FAN7621 データシートの表示（PDF） - Fairchild Semiconductor

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FAN7621

PFM Controller for Half-Bridge Resonant Converters

Fairchild Semiconductor 

Functional Description

1. Basic Operation: FAN7621 is designed to drive high- Gain

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.

1.8

Dead t ime

1.6

High-side

MOSFET

gate drive

1.4

f min

f normal f max

f ISS

Low-side

MOSFET

gate drve

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.

VREF

I CTC

ICTC 2ICTC

+

3V

-

1V

+

CT

-

SQ

R -Q

F/F

2V

RT

3

Counter

(1/4) Gate drive

Figure 17. Current Controlled Oscillator

1.2

1.0

Soft-sta rt

0.8

0.6

60

70

80

90

100 110 120 130 140 150

Frequency (kHz)

Figure 18. Resonant Converter Typical Gain Curve

VCC

Rmax

Rmin

RSS

CSS

LVCC

HVCC

RT

HO

CON

CTR

LO

CS

SG PG

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.

R sense

Figure 19. Frequency Control Circuit

The minimum switching frequency is determined as:

f min = 5.2kΩ ×100(kHz)

(1)

Rmin

Assuming the saturation voltage of opto-coupler

transistor is 0.2V, the maximum switching frequency is

determined as:

f max = (5.2kΩ + 4.68kΩ ) ×100(kHz)

(2)

Rmin

Rmax

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

© 2009 Fairchild Semiconductor Corporation

FAN7621 • Rev. 1.0.1

9

www.fairchildsemi.com

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