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

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FAN7621SSJX

PFM Controller for Half-Bridge Resonant Converters

Fairchild Semiconductor 

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

L V CC

V AR

(a) (b)

( a) (b) (a) (b)

V CssH

V CssL

ICr

t stop

tS /S

(a ) P r o te ction s a r e tr igge r e d, ( b ) F S F R- U S r e sta r ts

time

Figure 19. Frequency Sweeping of Soft-Start

4. Self Auto-restart: The FAN7621S can restart

automatically even if a built-in protection is triggered with

external supply voltage. As shown in Figure 20 and

Figure 21; once any protections are triggered, M1 switch

turns on and V-I converter is disabled. CSS starts to be

discharged until the VCss across CSS drops to VCssL. Then

all protections are reset, M1 turns off, and V-I converter

resumes. The FAN7621S starts switching again with soft-

start. If the protections occur while VCss is under VCssL

and VCssH level, the switching is terminated immediately,

VCss continues to increase until reaching VCssH, then CSS

is discharged by M1.

Figure 21. Self Auto-Restart Operation

5. Protection Circuits: The FAN7621S has several self-

protective functions, such as Over-Current Protection

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

Voltage Protection (OVP), and Thermal Shutdown (TSD).

These protections are auto-restart mode protections, as

shown in Figure 21.

Once a fault condition is detected, switching is

terminated and the MOSFETs remain off. When LVCC

falls to the LVCC stop voltage of 10V or the AR signal is

HIGH, the protection is reset. FAN7621S resumes normal

operation when LVCC reaches the start voltage of 12.5V.

Figure 20. Internal Block of AR Pin

After protections trigger, FAN7621S is disabled during

the stop-time, tstop, where VCss decreases and reaches to

VCssL. The stop-time of FAN7621S can be estimated as:

tstop=Css · Rss+Rmin || 5kΩ

(5)

For the soft-start time, ts/s it can be set as Equation (4).

Figure 22. Protection Blocks

5.1 Over-Current Protection (OCP): When the

sensing pin voltage drops below -0.58V, OCP is

triggered and the MOSFETs remain off. This protection

has a shutdown time delay of 1.5µs to prevent

premature shutdown during startup.

5.2 Abnormal Over-Current Protection (AOCP): If

the secondary rectifier diodes are shorted, large

current with extremely high di/dt can flow through the

MOSFET before OCP is triggered. AOCP is triggered

without shutdown delay when the sensing pin voltage

drops below -0.9V.

5.3 Over-Voltage Protection (OVP): When the LVCC

reaches 23V, OVP is triggered. This protection is used

when auxiliary winding of the transformer to supply VCC

to the controller is utilized.

5.4 Thermal Shutdown (TSD): If the temperature of

the junction exceeds approximately 130°C, the thermal

shutdown triggers.

© 2009 Fairchild Semiconductor Corporation

FAN7621S • Rev. 1.0.1

10

www.fairchildsemi.com

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