FAN7621SJ datasheet(9/17 Pages) FAIRCHILD

Part #	FAN7621SJ

Description	PFM Controller for Half-Bridge Resonant Converters

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Manufacturer	FAIRCHILD [Fairchild Semiconductor]

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FAN7621 • Rev. 1.0.1

9

Functional Description

1. Basic Operation

: FAN7621 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 t ime

time

Figure 16. MOSFETs Gate Drive Signal

2. Internal Oscillator

: FAN7621 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.

I

CTC

+

-

+

-

3V

1V

-Q

Q

R

S

F/F

2I

CTC

VREF

I

CTC

2V

Counter

(1/4)

R

T

3

Gate drive

C

T

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

Frequency (kHz)

f min

f normal

f max

f ISS

Soft-sta rt

Figure 18. Resonant Converter Typical Gain Curve

R

sense

VCC

LVCC

RT

CON

CS

SG

PG

CTR

HVCC

HO

LO

R

max

R

min

R

SS

C

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 startup, 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

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