[mA]

[V]

0.1

6V

Power On Reset

Range

7