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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
April 2009
FSQ0765RQ Green-Mode Fairchild Power Switch (FPSTM) for Quasi-Resonant Operation - Low EMI and High Efficiency
Features
! Optimized for Quasi-Resonant Converter (QRC) ! Low EMI through Variable Frequency Control and AVS
Description
A Quasi-Resonant Converter (QRC) generally shows lower EMI and higher power conversion efficiency than a conventional hard-switched converter with a fixed switching frequency. The FSQ-series is an integrated Pulse-Width Modulation (PWM) controller and SenseFET specifically designed for quasi-resonant operation and Alternating Valley Switching (AVS). The PWM controller includes an integrated fixed-frequency oscillator, Under-Voltage Lockout (UVLO), LeadingEdge Blanking (LEB), optimized gate driver, internal softstart, temperature-compensated precise current sources for a loop compensation, and self-protection circuitry. Compared with a discrete MOSFET and PWM controller solution, the FSQ-series can reduce total cost, component count, size, and weight; while simultaneously increasing efficiency, productivity, and system reliability. This device provides a basic platform that is well suited for cost-effective designs of quasi-resonant switching flyback converters.
(Alternating Valley Switching)
! High-Efficiency through Minimum Voltage Switching ! Narrow Frequency Variation Range over Wide Load
and Input Voltage Variation Power Consumption
! Advanced Burst-Mode Operation for Low Standby ! Simple Scheme for Sync Voltage Detection ! Pulse-by-Pulse Current Limit ! Various Protection functions: Overload Protection
! ! ! !
(OLP), Over-Voltage Protection (OVP), Abnormal Over-Current Protection (AOCP), Internal Thermal Shutdown (TSD) with Hysteresis, Output Short Protection (OSP) Under-Voltage Lockout (UVLO) with Hysteresis Internal Startup Circuit Internal High-Voltage Sense FET (650V) Built-in Soft-Start (17.5ms)
Applications
! Power Supply for LCD TV and Monitor, VCR, SVR,
STB, and DVD & DVD Recorder
! Adapter
Related Resources
Visit http://www.fairchildsemi.com/apnotes/ for:
! AN-4134: Design Guidelines for Offline Forward ! !
! ! ! !
!
Converters Using Fairchild Power Switch (FPSTM) AN-4137: Design Guidelines for Offline Flyback Converters Using Fairchild Power Switch (FPSTM) AN-4140: Transformer Design Consideration for Offline Flyback Converters Using Fairchild Power Switch (FPSTM) AN-4141: Troubleshooting and Design Tips for Fairchild Power Switch (FPSTM) Flyback Applications AN-4145: Electromagnetic Compatibility for Power Converters AN-4147: Design Guidelines for RCD Snubber of Flyback AN-4148: Audible Noise Reduction Techniques for Fairchild Power Switch Fairchild Power Switch(FPSTM) Applications AN-4150: Design Guidelines for Flyback Converters Using FSQ-Series Fairchild Power Switch (FPSTM)
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Ordering Information
Maximum Output Power(1) Product Number PKG.(5) Operating Temp. Current RDS(ON) Limit Max. 230VAC15%(2) Adapter(3)
80W
85-265VAC Adapter(3)
48W
Open Frame(4)
90W
Open Frame(4)
70W
Replaces Devices
FSCM0765R FSDM0765RB
FSQ0765RQWDTU TO-220F-6L -25 to +85C
3.5A
1.6
Notes: 1. The junction temperature can limit the maximum output power. 2. 230VAC or 100/115VAC with doubler. 3. Typical continuous power in a non-ventilated enclosed adapter measured at 50C ambient temperature. 4. Maximum practical continuous power in an open-frame design at 50C ambient. 5. For Fairchild's definition of "green" Eco Status, please visit: http://www.fairchildsemi.com/company/green/rohs_green.html. Eco Status: RoHS.
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 2
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Application Diagram
VO
AC IN Vstr PWM FB VCC Drain
Sync
GND
FSQ0765R Rev.00
Figure 1. Typical Flyback Application
Internal Block Diagram
Sync 5 Vstr 6 VCC 3 Drain 1
AVS
0.35/0.55 VBurst IFB 3R R tON < tOSP after SS SoftStart
OSC
Vref
VCC
Vref Idelay
VCC good 8V/12V
FB 4
PWM LEB 250ns
SQ RQ Gate driver
VOSP
LPF AOCP
VSD LPF VOVP
TSD
S
Q
2
VOCP (1.1V)
GND
RQ
VCC good FSQ0765R Rev.00
Figure 2. Internal Block Diagram
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 3
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Pin Configuration
6. Vstr 5. Sync 4. FB 3. VCC 2. GND 1. Drain
FSQ0765R Rev.00
Figure 3. Pin Configuration (Top View)
Pin Definitions
Pin #
1 2 3
Name
Drain GND VCC
Description
SenseFET drain. High-voltage power SenseFET drain connection. Ground. This pin is the control ground and the SenseFET source. Power Supply. This pin is the positive supply input. This pin provides internal operating current for both startup and steady-state operation. Feedback. This pin is internally connected to the inverting input of the PWM comparator. The collector of an opto-coupler is typically tied to this pin. For stable operation, a capacitor should be placed between this pin and GND. If the voltage of this pin reaches 6V, the overload protection triggers, which shuts down the FPS. Sync. This pin is internally connected to the sync-detect comparator for quasi-resonant switching. In normal quasi-resonant operation, the threshold of the sync comparator is 1.2V/1.0V. Startup. This pin is connected directly, or through a resistor, to the high-voltage DC link. At start-up, the internal high-voltage current source supplies internal bias and charges the external capacitor connected to the VCC pin. Once VCC reaches 12V, the internal current source is disabled. It is not recommended to connect Vstr and Drain together.
4
FB
5
Sync
6
Vstr
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 4
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
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. TA = 25C, unless otherwise specified.
Symbol
Vstr VDS VCC VFB VSync IDM ID EAS PD TJ TA TSTG ESD Vstr Pin Voltage Drain Pin Voltage Supply Voltage
Parameter
Min.
500 650
Max.
Unit
V V
20 -0.3 -0.3 TC = 25C TC = 100C 13.0 13.0 14.4 3.6 2.28 570 45 Internally limited -25 -55 Human Body Model Charged Device Model 2 2 +85 +150
V V V A A mJ W C C C kV
Feedback Voltage Range Sync Pin Voltage Drain Current Pulsed Continuous Drain Current(6) Single Pulsed Avalanche Energy(7) Total Power Dissipation(Tc=25C) Operating Junction Temperature Operating Ambient Temperature Storage Temperature Electrostatic Discharge Capability
Notes: 6. Repetitive rating: Pulse width limited by maximum junction temperature. 7. L=81mH, starting TJ=25C.
Thermal Impedance
TA = 25C unless otherwise specified.
Symbol
JA JC Junction-to-Ambient Thermal Junction-to-Case Thermal
Parameter
Resistance(8) Resistance(9)
Value
50 2.8
Unit
C/W C/W
Notes: 8. Free standing with no heat-sink under natural convection. 9. Infinite cooling condition - refer to the SEMI G30-88.
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 5
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Electrical Characteristics
TA = 25C unless otherwise specified.
Symbol
SENSEFET SECTION
BVDSS IDSS RDS(ON) COSS td(on) tr td(off) tf tON.MAX tB tW fS fS tAVS VAVS tSW IFB DMIN VSTART VSTOP tS/S VBURH VBURL Hysteresis
Parameter
Drain Source Breakdown Voltage Zero-Gate-Voltage Drain Current Drain-Source On-State Resistance Output Capacitance Turn-On Delay Time Rise Time Turn-Off Delay Time Fall Time Maximum On Time Blanking Time Detection Time Window Initial Switching Frequency Switching Frequency Variation(11) AVS Triggering Threshold(11) On Time Feedback Voltage TJ = 25C
Condition
VCC = 0V, ID = 100A VDS = 520V, VGS = 0V TJ = 25C, ID = 1.8A VGS = 0V, VDS = 25V, f = 1MHz
Min. Typ. Max. Unit
650 300 1.3 125 27 102 63 65 8.8 13.5 59.6 10.0 15.0 6.0 66.7 5 4.0 1.2 13.5 700 11 900 12 8 17.5 0.45 0.55 0.35 200 0.65 0.45 20.5 1100 0 13 9 7 75.8 10 11.2 16.5 1.6 V A pF ns ns ns ns s s s kHz % s V s A % V V ms V V mV
VDD = 325V, ID = 6.5A
CONTROL SECTION
TJ = 25C, Vsync = 5V TJ = 25C, Vsync = 0V -25C < TJ < 85C at VIN = 240VDC, Lm = 360H (AVS triggered when VAVS>spec & tAVSSwitching Time Variance by AVS(11) Feedback Source Current Minimum Duty Cycle UVLO Threshold Voltage Internal Soft-Start Time
BURST-MODE SECTION
Burst-Mode Voltages TJ = 25C, tPD = 200ns
(10)
0.25
Note: 10. Propagation delay in the control IC.
Continued on the following page...
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 6
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Electrical Characteristics (Continued)
TA = 25C unless otherwise specified.
Symbol
PROTECTION SECTION
ILIMIT VSD IDELAY tLEB tOSP VOSP tOSP_FB TSD Hys VSH1 VSL1 tsync VSH2 VSL2 VCLAMP VOVP tOVP
Parameter
Peak Current Limit Shutdown Feedback Voltage Shutdown Delay Current Leading-Edge Blanking Time(11) Threshold Time
Condition
TJ = 25C, di/dt = 460mA/s VCC = 15V VFB = 5V
Min. Typ. Max. Unit
3.08 5.5 4 3.50 6.0 5 250 1.2 1.8 2.0 125 2.0 2.5 140 60 1.0 0.8 4.3 4.0 0.0 7.4 1.0 1.2 1.0 230 4.7 4.4 0.4 8.0 1.7 1.4 1.2 5.1 4.8 0.8 8.6 2.4 3.0 155 1.4 3.92 6.5 6 A V A ns s V s C
Output Short Threshold Feedback Protection(11) Voltage
TJ = 25C OSP triggered when tONVOSP & lasts longer than tOSP_FB Feedback Blanking Time
Shutdown Temperature Thermal Shutdown(11) Hysteresis
SYNC SECTION
Sync Threshold Voltage 1 Sync Delay Time(11)(12) Sync Threshold Voltage 2 Low Clamp Voltage Over-Voltage Threshold Voltage Protection Blanking Time(11) Operating Supply Current (Control Part Only) Start Current Startup Charging Current Minimum VSTR Supply Voltage VCC = 15V, VFB = 2V ISYNC_MAX = 800A ISYNC_MIN = 50A VCC = 15V, VFB=2V VCC = 15V, VFB = 2V V ns V V V s
TOTAL DEVICE SECTION
IOP ISTART ICH VSTR VCC = 13V, VFB=0V VCC = 10V (before VCC reaches VSTART) VCC = 0V, VSTR = minimum 50V 1 350 0.65 3 450 0.85 26 5 550 1.00 mA A mA V
Notes: 11. Guaranteed by design, but not tested in production. 12. Includes gate turn-on time.
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 7
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Comparison Between FSDM0x65RNB and FSQ-Series
Function
Operation Method
FSDM0x65RE
Constant Frequency PWM Frequency Modulation
FSQ-Series
Quasi-Resonant Operation
FSQ-Series Advantages
! Improved efficiency by valley switching ! Reduced EMI noise ! Reduced components to detect valley point
EMI Reduction
! Valley switching Reduce EMI Noise ! Inherent frequency modulation ! Alternate valley switching
Hybrid Control Burst-Mode Operation Strong Protections TSD Burst-Mode Operation OLP, OVP 145C without Hysteresis
CCM or AVS Based on Load ! Improves efficiency by introducing hybrid control and Input Condition Advanced Burst-Mode Operation OLP, OVP, AOCP, OSP 140C with 60C Hysteresis
! Improved standby power by AVS in burst-mode ! Improved reliability through precise AOCP ! Improved reliability through precise OSP ! Stable and reliable TSD operation ! Converter temperature range
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Typical Performance Characteristics
These characteristic graphs are normalized at TA= 25C.
Normalized
1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 4. Operating Supply Current (IOP) vs. TA
Temperature [C]
Figure 5. UVLO Start Threshold Voltage (VSTART) vs. TA
Normalized
1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 6. UVLO Stop Threshold Voltage (VSTOP) vs. TA
Temperature [C]
Figure 7. Startup Charging Current (ICH) vs. TA
Normalized
1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 8. Initial Switching Frequency (fSW) vs. TA
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Temperature [C]
Figure 9. Maximum On Time (tON.MAX) vs. TA
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Typical Performance Characteristics (Continued)
These characteristic graphs are normalized at TA= 25C.
Normalized
1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 10. Blanking Time (tB) vs. TA
Temperature [C]
Figure 11. Feedback Source Current (IFB) vs. TA
Normalized
1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 12. Shutdown Delay Current (IDELAY) vs. TA
Temperature [C]
Figure 13. Burst-Mode High Threshold Voltage (Vburh) vs. TA
Normalized
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 14. Burst-Mode Low Threshold Voltage (Vburl) vs. TA
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 10
Temperature [C]
Figure 15. Peak Current Limit (ILIM) vs. TA
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Typical Performance Characteristics (Continued)
These characteristic graphs are normalized at TA= 25C.
Normalized
1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 16. Sync High Threshold Voltage 1 (VSH1) vs. TA
Temperature [C]
Figure 17. Sync Low Threshold Voltage 1 (VSL1) vs. TA
Normalized
1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 18. Shutdown Feedback Voltage (VSD) vs. TA
Temperature [C]
Figure 19. Over-Voltage Protection (VOV) vs. TA
Normalized
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Normalized
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -25 0 25 50 75 100 125
Temperature [C]
Figure 20. Sync High Threshold Voltage 2 (VSH2) vs. TA
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 11
Temperature [C]
Figure 21. Sync Low Threshold Voltage 2 (VSL2) vs. TA
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Functional Description
1. Startup: At startup, an internal high-voltage current source supplies the internal bias and charges the external capacitor (Ca) connected to the VCC pin, as illustrated in Figure 22. When VCC reaches 12V, the FPSTM begins switching and the internal high-voltage current source is disabled. The FPS continues its normal switching operation and the power is supplied from the auxiliary transformer winding unless VCC goes below the stop voltage of 8V.
VDC
2.2 Leading-Edge Blanking (LEB): At the instant the internal SenseFET is turned on, a high-current spike usually occurs through the SenseFET, caused by primary-side capacitance and secondary-side rectifier reverse recovery. Excessive voltage across the Rsense resistor would lead to incorrect feedback operation in the current-mode PWM control. To counter this effect, the FPS employs a leading-edge blanking (LEB) circuit. This circuit inhibits the PWM comparator for a short time (tLEB) after the SenseFET is turned on.
VCC Idelay Vref IFB
OSC
VO
VFB
FOD817A
4
CB
SenseFET
Ca
D1
D2 + VFB*
3R Gate driver
R
3
VCC
6 ICH
Vstr
KA431
-
VSD
OLP
Rsense
8V/12V
VCC good
Vref
FSQ0765R Rev. 00
Figure 23. Pulse-Width-Modulation (PWM) Circuit
Internal Bias
FSQ0765R Rev.00
Figure 22. Startup Circuit 2. Feedback Control: FPS employs current-mode control, as shown in Figure 23. An opto-coupler (such as the FOD817A) and shunt regulator (such as the KA431) are typically used to implement the feedback network. Comparing the feedback voltage with the voltage across the Rsense resistor makes it possible to control the switching duty cycle. When the reference pin voltage of the shunt regulator exceeds the internal reference voltage of 2.5V, the opto-coupler LED current increases, pulling down the feedback voltage and reducing the duty cycle. This typically happens when the input voltage is increased or the output load is decreased. 2.1 Pulse-by-Pulse Current Limit: Because currentmode control is employed, the peak current through the SenseFET is limited by the inverting input of PWM comparator (VFB*), as shown in Figure 23. Assuming that the 0.9mA current source flows only through the internal resistor (3R + R = 2.8k), the cathode voltage of diode D2 is about 2.5V. Since D1 is blocked when the feedback voltage (VFB) exceeds 2.5V, the maximum voltage of the cathode of D2 is clamped at this voltage, clamping VFB*. Therefore, the peak value of the current through the SenseFET is limited.
3. Synchronization: The FSQ-series employs a quasiresonant switching technique to minimize the switching noise and loss. The basic waveforms of the quasiresonant converter are shown in Figure 24. To minimize the MOSFET's switching loss, the MOSFET should be turned on when the drain voltage reaches its minimum value, which is indirectly detected by monitoring the VCC winding voltage, as shown in Figure 24.
Vds
VRO VRO
VDC
Vsync
tF Vovp (8V)
1.2V 1.0V 230ns Delay MOSFET Gate
ON
ON
FSQ0765R Rev.00
Figure 24. Quasi-Resonant Switching Waveforms
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 12
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
The switching frequency is the combination of blank time (tB) and detection time window (tW). In case of a heavy load, the sync voltage remains flat after tB and waits for valley detection during tW. This leads to a low switching frequency not suitable for heavy loads. To correct this drawback, additional timing is used. The timing conditions are described in Figures 25, 26, and 27. When the Vsync remains flat higher than 4.4V at the end of tB that is tX, the next switching cycle starts after internal delay time from tX. In the second case, the next switching occurs on the valley when the Vsync goes below 4.4V within tB. Once Vsync detects the first valley within tB, the other switching cycle follows classical QRC operation.
tB=15s
tX
IDS
IDS
VDS ingnore
4.4V
Vsync
1.2V 1.0V
tB=15s
tX
internal delay
FSQ0765R Rev. 00
Figure 27. After Vsync Finds First Valley
IDS IDS
VDS
4.4V
Vsync
1.2V 1.0V
internal delay
FSQ0765R Rev. 00
Figure 25. Vsync > 4.4V at tX
tB=15s tX
4. Protection Circuits: The FSQ-series has several self-protective functions, such as Overload Protection (OLP), Abnormal Over-Current Protection (AOCP), Over-Voltage Protection (OVP), and Thermal Shutdown (TSD). All the protections are implemented as autorestart mode. Once the fault condition is detected, switching is terminated and the SenseFET remains off. This causes VCC to fall. When VCC falls down to the Under-Voltage Lockout (UVLO) stop voltage of 8V, the protection is reset and the startup circuit charges the VCC capacitor. When the VCC reaches the start voltage of 12V, normal operation resumes. If the fault condition is not removed, the SenseFET remains off and VCC drops to stop voltage again. In this manner, the auto-restart can alternately enable and disable the switching of the power SenseFET until the fault condition is eliminated. Because these protection circuits are fully integrated into the IC without external components, the reliability is improved without increasing cost.
VDS
Power on Fault occurs Fault removed
IDS
IDS
VDS
VCC
4.4V
Vsync
12V 8V
1.2V 1.0V
internal delay
FSQ0765R Rev. 00
FSQ0765R Rev. 00
Normal operation
Fault situation
Normal operation
t
Figure 26. Vsync < 4.4V at tX
Figure 28. Auto Restart Protection Waveforms
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
AOCP FSQ0765R Rev.00
Figure 30. Abnormal Over-Current Protection
4.3 Output-Short Protection (OSP): If the output is shorted, steep current with extremely high di/dt can flow through the SenseFET during the LEB time. Such a steep current brings high voltage stress on the drain of SenseFET when turned off. To protect the device from such an abnormal condition, OSP is included in the FSQseries. It is comprised of detecting VFB and SenseFET turn-on time. When the VFB is higher than 2V and the SenseFET turn-on time is lower than 1.2s, the FPS recognizes this condition as an abnormal error and shuts down PWM switching until VCC reaches Vstart again. An abnormal condition output short is shown in Figure 31.
MOSFET Drain Current Rectifier Diode Current Turn-off delay
VFB
6.0V
FSQ0765R Rev.00
Overload protection
VFB
0
2.5V t12= CFB*(6.0-2.5)/Idelay
Minimum turn-on time D 1.2us
Vo
0
t1
t2
t
Io
FSQ0765R Rev. 00
0
Figure 29. Overload Protection 4.2 Abnormal Over-Current Protection (AOCP): When the secondary rectifier diodes or the transformer pins are shorted, a steep current with extremely high di/dt can flow through the SenseFET during the LEB time. Even though the FSQ-series has overload protection, it is not enough to protect the FSQ-series in that abnormal case, since severe current stress is imposed on the SenseFET until OLP triggers. The FSQ-series has an internal AOCP circuit shown in Figure 30. When the gate turn-on signal is applied to the power SenseFET, the AOCP block is enabled and monitors the current through the sensing resistor. The voltage across the resistor is compared with a preset AOCP level. If the sensing resistor voltage is greater than the AOCP level, the set signal is applied to the latch, resulting in the shutdown of the SMPS.
Figure 31. Output Short Waveforms 4.4 Over-Voltage Protection (OVP): If the secondaryside feedback circuit malfunctions or a solder defect causes an opening in the feedback path, the current through the opto-coupler transistor becomes almost zero. Then, VFB climbs up in a similar manner to the overload situation, forcing the preset maximum current to be supplied to the SMPS until the overload protection triggers. Because more energy than required is provided to the output, the output voltage may exceed the rated voltage before the overload protection triggers, resulting in the breakdown of the devices in the secondary side. To prevent this situation, an OVP circuit is employed. In general, the peak voltage of the sync signal is proportional to the output voltage and the FSQ-series
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 14
+
-
4.1 Overload Protection (OLP): Overload is defined as the load current exceeding its normal level due to an unexpected abnormal event. In this situation, the protection circuit should trigger to protect the SMPS. However, even when the SMPS is in the normal operation, the overload protection circuit can be triggered during the load transition. To avoid this undesired operation, the overload protection circuit is designed to trigger only after a specified time to determine whether it is a transient situation or a true overload situation. Because of the pulse-by-pulse current limit capability, the maximum peak current through the SenseFET is limited, and therefore the maximum input power is restricted with a given input voltage. If the output consumes more than this maximum power, the output voltage (VO) decreases below the set voltage. This reduces the current through the optocoupler LED, which also reduces the opto-coupler transistor current, thus increasing the feedback voltage (VFB). If VFB exceeds 2.5V, D1 is blocked and the 5A current source starts to charge CB slowly up to VCC. In this condition, VFB continues increasing until it reaches 6V, when the switching operation is terminated, as shown in Figure 29. The delay time for shutdown is the time required to charge CFB from 2.5V to 6V with 5A. A 20 ~ 50ms delay time is typical for most applications.
3R
OSC PWM LEB 250ns
S R Q Q
Gate driver
R
R sense VOCP
2 GND
ILIM
output short occurs
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
uses a sync signal instead of directly monitoring the output voltage. If the sync signal exceeds 8V, an OVP is triggered, shutting down the SMPS. To avoid undesired triggering of OVP during normal operation, there are two points to be considered, which are depicted in Figure 32. One is at the peak voltage of the sync signal should be designed below 6V and the other is that the spike of the sync pin should be as low as possible; not to get longer than tOVP by decreasing the leakage inductance shown at VCC winding coil.
VVcc_coil &VCC Absolue max VCC (20V) VCC VVcc_coil
FSQ0765R Rev.00
6. Burst Operation: To minimize power dissipation in standby mode, the FPS enters burst-mode operation. As the load decreases, the feedback voltage decreases. As shown in Figure 33, the device automatically enters burst-mode when the feedback voltage drops below VBURL (350mV). At this point, switching stops and the output voltages start to drop at a rate dependent on standby current load. This causes the feedback voltage to rise. Once it passes VBURH (550mV), switching resumes. The feedback voltage then falls and the process repeats. Burst-mode operation alternately enables and disables switching of the power SenseFET, thereby reducing switching loss in standby mode.
VO
VOset
VDC
Improper OVP triggering Vsync
VOVP (8V) tOVP VSH2 (4.8V) tOVP
Npri NVcc
VFB
0.55V 0.35V
IDS
VCLAMP
VDS
Figure 32. OVP Triggering
time
4.5 Thermal Shutdown with Hysteresis (TSD): The SenseFET and the control IC are built in one package. This allows the control IC to detect abnormally high temperature of the SenseFET. If the temperature exceeds approximately 140C, the thermal shutdown triggers IC shutdown. The IC recovers its operation when the junction temperature decreases 60C from TSD temperature and VCC reaches startup voltage (Vstart). 5. Soft-Start: The FPS has an internal soft-start circuit that increases PWM comparator inverting input voltage with the SenseFET current slowly after it starts up. The typical soft-start time is 17.5ms. The pulse width to the power switching device is progressively increased to establish the correct working conditions for transformers, inductors, and capacitors. The voltage on the output capacitors is progressively increased with the intention of smoothly establishing the required output voltage. This mode helps prevent transformer saturation and reduces stress on the secondary diode during startup.
FSQ0765R Rev.00
t1
Switching disabled
t2 t3
Switching disabled
t4
Figure 33. Waveforms of Burst Operation 7. Switching Frequency Limit: To minimize switching loss and Electromagnetic Interference (EMI), the MOSFET turns on when the drain voltage reaches its minimum value in quasi-resonant operation. However, this causes switching frequency to increases at light load conditions. As the load decreases or input voltage increases, the peak drain current diminishes and the switching frequency increases. This results in severe switching losses at light-load condition, as well as intermittent switching and audible noise. These problems create limitations for the quasi-resonant converter topology in a wide range of applications.
To overcome these problems, FSQ-series employs a frequency-limit function, as shown in Figures 34 and 35. Once the SenseFET is turned on, the next turn-on is prohibited during the blanking time (tB). After the blanking time, the controller finds the valley within the detection time window (tW) and turns on the MOSFET, as shown in Figures 34 and Figure 35 (Cases A, B, and C).
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 15
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FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
If no valley is found during tW, the internal SenseFET is forced to turn on at the end of tW (Case D). Therefore, the devices have a minimum switching frequency of 48kHz and a maximum switching frequency of 67kHz.
tsmax=21s
IDS
IDS A
8. AVS (Alternating Valley Switching): Due to the quasi-resonant operation with limited frequency, the switching frequency varies depending on input voltage, load transition, and so on. At high input voltage, the switching on time is relatively small compared to low input voltage. The input voltage variance is small and the switching frequency modulation width becomes small. To improve the EMI performance, AVS is enabled when input voltage is high and the switching on time is small.
Internally, quasi-resonant operation is divided into two categories; one is first-valley switching and the other is second-valley switching after blanking time. In AVS, two successive occurrences of first-valley switching and the other two successive occurrences of second-valley switching is alternatively selected to maximize frequency modulation. As depicted in Figure 35, the switching frequency hops when the input voltage is high. The internal timing diagram of AVS is described in Figure 36.
fs
67kHz 59kHz 53kHz 48kHz
tB=15s ts IDS IDS
B tB=15s ts
Assume the resonant period is 2us
IDS
IDS C
1 15s 1 17 s 1 19 s 1 21s
AVS trigger point
tB=15s ts
Constant frequency CCM
Variable frequency within limited range DCM AVS region
IDS
IDS
D
C
B
A
Vin
FSQ0765R Rev.00
tB=15s
D tW=6s
FSQ0765R Rev. 00
Figure 35. Switching Frequency Range
tsmax=21s
Figure 34. QRC Operation with Limited Frequency
Vgate
Synchronize Synchronize
Vgate continued 2 pulses 1st valley switching fixed
Vgate continued another 2 pulses 2nd valley switching fixed fixed fixed
Vgate continued 2 pulses 1st valley switching fixed
GateX2 One-shot AVS
fixed triggering
1st or 2nd is depend on GateX2
de-triggering triggering
1st or 2nd is dependent on GateX2
VDS
tB
tB
tB
tB
tB
tB
GateX2: Counting Vgate every 2 pulses independent on other signals . FSQ0765R Rev. 00
1st valley- 2nd valley frequency modulation. Modulation frequency is approximately 17kHz.
Figure 36. Alternating Valley Switching (AVS)
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 16 www.fairchildsemi.com
FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
PCB Layout Guide
Due to the combined scheme, FPS shows better noise immunity than conventional PWM controller and MOSFET discrete solution. Further more are internal drain current sense eliminates the possibility of noise generation caused by a sensing resistor. There are some recommendations for PCB layout to enhance noise immunity and suppress natural noise inevitable in powerhandling components. There are typically two grounds in the conventional SMPS: power ground and signal ground. The power ground is the ground for primary input voltage and power, while the signal ground is ground for PWM controller. In FPS, those two grounds share the same pin, GND. Normally the separate grounds do not share the same trace and meet only at one point, the GND pin. More, wider patterns for both grounds are good for large currents by decreasing resistance. Capacitors at the VCC and FB pins should be as close as possible to the corresponding pins to avoid noise from the switching device. Sometimes Mylar(R) or ceramic capacitors with electrolytic for VCC are better for smooth operation. The ground of these capacitors needs to connect to the signal ground (not power ground). The cathode of the snubber diode should be close to the drain pin to minimize stray inductance. The Y-capacitor between primary and secondary should be directly connected to the power ground of DC link to maximize surge immunity. Because the voltage range of feedback and sync line is small, it is affected by the noise of the drain pin. Those traces should not draw across or close to the drain line. When the heat sink is connected to the ground, it should be connected to the power ground. If possible, avoid using jumper wires for power ground and drain.
Figure 37. Recommended PCB Layout
Mylar(R) is a registered trademark of DuPont Teijin Films.
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 17
www.fairchildsemi.com
FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
Package Dimensions
TO-220F-6L (Forming)
MKT-TO220A06revB
Figure 38. 6-Lead, TO-220 Package
Package drawings are provided as a service to customers considering Fairchild components. Drawings may change in any manner without notice. Please note the revision and/or date on the drawing and contact a Fairchild Semiconductor representative to verify or obtain the most recent revision. Package specifications do not expand the terms of Fairchild's worldwide terms and conditions, specifically the warranty therein, which covers Fairchild products. Always visit Fairchild Semiconductor's online packaging area for the most recent package drawings: http://www.fairchildsemi.com/packaging/.
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 18
www.fairchildsemi.com
FSQ0765RQ -- Green-Mode Farichild Power Switch (FPSTM) for Quasi-Resonant Operation
(c) 2008 Fairchild Semiconductor Corporation FSQ0765RQ Rev. 1.0.1 19
www.fairchildsemi.com

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