FSFR1900 datasheet(11/18 Pages) FAIRCHILD

Part #	FSFR1900

Description	Fairchild Power Switch (FPS) for Half-Bridge Resonant Converters

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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FSFR series Rev.1.0.9

11

in Figure 19. FSFR-series also has an internal soft-start

for 3ms to reduce the current overshoot during the initial

cycles, which adds 40kHz to the initial frequency of the

external soft-start circuit, as shown in Figure 20. The

initial frequency of the soft-start is given as:

min

5.2

(

) 100 40 (

)

ISS

SS

kk

f

kHz

RR

ΩΩ

=+

×

+

(3)

It is typical to set the initial frequency of soft-start two ~

three times the resonant frequency (fO) of the resonant

network.

The soft-start time is three to four times of the RC time

constant. The RC time constant is as follows:

SS

TR

C

=⋅

(4)

f

s

time

Control loop

take over

40kHz

f ISS

Figure 20. Frequency Sweeping of Soft-start

4. Control Pin: The FSFR-series has a control pin for

protection, cycle skipping, and remote on/off. Figure 21

shows the internal block diagram for control pin.

Figure 21. Internal Block of Control Pin

Protection: When the control pin voltage exceeds 5V,

protection

is

triggered.

Detailed

applications

are

described in the protection section.

Pulse Skipping: FSFR-series stops switching when the

control pin voltage drops below 0.4V and resumes

switching when the control pin voltage rises above 0.6V.

To use pulse-skipping, the control pin should be

connected to the opto-coupler collector pin. The

frequency that causes pulse skipping is given as:

()

kHz

100

x

R

k

16

.

4

R

k

2

.

5

max

min

SKIP

+

=

(5)

Figure 22. Control Pin Configuration for

Pulse Skipping

Remote On / Off: When an auxiliary power supply is

used for standby, the main power stage using FSFR-

series can be shut down by pulling down the control pin

voltage, as shown in Figure 23. R1 and C1 are used to

ensure soft-start when switching resumes.

OP1

R

min

FPS

Main

Output

Main Off

Aux

Output

R1

C1

OP1

R

T

CON

Figure 23. Remote On / Off Circuit

4. Current Sensing Method

Current Sensing Using Resistor: FSFR-series senses

drain current as a negative voltage, as shown in Figure

24 and Figure 25. Half-wave sensing allows low power

dissipation in the sensing resistor, while full-wave

sensing has less switching noise in the sensing signal.

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FSFR1900 Datasheet(PDF) 11 Page - Fairchild Semiconductor

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