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? semiconductor components industries, llc, 2003 february, 2003 - rev. 2 1 publication order number: and8099/d and8099/d 5.0 v, 2.0 a flyback converter prepared by: kristie valdez on semiconductor the design of a switching power supply is an iterative process which involves many variables that have to be adjusted in order to obtain an optimized solution. however, there are trade-offs which allow for a simple low cost, low component, single sided board design method. this application note provides a simple approach to designing a converter utilizing the on semiconductor ncp1055 high voltage switch regulator. the easy-to-follow step-by-step procedure guides the user into designing the different blocks that constitute the power supply, mainly the input block, the power stage, the magnetics, the snubber, the output block, and the feedback loop. the circuit diagram, bill of material, and pcb layout are also included at the end of the application note. this power supply is specifically designed for a 5.0 v, 2.0 a output and a maximum duty cycle of 48%. it meets iec and ul requirements. emi is minimal and a 70% achievable efficiency or greater is possible. the ncp1055 is a family of monolithic high voltage switching regulators designed to work in rectified ac line sources and flyback converter applications. they are capable of providing an output power ranging from 6.0 w to 40 w with a fixed ac input of 100 v, 115 v, or 230 v and 3.0 w to 20 w with a variable ac input ranging from 85 v to 265 v. this device features an on-chip 700 v sensefet ? power switch circuit, an active startup regulator circuit which eliminates the need for an auxiliary bias winding on the converter transformer, fault logic with a programmable timer for converter overload protection. protective features provide power switch current limiting, input under voltage lockout with hysteresis, thermal shutdown, and restart fault detection. for more information, please contact an on semiconductor sales representative or log on to www.onsemi.com . design parameters the first step in designing a power supply is to define and predetermine the input and output parameters. universal input voltage range: v in(min) 85 vac, v in(max) 265 vac output specifications: v out 5.0 v 2%, i out 2a input power: p in p out est.eff , an efficiency of 0.78 is a good starting point for a flyback converter using mosfet technology: p out 52.0 10 w p in 10 0.78 12.82 w dc rail voltages at low line and high line: v peak(min) v in(min) 2 85 2 120.21 vdc v peak(max) v in(max) 2 265 2 374.77 vdc average input current at low line: i in(avg) p in v in(low) , where v in(low) v peak(min) v ripple v diode; i in(avg) 12.82 80.2 0.160 a v ripple 32% v peak(min) input peak current: i peak 2i in(avg) t sw t on i peak 2 0.160 10 s 4.8 s 0.667 a application note http://onsemi.com
and8099/d http://onsemi.com 2 the component losses can be evaluated and budgetized with the following formula: p loss =p in (1 - eff) p % where p % is the percentage loss of the desired circuit section per total power supply loss. usually, 35% of the losses come from the power mosfet, 60% from the output rectifier, 5% from the magnetics, and 5% from miscellaneous sources. estimated power loss = p in - p out = 12.82 - 10 = 2.82 w mosfet power losses = 2.82 35% = 0.987 w rectifier power losses = 2.82 60% = 1.692 w circuit description input block the input block of the power supply consists of a fuse, an emi filter, a diode bridge rectifier, and an input bulk capacitor. fuse the fuse f1 is protecting the circuit from current surges occurring at turn on. in this application, f1 is rated for 2.0 a, 125 vac. emi filter the emi filter is suppressing common mode and differential mode noise and is very dependent upon board layout, component selection, etc. an x capacitor c1 and a common mode choke l1 are placed across the ac lines to attenuate differential mode noise, see figure 1. the emi inductor is slowing down any transient voltage surge to reduce high frequency noise. both the capacitor and choke should be placed before the diode bridge and as close to the ac line input as possible to minimize rfi. diode bridge rectifier in order to choose the right diode bridge rectifier, the values of the forward and surge currents and dc blocking voltage must be considered. the surge current can reach values up to five times that of the average input rms current. it is therefore necessary to select a rectifier capable of handling such large currents. dc blocking voltage is calculated at high line: v r v peak(max) v in(max) 2 375 vdc forward current: i f 1.5 i in(avg) 1.5 0.160 0.240 a surge current: i fsm 5i f 5 0.240 1.2 a input bulk capacitor the purpose of the input bulk capacitor c2 is to hold up the rectified line voltage and also to filter out common mode noise. it is placed between the bridge rectifier output and ground. the size of the bulk capacitor depends on peak rectified input voltage and the ripple voltage magnitude. a larger capacitor will lower the ripple voltage on the dc input line, but will induce a larger surge current when the supply is powered up. assuming a ripple magnitude of about 32% of the peak rectified voltage at low line, c bulk can then be calculated using: c bulk p in fac ( v peak(min) 2 v in(low) 2 ) 12.82 60 (120 2 80.2 2 ) 27 f select the closest standard capacitor of 33 f with low esr. aluminum electrolytics are preferred because of their sturdiness and high reliability. power stage at the heart of the power stage is the on semiconductor ncp1055. the ncp1055 is a high voltage switching regulator that uses a fixed-frequency, duty cycle controlled oscillator. rectified ac line voltage is applied to the startup circuit pin 5 through the primary winding of the transformer. the circuit then routes current to the supply capacitor c5 which is typically connected to pin 1. a switching cycle begins when the oscillator charges and discharges an on chip timing capacitor which generates a square wave signal used to pulse width modulate the power switch circuit. the control input pin is monitoring source or sink current drawn by an optocoupler. when the power supply output is greater than the reference voltage, the optocoupler begins to conduct pulling on the control input. the output of the control input is then sampled continuously during t on and has the ability to either turn the power switch circuit on or off at any time within t on . magnetics calculations the next step is the design of the flyback transformer. the design of the magnetics block is the most important and delicate part of the whole design process because it will determine how well the power supply will perform. the flyback-mode transformer functions by first conducting current in the primary winding, thus storing energy in the core of the transformer. the core energy is then transferred to the secondary winding when the primary side is turned off. the core and bobbin are standard efd20 sizes.
and8099/d http://onsemi.com 3 in order for the regulator to operate in discontinuous mode under worse case conditions and to maximize power, the maximum on time is 48% of the full period, therefore the the maximum primary inductance is calculated based on a maximum duty cycle of 48%. using a larger inductance than calculated will cause the power supply output to fall out of regulation. l pri v in(low)
max i peak f op , where
max is the maximum duty cycle l pri 80.2 0.48 0.667 * 100 10 3 0.577 mh primary flyback voltage: v fb v in(low) t on t off 80.2 4.8 10 -6 5.2 10 -6 74.03 v, where t on is 4.8 s primary to secondary turns ratio: n pri n sec v fb v out v f 74.03 5 0.525 13.4 13 turns by rearranging the above equation and solving for n sec yields 1 turn. energy entering the core during on-time (when the power switch is conducting): e stored l pri i peak 2 2 , where the stored energy is measured in joules. e stored 0.577 10 -3 0.667 2 2 1.28 10 -4 joules one can double check if the power capability of the transformer is large enough to supply enough power to the output with the following equation: p in(core) l pri i peak 2 2 f op p out p in(core) 1.28 10 -4 100 khz 12.8 w p out 10 w input snubber because of the high dv/dt characteristic of the power transistor drain voltage and of the transformer leakage inductance, voltage spikes and ringing occur at the drain when the power switch is turned off. resistor r1, c3, d5 compromise an rcd snubber. in parallel to the primary winding are r2 and c4 which compromise an rc ringing damper which slows down the dv/dt and reduces the peak voltage therefore decreasing the ringing due to high frequency noise. since i c dv dt , increasing the capacitance will also reduce the magnitude of the voltage ripple. the snubber and ringing damper act together to protect the ic from voltage transients greater than 700 v and reduce radiated noise. output block the output consists of a diode rectifier, a pi-filter, and a voltage regulator. for a flyback converter with an output voltage less than 7.5 v, a schottky rectifier provides the maximum efficiency and is therefore the best choice. the schottky rectifier used is the on semiconductor 1n5822, in which v r = 40 v, i f = 3.0 a, and v f = 0.525 v. the main purpose of this rectifier is to take the secondary voltage and convert it to a dc voltage. the following equations are used in selecting the schottky rectifier: maximum reverse peak voltage (calculated at high line): v rout v out v peak(max) n sec n pri v rout 5 375 1 13 33.85 v for discontinuous mode, the maximum forward peak current can be approximated using: i fout 4i out 8a diode d6 along with c7, c8, c9, l2, and c11 rectify the transformer secondary and filter the output in order to provide a tightly regulated dc output . capacitor c7, c8 and c9 are placed in parallel in order to reduce esr. in addition, the voltage rating of c7, c8 and c9 should be high enough for them to withstand the voltage spikes and the output voltage. l2 and c11 form a low pass filter that attenuates high frequency noise. output filter capacitor: c out i out(max) t off(max) v ripple(desired) ,
min 1 f op where t off(max) and v ripple(desired) 40 mv c out 8 0.52 1 100 10 3 0.040 1040 f output filter choke (designed for a break frequency of 4.0 khz): l 1 2 f c 2 , where c c11 330 f and f is the corner frequency. l 1 2 4k 330 f 2 4.8 h
and8099/d http://onsemi.com 4 feedback loop the feedback loop is composed of an optocoupler, a shunt regulator, a compensation capacitor and a resistor divider. the optocoupler isolates the ac input from the dc output. as a shunt regulator, the on semiconductor tl431 is used to regulate the output voltage. this monolithic voltage reference is programmable from v ref to 36 v using two external resistors. it exhibits a wide current range of 1.0 ma to 100 ma and is an excellent replacement for zener diodes. the reference voltage of the tl431 is set at 2.5 v, for a 5.0 v output voltage, by a resistor divider r5 and r6 (low tolerance, 2.0 k resistors). the tl431 monitors the 5.0 v output voltage and compares the divided down voltage to its 2.5 v internal reference. a small increase in the output voltage will cause the shunt regulator to start conducting, thus sinking current through the optocoupler's led. in turn, the optocoupler transistor becomes forward biased and starts driving current into the control input pin of the ncp1055. the power switch duty cycle is then adjusted accordingly. a compensation capacitor c10 of 0.1 f is placed between the cathode and the reference pin of the tl431 for improved stability. the resistor r3 limits the current going through the optocoupler to a safe level and prevents damage to the optocoupler. + + + + + c11 330 r6 2.0 k r5 2.0 k c10 0.1 r3 47 v in 85-265 vac f1 2.0 a c1 0.1 1 l1 10 mh d1 d2 d3 d4 r1 91 k + c2 33 c3 220 p c4 47 p r2 2.2 k d5 2 3 4 8 7 6 5 d6 t1 c7 330 c8 330 c9 330 c6 100 p c5 10 + ic2 r4 1.0 k ic3 5.0 v, 2.0 a c12 1.0 figure 1. circuit diagram ncp1055p100 x c13 1n l2 5.0 h
and8099/d http://onsemi.com 5 table 1. bill of materials ref. component value qty. part number manufacturer ic1 ic, 680 ma, 100 khz, dip8 1 ncp1055p100 on semiconductor ic2 optocoupler, dip 1 sfh615a-4 isocom ic3 2.5 v shunt reg., to-92 1 tl431ailp on semiconductor d1-4 1.0 a, 800 v, gen. purp. 4 1n4006 on semiconductor d5 1.0 a, 600 v, ultra fast 1 mur160tr on semiconductor d6 3.0 a, 40 v, schottky 1 1n5822 on semiconductor t1 flyback transformer 1 31592 midcom l1 choke, common mode, 10 mh 1 40479 midcom l2 choke, power, 5.0 h 1 40480 midcom c1 0.1 f, film, radial 1 xsc275v104-m15s nissei c2 33 f, 400 v, radial 1 400v33-hs united chem-con c3 220 pf, 1.0 kv, 10%, disc 1 ncd221k1kvy5f nic components c4 47 pf, 1.0 kv, 10%, disc 1 ncd470k1kvsl nic components c5 10 f, 16 v, 20%, radial 1 16v10 united chem-con c6 100 pf, 1.0 kv, 10%, disc 1 ncd101k1kvy5fab2 nic components c7-9, c11 330 f, 10 v, radial 4 10v330-hoss united chem-con c10 0.1 f, 50 v, cer., radial, 10% 1 sr215c104kaa avx c12 1.0 f, 35 v, tant., radial, 10% 1 105x9050 avx c13 0.001 f, 50 v, cer., radial, 10% 1 sr211c102kaa avx r1 91 k , 1.0 w 1 mo-1-91k-5tr sei r2 2.2 k , 1/2 w, 5% 1 1/2w-2.2k-5b sei r3 47 , 1/4 w, 5% 1 1/4w-47r-5 sei r4 1.0 k , 1/4 w, 5% 1 1/4w-1k-5b sei r5, r6 2.0 k , 1/4 w, 1% 2 ccf-55-2001ftr sei f1 2.0 a, axial 1 251002tr1 littlefuse j1 2 term block 2 pos plug and header 1 1757242 phoenix contact j1-2 term. block, 2 pos., plug and header 1 1757019 phoenix contact j3 4 term block 2 pos inv plug and header 1 1786404 phoenix contact j3-4 term. block, 2 pos., inv. plug and header 1 1786174 phoenix contact
and8099/d http://onsemi.com 6 table 2. converter test data and results test conditions data results line regulation vin = 85 vac; iout = 2.0 a vin = 265 vac; iout = 2.0 a vout = 4.971 v vout = 4.982 v 11 mv load regulation vin = 85 vac; iout = 0 = 0.200 a = 1.0 a = 2.0 a vin = 110 vac; iout = 0 = 0.200 a = 1.0 a = 2.0 a vin = 230 vac; iout = 0 = 0.200 a = 1.0 a = 2.0 a vin = 265 vac; iout = 0 = 0.200 a = 1.0 a = 2.0 a vout = 4.994 v = 4.991 v = 4.985 v = 4.971 v vout = 4.993 v = 4.991 v = 4.986 v = 4.976 v vout = 4.993 v = 4.991v = 4.987v = 4.981v vout = 4.994 v = 4.991 v = 4.987 v = 4.982 v 23 mv 17 mv 12 mv 12 mv efficiency vin = 85 vac = 110 vac = 230 vac = 265 vac vout = 4.971 v; iout = 2.0 a; pout = 9.942, pin = 13.64 w vout = 4.976 v; iout = 2.0 a; pout = 9.952, pin = 13.41 w vout = 4.981 v; iout = 2.0 a; pout = 9.942, pin = 13.47 w vout = 4.982 v; iout = 2.0 a; pout = 9.964, pin = 13.67 w 72.8 % 74.2 % 73.8 % 72.8 % output ripple voltage vin = 85 vac; iout = 2.0 a vin = 265 vac; iout = 2.0 a 100 mvp-p 230 mvp-p
and8099/d http://onsemi.com 7 figure 2. pcb metal layer figure 3. pcb silk screen
and8099/d http://onsemi.com 8 references 1. brown, marty, power supply cookbook , butterworth-heinemann, 1994. 2. pressman, abraham i., switching power supply design, second edition, mcgraw-hill, 1998. 3. motorola, inc., handling emi in switch mode power supply design , an-smps-emi, motorola, inc., 1998. on semiconductor and are registered trademarks of semiconductor components industries, llc (scillc). scillc reserves the right to mak e changes without further notice to any products herein. scillc makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does scillc assume any liability arising out of the application or use of any product or circuit, and s pecifically disclaims any and all liability, including without limitation special, consequential or incidental damages. typicalo parameters which may be provided in scillc data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. all operating parameters, including typicalso must be validated for each customer application by customer's technical experts. scillc does not convey any license under its patent rights nor the rights of others. scillc products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body , or other applications intended to support or sustain life, or for any other application in which the failure of the scillc product could create a sit uation where personal injury or death may occur. should buyer purchase or use scillc products for any such unintended or unauthorized application, buyer shall indem nify and hold scillc and its of ficers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and re asonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized u se, even if such claim alleges that scillc was negligent regarding the design or manufacture of the part. scillc is an equal opportunity/affirmative action employ er. publication ordering information japan : on semiconductor, japan customer focus center 2-9-1 kamimeguro, meguro-ku, tokyo, japan 153-0051 phone : 81-3-5773-3850 on semiconductor website : http://onsemi.com for additional information, please contact your local sales representative. and8099/d sensefet is a trademark of semiconductor components industries, llc (scillc) literature fulfillment : literature distribution center for on semiconductor p.o. box 5163, denver, colorado 80217 usa phone : 303-675-2175 or 800-344-3860 toll free usa/canada fax : 303-675-2176 or 800-344-3867 toll free usa/canada email : onlit@hibbertco.com n. american technical support : 800-282-9855 toll free usa/canada

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