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july 2010 ? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 FAN7621S ? pfm controller for half-bridge resonant converters FAN7621S pfm controller for half-bridge resonant converters features ? variable frequency control with 50% duty cycle for half-bridge resonant converter topology ? high efficiency through zero voltage switching (zvs) ? fixed dead time: 350ns ? up to 300khz operating frequency ? auto-restart operation for all protections with an external lv cc ? protection functions: over-voltage protection (ovp), over-current protection (ocp), abnormal over-current protection (aocp), internal thermal shutdown (tsd) applications ? pdp and lcd tvs ? desktop pcs and servers ? adapters ? telecom power supplies ? video game consoles description the FAN7621S is a pulse frequency modulation controller for high-efficiency half-bridge resonant converters. offering everything necessary to build a reliable and robust resonant converter, the FAN7621S simplifies designs and improves productivity, while improving performance. the FAN7621S includes a high- side gate-drive circuit, an accurate current-controlled oscillator, frequency-limit circuit, soft-start, and built-in protection functions. the high-side gate-drive circuit has a common-mode noise cancellation capability, which guarantees stable operation with excellent noise immunity. using the zero-voltage-switching (zvs) technique dramatically reduces the switching losses and significantly improves efficiency. the zvs also reduces the switching noise noticeably, which allows a small- sized electromagnetic interference (emi) filter. the FAN7621S can be applied to various resonant converter topologies; such as series resonant, parallel resonant, and llc resonant converters. related resources an4151 ? half-bridge llc resonant converter design using fsfr-series fairchild power switch (fps tm ) ordering information part number operating junction temperature package packaging method FAN7621Ssj -40 c to +130 c 16-lead, small outline package (sop) tube FAN7621Ssjx tape & reel
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 2 FAN7621S ? pfm controller for half-bridge resonant converters application circuit diagram figure 1. typical application circuit ( llc resonant half-bridge converter) block diagram figure 2. internal block diagram lv cc r t ar cs sg pg hv cc v o r max r min r ss c ss v cc v in cr FAN7621S ho ctr lo 6 1 12 3 ho cs ar lv cc hvcc ctr r t pg 2 8 16 9 10 sg 14 lo huv+ /huv- s r q high side gate driver low side gate driver 2v v ref in tern al bias level shifter b alancin g delay divider 1v i rt i rt 2i rt v ovp v cssh /v cssl lv cc s r q -1 delay 1.5s v aocp delay 50ns tsd luv+ /luv- v ocp v ref v ref 3v time delay time delay 350ns 350ns lv cc good lv cc good shutdown without delay 5k
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 3 FAN7621S ? pfm controller for half-bridge resonant converters pin configuration figure 3. package diagram pin definitions pin # name description 1 hv cc this is the supply voltage of the high-side gate-drive circuit ic. 2 ctr this is the drain of the low-side mosfet. typically, a transformer is connected to this pin. 3 ho this is the high-side gate driving signal. 4 nc no connection 5 nc no connection 6 ar this pin is for discharging the external soft-start capacitor when any protection is triggered. when the voltage of this pin drops to 0.2v, all protections are reset and the controller starts to operate again. 7 nc no connection 8 r t this pin programs the switching frequency. typically, an opto-coupler is connected to control the switching frequency for the output voltage regulation. 9 cs this pin senses the current flowing through the low-side mosfet. typically, negative voltage is applied on this pin. 10 sg this pin is the control ground. 11 nc no connection 12 lv cc this pin is the supply voltage of the control ic. 13 nc no connection 14 lo this is the low-side gate driving signal. 15 nc no connection 16 pg this pin is the power ground. this pin is connected to the source of the low-side mosfet. (3) ho (4) nc pg (16) FAN7621S nc (13) nc (15) (5) nc (6) ar (7) nc lo (14) lv cc (12) cs (9) nc (11) sg (10) (2) ctr (1) hv cc (8) r t
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 4 FAN7621S ? pfm controller for half-bridge resonant converters 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 high-side gate driving voltage v ctr -0.3 hv cc v v lo low-side gate driving voltage -0.3 lv cc lv cc low-side supply voltage -0.3 25.0 v hv cc to v ctr high-side v cc pin to center voltage -0.3 25.0 v v ctr center voltage -0.3 600.0 v v ar auto-restart pin input voltage -0.3 lv cc v v cs current sense (cs) pin input voltage -5.0 1.0 v v rt r t pin input voltage -0.3 5.0 v dv ctr /dt allowable center voltage slew rate 50 v/ns p d total power dissipation 1.13 w t j maximum junction temperature (1) +150 c recommended operating junction temperature (1) -40 +130 t stg storage temperature range -55 +150 c note: 1. the maximum value of the recommended operating junction temperature is limited by thermal shutdown. thermal impedance symbol parameter value unit ja junction-to-ambient thermal impedance 110 oc/w
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 5 FAN7621S ? pfm controller for half-bridge resonant converters electrical characteristics t a =25 c and lv cc =17v unless otherwise specified. symbol parameter test conditions min. typ. max. unit supply section i lk offset supply leakage current hv cc =v ctr 50 a i q hv cc quiescent hv cc supply current (hv cc uv+) - 0.1v 50 120 a i q lv cc quiescent lv cc supply current (lv cc uv+) - 0.1v 100 200 a i o hv cc operating hv cc supply current (rms value) f osc =100khz, c load =1nf 5 8 ma no switching 100 200 a i o lv cc operating lv cc supply current (rms value) f osc =100khz, c load =1nf 6 9 ma no switching 2 4 ma uvlo section lv cc uv+ lv cc supply under-voltage positive-going threshold (lv cc start) 11.2 12.5 13.8 v lv cc uv- lv cc supply under-voltage negative-going threshold (lv cc stop) 8.9 10.0 11.1 v lv cc uvh lv cc supply under-voltage hysteresis 2.5 v hv cc uv+ hv cc supply under-voltage positive-going threshold (hv cc start) 8.2 9.2 10.2 v hv cc uv- hv cc supply under-voltage negative-going threshold (hv cc stop) 7.8 8.7 9.6 v hv cc uvh hv cc supply under-voltage hysteresis 0.5 v oscillator & feedback section v rt v-i converter threshold voltage r t =5.2k 1.5 2.0 2.5 v f osc output oscillation frequency 94 100 106 khz dc output duty cycle 48 50 52 % f ss internal soft-start initial frequency f ss =f osc +40khz, r t =5.2k 140 khz t ss internal soft-start time 2 3 4 ms output section i source peak sourcing current hv cc =17v 250 360 ma i sink peak sinking current hv cc =17v 460 600 ma t r rising time c load =1nf, hv cc =17v 65 ns t f falling time 35 ns v hoh high level of high-side gate driving signal (v hvcc -v ho ) i o =20ma 1.0 v v hol low level of high-side gate driving signal 0.6 v v loh high level of high-side gate driving signal (v lvcc -v lo ) 1.0 v v lol low level of high-side gate driving signal 0.6 v
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 6 FAN7621S ? pfm controller for half-bridge resonant converters electrical characteristics (continued) t a =25 c and lv cc =17v unless otherwise specified. symbol parameter test conditions min. typ. max. unit protection section v cssh beginning voltage to discharge c ss 0.9 1.0 1.1 v v cssl beginning voltage to charge c ss and reset protections 0.16 0.20 0.24 v v ovp lv cc over-voltage protection lv cc > 21v 21 23 25 v v aocp aocp threshold voltage v/ t=-0.1v/s -1.0 -0.9 -0.8 v t bao aocp blanking time (2) v cs < v aocp ; v/ t=-0.1v/s 50 ns v ocp ocp threshold voltage v/ t=-1v/s -0.64 -0.58 -0.52 v t bo ocp blanking time (2) v cs < v ocp ; v/ t=-1v/s 1.0 1.5 2.0 s t da delay time (low-side) detecting from v aocp to switch off (2) v/ t=-1v/s 250 400 ns t sd thermal shutdown temperature (2) 110 130 150 c dead-time control section d t dead time (3) 350 ns notes: 2. these parameters, although guaranteed, are not tested in production. 3. these parameters, although guaranteed, are tested only in eds (wafer test) process.
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 7 FAN7621S ? pfm controller for half-bridge resonant converters typical performance characteristics these characteristic graphs are normalized at t a =25oc. 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c temp ( o c) 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 normalized at 25 o c figure 4. low-side mosfet duty cycle vs. temperature figure 5. switching frequency vs. temperature 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c figure 6. high-side v cc (h v cc ) start vs. temperature figure 7. high-side v cc (h v cc ) stop vs. temperature 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c figure 8. low-side v cc (l v cc ) start vs. temperature figure 9. low-side v cc (l v cc ) stop vs. temperature
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 8 FAN7621S ? pfm controller for half-bridge resonant converters typical performance characteristics (continued) these characteristic graphs are normalized at t a =25oc. 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c figure 10. lv cc ovp voltage vs. temperature figure 11. r t voltage vs. temperature figure 12. v cssl vs. temperature figure 13. v cssh vs. temperature 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 temp ( o c) normalized at 25 o c figure 14. ocp voltage vs. temperature 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 normalized at 25 temp( ) 0.9 0.95 1 1.05 1.1 -50 -25 0 25 50 75 100 normalized at 25 temp( )
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 9 FAN7621S ? pfm controller for half-bridge resonant converters functional description 1. basic operation : FAN7621S is designed to drive high-side and low-side mosfets complementarily with 50% duty cycle. a fixed dead time of 350ns is introduced between consecutive transitions, as shown in figure 15. figure 15. mosfets gate drive signal 2. internal oscillator : FAN7621S employs a current- controlled oscillator, as shown in figure 16. 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. figure 16. current controlled oscillator 3. frequency setting : figure 17 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 18 shows the typical circuit configuration for r t pin, where the opto- coupler transistor is connected to the r t pin to modulate the switching frequency. the minimum switching frequency is determined as: min min 5.2 100( ) k fkhz r = (1) assuming the saturation voltage of the opto-coupler transistor is 0.2v, the maximum switching frequency is determined as: max min max 5.2 4.68 ( ) 100( ) kk fkhz rr ? =+ (2) figure 17. resonant converter typical gain curve FAN7621S figure 18. frequency control circuit 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 soft-start is implemented by sweeping down the switching frequency from an initial high frequency ( f iss ) until the output voltage is established. the soft-start circuit is made by connecting r-c series network on the r t pin, as shown in figure 18. FAN7621S also has an internal soft-start of 3ms to reduce the current overshoot during the initial cycles, which adds 40khz to the initial frequency of the external soft-start circuit, as shown in figure 19. the initial frequency of the soft-start is given as: min 5.2 5.2 ()10040() iss ss kk fkhz rr =++ (3) it is typical to set the initial (soft-start) frequency two ~ three times the resonant frequency ( f o ) of the resonant network. i c tc + - + - 3v 1v -q q r s f/ f 2i ctc v re f i ctc 2v + - c oun t er (1/4) r t 8 gate dri ve c t
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 10 FAN7621S ? pfm controller for half-bridge resonant converters the soft-start time is three to four times the rc time constant. the rc time constant is as follows: ss ss ss c r t ? = (4) 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. c ss starts to be discharged until the v css across c ss drops to v cssl . 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 v css is under v cssl and v cssh level, the switching is terminated immediately, v css continues to increase until reaching v cssh , then c ss is discharged by m1. figure 20. internal block of ar pin after protections trigger, FAN7621S is disabled during the stop-time, t stop , where v css decreases and reaches to v cssl . the stop-time of FAN7621S can be estimated as: t stop = c ss r ss + r min || 5 k ? (5) for the soft-start time, t s/s it can be set as equation (4). 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 lv cc falls to the lv cc stop voltage of 10v or the ar signal is high, the protection is reset. FAN7621S resumes normal operation when lv cc reaches the start voltage of 12.5v. 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 1.5s 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 lv cc reaches 23v, ovp is triggered. this protection is used when auxiliary winding of the transformer to supply v cc to the controller is utilized. 5.4 thermal shutdown (tsd) : if the temperature of the junction exceeds approximately 130 c, the thermal shutdown triggers. f s time control loop take over 40khz f iss lv cc i cr v ar t stop t s/s v cssh (a) (a) (a) (b) (b) (a) protections are triggered, (b) fsfr-us restarts v cssl (b)
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 11 FAN7621S ? pfm controller for half-bridge resonant converters 6. current sensing using resistor: FAN7621S senses drain current as a negative voltage, as shown in figure 23 and figure 24. half-wave sensing allows low power dissipation in the sensing resistor, while full-wave sensing has less switching noise in the sensing signal. figure 23. half-wave sensing figure 24. full-wave sensing 7. pcb layout guidelines : duty imbalance problems may occur due to the radiated noise from the main transformer, the inequality of the secondary-side leakage inductances of main transformer, and so on. 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 high- and low-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 turn-on 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. error! reference source not found. shows an example for the duty-balanced case. the yellow and blue lines show the primary current flows when the lower-side and higher- side mosfets turn on, respectively. the primary current does not enclose any component of controller. figure 25. example for duty balancing
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 12 FAN7621S ? pfm controller for half-bridge resonant converters physical dimensions figure 26. 16-lead 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 ver ify 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/ .
? 2009 fairchild semiconductor corporation www.fairchildsemi.com FAN7621S ? rev. 1.0.1 13 FAN7621S ? pfm controller for half-bridge resonant converters

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