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 

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 19. FAN7621 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:

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

(3)

Rmin

RSS

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

three times the resonant frequency (fO) of the resonant

network.

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

constant. The RC time constant is as follows:

TSS = RSS ⋅ CSS

(4)

fs

f ISS

40kHz

Control loop

take over

VCC

Rmax

Rmin

RSS

CSS

L V CC

HV CC

RT

HO

CON

CTR

LO

CS

SG PG

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 FAN7621

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.

time

Figure 20. Frequency Sweeping of Soft-Start

4. Control Pin: The FAN7621 has a control pin for

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

shows the internal block diagram for control pin.

CON 6

LVCC

IOLP

-

0.4 / 0.6V

+

+

OLP

5V

-

LVCC

+

23V

-

OVP

LVCC good

Stop Switching

SQ

R -Q

Auto-restart

protection

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: FAN7621 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:

SKIP = 5.2k + 4.16k x100(kHz)

(5)

R min R max

R1

C1

FAN7621

RT

Rmin

OP1

Main

Output

Main Off

Aux

Output

CON

OP1

Figure 23. Remote On / Off Circuit

5. Protection Circuits: The FAN7621 has several self-

protective functions, such as Overload Protection (OLP),

Over-Current Protection (OCP), Abnormal Over-Current

Protection (AOCP), Over-Voltage Protection (OVP), and

Thermal Shutdown (TSD). OLP, OCP, and OVP are

auto-restart mode protections; while AOCP and TSD are

latch-mode protections, as shown in Figure 24.

Auto-Restart Mode Protection: Once a fault condition

is detected, switching is terminated and the MOSFETs

remain off. When LVCC falls to the LVCC stop voltage of

11.3V, the protection is reset. FAN7621 resumes normal

operation when LVCC reaches the start voltage of 14.5V.

© 2009 Fairchild Semiconductor Corporation

FAN7621 • Rev. 1.0.1

10

www.fairchildsemi.com

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