FAN7621SSJX データシートの表示（PDF） - ON Semiconductor

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FAN7621SSJX

PFM Controller for Half-Bridge Resonant Converters

ON Semiconductor 

Functional Description

1. Basic Operation: FAN7621S 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 15.

Figure 15. MOSFETs Gate Drive Signal

2. Internal Oscillator: FAN7621S employs a current-

controlled oscillator, as shown in Figure 16. 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. Resonant Converter Typical Gain Curve

V REF

I CTC

I CTC 2I CTC

RT

8

+

- 2V

+

3V

-

1V

+

CT

-

SQ

R -Q

F /F

C oun ter

(1/4)

G ate drive

Figure 18. Frequency Control Circuit

Figure 16. Current Controlled Oscillator

3. Frequency Setting: Figure 17 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 18 shows

the typical circuit configuration for RT pin, where the opto-

coupler transistor is connected to the RT pin to modulate

the switching frequency.

The minimum switching frequency is determined as:

f min = 5.2kΩ ×100 (kHz)

(1)

Rmin

Assuming the saturation voltage of the 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

implemented by sweeping down the switching frequency

from an initial high frequency (f I S S ) until the output

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

connecting R-C series network on the RT pin, as shown

in Figure 18. FAN7621S also has an internal soft-start of

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 19. The

initial frequency of the soft-start is given as:

f ISS = (5.2kΩ + 5.2kΩ ) ×100 + 40 (kHz)

(3)

Rmin

RSS

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

three times the resonant frequency (fO) of the resonant

network.

© 2009 Fairchild Semiconductor Corporation

FAN7621S • Rev. 1.0.1

9

www.fairchildsemi.com

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