www.fairchildsemi.com ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 1 AN-8024 applying fairchild power switch (fps tm ) fsbh-series to standby auxiliary power supply 1. introduction the highly integrated fsbh-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 switch-mode power supplies (smps) with minimal external components. the integrated pwm controller features include a proprietary green-mode function that provides 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 and burst modes function with a low operating current (2.5ma in green mode) to maximize the light load efficiency so that the power supply can meet stringent standby power regulations. the fsbh-series has 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, from 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), brownout protection, and over-temperature protection (otp); are fully integrated into fsbh-series, which improves the smps reliability without increasing the system cost. this application note presents design consideration to apply fsbh-series to a standby auxiliary power supply with single output. it covers designing the transformer, selecting the components, feedback loop design, and design tips to maximize efficiency. for multi-output applications, refer to fairchild application note an-4137. figure 1. typical application circuit
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 2 2. design cons iderations flyback converters have two kinds of operation modes; continuous conduction mode (ccm) and discontinuous conduction mode (dcm). ccm and dcm each has advantages and disadvantages. in general, dcm provides better switching conditions 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 lo w compared to ccm. however, dcm inherently causes high rms current, which increases the conduction loss of the mosfet severely for low line condition. thus, especially for standby auxiliary power supply applications with low output voltage where schottky diode without reverse recovery can be used; it is typical to design the converter such that the converter operates in ccm to maximize efficiency. in this section, a design procedure is presented using figure 1 as a reference. an offline smps with 20w/5v nominal output power has been selected as a design example. [step-1] define the system specifications when designing a power supply with peak load current profile, the following specifications should be determined: ? `line voltage range ( v line min and v line max ) ? `line frequency ( f l ) ? `nominal output power ( p o ) ? `estimated efficiencies for nominal load ( ): the power conversion efficiency must be estimated to calculate the input powers for nominal load condition. if no reference data is available, set = 0.7~0.75 for low-voltage output applications and = 0.8~0.85 for high-voltage output applications. with the estimated efficiency, the input power for peak load condition is given by: o in p p = (1) (design example) the specifications of the target system are: ? v line min =90v ac and v line max =264vac ? line frequency f l = 60hz (90v ac ) and 50hz (264v ac ) ? nominal output power p o = 20w (5v/4a) ? estimated efficiency: = 0.77 20 26 0.77 o in p p w == = [step-2] determine the input capacitor (c in ) and the input voltage range it is typical to select the input capacitor as 2~3 f per watt of peak input power for universal input range (85-265v ac ) and 1 f per watt of peak input power for european input range (195v-265v ac ). with the input capacitor chosen, the minimum input capacitor voltage at nominal load condition is obtained as: 2 (1 ) 2( ) ?? =? ? ? min min in ch in line in l pd vv cf (2) where d ch is the input capacitor charging duty ratio defined as shown in figure 2, which is typically about 0.2. the maximum input capacitor voltage is given as: 2 = max max in line vv (3) figure 2. input capacitor voltage waveform (design example) by choosing 100 f capacitor for input capacitor, the minimum input voltages for nominal load is obtained as: 2 2 6 (1 ) 2( ) 26 (1 0.2) 2 (90) 113 100 10 60 min min in ch in line in l pd vv cf v ? ?? =? ? ? ?? =? ? = ? the maximum input voltage is obtained as: 2 2 264 373 max max in line vv v =? =? = [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 0. with a given v ro , the maximum duty cycle ( d max ), and the maximum nominal mosfet voltage ( v ds nom ) are obtained as: ro max min ro in v d vv = + (4)
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 3 nom max d sinro vvv =+ (5) () max nom in o f d oo ro vvv vv v ?+ =+ (6) figure 3. 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 . this, however, 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 trade-off between the voltage stresses of mosfet and diode. especially for low output voltage application, 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 1 shows the forward-voltage drops for schottky diodes with different voltage ratings. because the actual drain voltage and diode voltage rise above the nominal voltage due to the leakage inductance of the transformer, as shown in 0, it is typical to set v ro such that v ds nom and v do nom are 60~70% of voltage ratings of mosfet and diode, respectively. table 1. diode forward-vol tage 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 1, it is necessary to use a rectifier diode with 40v voltage rating to maximize efficiency. assuming that the nominal voltages of mosfet and diode are less than 68% of their voltage rating, the reflected output voltage is given as: () 373 (5 0.5) 5 0.68 40 27.2 max nom in o f do o ro ro vvv vv v v ?+ =+ ?+ =+ = 0.68 700 476 nom max ds in ro vvv =+ AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 4 transformer size. from a practical point of view, it is reasonable to set k rf = 0.3~0.6 for the universal input range and k rf = 0.4~0.8 for the european input range. once l m is calculated by determining k rf from equation (7), the peak current and rms current of the mosfet for minimum input voltage and nominal load condition are obtained as: ds 2 pk edc i ii =+ (8) 22 3( ) ( ) 23 rms max ds edc d i ii ?? =+ ?? ?? (9) where in edc min in max p i vd = ? (10) and min in max msw vd i lf = (11) i 2 rf edc i k i = s pk d i figure 4. mosfet current and ripple factor (k rf ) (design example) determining the ripple factor as 0.6: 2 2 3 () (113 0.47) 2 2 26 100 10 0.6 900 min in max m in sw rf vd l pfk h ? ? == ?? ? = 26 0.49 113 0.47 in edc min in max p i a vd === ? ? 63 113 0.47 0.59 900 10 100 10 min in max msw vd i a lf ? ? = = = ? ds 0.49 0.295 0.78 2 pk edc i i ia =+=+ = 22 22 3( ) ( ) 23 0.47 3(0.49) (0.295) 0.36 3 rms max ds edc d i ii a ?? =+ ?? ?? ?? =+ = ?? [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 fsbh-series lineup with power ratings is summarized in table 2. table 2. lineup of fsbh-series with power ratings product i lim maximum output power for universal input range and open frame fsbh0f70 0.73a 8w fsbh0170 0.80a 13w fsbh0270 1.00a 16w fsbh0370 1.20a 19w (design example) fsbh0370 is selected. [step-6] determine the minimum primary turns with a given core, the minimum number of turns for the transformer primary side to avoid core saturation is given by: min 6 10 mlim p sat e li n ba = (12) where a e is the cross-sectional area of the core in mm 2 , i lim is the pulse-by-pulse current limit level, and b sat is the saturation flux density in tesla. the pulse-by-pulse current limit level is included in equation (12) because the inductor current reaches the pulse-by-pulse current limit level during the load transient or overload condition. error! reference source not found. shows the typical characteristics of ferrite core from tdk (pc40). since the saturation flux density ( b sat ) decreases as the temperature increases, the high temperature characteristics should be consider ed. if there is no reference data, use b max =0.3 t. figure 5. typical b-h characteristics of ferrite core (tdk/pc40)
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 5 (design example) eel-19 core is selected, whose effective cross-sectional area is 25mm 2 . choosing the saturation flux density as 0.3 t, the minimum number of turns for the primary side is obtained as: min 6 6 6 10 900 10 1.2 10 144 0.3 25 mlim p sat e li n ba ? ? = ? == ? [step-7] determine the number of turns for each winding figure 6 shows the simplified diagram of the transformer. first, calculate the turn ratio (n) between the primary side and the secondary side from the reflected output voltage, determined in step 3, as: ro p sof v n n nvv == + (13) where n p and n s are the number of turns for primary side and secondary side, respectively; v o is the output voltage; and v f is the diode ( d o ) forward-voltage drop. then, determine the proper integer for n s , such that the resulting n p is larger than n p min obtained from equation (12). the number of turns for the auxiliary winding for v dd supply is determined as: * 1 dd fa as of vv nn vv + =? + (14 ) where v dd * is the nominal value of the supply voltage and v fa is the forward-voltage drop of d dd as defined in figure 6. since v dd increases as the output load increases, it is proper to set v dd at 3~5v higher than v dd uvlo level (8v) to avoid the over-voltage protection condition during the peak load operation. figure 6. simplified transformer diagram (design example) assuming the diode forward- voltage drop is 0.5v, the turn ratio is obtained as: 100 18.18 50.5 ro p sof v n n nvv == = = ++ then, determine the proper integer for n s , such that the resulting n p is larger than n p min as: min 8, 146 sps p nnnn n ==?=> setting v dd * as 15v, the number of turns for the auxiliary winding is obtained as: * 15 1.2 824 50.5 + + = ?= ?= ++ dd fa as of vv nn vv [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 rms rms max sec ds max d ini d ? =? (15) the current density is typically 3~5a/mm 2 when the wire is long (>1m). when the wire is short with a small number of turns, a current density of 5~10a/mm 2 is also acceptable. avoid using wire with a diameter larger than 1mm to avoid severe eddy current losses as well as to make winding easier. for high-current output, it is better to use parallel windings with multiple strands of thinner wire to minimize skin effect. (design example) the rms current of primary-side winding is obtained from step 4 as 0.36a. the rms current of secondary-side winding is calculated as: 1 10.47 18.18 0.36 6.9 0.47 rms rms max sec ds max d ini d a ? =? ? =? = 0.3mm (5a/mm 2 ) and 0.65mm2 (10a/mm 2 ) diameter wires are selected for primary and secondary windings, respectively. [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: max in do o v vv n =+ (16) 1 rms rms max do ds max d ini d ? =? (17)
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 6 the typical voltage and current margins for the rectifier diode are: 1.3 r rm do vv >? (18) 1.5 r ms fdo ii >? (19) where v rrm is the maximum reverse voltage and i f is the current rating of the diode. (design example) the diode voltage and current are calculated as: 373 5 25.5 18.18 max in do o v vv v n =+ =+ = 1 10.47 18.18 0.36 6.9 0.47 rms rms max do ds max d ini d a ? =? ? =? = two 5a and 40v diodes in parallel are selected for the rectifier diode. [step-10] feedback circuit configuration since fsbh-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: 3.2 lim lim sat fb i i k v == (20) where i lim is the pulse-by-pulse current limit and v fb sat is the feedback saturation voltage. which is typically 3.2v. 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: ? ? (/) (1 / )(1 / ) 2(1/) ? = ?? +? =? ++ o vc fb lin p s z rz ro in p v g v kr v n n ss vv s (21) where r l is the load resistance and the pole and zeros of equation (21) are obtained as: 2 2 (1 ) 1(1) , (/) l zrz p co m s p lo rd d and r cdlnn rc ? ? + == = w here 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 7 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 together 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 7. cc m flyback converter control-to output trans- fer function variation for different input voltages figure 8 shows the variation of a ccm flyback converter control-to-output transfer function for different loads. this figure shows that the low frequency gain does not change for different loads and the rhp zero is lowest at the full load condition. figure 8. 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, vary according to the operating conditions. moreover, 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,
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 7 changing the system transfer functions as the load current decreases and/or input voltage increases. 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 phase margin in low input voltage and full load condition, the stability over all the operating ranges can be guaranteed. figure 9 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 adjusted 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 as: ?? ?> oopdka f b d vv v ctr i r (22) where v opd is the drop voltage of the photodiode, about 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. figure 9. feedback circuit the feedback compensation network transfer function of figure 9 is obtained as: ? 1/ ? 1/ z c fb i opc s v vss + =? ? + (23) where 1 = fb i d f r r rc , 1 1 () zc f f r rc = + , and 1 = pc f bfb r c . and r fb is the equivalent feedback bias resistor of fsbh- series (5k ); and r 1 , r d , r f , c f and c fb are shown in figure 10. (design example) assuming ctr is 100%, 3 33 110 51.2 2.5 1.3 110 110 ? ?? ? ? ?> ?? ?? < == oopdka d oopdka d vv v ctr r vv v r k the minimum cathode current for ka431 is 1ma. 3 1.2 110 ? < = opd bias v r k 1k resistor is selected for r bias . the voltage divider resistors r 1 and r 2 for v o sensing are selected as 20k and 20k . [step-11] design input voltage sensing circuit figure 10 shows a resistive voltage divider with low-pass filter for line-voltage detection of the vin pin. the v in voltage is used for brownout protection, which triggers when the v in voltage drops below 0.6v. a 500ms debounce time is introduced for brownout protection to prevent false triggering by the voltage ripple on the input capacitor. fsbh-series devices start up when the v in voltage reaches 1.1v. it is typical to use 100:1 voltage divider for v in level. figure 10. input voltage sensing
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 8 design summary figure 11 shows the final schematic of the 20w power supply of the design example. figure 11. final schematic of design example
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 9 eel-19 n p /2 n 5v n a 1 2 3 4 5 6 10 n p /2 figure 12. transformer specification core: eel-19 (ae=25mm 2 ) bobbin: eel-19 pin (s f) wire turns winding method n a 4 5 0.3 1 24 solenoid winding insulation: polyester tape t = 0.025mm, 1 layer n p /2 3 2 0.3 1 73 solenoid winding insulation: polyester tape t = 0.025mm, 2 layers n 5v 6 10 0.65 3 8 solenoid winding insulation: polyester tape t = 0.025mm, 2 layers n p /2 2 1 0.3 1 73 solenoid winding insulation: polyester tape t = 0.025mm, 2 layers pin specifications remark inductance 1 3 900 h 10% 100 khz, 1 v leakage 1 3 < 30 h max. short all other pins
AN-8024 application note ? 2009 fairchild semiconductor corporation www.fairchildsemi.com rev. 1.0.1 ? 9/18/09 10 related datasheets fsbh0f70a, fsbh0170/a, fsbh0270/a, fsbh0370 ? 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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