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FSFR2000

Fairchild Power Switch (FPS™) for Half-Bridge Resonant Converter

Fairchild Semiconductor 

Latch-Mode Protection: Once this protection is

triggered, switching is terminated and the MOSFETs

remain off. The latch is reset only when LVcc is

discharged below 5V.

LVcc

7

+ LVcc good

Vref

11 / 14 V -

Internal

Bias

OCP

OLP

OVP

LVcc good

CON

20k

Auto-restart

protection

SQ

R -Q

F/F

Latch

protection

QS

-Q R

F/F

Shutdown

AOCP

TSD

LVcc < 5V

Current Sensing Using Resonant Capacitor Voltage:

For high-power applications, current sensing using a

resistor may not be available due to the severe power

dissipation in the resistor. In that case, indirect current

sensing using the resonant capacitor voltage can be a

good alternative because the amplitude of the resonant

capacitor voltage (Vcrp-p) is proportional to the resonant

current in the primary side (Ipp-p) as:

V p−p

Cr

=

I p−p

p

2π fsCr

(6)

To minimize power dissipation, a capacitive voltage

divider is generally used for capacitor voltage sensing,

as shown in Figure 27.

Figure 24. Protection Blocks

Current Sensing Using Resistor: FSFR-series senses

drain current as a negative voltage, as shown in Figure

25 and Figure 26. Half-wave sensing allows low power

dissipation in the sensing resistor while full-wave sensing

has less switching noise in the sensing signal.

Cr

Np

Ns

Control

IC

VCS

CS

SG

PG

Rsense

Ids

Ns

Ids

VCS

Figure 25. Half-wave Sensing

Ids

VCS

Control

IC

VCS

CS

Cr

Np

Ns

SG

PG

Rsense

Ns

Ids

Figure 26. Full-wave Sensing

CON

Cd

Rd

Control

IC

SG

VSENSE

CSENSE

Ip

Np

Ip

PG

100

CB

Ns

Ns

Cr

VCr

VCrp-p

Vsense

V pk

sense

V p−p

Cr

=

CB

Csense + CB

V pk

sense

2

= VCON

VCON

Tdelay = Rd Cd

Vsensepk

Vsensepk

Figure 27. Current Sensing Using Resonant

Capacitor Voltage

5.1 Over-Current Protection (OCP): When the sensing

pin voltage drops below -0.6V, OCP is triggered and

MOSFETs remain off. This protection has a shutdown

time delay of 1.5µs to prevent premature shutdown

during start-up.

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 or OLP is triggered. AOCP is triggered

without shutdown delay when the sensing pin voltage

drops below -0.9V. This protection is latch mode and

reset when LVcc is pulled down below 5V.

© 2007 Fairchild Semiconductor Corporation

FSFR2100 • 1.0.0

11

www.fairchildsemi.com

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