application note a 25w off-line flyback switching regulator introduction this note describes a low cost switching power sup- ply for applications requiring multiple output volt- ages, e.g. personal computers, instruments, etc... the discontinuous mode flyback regulator used in this application provides good voltage tracking be- tween outputs, which allows the use of primary side voltage sensing. this sensing technique reduces costs by eliminating the need for an isolated secon- dary feedback loop. the low cost, (8 pin) uc1842 current mode control chip employed in this power supply provides per- formance advantages such as : 1) fast transient response 2) pulse by pulse current limiting 3) stable operation to simplify drive circuit requirements, a to-220 power mos sgsp369 is utilized for the power switch. this switch is driven directly from the output of the control chip. power supply specifications 1. input voltage : 95vac to 130vac (50 hz/60 hz) 2. output voltage : a. + 5 v, 5 % : 1 a to 4 a load ripple voltage : 50 mv p-p max b. + 12 v, 3 % : 0.1 a to 0.3 a load ripple voltage : 100 mv p-p max c. 12 v, 3 % : 0.1 a to 0.3 a load ripple voltage : 100 mv p-p max 3. line isolation : 3750 v 4. switching frequency : 40 khz 5. efficiency @ full load : 70 % basic circuit operation the 117vac input line voltage is rectified and smoothed to provide dc operating voltage for the circuit. when power is initially applied to the circuit, capacitor c2 charges through r2. when the voltage across c2 reaches a level of 16 v the output of ic1 is enabled, turning on power mos q1. during the on time of q1, energy is stored in the air gap of transformer (inductor) t1. at this time the po- larity of the output windings is such that all output rectifiers are reverse biased and no energy is trans- ferred. primary current is sensed by a resistor, r10, and compared to a fixed 1 v reference inside ic1. when this level is reached, q1 is turned off and the polarity of all transformer windings reverses, for- ward biasing the output rectifiers. all the energy stored is now transferred to the output capacitors. many cycles of this store/release action are needed to charge the outputs to their respective voltages. note that c2 must have enough energy stored in- itially to keep the control circuitry operating until c4 is charged to a level of approximately13 v. the volt- age across c4 is fed through a voltage divider to the error amplifier (pin 2) and compared to an internal 2.5 v reference. energy stored in the leakage inductance of t1 causes a voltage spike which be added to the nor- mal reset voltage across t1 when q1 turns off. the clamp consisting of d4, c9 and r12 limits this volt- age excursion from exceeding the bvdss rating of q1. in addition, a turn-off snubber made up of d5, c8 and r11 keeps power dissipation in q1 low by delaying the voltage rise until drain current has de- creased from its peak value. this snubber also damps out any ringing which may occur due to parasitics. less than 3.5 % line and load regulation is achieved by loading the output of the control winding nc, with r9. this resistor dissipates the leakage energy as- sociated with this winding. note that r9 must be iso- lated from r2 with diode d2, otherwise c2 could not charge to the 16 v necessary for initial start-up. a small filter inductor in the 5 v secondary is added to reduce output ripple voltage to less than 50 mv. this inductor also attenuates any high frequency noise. AN247/1188 1/6
figure 1 : 25w off-line flyback regulator. notes : 1. all resistors are 1/4 w unless noted. 2. see appendix for construction details. 3. l1 = ferroxcube 204t50-3c8 (toroid), n turns : 4, wir e gauge : 1 mm. (18 awg). application note 2/6
figure 2 : block diagram : uc1842 current mode controller. typical switching waveforms upper trace : q1 - gate to source voltage lower trace : q1 - gate current application note 3/6
typical switching waveforms upper trace : q1 - drain to source voltage lower trace : primary current - i d upper trace : + 5v charging current lower trace : + 5v output ripple voltage performance data conditions 5 v out 12 v out 12 v out low line (95 vac) 12 @ 100 ma 12 @ 300 ma + 5 v @ 1.0 a 4.0 a + 5 v @ 1.0 a 4.0 a 5.211 4.854 5.199 4.950 12.05 12.19 11.73 11.68 12.01 12.14 11.69 11.63 nominal line (120 vac) 12 @ 100 ma 12 @ 300 ma + 5 v @ 1.0 a 4.0 a + 5 v @ 1.0 a 4.0 a 5.220 4.875 5.208 4.906 12.07 12.23 11.73 11.67 12.03 12.18 11.68 11.62 high line (130 vac) 12 @ 100 ma 12 v @ 300 ma + 5 v @ 1.0 a 4.0 a + 5 v @ 1.0 a 4.0 a 5.207 4.855 5.200 4.902 12.06 12.21 11.71 11.66 12.02 12.15 11.67 11.61 overall line and load regulation 3.5 % 2.3 % 2.4 % application note 4/6
appendix power transformer - t1 core : ferroxcube ec-35/3c8 gap : 0.25mm. in each outer leg note : for reduced emi put gap in center leg only. use 0.5mm. transformer construction application note 5/6
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specification mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics printed in italy all rights reserved sgs-thomson microelectronics group of companies australia - brazil - canada - china - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the nether- lands - singapore - spain - sweden - switzerland - taiwan - thailand - united kingdom - u.s.a. application note 6/6
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