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September 2015
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
FAN6224
Synchronous Rectification Controller for Flyback and
Forward Freewheeling Rectification
Features
mWSaver™ Technology:
- Internal Green Mode to Stop SR Switching for
Lower No-Load Power Consumption
- 300 A Ultra-Low Green Mode Operating
Current
Synchronous Rectification Controller
Suited for High-Side and Low-Side of Flyback
Converters in QR, DCM, and CCM Operation
Suited for Forward Freewheeling Rectification
PWM Frequency Tracking with Secondary-Side
Winding Voltage Detection
140 kHz Maximum Operation Frequency
VDD Pin Over-Voltage Protection (OVP)
LPC Pin Open/Short Protection
RES Pin Open/Short Protection
RP Pin Open/Short Protection
Internal Over-Temperature Protection (OTP)
SOP-8 Package Available
Applications
AC-DC NB Adapters
Open-Frame SMPS
Description
FAN6224 is a secondary-side Synchronous Rectification
(SR) controller to drive SR MOSFET for improved
efficiency. The IC is suitable for flyback converters and
forward freewheeling rectification.
FAN6224 can be applied in Continuous or
Discontinuous Conduction Mode (CCM and DCM) and
Quasi-Resonant (QR) flyback converters based on a
proprietary linear-predict timing-control technique. The
benefits of this technique include a simple control
method without current-sense circuitry to accomplish
noise immunity.
With PWM frequency tracking and secondary-side
winding voltage detection, FAN6224 can operate in both
fixed- and variable-frequency systems up to 140kHz.
FAN6224 detects output load condition and determines
adjustable loading levels for Green Mode. In Green
Mode, the SR controller stops all SR switching operation
to reduce the operating current. Power consumption is
maintained at a minimum level in light-load condition.
Ordering Information
Part Number
Operating
Temperature Range
Package
Packing
Method
FAN6224M
-40°C to +105°C
8-Lead, Small Outline Package (SOP-8)
Tape & Reel
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 2
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Typical Application Diagrams
VIN
Q1
VDD
RES
AGND
GND
LPC
VOUT
FAN6224
R1
R2
R3
GATE
8
3 5
7
64
Q2
R4
ISR
VDET
VLPC VRES
1
RRP
CRP
RP
N1N2
ISR VDET
VLPC
VRES
VOUT
VIN
Q1
N1Q2
N2
N3
VDD
RES
AGNDGND
LPC
FAN6224
R3
R4
R1
GATE
8
35
7
64 R2
1
RRP
CRP
RP
Figure 2. Flyback High-Side SR
VDD
RES
AGNDGND
LPC
VOUT
VIN
FAN6224
R1
R2
R3
Q1
GATE
8
3 5
7
64
Q3
R4
Q21
RRP
RP
CRP
VLPC VRES
VDET
ISR
Figure 3. Forward Freewheeling Rectification
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 3
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Internal Block Diagram
+
-
1µA/V
VDD
LPC
Enable
0.256µA/V
S
R
Q
Q
3
4
GND
GATE
Drive
PWM Block
OVP
Green Mode
Causal
Function
RESET
2
6
AGND
AGND
7
RES
Calculate
VLPC-EN
+
-
+
-
+
-blanking
RESET
5
RP 1
GATE
Maximum
Period
Gate Expand Limit
8
Internal OTP
Fault Timing
Protection
S&H
tLPC-EN
VLPC-EN
Setting High/Low
Frequency Mode
tGREEN-ON/OFF
+
-
Protection
iCHR
CT
iDISCHR
VCT
27.5V/26V
10.5V/10.1V
Internal Bias
Timing
Calculation
Adjustable
Green Mode
0.35V
2.5V
1.45V
S&H
Protection
RESET
tGREEN-ON/OFF
S&H
VCT
Figure 4. Block Diagram
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 4
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Marking Information
ZXYTT
6224
TM
Figure 5. Top Mark
Pin Configuration
VDD
FAN6224
1 2 3 4
5678
LPC AGNDRES
RP GATEAGND GND
Figure 6. Pin Configuration
Pin Definitions
Pin #
Name
Description
1
RP
Programmable. A resistor paralleled with a capacitor is connected to RP pin and reference
ground externally. The timing to enter / exit Green Mode is programmable by the resistor, while
the range of operating frequency is programmable by the capacitor.
2, 6
AGND
Signal Ground.
3
GATE
Driver Output. The totem-pole output driver for driving the power MOSFET.
4
GND
Ground. MOSFET source connection.
5
VDD
Power Supply. The threshold voltages for startup and turn-off are 10.5 V and 10.1 V,
respectively.
7
RES
Reset Control of Linear Predict. RES pin is used to detect output voltage level through a
voltage divider. An internal current source, IDISCHR, is modulated by this voltage level on the
RES pin.
8
LPC
Winding Detection. This pin is used to detect the voltage on the winding during the on-time
period of the primary GATE.
: Fairchild Logo
Z: Plant Code
X: Year Code
Y: Week Code
TT: Die Run Code
T: Package Type (M = SOP)
M: Manufacturing Flow Code
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 5
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Absolute Maximum Ratings
Stresses exceeding the absolute maximum ratings may damage the device. The device may not function or be
operable above the recommended operating conditions and stressing the parts to these levels is not recommended.
In addition, extended exposure to stresses above the recommended operating conditions may affect device reliability.
The absolute maximum ratings are stress ratings only.
Symbol
Parameter
Min.
Max.
Unit
VDD
DC Supply Voltage
30
V
VLPC
Voltage on LPC Pin (TA=25°C)
-0.3
7.0
V
VRES
Voltage on RES Pin (Continuously in -0.5 V) (TA=25°C)
-1.5
7.0
V
VRP
Voltage on RP Pin (TA=25°C)
-0.3
7.0
V
PD
Power Dissipation (TA=25°C)
0.8
W
ΘJA
Thermal Resistance (Junction-to-Air)
151
°C/W
ΘJC
Thermal Resistance (Junction-to-Case)
58
°C/W
TSTG
Storage Temperature Range
-55
150
°C
TL
Lead Temperature (Soldering) 10s
260
°C
ESD
Electrostatic Discharge
Capability
Human Body Model, JESD22-A114
5500
V
Charged Device Model, JESD22-C101
2000
Notes:
1. Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device.
2. All voltage values, except differential voltages, are given with respect to GND pin.
Recommended Operating Conditions
The Recommended Operating Conditions table defines the conditions for actual device operation. Recommended
operating conditions are specified to ensure optimal performance to the datasheet specifications. Fairchild does not
recommend exceeding them or designing to Absolute Maximum Ratings.
Symbol
Parameter
Condition
Min.
Max.
Unit
VLPC
Voltage on LPC Pin
Continuous Operation
4.8
V
VRES
Voltage on RES Pin
4.8
V
VRP
Voltage on RP Pin
0.5
2.5
V
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 6
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Electrical Characteristics
VDD=15 V and TA=25°C, unless otherwise noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Unit
VOP
Continuously Operating Voltage
VDD-OFF
VDD-OVP
V
VDD-ON
Turn-On Threshold Voltage
9.5
10.5
11.5
V
VDD-OFF
Turn-Off Threshold Voltage
9.1
10.1
11.1
V
VDD-HYST
Hysteresis Voltage for Turn-On /
Turn-Off Threshold
0.1
0.7
V
IDD-OP
Operating Current
VDD=15 V, LPC=65 kHz,
CL=6000 pF
7
8
mA
IDD-GREEN
Operating Current in Green Mode
VDD=15 V
300
400
µA
VDD-OVP
VDD Over-Voltage Protection
26.0
27.5
29.0
V
VDD-OVP-HYST
Hysteresis Voltage for VDD OVP
1.1
1.5
1.9
V
tVDD-OVP
VDD OVP Debounce Time(3)
100
µs
Output Driver for internal SR Mosfet Section
VZ
Output Voltage Maximum
(Clamp)
10
12
14
V
VOL
Output Voltage LOW
VDD=12 V, IO=50 mA
0.5
V
VOH
Output Voltage HIGH
VDD=12 V, IO=50 mA
9
V
tR
Rising Time
VDD=12 V, CL=6 nF,
GATE=2 V~9 V
30
70
120
ns
tF
Falling Time
VDD=12 V, CL=6 nF,
GATE=9 V~2 V
20
50
100
ns
tPD_HIGH_LPC
Propagation Delay to GATE
HIGH (LPC Trigger)
tR:0%~10%, VDD=12 V
150
250
ns
tPD_LOW_LPC
Propagation Delay to GATE LOW
(LPC Trigger(3)
tF:100%~90%,VDD=12 V
150
ns
tMAX-PERIOD
Limitation between LPC Rising
Edge to Gate Falling Edge
fs=65 kHz
24.0
29.5
35.0
µs
fs=140 kHz
12.5
15.5
18.5
LPC Section
tBNK
Blanking Time for Charging CT(3)
150
ns
tLPC-SMP
LPC Sampling Timing of Previous
Cycle
fs=65 kHz,
RRP=75 k~200 k,
CRP=100 nF
0.9
1.1
1.3
µs
fs=140 kHz,
RRP=75 k~200 k,
CRP=1 nF
0.5
0.6
0.7
µs
VLPC-SOURCE
Lower Clamp Voltage
Source ILPC=10 µA
0
0.1
0.2
V
VLPC-HIGH-EN
Threshold Voltage for LPC to
Enable SR
VLPC-HIGH>VLPC-HIGH-EN, SR
Enable
1.38
1.45
1.54
V
VEN-CLAMP
SR Enable Threshold Clamp
Voltage(3)
VLPC-EN=2.5 V at VLPC-HIGH
>3 V
2.5
V
VLPC-TH-HIGH
Threshold Voltage on LPC Rising
Edge(3)
1.22
V
VLPC-CLAMP-H
VLPC High Clamping Voltage
VLPC>VLPC-CLAMP-H
5.7
6.2
6.7
V
VLPC-DIS
Threshold Voltage of VLPC to
Disable SR Gate Switching
VLPC>VLPC-DIS
4.8
5.5
V
tLPC-EN-RES
No LPC Signal, Reset VLPC-EN(3)
95
s
Continued on the following page…
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 7
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Electrical Characteristics (Continued)
VDD=15 V and TA=25°C, unless otherwise noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Unit
RES Section
tRES-SMP
VRES Sampling Time(3)
tSR_gate=5 µs
2.5
µs
VRES-EN
Threshold Voltage of VRES to
Enable SR Gate Switching
VRES>VRES-EN
1.3
1.6
2.0
V
VRES-CLAMP-H
VRES High Clamping Voltage
VRES>VRES-CLAMP-H
5.7
6.2
6.7
V
KRES-DROP
VRES Drop Protection Ratio(3)
VRES[n+1]<VRES[n] x KRES-DROP
85
%
VRES-SOURCE
VRES Low Clamping Voltage
IRES=10 µA, VDD=15 V
0
0.2
0.4
V
Linear Prediction Section
RatioLPC
Transfer Ratio of VLPC to ILPC(3)
1
µA/V
RatioRES
Transfer Ratio of VRES to IRES(3)
0.256
µA/V
RatioLPC-RES
RatioLPC/RatioRES
VRES=3 V,VLPC=3 V CRP=100 nF
3.65
3.90
4.15
tLPC-EN
Debounce Time for VLPC>VLPC-
EN=0.875 x VLPC-HIGH
fs=65 kHz, RRP=75 k~200 k,
CRP=100 nF
0.9
1.1
1.3
µs
fs=140 kHz, RRP=75 k~200 k,
CRP=1 nF
0.5
0.6
0.7
RatioSR-LMT
Maximum Ratio of SR Gate On
Time(3)
RatioSR-LMT < tON-SR[n+1]/ tON-SR[n]
120
%
tLPC-EXP-LMT
LPC Pulse Width Expansion Limit
tLPC-EXP-LMT < tLPC[n+1]- tLPC[n]
0.5
0.7
0.9
µs
tLPC-SRK-LMT
LPC Pulse Width Shrink Limit
tLPC-SRK-LMT < tLPC[n]- tLPC[n+1]
0.6
0.8
1.0
µs
Green Mode Section
tGREEN-OFF
SR Gate On Time to Exit Green
Mode
RRP=200 k, CRP=100 nF
5.5
5.9
6.3
µs
RRP=75 k,CRP=1 nF
3.0
3.3
3.6
tGREEN-ON
SR Gate On time to Enter Green
Mode
RRP=200 k, CRP=100 nF
4.0
4.4
4.8
µs
RRP=75 k,CRP=1 nF
1.6
1.9
2.2
tGREEN-
HYST(65kHz)
Hysteresis Voltage for tGREEN-
On/tGREEN-Off Threshold(3)
RRP=200 k, CRP=100 nF
1.5
µs
tGREEN-
HYST(140kHz)
Hysteresis Voltage for tGREEN-
On/tGREEN-Off Threshold(3)
RRP=75 k,CRP=1 nF
1.4
µs
nGREEN-OFF
Number of Switching Cycles to
Exit Green Mode(3)
SR Gate On Time > tGREEN-OFF
15
times
nGREEN-ON
Number of Switching Cycles to
Enter Green Mode(3)
SR Gate On Time < tGREEN-ON
3
times
VRP-OPEN
Threshold Voltage for RP Pin Pull
High Protection
3.0
3.5
4.0
V
VRP-SHORT
Threshold Voltage for RP Pin Pull
Low Protection
0.30
0.35
0.40
V
tGREEN-ENTER
No Gate Signal to Enter Green
Mode(3)
75
s
Continued on the following page…
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 8
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Electrical Characteristics
VDD=15 V and TA=25℃, unless otherwise noted.
Symbol
Parameter
Condition
Min.
Typ.
Max.
Unit
Operation Frequency Setting Section
VCRP-TH
Threshold Voltage for High / Low
Frequency Determination(3)
Set VRP> VCRP-th for Higher
Operating Frequency
0.35
V
tCRP-TH
Debounce Time for High / Low
Frequency Determination(3)
170
µs
IRP-SOURCE
RP Pin Source Current
8.5
9.5
10.5
µA
Casual Function Section
tDEAD-CAUSAL
SR Turn-Off Dead Time by Causal
Function
fS=65 kHz,
(RRP=75 k~200 k,
CRP=100 nF)
480
680
880
ns
fS=140 kHz,
(RRP=75 k~200 k,
CRP=1 nF)
350
500
650
ns
tCAUSAL-FAULT
If tS-PWM(n+1) > tCAUSALx tS-PWM(n),
SR Stops Switching & Enters
Green Mode
fS=65 kHz to 140 kHz
130
150
170
%
tCAUSAL_LEAVE
(Assume SR Triggers Fault
Causal Protection) If LPC Rises
Twice during tCAUSAL_LEAVE and
Previous On-Time of VLPC-HIGH is
Longer than tLPC-EN, then SR
Leaves Fault Causal Protection(3)
5.3
µs
tDEAD-CFR
Once CFR is Triggered, SR
Terminates & Forces SR to Enter
Green Mode (The Last Time from
SR Gate Falling to LPC Rising)(3)
Causal Function Regulator
(CFR)
70
ns
Internal Over-Temperature Protection for OTP
TOTP
Internal Threshold Temperature
for OTP(3)
140
°C
TOTP-HYST
Hysteresis Temperature for
Internal OTP(3)
20
°C
Note:
3. Guaranteed by Design
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 9
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Typical Performance Characteristics
Figure 7. VDD-ON vs. Temperature
Figure 8. VDD-OFF vs. Temperature
Figure 9. tGREEN-OFF vs. Temperature
Figure 10. tGREEN-OFF vs. Temperature
Figure 11. IRP-SOURCE vs. Temperature
Figure 12. IDD-GREEN vs. Temperature
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 10
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Typical Performance Characteristics (Continued)
Figure 13. tDEAD-CAUSAL vs. Temperature
Figure 14. tDEAD-CAUSAL vs. Temperature
Figure 15. VRES-EN vs. Temperature
Figure 16. RatioLPC-RES vs. Temperature
Figure 17. tLPC-EN vs. Temperature
Figure 18. tLPC-EN vs. Temperature
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 11
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Typical Performance Characteristics (Continued)
Figure 19. tMAX-PERIOD vs. Temperature
Figure 20. tMAX-PERIOD vs. Temperature
Figure 21. VLPC-SOURCE vs. Temperature
Figure 22. VRES-SOURCE vs. Temperature
Figure 23. tGREEN-ON vs. RRP
Figure 24. tGREEN-OFF vs. RRP
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 12
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Functional Description
VOUT
VDET
IM
VIN/n
Primary
MOSFET
VGS Synchronous Rectifier
MOSFET
VCT
VLPC
tPM.ON
IDS ISR /n
VIN/n+VOUT
tL.DIS
tCT.DIS
Body diode of
SR MOSFET Body diode of
SR MOSFET
VLPC-TH-HIGH
0.875VLPC-HIGH
VLPC-HIGH
IDS
Primary
MOSFET
IM,av
IM,max
IM,min
VOUT
VDET
IM
VIN/n
Primary
MOSFET
VGS Synchronous Rectifier
MOSFET
VCT
VLPC
tPM.ON
IDS ISR /n
VIN/n+VOUT
tL.DIS
tCT.DIS
Body diode of
SR MOSFET Body diode of
SR MOSFET
VLPC-TH-HIGH
0.875VLPC-HIGH
VLPC-HIGH Blanking Time
(tLPC-EN)
IM,max
IM,min
VRES
VRES
VRES-EN
VRES-EN
Figure 25. Waveforms of Linear-Predict Timing Control in CCM and DCM / QR Flyback
for Low-Side Application
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 13
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
VOUT
VDET
IM
VIN/n
Primary
MOSFET
VGS Synchronous Rectifier
MOSFET
VCT
VLPC
tPM.ON
IDS ISR /n
VIN/n+VOUT
tL.DIS
tCT.DIS
Body diode of
SR MOSFET Body diode of
SR MOSFET
VLPC-TH-HIGH
VLPC-HIGH
IDS
Primary
MOSFET
IM,av
IM,max
IM,min
VOUT
VDET
IM
VIN/n
Primary
MOSFET
VGS Synchronous Rectifier
MOSFET
VCT
VLPC
tPM.ON
IDS ISR /n
VIN/n+VOUT
tL.DIS
tCT.DIS
Body diode of
SR MOSFET Body diode of
SR MOSFET
VLPC-TH-HIGH
VLPC-EN=
0.875VLPC-HIGH
VLPC-HIGH Blanking Time
(tLPC-EN)
IM,max
IM,min
VRES
VRES
VRES-EN
VRES-EN
tRES-SMP
tRES-SMP
VLPC-EN=
0.875VLPC-HIGH
Figure 26. Waveforms of Linear-Predict Timing Control in CCM and DCM / QR Flyback for
High-Side Application
Linear Predict Timing Control
The SR MOSFET turn-off timing is determined by
linear-predict timing control and the operation principle
is based on the volt-second balance theorem, which
states: the inductor average voltage is zero during a
switching period in steady state, so the charge voltage
and charge time product is equal to the discharge
voltage and discharge time product. In flyback
converters, the charge voltage on the magnetizing
inductor is input voltage (VIN), while the discharge
voltage is reflected output voltage (nVOUT), as the
typical waveforms show in Figure 25. The following
equation can be drawn:
..IN PM ON OUT L DIS
V t n V t
(1)
where tPM,ON is inductor charge time; tL,DIS is inductor
discharge time; and n is turn ratio of primary windings
(N1) to secondary windings (N2).
FAN6224 uses the LPC and RES pins with two sets of
voltage dividers to sense DET voltage (VDET) and output
voltage (VOUT), respectively; so VIN/n, tPM.ON, and VOUT
can be obtained. As a result, tL,DIS, which is the on-time
of SR MOSFET, can be predicted by Equation 1. As
shown in Figure 25, the SR MOSFET is turned on when
the SR MOSFET body diode starts conducting and DET
voltage drops to zero. The SR MOSFET is turned off by
linear-predict timing control.
Circuit Realization
The linear-predict timing-control circuit generates a
replica (VCT) of the magnetizing current of the flyback
transformer using an internal timing capacitor (CT), as
shown in Figure 27. Using the internal capacitor voltage,
the inductor discharge time (tL.DIS) can be detected
indirectly, as shown in Figure 25. When CT is discharged
to zero, the SR controller turns off the SR MOSFET.
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 14
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
1µA/V
VCT
8
LPC
0.256µA/V
S
R
Q
Q
+
-
CT
iCHR iDISCHR
VLPC-TH
7RES
VCT Turn off
SR Gate
Turn on SR Gate at
the falling edge
VLPC
SR Gate
R1
R2
R3
R4
VDET VOUT
Figure 27. Simplified Linear-Predict Block
The voltage-second balance equation for the primary-
side inductance of the flyback converter is given in
Equation (1). Inductor current discharge time is given as:
.
.IN PM ON
L DIS OUT
Vt
tnV
(2)
The voltage scale-down ratio between RES and LPC is
defined as K below:
4 3 4
2 1 2
/
/
R R R
KR R R
(3)
During tPM.ON, the charge current of CT is iCHR-iDICHR,
while during tL.DIS, the discharge current is iDICHR. As a
result, the current-second balance equation for internal
timing capacitor (CT) can be derived from:
..
3.9
( ( ) )
IN OUT OUT PM ON OUT CT DIS
VV V t V t
Kn
(4)
Therefore, the discharge time of CT is given as:
.
.
3.9
( ( ) )
IN OUT OUT PM ON
CT DIS OUT
VV V t
Kn
tV
(5)
When the voltage scale-down ratio between LPC and
RES (K) is 3.9, the discharge time of CT (tCT.DIS) is the
same as inductor current discharge time (tL.DIS).
However, considering the tolerance of voltage divider
resistors and internal circuit, the scale-down ratio (K)
should be larger than 3.9 to guarantee that tCT.DIS is
shorter than tL.DIS. It is typical to set K around 4.0~4.5.
Referring to Figure 25, when LPC voltage is higher
than VLPC-EN over a period of blanking time (tLPC-EN)
and lower than VLPC-TH-HIGH (1.22 V), then SR
MOSFET can be triggered. Therefore, VLPC-EN must be
lager than VLPC-TH-HIGH or the SR MOSFET cannot be
turned on. As a result, when designing the voltage
divider of the LPC, considering the tolerance, R1 and
R2 should satisfy the equation:
541
21
2.)( .
OUT
MININ V
n
V
RR R
(6)
On the other hand, there is also a threshold voltage,
VRES-EN, for RES pin to enable SR switching, hence R3
and R4 must satisfy:
3
34 2
OUT
RV
RR

(7)
In addition, considering the linear operating range, LPC
and RES voltage should be under 4.8 V, and therefore:
2.
12
( ) 4.8
IN MAX OUT
RV V
R R n
(8)
4
34 4.8
OUT
RV
RR

(9)
For high-side applications, as shown in Figure 2, an
extra auxiliary winding (N3) is used to supply voltage for
controller. To detect output voltage, the RES pin is
connected to the auxiliary winding through a set of
voltage dividers. As Figure 26 shows, VRES is
proportional to VOUT when SR MOSFET or its body
diode conducts. Therefore, information of VOUT is
sampled at tRES-SMP after the primary-side MOSFET
turns off. As a result, Equation (4) can be rewritten as:
..
3.9
( ( ) )
'IN OUT OUT PM ON OUT CT DIS
VV V t V t
K n n
(10)
where n’ is the turn ratio of auxiliary windings (N3) to
secondary windings (N2).
The discharge time of CT can be obtained as:
.
.
3.9
( ( ) )
'IN OUT OUT PM ON
CT DIS OUT
VV V t
K n n
tV
(11)
Therefore, when the voltage scale-down ratio (K) and
turn ratio (n’) product is 3.9; the discharge time, tCT.DIS,
is the same as inductor current discharge time, tL.DIS. To
guarantee tCT.DIS is shorter than tL.DIS, the K and n’
product should be larger than 3.9. It is typical to set the
product around 4.0~4.5. When designing the voltage
divider of LPC, the consideration is the same as that of
low-side application, which means that the linear
operating range, Equations (6) and (8) must be
satisfied. However, when determining the voltage
divider of RES, note that turn ratio n’ must be taken into
consideration and so that Equation (7) and (9) are
modified as:
4
34
'2
OUT
RnV
RR
(12)
4
34
' 4.8
OUT
RnV
RR
(13)
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 15
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
CCM Operation
The typical waveforms of CCM operation in steady state
are shown as right side of Figure 25 and Figure 26.
When the primary-side MOSFET is turned on, the
energy is stored in Lm. During the on-time of the
primary-side MOSFET (tPM.ON), the magnetizing current
(IM) increases linearly from IM,min to IM,max. Meanwhile,
internal timing capacitor (CT) is charged by current
source (iCHR-iDICHR) proportional to VIN, so VCT also
increases linearly.
When the primary-side MOSFET is turned off, the
energy stored in Lm is released to the output. During the
inductor discharge time (tL.DIS), the magnetizing current
(IM) decreases linearly from IM,max to IM,min. At the same
time, the internal timing capacitor (CT) is discharged by
current source (iDISCHR) proportional to VOUT, so VCT also
decreases linearly. To guarantee the proper operation
of SR, it is important to turn off the SR MOSFET just
before SR current reaches IM,min so that the body diode
of the SR MOSFET is naturally turned off.
DCM / QR Operation
In DCM / QR operation, when primary-side MOSFET is
turned off, the energy stored in Lm is fully released to
the output at the turn-off timing of primary-side
MOSFET. Therefore, the DET voltage continues
resonating until the primary-side MOSFET is turned on,
as depicted in Figure 25. While DET voltage is
resonating, DET voltage and LPC voltage drop to zero
by resonance, which can trigger the turn-on of the SR
MOSFET. To prevent fault triggering of the SR
MOSFET in DCM operation, a blanking time is
introduced to LPC voltage. The SR MOSFET is not
turned on even when LPC voltage drops below VLPC-TH-
HIGH unless LPC voltage stays above 0.875 VLPC-HIGH
longer than the blanking time (tLPC-EN). The turn-on
timing of the SR MOFET is inhibited by gate inhibit time
(tINHIBIT), once the SR MOSFET turns off, to prevent
fault triggering.
mWSaver™ Technology
Green-Mode Operation
To minimize the power consumption at light-load
condition, the SR circuit is disabled when the load
decreases. As illustrated in Figure 28, the discharge
times of the inductor and internal timing capacitor
decrease as load decreases. If the discharge time of
the internal timing capacitor (tCT.DIS) is shorter than
tGREEN-ON for more than three cycles, then the SR circuit
enters Green Mode. Once FAN6224 enters Green
Mode, the SR MOSFET stops switching and the major
internal block is shut down to further reduce the
operating current of the SR controller. In Green Mode,
the operating current reduces to 300 µA. This allows
power supplies to meet stringent power conservation
requirements. When the discharge time of the internal
capacitor is longer than tGREEN-OFF for more than fifteen
cycles, the SR circuit is enabled and resumes the
normal operation, as shown in Figure 29.
To enhance flexibility of design, tGREEN-ON and tGREEN-OFF
are adjustable by the external resistor of the RP pin
within a certain range. As shown in Figure 30, larger
RRP resistance corresponds to longer tGREEN-ON and
tGREEN-OFF, and vice versa. Therefore, by setting
different resistance of RRP, the loading of entering and
exiting Green Mode is adjustable.
IM
SR Gate
1.9µs~4.4µs
3 Times
Green Mode
Normal Mode
1.9µs~4.4µs 1.9µs~4.4µs t
t
Figure 28. Entering Green Mode
t
IM
SR Gate
3.3µs~5.9µs
……
15 Times
Green Mode Normal Mode
3.3µs~5.9µs
3.3µs~5.9µs
t
Figure 29. Resuming Normal Operation
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
50 70 90 110 130 150 170 190 210 230
tGREEN-ON
tGREEN-OFF
RRP (kΩ)
tCT.DIS (s)
Figure 30. Adjustable tGREEN-ON and tGREEN-OFF
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 16
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
Selection of Operating Frequency
For different operating frequency range, internal
parameters of the SR controller should be different to
optimize signal processing. The capacitor of the RP pin
(CRP) is used to determine the operating frequency
range of the SR controller. For low switching frequency
systems (<100 kHz), CRP is recommended as 10 nF; for
high switching frequency systems (100k~140 kHz), CRP
is recommended as 1nF.
Causal Function
Causal function is utilized to limit the time interval (tSR-
MAX) from the rising edge of VLPC to the falling edge of
the SR Gate. As shown in Figure 31, tSR-MAX is limited to
previous switching period (tS-PWM) minus a dead time,
say tDEAD-CAUSAL. When the system operates at fixed
frequency, whether voltage-second balance theorem
can be applied or not, causal function can guarantee
reliable operation.
t
VLPC
SR Gate
VCT
SR On-Time
tS-PWM
tSR-MAX= tS-PWM – tDEAD-CAUSAL
SR Gate is
turned off by
causal function
t
t
Figure 31. Causal Function Operation
Fault Causal Timing Protection
Fault causal timing protection is utilized to disable the
SR Gate under some abnormal conditions. Once the
switching period (tS-PWM[n]) is longer than 150% of
previous switching period (tS-PWM[n-1]), the SR Gate is
disabled and enters Green Mode, as shown in Figure
32. Since the rising edge of VLPC among switching
periods (tS-PWM) is tracked for causal function, the
accuracy of switching period is important. Therefore, if
the detected switching period has a serious variation,
the SR Gate is terminated to prevent fault trigger.
t
VLPC
SR Gate Disable
SR Gate &
enter Green
Mode
tS-PWM [n-1] tS-PWM [n] > 1.5xtS-PWM [n-1]
t
Figure 32. Fault Causal Timing Protection
Gate Expansion Limit Protection
Gate expansion limit protection controls the on-time
expansion of the SR MOSFET. Once the discharge
time of the internal timing capacitor (tDIS.CT) is longer
than 120% of the previous on-time of the SR MOSFET
(ton-SR[n-1]); ton-SR[n] is limited to 120% of ton-SR[n-1], as
shown in Figure 33. When output load changes rapidly
from light load to heavy load, voltage-second balance
theorem may not be applied. In this transient state, gate
expand limit protection is activated to prevent overlap
between the SR Gate and the PWM gate.
ton-SR [n-1]
VLPC
SR Gate
VCT
ton-SR [n]= ton-SR [n-1]*120%
tDIS.CT [n]
tDIS.CT [n-1]
t
t
t
SR Gate is limited to 120% of ton-SR[n-1]
Figure 33. Gate Expand Limit Protection
RES Dropping Protection
RES dropping protection prevents VRES dropping too
much within a cycle. The VRES is sampled as a
reference voltage, VRES’, on VLPC rising edge. Once VRES
drops below 85% of VRES’, the SR Gate is turned off
immediately, as shown in Figure 34. When output
voltage drops rapidly within a switching cycle, voltage-
second balance may not be applied; RES dropping
protection is activated to prevent overlap.
VLPC
SR Gate
VRES
VRES’
SR Gate is turned
off immediately
0.85*VRES’
t
t
t
Figure 34. VRES Dropping Protection
LPC Width Expansion / Shrink Protection
LPC width expansion and shrink protection is utilized to
disable the SR MOSFET switching under some
abnormal conditions. As Figure 35 shows, once the
LPC pulse width (tLPC[n]) is longer than that of previous
cycle (tLPC[n-1]) for tLPC-EXP-LMT, the LPC width
expansion protection is triggered and SR MOSFET
switching is terminated immediately. Figure 36 shows
the timing diagram of LPC width shrink protection. Once
tLPC[n] is shorter than tLPC[n-1], the SR MOSFET
switching also shuts down immediately.
© 2013 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN6224 • Rev. 1.4 17
FAN6224 — Synchronous Rectification Controller for Flyback and Forward Freewheeling Rectification
t
VLPC
SR Gate
tLPC[n-1] tLPC[n]
SR Gate off
t
Figure 35. VLPC Width Expand Protection
t
VLPC
SR Gate
tLPC[n-1] tLPC[n]
SR Gate off
t
Figure 36. VLPC Width Shrink Protection
Over-Time Protection
Generally, the minimum operating frequency of PWM
controller in normal status is above 65 kHz
(65~140 kHz). In FAN6224, there are two over-time
protections that force the SR controller to go into green
mode. As shown in upper part of Figure 37, the first one
is when the time between LPC pulses (from LPC falling
edge to rising edge) is longer than 95 us. This is
typically triggered when the primary side controller
operates in burst mode operation. To minimize the
power consumption, FAN6224 enters into green mode
in this condition. This green mode is also triggered
when the LCP voltage divider is malfunctioning.
Another condition is when the time duration from SR
turn-off to SR turn-on is longer than 75us as shown in
lower part of Figure 37. This happens when the
PWM controller in the primary side goes into burst
mode operation at light load condition.
t
VLPC
Disable SR Gate &
enter Green Mode
t
tSR-Gate < 75uS tSR-Gate > 75uS
tLPC < 95uS tLPC > 95uS
VSR-GATE
VLPC
VSR-GATE
VCT
Figure 37. Over-Time Protection
LPC Pin Open / Short Protection
LPC-Open Protection: If VLPC is higher than VLPC-DIS
for longer than debounce time tLPC-HIGH, FAN6224 stops
switching immediately and enters Green Mode. VLPC is
clamped at 6.2 V to avoid LPC pin damage.
LPC-Short Protection: If VLPC is pulled to ground and
the charging current of timing capacitor (CT) is near
zero, SR Gate is not output.
RES Pin Open / Short Protection
RES-Open Protection: If VRES is pulled to HIGH level,
the gate signal is extremely small and FAN6224 enters
Green Mode. In addition, VRES is clamped at 6.2 V to
avoid RES pin damage.
RES-Short Protection: If VRES is lower than VRES-EN
(1.6 V), FAN6224 stops switching immediately and
enters Green Mode.
Under-Voltage Lockout (UVLO)
The power ON and OFF VDD threshold voltages are
fixed at 10.5 V and 10.1 V, respectively. The FAN6224
can be used in various output voltage applications.
VDD Pin Over-Voltage Protection (OVP)
Over-voltage conditions are usually caused by an open
feedback loop. VDD over-voltage protection prevents
damage to the SR MOSFET. When the voltage on the
VDD pin exceeds 27.5 V; the SR controller stops
switching the SR MOSFET.
Over-Temperature Protection (OTP)
To prevent the SR Gate from fault triggering in high
temperatures, internal over-temperature protection is
integrated in FAN6224. If the temperature is over
140°C, the SR Gate is disabled until the temperature
drops below 120°C.
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