FSFR1900 datasheet(10/17 Pages) FAIRCHILD

Part #	FSFR1900

Description	Power Switch (FPS?? for Half-Bridge Resonant Converters

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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FSFR series • 1.0.3

10

Functional Description

1. Basic Operation:

FSFR-series is designed to drive

high-side and low-side MOSFETs complementarily with

50% duty cycle. A fixed dead time of 350ns is introduced

between consecutive transitions, as shown in Figure 16.

High side

MOSFET

gate drive

Low side

MOSFET

gate drve

Dead time

time

Figure 16. MOSFETs Gate Drive Signal

2. Internal Oscillator

: FSFR-series employs a current-

controlled oscillator, as shown in Figure 17. Internally,

the voltage of RT pin is regulated at 2V and the

charging/discharging current for the oscillator capacitor,

CT, is obtained by copying the current flowing out of RT

pin (ICTC) using a current mirror. Therefore, the switching

frequency increases as ICTC increases.

Figure 17. Current Controlled Oscillator

3. Frequency Setting

: Figure 18 shows the typical

voltage gain curve of a resonant converter, where the

gain is inversely proportional to the switching frequency

in the ZVS region. The output voltage can be regulated

by modulating the switching frequency. Figure 19 shows

the typical circuit configuration for RT pin, where the

opto-coupler transistor is connected to the RT pin to

modulate the switching frequency.

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Gain

140

150

60

70

80

90

100

110

120

130

freq (kHz)

f min

f normal

f max

f ISS

Soft-start

Figure 18. Resonant Converter Typical Gain Curve

Control

IC

VDL

LVcc

RT

CON

SG

PG

R

min

R

max

C

ss

R

ss

Figure 19. Frequency Control Circuit

The minimum switching frequency is determined as:

min

5.2

100(

)

k

f

kHz

R

Ω

=×

(1)

Assuming the saturation voltage of opto-coupler

transistor is 0.2V, the maximum switching frequency is

determined as:

max

min

max

5.2

4.68

(

) 100(

)

kk

fkHz

RR

Ω

=+

×

(2)

To prevent excessive inrush current and overshoot of

output voltage during start-up, increase the voltage gain

of the resonant converter progressively. Since the

voltage gain of the resonant converter is inversely

proportional to the switching frequency, the soft-start is

implemented by sweeping down the switching frequency

from an initial high frequency (f

IS S ) until the output

voltage is established. The soft-start circuit is made by

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