FAN7621 PFM Controller for Half-Bridge Resonant Converters

Transcription

1 July 200 FAN762 PFM Controller for HalfBridge Resonant Converters Features Variable Frequency Control with 50% Duty Cycle for Halfbridge Resonant Converter Topology High Efficiency through Zero Voltage Switching (ZVS) Fixed Dead Time (350ns) Up to 300kHz Operating Frequency Pulse Skipping for Frequency Limit (Programmable) at LightLoad Condition Remote On/Off Control Using Pin Protection Functions: OverVoltage Protection (OVP), Overload Protection (OLP), OverCurrent Protection (OCP), Abnormal OverCurrent Protection (AOCP), Internal Thermal Shutdown (TSD) Applications PDP and LCD TVs Desktop PCs and Servers Adapters Telecom Power Supplies Video Game Consoles Description The FAN762 is a pulse frequency modulation controller for highefficiency halfbridge resonant converters. Offering everything necessary to build a reliable and robust resonant converter, the FAN762 simplifies designs and improves productivity, while improving performance. The FAN762 includes a highside gatedrive circuit, an accurate current controlled oscillator, frequency limit circuit, softstart, and builtin protection functions. The highside gatedrive circuit has a commonmode noise cancellation capability, which guarantees stable operation with excellent noise immunity. Using the zerovoltageswitching (ZVS) technique dramatically reduces the switching losses and efficiency is significantly improved. The ZVS also reduces the switching noise noticeably, which allows a smallsized Electromagnetic Interference (EMI) filter. The FAN762 can be applied to various resonant converter topologies; such as series resonant, parallel resonant, and LLC resonant converters. Related Resources AN45 Halfbridge LLC Resonant Converter Design using FSFRseries Fairchild Power Switch (FPS TM ) FAN762 PFM Controller for HalfBridge Resonant Converters Ordering Information Part Number Operating Junction Temperature Package Packaging Method FAN762N 6Lead, Dual Inline Package (DIP) Tube FAN762SJ 40 C ~ 30 C 6Lead, SmallOutline Package (SOP) Tube FAN762SJX 6Lead, SmallOutline Package (SOP) Tape & Reel FAN762 Rev..0.3

2 Application Circuit Diagram V IN C DL Block Diagram R T 8 V REF I CTC V CC R sense RT CS LVcc FAN762 SG HV CC HO CTR LO PG Cr L lk Lm Np Ns Ns D D2 KA43 Figure. Typical Application Circuit (LLC Resonant HalfBridge Converter).3 / 4.5V 2 Time Delay good Internal Bias V REF HighSide Gate Drive C F V O R F HV CC 3 HO 2 CTR FAN762 PFM Controller for HalfBridge Resonant Converters 6 I OLP I CTC 2I CTC 2V 3V V S R F/F Q Q Counter (/4) 350ns LevelShift HV CC good 8.7 / 9.2V 0.4 / 0.6 V Time Delay 350ns Balancing Delay LowSide Gate Drive 4 LO OLP 5V 23 V OVP good S R Q Q Autorestart Protection Q Q S R Latch Protection Shutdown without delay 50ns Delay <5V TSD V VAOCP 6 PG Delay.5µs V OCP 0.58 V 0 SG Figure 2. Internal Block Diagram 9 CS FAN762 Rev

3 Pin Configuration Pin Definitions () HV CC (2) CTR (3) HO (4) NC (5) NC (6) (7) NC (8) R T FAN762 Figure 3. Package Diagram Pin # Name Description PG (6) NC (5) LO (4) NC (3) (2) NC () SG (0) CS (9) HV CC This is the supply voltage of the highside gatedrive circuit IC. 2 CTR This is the drain of the lowside MOSFET. Typically, a transformer is connected to this pin. 3 HO This is the highside gate driving signal. 4 NC No connection. 5 NC No connection. 6 This pin is for a protection and enabling/disabling the controller. When the voltage of this pin is above 0.6V, the IC operation is enabled. When the voltage of this pin drops below 0.4V, gate drive signals for both MOSFETs are disabled. When the voltage of this pin increases above 5V, protection is triggered. 7 NC No connection. 8 R T This pin programs the switching frequency. Typically, an optocoupler is connected to control the switching frequency for the output voltage regulation. 9 CS This pin senses the current flowing through the lowside MOSFET. Typically, negative voltage is applied on this pin. 0 SG This pin is the control ground. NC No connection. 2 This pin is the supply voltage of the control IC. 3 NC No connection. 4 LO This is the lowside gate driving signal. 5 NC No connection. 6 PG This pin is the power ground. This pin is connected to the source of the lowside MOSFET. FAN762 PFM Controller for HalfBridge Resonant Converters FAN762 Rev

4 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. T A=25 C unless otherwise specified. Symbol Parameter Min. Max. Unit V HO HighSide Gate Driving Voltage V CTR0.3 HV CC V V LO LowSide Gate Driving Voltage 0.3 LowSide Supply Voltage V HV CC to V CTR HighSide V CC Pin to Center Voltage V V CTR Center Voltage V V Control Pin Input Voltage 0.3 V V CS Current Sense (CS) Pin Input Voltage V V RT R T Pin Input Voltage V dv CTR/dt Allowable Center Voltage Slew Rate 50 V/ns P D T J Total Power Dissipation 6DIP.56 W 6SOP.3 W Maximum Junction Temperature () 50 Recommended Operating Junction Temperature () T STG Storage Temperature Range C Note:. The maximum value of the recommended operating junction temperature is limited by thermal shutdown. Thermal Impedance C FAN762 PFM Controller for HalfBridge Resonant Converters Symbol Parameter Value Unit θ JA JunctiontoAmbient Thermal Impedance 6DIP 80 6SOP 0 ºC/W FAN762 Rev

5 Electrical Characteristics T A=25 C and =7V unless otherwise specified. Symbol Parameter Test Conditions Min. Typ. Max. Unit Supply Section I LK Offset Supply Leakage Current HV CC=V CTR 50 μa I QHV CC Quiescent HV CC Supply Current (HV CCUV) 0.V μa I Q Quiescent Supply Current (UV) 0.V μa I OHV CC I O UVLO Section Operating HV CC Supply Current (RMS Value) Operating Supply Current (RMS Value) f OSC=00kHz, V > 0.6V, C Load=nF 5 8 ma No Switching, V < 0.4V μa f OSC=00kHz, V > 0.6V, C Load=nF 6 9 ma No Switching, V < 0.4V 2 4 ma UV Supply UnderVoltage Positive Going Threshold ( Start) V UV Supply UnderVoltage Negative Going Threshold ( Stop) V UVH Supply UnderVoltage Hysteresis 3.2 V HV CCUV HV CC Supply UnderVoltage Positive Going Threshold (HV CC Start) V HV CCUV HV CC Supply UnderVoltage Negative Going Threshold (HV CC Stop) V HV CCUVH HV CC Supply UnderVoltage Hysteresis 0.5 V Oscillator & Feedback Section V DIS Control Pin Disable Threshold Voltage V V EN Control Pin Enable Threshold Voltage V V RT VI Converter Threshold Voltage V f OSC Output Oscillation Frequency R T=5.2kΩ khz DC Output Duty Cycle % f SS Internal SoftStart Initial Frequency f SS=f OSC40kHz, R T=5.2kΩ 40 khz t SS Internal SoftStart Time ms Output Section I source Peak Sourcing Current HV CC=7V ma I sink Peak Sinking Current HV CC=7V ma t r Rising Time C Load=nF, HV CC=7V 65 ns t f Falling Time 35 ns V HOH V HOL V LOH V LOL High Level of HighSide Gate Driving Signal (V HVCCV HO) Low Level of HighSide Gate Driving Signal High Level of HighSide Gate Driving Signal (V LVCCV LO) Low Level of HighSide Gate Driving Signal I O=20mA.0 V 0.6 V.0 V 0.6 V FAN762 PFM Controller for HalfBridge Resonant Converters FAN762 Rev

6 Electrical Characteristics (Continued) T A=25 C and =7V unless otherwise specified. Symbol Parameter Test Conditions Min. Typ. Max. Unit Protection Section I OLP OLP Delay Current V =4V μa V OLP OLP Protection Voltage V > 3.5V V V OVP OverVoltage Protection > 2V V V AOCP AOCP Threshold Voltage V t BAO AOCP Blanking Time 50 ns V OCP OCP Threshold Voltage V t BO OCP Blanking Time (2) μs t DA Delay Time (LowSide) Detecting from (2) ns V AOCP to Switch Off T SD Thermal Shutdown Temperature (2) C I SU V PRSET Protection Latch Sustain Supply Current Protection Latch Reset Supply Voltage DeadTime Control Section =7.5V μa 5 V D T Dead Time 350 ns Note: 2. These parameters, although guaranteed, are not tested in production. FAN762 PFM Controller for HalfBridge Resonant Converters FAN762 Rev

7 Typical Performance Characteristics These characteristic graphs are normalized at T A=25ºC Figure 4. LowSide MOSFET Duty Cycle vs. Temperature Figure 5. Switching Frequency vs. Temperature..05 FAN762 PFM Controller for HalfBridge Resonant Converters Figure 6. HighSide V CC (HV CC) Start vs. Temperature Figure 7. HighSide V CC (HV CC) Stop vs. Temperature Figure 8. LowSide V CC () Start vs. Temperature Figure 9. LowSide V CC () Stop vs. Temperature FAN762 Rev

8 Typical Performance Characteristics (Continued) These characteristic graphs are normalized at T A=25ºC Figure 0. OLP Delay Current vs. Temperature Figure. OLP Protection Voltage vs. Temperature FAN762 PFM Controller for HalfBridge Resonant Converters Figure 2. OVP Voltage vs. Temperature Figure 3. R T Voltage vs. Temperature Figure 4. Pin Enable Voltage vs. Temperature Figure 5. OCP Voltage vs. Temperature FAN762 Rev

9 Functional Description. Basic Operation FAN762 is designed to drive highside and lowside MOSFETs complementarily with 50% duty cycle. A fixed dead time of 350ns is introduced between consecutive transitions, as shown in Figure 6. Highside MOSFET gate drive Lowside MOSFET gate drve Dead t ime Figure 6. MOSFETs Gate Drive Signal 2. Internal Oscillator FAN762 employs a currentcontrolled oscillator, as shown in Figure 7. Internally, the voltage of R T pin is regulated at 2V and the charging / discharging current for the oscillator capacitor, C T, is obtained by copying the current flowing out of R T pin (I CTC) using a current mirror. Therefore, the switching frequency increases as I CTC increases. V REF I CTC I CTC 2I CTC C T 3V time R Q V S F/F Q Gain.8 f min f normal f max f ISS Softsta rt Frequency (khz) Figure 8. Resonant Converter Typical Gain Curve V CC RT R max R min R SS C SS HV CC FAN762 HO CTR LO CS FAN762 PFM Controller for HalfBridge Resonant Converters 2V R Counter T (/4) 8 Gate drive SG PG Figure 7. Current Controlled Oscillator 3. Frequency Setting Figure 8 shows the typical voltage gain curve of a resonant converter, where the gain is inversely proportional to the switching frequency in the ZVS region. The output voltage can be regulated by modulating the switching frequency. Figure 9 shows the typical circuit configuration for R T pin, where the optocoupler transistor is connected to the R T pin to modulate the switching frequency. R sense Figure 9. Frequency Control Circuit The minimum switching frequency is determined as: min 5.2kΩ f = 00( khz) () R min Assuming the saturation voltage of optocoupler transistor is 0.2V, the maximum switching frequency is determined as: max 5.2kΩ 4.68kΩ f = ( ) 00( khz) (2) R R min max To prevent excessive inrush current and overshoot of output voltage during startup, increase the voltage gain of the resonant converter progressively. Since the voltage gain of the resonant converter is inversely proportional to the switching frequency, the softstart is FAN762 Rev

10 implemented by sweeping down the switching frequency from an initial high frequency (f ISS ) until the output voltage is established. The softstart circuit is made by connecting RC series network on the R T pin, as shown in Figure 9. FAN762 also has an internal softstart for 3ms to reduce the current overshoot during the initial cycles, which adds 40kHz to the initial frequency of the external softstart circuit, as shown in Figure 20. The initial frequency of the softstart is given as: ISS 5.2kΩ 5.2kΩ f = ( ) ( khz) (3) R R min SS It is typical to set the initial (softstart) frequency of two ~ three times the resonant frequency (f O) of the resonant network. The softstart time is three to four times the RC time constant. The RC time constant is as follows: T = R C (4) SS SS SS f s f ISS 40kHz Control loop take over time Figure 20. Frequency Sweeping of SoftStart 4. Control Pin The FAN762 has a control pin for protection, cycle skipping, and remote on/off. Figure 2 shows the internal block diagram for control pin. V CC R max R min R SS C SS RT CS FAN762 SG HV CC CTR 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 FAN762 can be shut down by pulling down the control pin voltage, as shown in Figure 23. R and C are used to ensure softstart when switching resumes. FAN762 R C HO LO PG OP Main Off Main Output FAN762 PFM Controller for HalfBridge Resonant Converters 6 IOLP 0.4 / 0.6V R T R min Aux Output OLP 5V 23V OVP good S R Q Q Autorestart protection Stop Switching Figure 2. 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: FAN762 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 pulseskipping, the control pin should be connected to the optocoupler collector pin. The frequency that causes pulse skipping is given as: SKIP 5.2k 4.6k = x00( khz) (5) R min R max OP Figure 23. Remote On / Off Circuit 5. Protection Circuits The FAN762 has several selfprotective functions, such as Overload Protection (OLP), OverCurrent Protection (OCP), Abnormal OverCurrent Protection (AOCP), OverVoltage Protection (OVP), and Thermal Shutdown (TSD). OLP, OCP, and OVP are autorestart mode protections; while AOCP and TSD are latchmode protections, as shown in Figure AutoRestart Mode Protection: Once a fault condition is detected, switching is terminated and the MOSFETs remain off. When falls to the stop voltage of.3v, the protection is reset. FAN762 FAN762 Rev

11 resumes normal operation when reaches the start voltage of 4.5V. 5.2 LatchMode Protection: Once this protection is triggered, switching is terminated and the gate output signals remain off. The latch is reset only when is discharged below 5V. OCP OLP / 4V Figure 24. Protection Blocks 5.3 Current Sensing Using Resistor: FAN762 senses drain current as a negative voltage, as shown in Figure 25 and Figure 26. Halfwave sensing allows low power dissipation in the sensing resistor, while fullwave sensing has less switching noise in the sensing signal. C DL 2 OVP good 20k V CS RT CS FAN762 LVCC good HV CC HO CTR LO VRE F Autorestart protection S Q R Q F/F I ds In ternal Bias La tch protection Q S Q R F/F Shutdown AOCP TSD <5V 5.4 Current Sensing Using Resonant Capacitor Voltage: For highpower 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 (V pp cr ) is proportional to the resonant current in the primary side (I pp p ) as: p p p p I p VCr = (6) 2π f C s r To minimize power dissipation, a capacitive voltage divider is generally used for capacitor voltage sensing, as shown in Figure 27. I p V Cr C DL RT FAN762 CS SG V sense HV CC HO CTR LO PG C sense 00 I p C B C r V Cr pp FAN762 PFM Controller for HalfBridge Resonant Converters SG PG V CS V sense pk Vsense C B = V p p VCr Csense CB 2 pk sense = V R sense I ds V sense pk V Figure 25. HalfWave Sensing pk Vsense I ds t Delay =R d C d C DL V CS RT V CS HV CC FAN762 HO CTR LO CS SG PG R sense I ds Figure 26. FullWave Sensing Figure 27. Current Sensing Using Resonant Capacitor Voltage 5.5 OverCurrent Protection (OCP): When the sensing pin voltage drops below 0.6V, OCP is triggered and the MOSFETs remain off. This protection has a shutdown time delay of.5µs to prevent premature shutdown during startup. 5.6 Abnormal OverCurrent 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 V. This protection is latch mode and reset when is pulled down below 5V. FAN762 Rev..0.3

12 5.7 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 power supply. However, even when the power supply is in the normal condition, the overload situation can occur during the load transition. To avoid premature triggering of protection, the overload protection circuit should be designed to trigger only after a specified time to determine whether it is a transient situation or a true overload situation. Figure 27 shows a typical overload protection circuit. By sensing the resonant capacitor voltage on the control pin, the overload protection can be implemented. Using RC time constant, shutdown delay can be also introduced. The voltage obtained on the control pin is given as: V CB = V 2( C C ) B sense p p Cr where V Cr pp is the amplitude of the resonant capacitor voltage. 5.8 OverVoltage Protection (OVP): When the reaches 23V, OVP is triggered. This protection is used when auxiliary winding of the transformer to supply V CC to the controller is utilized. 5.9 Thermal Shutdown (TSD): If the temperature of the junction exceeds approximately 30 C, the thermal shutdown triggers. (7) 6. PCB Layout Guideline Duty imbalance problems may occur due to the radiated noise from main transformer, the inequality of the secondaryside leakage inductances of main transformer, and so on. Among them, it is one of the dominant reasons that the control components in the vicinity of R T pin are enclosed by the primary current flow pattern on PCB layout. The direction of the magnetic field on the components caused by the primary current flow is changed when the highandlow side MOSFET turns on by turns. The magnetic fields with opposite direction from each other induce a current through, into, or out of the R T pin, which makes the turnon duration of each MOSFET different. It is strongly recommended to separate the control components in the vicinity of R T pin from the primary current flow pattern on PCB layout. Figure 28 shows an example for the dutybalanced case. The yellow and blue lines show the primary current flows when the lowerside and higherside MOSFETs turns on, respectively. The primary current does not enclose any component of controller. It is helpful to reduce the duty imbalance to make the loop configured between pin and optocoupler as small as possible, as shown in the red line in Figure 28. FAN762 PFM Controller for HalfBridge Resonant Converters Figure 28. Example for Duty Balancing FAN762 Rev

13 Typical Application Circuit (HalfBridge LLC Resonant Converter) Application Device Input Voltage Range Rated Output Power LCD TV FAN V DC (340~400V DC) Features High efficiency ( >94% at 400V DC input) Reduced EMI noise through zerovoltageswitching (ZVS) Enhanced system reliability with various protection functions FAN W Output Voltage (Rated Current) 24V8.3A FAN762 PFM Controller for HalfBridge Resonant Converters Figure 29. Typical Application Circuit FAN762 Rev

14 Typical Application Circuit (Continued) Usually, LLC resonant converters require large leakage inductance value. To obtain a large leakage inductance, sectional winding method is used. Core: EC35 (Ae=06 mm 2 ) Bobbin: EC35 (Horizontal) Transformer Model Number: SNX N p EC N s2 N s Figure 30. Transformer Construction Pins (S F) Wire Turns Note N p φ 88 (Litz Wire) 36 N s φ 234 (Litz Wire) 4 Bifilar Winding N s φ 234 (Litz Wire) 4 Bifilar Winding FAN762 PFM Controller for HalfBridge Resonant Converters Pins Specifications Remark PrimarySide Inductance (L p) μH ± 0% 00kHz, V PrimarySide Effective Leakage (L r) 26 0μH ± 0% Short one of the secondary windings For more detailed information regarding the transformer, visit or contact sales@santronicsusa.com or (Sunnyvale, California USA). FAN762 Rev

15 Physical Dimensions 2.54 A (0.40) TOP VIEW MIN MAX A SIDE VIEW FAN762 PFM Controller for HalfBridge Resonant Converters NOTES: UNLESS OTHERWISE SPECIFIED A THIS PACKAGE FORMS TO JEDEC MS00 VARIATION BB B) ALL DIMENSIONS ARE IN MILLIMETERS. C) DIMENSIONS ARE EXCLUSIVE OF BURRS, MOLD FLASH, AND TIE BAR PROTRUSIONS D) FORMS TO ASME Y4.5M994 E) DRAWING FILE NAME: N6EREV Figure 3. 6Lead Dual Inline Package (DIP) 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: FAN762 Rev

16 Physical Dimensions FAN762 PFM Controller for HalfBridge Resonant Converters Figure 32. 6Lead Small Outline Package (SOP) 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: FAN762 Rev

17 FAN762 Rev FAN762 PFM Controller for HalfBridge Resonant Converters