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www.fairchildsemi.com ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 AN-9719 applying fairchild power switch (fps?) fsl1x7 to low- power supplies 1. introduction the highly integrated fsl-series consists of an integrated current-mode pulse width modulator (pwm) and an avalanche-rugged 700v sensefet. it is specifically designed for high-performance offline switched mode power supplies (smps) with minimal external components. the features of the integrated pwm controller include a proprietary green-mode function providing off-time modulation to linearly decrease the switching frequency at light-load conditions to minimize standby power consumption. the pwm controller is manufactured using the bicmos process to further reduce power consumption. the green mode and burst mode functions with a low operating current (2ma in green mode) maximize light-load efficiency so that the power supply can meet stringent standby power regulations. the fsl-series has a built-in synchronized slope compensation to achieve stable peak-current-mode control. the proprietary external line compensation ensures constant output power limit over a wide ac input voltage range, 90v ac to 264v ac , and helps optimize the power stage. many protection functions; such as open?loop / overload protection (olp), over-voltage protection (ovp), and over- temperature protection (otp); ar e fully integrated into fsl- series. these features improve the smps reliability without increasing system cost. compared to a discrete mosfet and controller or rcc switching converter solution, the fsl-series reduces total component count, converter size, and weight while increasing efficiency, productivity, and system reliability. these devices provide a basic platform for design of cost- effective flyback converters. figure 1. typical application circuit
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 2 2. device block description 2.1 startup circuit for startup, the hv pin is connected to the line input or bulk capacitor through external resistor r hv , as shown in figure 2. typical startup current is 3.5ma and it charges the v dd capacitor (c dd ) through resistor r hv . the startup current turns off when the v dd capacitor voltage reaches v dd-on . the v dd capacitor maintains v dd until the auxiliary winding of the transformer provides the operating current. figure 2. startup circuit 2.2 soft-start the fsl-series has internal soft-start circuit that slowly increases the sensefet current during startup. the typical soft-start time is 5ms, during which the v limit level is increased in six steps to smoothly establish the required output voltage, as shown in figure 3. it also helps prevent transformer saturation and reduce stress on the secondary diode during startup. figure 3. soft-start function 2.3 green mode operation the fsl-series uses feedback voltage (v fb ) as an indicator of the output load and modulates the pwm frequency, as shown in figure 4, such that the switching frequency decreases as load decreases. in heavy-load conditions, the switching frequency is 100khz. once v fb decreases below v fb-n (2.5v), the pwm frequency starts to linearly decrease from 100khz to 18khz to reduce the switching losses. as v fb decreases below v fb-g (2.4v), the switching frequency is fixed at 18khz and fsl-series enters ?deep? green mode to reduce the standby power consumption. as v fb decreases below v fb-zdc (2.1v), fsl-series enters into burst-mode operation. when v fb drops below v fb-zdc , fsl-series stops switching and the output voltage starts to drop, which causes the feedback voltage to rise. once v fb rises above v fb-zdc , switching resumes. burst mode alternately enables and disables switching, thereby reducing switching loss to improve power saving, as shown in figure 5. figure 4. pwm frequency figure 5. bust-mode operation
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 3 2.4 constant power control to constantly limit the output power of the converter, high / low line compensation is included. sensing the converter input voltage through the vin pin, the high / low line compensation function generates a line-voltage-dependent peak-current-limit threshold voltage for constant power control, as shown in figure 6. figure 6. constant power control 2.5 protection functions the fsl-series provides full protection functions to prevent the power supply and the load from being damaged. the protection features is shown in table 1. table 1. protection functions fsl127h fsl137h ovp latch latch otp latch latch olp auto restart auto restart vin-h latch latch vin-l auto restart auto restart 2.5.1 v dd over-voltage protection (ovp) v dd over-voltage protection prevents ic damage caused by over voltage on the v dd pin. the ovp is triggered when v dd reaches 28v. it has debounce time (typically 130s) to prevent false trigger by switching noise. 2.5.2 over-temperature protection (otp) the sensefet and the control ic integration make temperature detection of the sensefet easier. when the junction temperature exceeds approximately 142c, thermal shutdown is activated. 2.5.3 overload protection (olp) when the upper branch of the voltage divider for the shunt regulator (ka431 shown) is open circuit, as shown in figure 7, or an output over-current or short occurs; there is no current flowing through the opto-coupler transistor. v fb (feedback voltage) pulls up to 6v. when the feedback voltage is above 4.6v for longer than 56ms, olp is triggered. this protection is also triggered when the smps output drops below the nominal value longer than 56ms due to the overload condition. figure 7. olp operator
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 4 3. design example flyback converters have two kinds of operation modes; continuous conduction mode (ccm) and discontinuous conduction mode (dcm). each has its own advantages and disadvantages. in general, dcm generates lower stress for the rectifier diodes, since the diodes are operating at zero current just before becoming reverse biased and the reverse recovery loss is minimized. the transformer size can be reduced using dcm because the average energy storage is low compared to ccm. however, dcm causes high rms current, which severely increases the conduction loss of the mosfet for low line condition. for standby auxiliary power supply applications with low output voltage and minimal reverse recovery of schottky diode, it is typical to design the converter such that the converter operates in ccm to maximize efficiency. this section presents a design procedure using the figure 1 schematic as a reference. an offline smps with 12w nominal output power has been selected as the example. [step-1] define the system specifications when designing a power supply, the following specifications should be determined first: ? line voltage range ( v v ? line frequency ( f l ? nominal output power ( p o ? estimate efficiencies for nominal load ( (design example) the specifications of the target system are: ? v 90v v 264v ? line frequency ( f ? nominal output power ( p ? estimated efficiency ( 12 0.8 15 [step-2] determine input capacitor (c in ) and input voltage range it is typical to select the input capacitor as 2~3 2 1 2 figure 8. input capacitor voltage waveform (design example) by choosing 20 2 1 2 90 15 1 0.2 20 10 60 79 2 2 264 373 [step-3] determine the reflected output voltage (v ro ) when the mosfet is turned off, the input voltage (v in ), together with the output voltage reflected to the primary (v ro ), are imposed across the mosfet, as shown in figure 9. with a given v ro , the maximum duty cycle (d max ) and the nominal mosfet voltage (v ds nom ) are obtained as:
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 5 figure 9. output voltage reflected to the primary as can be seen in equation 5, the voltage stress across mosfet can be reduced by reducing v ro . however, this increases the voltage stresses on the rectifier diodes in the secondary side, as shown in equation 6. therefore, v ro should be determined by a balance between the voltage stresses of mosfet and diode. especially for low output voltage applications, the rectifier diode forward-voltage drop is a dominant factor determining the power supply efficiency. therefore, the reflected output voltage should be determined such that rectifier diode forward voltage can be minimized. table 2 shows the forward-voltage drop for schottky diodes with different voltage rating. the actual drain voltage and diode voltage rise above the nominal voltage is due to the leakage inductance of the transformer as shown in figure 9. it is typical to set v ro such that v ds nom and v do nom are 70~80% of voltage ratings of mosfet and diode, respectively. table 2. diode forward-voltage drop for different voltage ratings (3a schottky diode) part name v rrm v f sb320 20v 0.5v sb330 30v sb340 40v sb350 50v 0.74v sb360 60v sb380 80v 0.85v sb3100 100v (design example) as can be seen in table 2, it is recommended to use rectifier diode with 100v voltage rating to maximize efficiency. assuming that the nominal voltages of mosfet and diode are less than 80% of their voltage rating, the reflected output voltage is given as: 373 12 0.85 12 0.8 100 80 ? 373 12 0.85 68 70.5 0.8 700 560 ? 560 373 187 by determining 74 74 79 0.48 373 74 447 373 12 0.85 74 12 76.8 [step-4] determine the transformer primary-side inductance (l m ) the transformer primary-side inductance is determined for the minimum input voltage and nominal-load condition. with the d max from step-3, the primary-side inductance (l m ) of the transformer is obtained as: 2
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 6 once l m is calculated from equation (7) after choosing k rf , the peak and rms current of the mosfet for the minimum input voltage and nominal load condition are obtained as: ? 2 3 ? 2 3 ? i rf e dc i k i = pk d i figure 10. mosfet current and ripple factor (k rf ) (design example ) choosing the ripple factor as 0.88, 2 79 0.48 2 15 100 10 0.88 540 15 79 0.48 0.4 ? 79 0.48 540 10 100 10 0.7 ? 2 0.4 0.35 0.75 3 ? 2 3 3 0.4 0.35 0.48 3 0.31 [step-5] choose the proper fps considering input power and peak drain current with the resulting maximum peak drain current of the mosfet (i ds pk ) from equation 8, choose the proper fps for which the pulse-by-pulse current limit level (i lim ) is higher than i ds pk . since fps has 10% tolerance of i lim , there should be some margin when choosing the proper fps device. the fsl-series lineup with proper power rating is summarized in table 3. table 3. lineup of fsl1x7-series with power rating product i lim at v in =1.2v 85-265v ac open frame min. typ. max. fsl127h 0.51a 0.61a 0.71a 16w fsl137h 0.74a 0.84a 0.94a 19w (design example) 0.75 0.84 . [step-6] determine the minimum primary turns with a given magnetic core, the minimum number of turns for the transformer primary side to avoid the core saturation is given by: 10 figure 11. typical b-h characteristics of ferrite core (tdk/pc40)
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 7 ( design example) ee-16 core is selected, whose effective cross-sectional area is 19.2mm 2 . choosing the saturation flux density as 0.3t, the minimum number of turns for the primary-side is obtained as: 10 540 10 0.8 0.3 19.2 10 75 [step-7] determine the number of turns for each winding figure 12 shows a simplified diagram of the transformer. first calculate the turn ratio (n) between the primary-side and the secondary-side from the reflected output voltage (v ro ) determined in step-3 as: figure 12. simplified transformer diagram (design example) assuming the diode forward- voltage drop is 0.85v, the turn ratio is obtained as: 74 12 0.85 5.8 n s such that the resulting n p is larger than n p min as: 13, 75 12 0.85 12 0.5 13 13 [step-8] determine the wire diameter for each winding based on the rms current of winding the maximum rms current of the secondary winding is obtained as: 1 (design example) the rms current of primary-side winding is obtained from step-4 as 0.31a. the rms current of secondary-side winding is calculated as: 1 5.8 0.31 1 0.48 0.48 1.87 [step-9] choose the rectifier diode in the secondary- side based on the voltage and current ratings the maximum reverse voltage and the rms current of the rectifier diode are obtained as: 1
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 8 the typical voltage and current margins for the rectifier diode are as follows: 1.2 1.8 (design example) the diode voltage and current are calculated as: 12 373 5.8 76.3 1 5.8 0.31 1 0.48 0.48 1.87 [step-10] feedback circuit configuration since fsl-series employs current-mode control, the feedback loop can be implemented with a one-pole and one- zero compensation circuit. the current control factor of fps, k, is defined as: 2.5 i lim is the pulse-by-pulse current limit and v fb sat is the feedback saturation voltage, typically 2.5v. as described in step-4, it is typical to design the flyback converter to operate in ccm for heavy-load condition. for ccm operation, the control-to-output transfer function of a flyback converter using current mode control is given by: ? 2 1 ? 1 ? 1 ? r l is the load resistance. the pole and zeros of equation 21 are obtained as: 1 , 1 ? 1 d is the duty cycle of the fps and r c is the esr of c o . notice that there is a right-half-plane (rhp) zero ( rz ) in the control-to-output transfer function of equation 21. because the rhp zero reduces the phase by 90 degrees, the crossover frequency should be placed below the rhp zero. figure 13 shows the variation of a ccm flyback converter control-to-output transfer function for different input voltages. this figure shows the system poles and zeros with the dc gain change for different input voltages. the gain is highest at the high input voltage condition and the rhp zero is lowest at the low input voltage condition. figure 13. ccm flyback converter control-to-output transfer function variation for different input voltage figure 14 shows the variation of a ccm flyback converter control-to-output transfer function for different loads. note that the dc gain changes for different loads and the rhp zero is the lowest at the full-load condition. figure 14. ccm flyback converter control-to-output transfer function variation for different loads when the input voltage and the load current vary over a wide range, it is not easy to determine the worst case for the feedback loop design. the gain, together with zeros and poles, varies according to the operating conditions. even though the converter is designed to operate in ccm or at the boundary of dcm and ccm in the minimum input voltage and full-load condition; the converter enters into dcm, changing the system transfer functions as the load current decreases and/or input voltage increases.
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 9 one simple and practical way to address this problem is designing the feedback loop for low input voltage and full- load condition with enough phase and gain margin. when the converter operates in ccm, the rhp zero is lowest in low input voltage and full-load condition. the gain increases only about 6db as the operating condition is changed from the lowest input voltage to the highest input voltage condition under universal input condition. when the operating mode changes from ccm to dcm, the rhp zero disappears, making the system stable. therefore, by designing the feedback loop with more than 45 degrees of phase margin in low input voltage and full-load condition, the stability over all the operating ranges can be guaranteed. figure 15 is a typical feedback circuit mainly consisting of a shunt regulator and a photo-coupler. r 1 and r 2 form a voltage divider for output voltage regulation. r f and c f are for control-loop compensation. the maximum source current of the fb pin is about 1ma. the phototransistor must be capable of sinking this current to pull the fb level down at no load. the value of r d , is determined by: v opd is the forward-voltage drop of the photodiode (~1.2v); v ka is the minimum cathode-to-anode voltage of ka431 (2.5v); and ctr is the current transfer rate of the opto-coupler. 6v + - figure 15. feedback circuit the feedback compensation network transfer function of figure 15 is obtained as: 1 ? 1 ? , 1 1 (design example) assuming ctr is 100%, 110 110 121.22.5 110 8.3? 1.2? r bias . the voltage divider resistors r 1 and r 2 should be designed to provide 2.5v to the reference pin of the ka431.the relationship between r 1 and r 2 is given as: 2.5 2.5 2.5 12 2.5 3.8 r 1 , r 2 . [step-11] design input voltage sensing circuit figure 16 shows a resistive voltage divider with low-pass filter for line-voltage detection of the vin pin. fsl-series devices start and enable the latch function when the v in voltage reaches 1.03v. if latch protection is triggered, the v in voltage is used for release latch protection as the v in voltage drops below 0.7v. it is typical to use 100:1 voltage divider for v in level. figure 16. input voltage sensing
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 10 4. printed circuit board layout high-frequency switching current / voltage makes printed circuit board layout a very important design task. good pcb layout minimizes excessive emi and helps the power supply survive during surge/esd tests. 4.1 guidelines to get better emi performance and reduce line-frequency ripple, the output of the bridge rectifier should be connected to capacitor c dc first, then to the switching circuits. ? the high-frequency current loop is in c dc ? transformer ? drain pin ? c dc . the area enclosed by this current loop should be as small as possible. keep the traces (especially 2 1 in figure 17) short, direct, and wide. high-voltage traces related the drain and rcd snubber should be kept far away from control circuits to prevent unnecessary interference. ? as indicated by 2 , the ground of control circuits should be connected first, then route other traces. ? as indicated by 3 , the area enclosed by the transformer auxiliary winding, d dd and c dd should also be kept small. place c dd close to the controller for good decoupling. ? gnd 2 3 1 are suggestions for esd tests where the earth ground is not available on the power supply. in the esd discharge path, the charge goes from the secondary, through the transformer stray capacitance, to gnd 3 , then gnd 1 , and back to mains. it should be noted that control circuits should not be placed on the discharge path. point discharge for common-mode choke ( see figure 17 ) can decrease high-frequency impedance and increase esd immunity. ? should a y-cap between primary and secondary be required, connect this y-cap to the positive terminal of c dc . if this y-cap is connected to primary ground, it should be connected to the negative terminal of c dc (gnd 1 ) directly. point discharge of this y-cap also helps for esd. in addition, the creepage distance between these two pointed ends should be at least 5mm according to safety requirements. figure 17. layout considerations
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 11 5. design summary 5.1 schematic figure 18 shows the final schematic of the 12w power supply of the design example. figure 18. design example 5.2 transformer specification figure 19. transformer specification ? core: eel-16 (a e =19.2mm 2 ) ? bobbin: eel-16
AN-9719 application note ? 2010 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.0 ? 11/2/10 12 pin (s f) wire turns n a 5 pin specification remark inductance 1 related datasheets fsl127h ? green mode fairchild power switch (fps ? ) fsl137h? green mode fairchild power switch (fps ? ) disclaimer fairchild semiconductor reserves the right to make changes without further notice to any products herein to improve reliability, function, or design. fairchild does not assume any liability arising out of the application or use of any product or circuit described herein; neither does it convey any license under its patent rights, nor the rights of others. life support policy fairchild?s products are not authorized for use as critical components in life support devices or systems without the express written approval of the president of fairchild semiconductor corporation. as used herein: 1. life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. a critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.

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