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december 2001 1/32 g. bacchin - c. spini AN1487 application note 35w flyback converters for set-top box applications using the viper100a ? abstract this document describes a reference design of a 35w switch mode power supply dedicated to set-top box application. the board accepts full range input voltage (90 to 265vrms) and delivers 5 outputs. it is based on the viper100a integrating the controller and a power mosfet, working at fixed frequency, pwm mode. 1.0 introduction set-top boxes growth is very fast in all the countries either for satellite or terrestrial decoding and so the market asks for solutions with good performances and high cost effectiveness in small volumes. the integrated monolithic solution like the viper makes it a very suitable device. it is able to satisfy all the requirements of a compact and flexible design with only few external components, integrating all the necessary functions to have a robust design. a specific application circuit is here proposed. 1.1 main characteristics the main characteristics of the smps are listed here below: table 1: input parameters table 2: output parameters notes: (a)dedicated to 5v digital circuitry and to 3.3v local post regulators (b)dedicated to scart and general purpose circuits (c)dedicated to tuner (d)dedicated to lnb regulator (e)the output voltage value is selectable via the signal ocontrolo - ttl comp. or open coll. 1 = 22v - 0 = 17v symbol parameter value unit vin input voltage 90 - 264 vrms f frequency 45-66 hz vout (v) iout (a) pout (w) stability notes 3.3 4 13.2 +/- 2% 5 1.5 7.5 +/- 2% (a) 12 0.4 4.8 +/- 5% (b) 35 0.015 0.525 +/- 8% (c) 22/17 0.35 7.7 +/- 2% (d-e) p out = 33.725 (w)
AN1487 - application note 2/32 1.1.1 stand-by no stand-by mode is foreseen by equipment 1.1.2 overcurrent protection on all outputs, with auto-restart at short protection 1.1.3 pcb type & size cu single side 70 m m, fr-4, 131 x 100 mm 1.1.4 safety in accordance with en60950, creepage and clearance minimum distance 6.4mm 1.1.5 emi in accordance with en50022 class b 2.0 principle of operation the core of the design is a viper100a, integrating the controller and a mosfet in a standard to-220fp insulated. the switching frequency (~65 khz) has been chosen to get a compromise between the transformer size and the harmonics of the switching frequency, in order to optimise the input filter size and its cost. the transformer reflected voltage is ~50v, providing enough room for the leakage inductance voltage spike with still margin for reliability. the diodes d1 and d15 clamp the peak of the leakage inductance voltage spike at a safe level for reliable operation of the device. the viper100a is activated by an internal current source that draws current from the dc bus and charges the capacitor c12. thanks to this circuit, the start-up time is short and independent from the input mains voltage. during the normal operation, the controller is powered by the transformer via the diode d6. the network q2, c33, r27 acts as a lead edge blanking, deleting the spikes at mosfet turn-off. this improves the auxiliary voltage stability and the circuit performance during short circuits. r2 and c2 determine the switching frequency and the maximum duty cycle while r1 and c3 are a protection against discharges. the capacitor c4 is part of the compensation loop, while the zener d11 is a cheap solution for clamping the e/a output voltage, so the maximum power delivered by the circuit. this zener plays an important role during the output short circuit protection of the circuit. the output rectifiers have been chosen in accordance with the maximum reverse voltage and power dissipation. the rectifier for 3.3v is a schottky barrier, type stps10l60cf. this diode has low forward voltage drop, hence dissipating less power with respect to a standard type. due to the high current delivered by the output, it needs to be mounted on a heat sink. the p/n is indicated on the bom. a small lc filter has been added on the +3.3v in order to filter the high frequency ripple without increasing the output capacitors size. the other two output rectifiers are axial, fast recovery. the output voltage regulation is performed by the secondary feedback on the 3.3v output. the feedback network is the classical one using a tl431 driving an optocoupler, in this case a tcdt1102g, insuring the required insulation between primary and secondary. the opto-transistor drives directly the comp pin of the viper. the 5v output stability is assured by the dc/dc converter based on the st l4971 (ic6) while a standard linear regulator assures the 12v stability at low cost. a dedicated dc/dc converter based on the st34063a provides the suitable voltage dedicated to the lnb of the satellite parabola. the input of this converter is connected to the +35v output dedicated to the tuner. the stability of the 35v is guarantee by the transformer coupling. the input emi filter is a classical pi-filter, 1-cell for differential and common mode noise. a ntc limits the inrush current produced by the capacitor charging at plug-in and a standard 5*20 fuse protects from catastrophic failures. the transformer is slot type, manufactured by orega, in accordance with the en60950. the figures 2 and 3 show the drain voltage and current at the nominal input mains voltage during normal operation at full load. as visible the working frequency is almost 66khz. at low mains the transformer works at the boundary of the ccm.
AN1487 - application note 3/32 r1 39r r2 2k2 c2 12n c3 22n osc 1 ic1 viper100afp ic2 tcdt1102g c4 22n r4 2k7 r5 10k c10 100uf- 400v d1 stta106 c12 47uf-25vyxf d2 stta106 d4 byw98-200 c16 2200uf-6.3v yxf l1 2.7uh elc08d c19 470uf-6.3v yxf vin 1 out 3 ic5 l7812acv c20 1000uf-6.3v yxf c21 n56 r8 0r68-1w l2 1mh elc08d r9 20k r10 1k2 c15 220uf-50v yxf r12 390 q1 bc548a r11 4k7 r13 1k5 control +17v/22v +35v +12v +5v d3 byv10-60 +3.3v c22 2n2-y2 c18 2200uf-6.3v yxf r15 24k r16 10k l3 100uh we-fi r18 2k7-1% r14 4k7-1% d5 1n5822 r19 82r-1% r17 220r - 1% vcc 6 tc 3 ipk 7 cii 5 swe 2 swc 1 drc 8 gnd 4 ic4 st34063a 1 3 d8 df06m r20 ntc_16r 3 2 14 l4 b82732- 27mh 1 2 jp1 molex2 c29 1n0 c28 1n0 f2 fuse1 f1 fuse 2a - 5*20 d7 stps1 0l60cf t1 orega g6988-01 4 2 3 14-15 12-13 17 16 10 11 6 7 1 2 3 4 5 6 7 8 9 10 11 12 jp2 stocko-12 +3.3v-rtn fb 8 out 4 vcc 5 osc 3 ic6 l4971_dip8 +5v-rtn +3.3v +3.3v-rtn +5v-rtn +5v +12v control +17v/22v +35v +17v/22v rtn +17v/22v rtn c5 100n wurth elektronik c27 100n c26 100n 88-264vrms d6 byw100-200 q2 bc556 r27 2k2 c33 1n0 ic3 tl431acz c9 150n c25 220uf-63v yxf c13 47uf-25v yxf c14 470uf-35v yxf r26 10m - vr37 c34 1n0 c6 22n c7 22n c23 2n2 c24 220n c8 100n q4 bc548a r23 12k q3 bc548a r21 1k0 d12 1n4148 d13 1n4148 d14 1n4148 r22 1k0 d11 bzx79-c3v9 c35 100p-1kv r24 270r r25 4k7 c36 100n c37 100n c39 100n c40 100n r28 680r r29 33r d15/r6 1.5ke200a control 17/22v @0.35 35v @15 ma 3v3 @4 a 5v @1.5 a 12v @0.4 a gnd gnd gnd gnd gnd 3v3 @4 a figure 1: electrical schematic
AN1487 - application note 4/32 here following some waveforms during the normal operation at full load: figure 4 gives the measurement of the drain peak voltage at full load and maximum input mains voltage. the maximum voltage peak is 596v, assuring a reliable operation of the viper100a with a good margin against the maximum bvdss. figure 4: vds & id at full load (vin = 265 vrms - 50 hz) the maximum piv of the diodes has been measured during the worst operating condition and it is indicated on the right of figure 5 and figure 6. the margin, with respect to the maximum voltage sustained by the diodes, assures a safe operating condition for the devices. here following (figure 7 and figure 8), the most salient controller ic signals are depicted. in both the pictures is possible to distinguish clean waveforms free of hard spikes or noise that could affect the controller correct operation. figure 2: vds & id at full load (vin = 115 vrms - 50 hz) ch1:drain voltage ch4:drain current figure 3: vds & id at full load (vin = 220 vrms - 50 hz) ch1:drain voltage ch4:drain current ch1:drain voltage
AN1487 - application note 5/32 2.1 5v dc/dc converter in figure 9, the most relevant waveforms of the 5v-buck converter at full load are shown. it is based on the l4971 a monolithic ic integrating the controller and a n-channel mosfet. as indicated, the working frequency of this converter is fixed, 94 khz (100khz nominal) and it is programmable by the rc network r15/c23. the output voltage is selectable by the resistor divider r18/r14 and a soft-start time is programmable by c7. thanks to the high frequency operation allowed by the l4971, the output inductor is a small, standard 100mh toroid. this value provides for the continuous mode operation of the inductor and a small ripple current, so requiring a capacitor without a very small esr cheaper. the diode is axial, schottky type, rated to sustain the current in case of a long term short of the output. figure 5: diode piv at vin = 265 vrms - 50 hz, full load ch2:+3.3v diode: anode voltage figure 6: diode piv at vin = 265 vrms - 50 hz, full load ch3:+15v diode: anode voltage ch4:+35 diode: anode voltage figure 7: vin = 115 vrms - 50 hz ch2:vpin5 - vcomp ch3:vpin2 - vdd ch4:vpin1 - vosc figure 8: vin = 220 vrms - 50 hz ch2:vpin5 - vcomp ch3:vpin2 - vdd ch4:vpin1 - vosc
AN1487 - application note 6/32 figure 9: 5v-buck converter at full load waveforms 2.2 17/25v dc/dc converter for lnb the lnb converter is a buck, based on the st34063, a monolithic chip integrating the controller and a bjt as switch. it is a low cost device requiring just few components around it. the operation of this converter is variable frequency. the duty is imposed by the input/output voltage ratio like in any buck converter. the operating frequency is imposed by the inductor charging/discharging time, hence by its value. the control loop is hysteretic, using an internal comparator. this converter is dedicated to the lnb linear regulators. changing the converter output voltage by means of the control signal it is possible to keep a constant voltage drop on the linear element, avoiding large heat sinks and dissipation. in figure 10, there are some significant waveforms, the oscillator saw tooth and the chopped voltage. in figure 11 there is the output voltage variation in accordance with the control pin. the output current of the converter is limited in case of short circuits. ch1:vpin3 - vosc ch2:vpin7 - vcomp ch3:vpin4 - vout (free wheeling diode) figure 10: st34063 converter oscillator and chopped voltage ch2:vpin3 - vtc at 22v r2:vpin3 - vtc at 17v ch3:vpin2 - vswe at 22v r3:vpin2 - vswe at 17v figure 11: 17/25v output and set top box control voltage ch2:vout at full load ch3:vcontrol
AN1487 - application note 7/32 2.3 cross regulation in the following tables the output voltage cross regulation is measured with static loads and the overall efficiency of the converter measured at different input voltages. if not differently mentioned, all the output voltages have been measured after the output connector and a 100 mm length cable, simulating the connection with the stb motherboard. table 3: full load at vin=115vrms, iin=0.67arms table 4: full load at vin=220vrms, iin=0.4arms the efficiency of the converter is rather high thanks to the viper100a characteristics but it is affected by 5v and lnb dc/dc converters and by the 12v linear regulator. table 5: reduced load 1 at vin=115vrms, iin=0.5arms table 6: reduced load 1 at vin=220vrms, iin=0.29arms vout [v] = 34.78 5.057 22.47 3.29 12.12 iout [a] = 0.016 1.500 0.352 4.057 0.402 pout tot [w] pin [w] eff. pout [w] = 0.550 7.586 7.909 13.348 4.872 34.264 48.5 70.65% v c14 [v] = 15.1 v c12 [v] = 12.37 vout [v] = 34.67 5.056 22.46 3.29 12.12 iout [a] = 0.016 1.500 0.352 4.057 0.402 pout tot [w] pin [w] eff. pout [w] = 0.548 7.584 7.906 13.348 4.872 34.257 46.6 73.51% v c14 [v] = 15.08 v c12 [v] = 12.35 vout [v] = 34.6 5.08 22.46 3.32 12.13 iout [a] = 0.007 1.00 0.25 3.06 0.3 pout tot [w] pin [w] eff. pout [w] = 0.255 5.080 5.615 10.159 3.639 24.748 34.7 71.32% v c14 [v] = 15.03 v c12 [v] = 12.25 vout [v] = 34.6 5.08 22.46 3.32 12.13 iout [a] = 0.007 1.00 0.25 3.06 0.3 pout tot [w] pin [w] eff. pout [w] = 0.255 5.080 5.615 10.159 3.639 24.748 34.0 72.79% v c14 [v] = 15.00 v c12 [v] = 12.24
AN1487 - application note 8/32 table 7: reduced load 2 at vin=115vrms, iin=0.35arms table 8: reduced load 2 at vin=220vrms, iin=0.21arms table 9: reduced load - for cable stb, without the lnb and tuner loads at vin=115vrms, iin=0.53arms table 10: reduced load - for cable stb, without the lnb and tuner loads at vin=220vrms, iin=0.32arms the above tables shown the output voltage measured applying the same loads that we could have in case of a different set-top box type is powered (e.g. a terrestrial or cable) without the lnb block of the satellite antenna and the 35v for the tuner. like before all the regulated output voltages are within the tolerances. vout [v] = 34.17 5.1 22.46 3.36 12.15 iout [a] = 0.007 0.75 0.18 1.98 0.203 pout tot pin [w] eff. pout [w] = 0.248 3.825 4.043 6.653 2.466 17.235 24.0 71.81% v c14 [v] = 12.01 v c12 [v] = 14.82 vout [v] = 34.2 5.1 22.46 3.36 12.15 iout [a] = 0.007 0.75 0.18 1.98 0.203 pout tot pin [w] eff. pout [w] = 0.249 3.825 4.043 6.653 2.466 17.236 23.8 72.42% v c14 [v] = 12.43 v c12 [v] = 15.29 vout [v] = 42.2 5.06 22.49 3.29 12.11 iout [a] = 0 1.5 0.000 4.01 0.4 pout tot pin [w] eff. pout [w] = 0.000 7.590 0.000 13.193 4.844 25.627 37.9 67.62% v c14 [v] = 15.3 v c12 [v] = 12.44 vout [v] = 42.5 5.06 22.49 3.28 12.11 iout [a] = 0 1.5 0.000 4.01 0.4 pout tot pin [w] eff. pout [w] = 0.000 7.590 0.000 13.153 4.844 25.587 37.0 69.15% v c14 [v] = 12.43 v c12 [v] = 15.29
AN1487 - application note 9/32 table 11: no-load (output connector unplug) at vin=115vrms table 12: no-load (output connector unplug) at vin=220vrms 2.4 viper100 waveforms at no load unplugging the output connector the circuit is still able to maintain all the voltages perfectly under control and within the tolerance. hence, a perfect functionality of the circuit is achieved also in this abnormal condition. during the no load operation at higher mains range the circuit works in burst mode and, thanks to this controller functionality, it keeps the input power almost constant. this makes it suitable to support stand-by operation with low consumption from the mains. it has to be kept into account that this circuit it has not been optimised for the stand-by operation hence it could be improved. for example, switching off the dc-dc converters and replacing the linear regulator with a similar one with disable, a power consumption reduction is achieved. of course, the consumption of the other output voltage loads has to be decreased in other ways, like switches or by dedicated disable pins. vout [v] = 33 5.13 22.49 3.4 12.17 iout [a] = 0 0 0 0 0 pout tot [w] pin [w] pout [w] = 0.000 0.000 0.000 0.000 0.000 0.000 1.8 v c14 [v] = 14.41 v c12 [v] = 9.33 all voltages are within tolerance vout [v] = 33 5.13 22.49 3.4 12.17 iout [a] = 0 0 0 0 0 pout tot [w] pin [w] pout [w] = 0.000 0.000 0.000 0.000 0.000 0.000 2.0 v c14 [v] = 14.41 v c12 [v] = 9.33 all voltages are within tolerance figure 12: vin = 115 vrms - 50 hz ch1:vpin3 - vds ch2:vpin5 - vcomp ch3:vpin2 - vdd ch4:vpin1 - vosc figure 13: vin = 220 vrms - 50 hz ch1:vpin3 - vds ch2:vpin5 - vcomp ch3:vpin2 - vdd ch4:vpin1 - vosc
AN1487 - application note 10/32 3.0 output voltage ripple at full load in figure 14 and figure 15, the output voltage ripple at switching and mains frequency are shown. as per the previous measures, the probes have been connected on test points after the connection cable. 4.0 line frequency ripple thanks to the current mode and its voltage feed-forward, the low frequency residual ripple compared with the ripple across c10 (input elcap) shows an excellent rejection of the control loop (>80 db). in figure 16, the ch2 trace shows the rejection for the +35v output that is not regulated but it depends only from the transformer coupling. at 220vac the rejection is even better, due to the lower ripple across the input capacitor. figure 14: output voltage ripple 115 vrms - 50hz ch2:+5 vout ch3:+3.3 vout ch4:+12 vout figure 15: output voltage ripple 115 vrms - 50hz ch2:+35 vout ch3:+17/22 vout figure 16: line frequency ripple at v115 vrms 50hz ch1:vpin3 - vds ch2:+35 vout ch3:+3.3 vout figure 17: 3.3v regulated output dinamic load variation at 220 vrms 50 hz ch2:+3.3 vout at test points ch3:+3.3 vout before l1 ch4:+3.3 iout
AN1487 - application note 11/32 5.0 dynamic load tests 5.1 +3.3v regulated output figures 17, 18 and 19 show the output voltage regulation against a dynamic load variation of the feed backed voltage, at the nominal input voltage values. as shown in figure 17, the response after the connector is not very good, even if the response is quite fast. making the same measure before the filter inductor (l1), at the feed back divider connection points, the response is much better (o2.2%). this means that the filter inductor heavily affect the response. to avoid any expensive solution to improve it, the better way is to measure the voltage regulation during the normal operation, powering the real load circuitry. this, because there are some local capacitors or filters helping a lot the regulation. besides, normally the dynamic load changes are less than the testing value indicated. the regulation for all the other output voltage is good. the +15v-unregulated output has a modulation not critical for both the linear and the switching regulator while it is more depth for the +35v output. anyway, this variation is not critical for the varicap zener diodes dedicated to tuning. the measures have been done at both the nominal mains voltages giving the same results. 5.2 +5v regulated output the above test shows the response of the +5v output voltage varying its load as indicated. as visible in figure 20, the dc/dc converter provides for an excellent stability of the output voltage against the load variation. the variation of the 15v unregulated is around to 200 mv, then not critical for the 12v regulator. the regulation has been tested at both the nominal mains voltages. at 115vac the waveforms have the same amplitude. load condition: +5v, +12v, +17/22v, +35v full load +3,3v load 50% 100%, 50hz load condition: +3.3v, +12v, +17/22v, +35v: full load +5v: load 50% 100%, 50hz figure 18: 3.3v regulated output dinamic load variation at 220 vrms - 50hz ch2:+15v - d4 anode ch3:+35 vout ch4:+3.3 iout figure 19: 3.3v regulated output dinamic load variation at 220 vrms - 50hz ch1:+5 vout ch2:+12 vout ch3:+17/22 vout ch4:+3.3 iout
AN1487 - application note 12/32 5.3 +17/22v regulated output even the lnb converter has a very good regulation against load variations. as in the figure 21, the voltage variation is only 75 mv. 6.0 start-up behaviors at full load figure 22: 220 vac - 50hz load condition: +3.3v, +12v, +5v, +35v: full load +17/22v: load 50% 100%, 50hz figure 20: 5v regulated output dinamic load variation at 220 vrms - 50hz ch1: +5v vout ch2:+12v vout ch3:+35v vout ch4:+5v iout figure 21: 17/22v regulated output dinamic load variation at 220 vrms - 50hz ch2:+17/22v vout ch3:+35v vout ch4:+17/22v iout ch1:+17/22 vout ch2:+12 vout ch3:+3.3 vout ch4:+5 vout
AN1487 - application note 13/32 in figure 22, there are the rising slopes at full load of the more significant output voltages at 220vac input mains voltage. as shown in the picture, the rising times are constant and there is only a difference for the 5v and 17/22v rise time, with respect to the other outputs. the phenomenon is due to the delay introduced by the two dc/dc converter that need time for start-up. the circuit has been tested at minimum, nominal and maximum input mains voltage. the behavior is the same in all conditions with a monotonic slope and free of overshoot. 7.0 wake-up time in figures 23 and 24, there are the waveforms with the wake-up time measured at minimum and maximum input mains. as measured, thanks to the viper100a internal current source, the capacitor c12 is charged with a constant current, independent from the input mains value. this means that the power supply wake- up time is perfectly constant and it can be smaller and so cheaper. thus, the annoying problem of a very long start-up time, especially at low mains, is solved without to add any additional extra component like with controller not integrating the hv start-up functionality. besides, it is a key circuit during stand-by operation because it is disconnected from the mains saving power. the measured time at 85 and 265 vac is 237 ms, which is rather fast for this kind of power supplies. 8.0 turn-off at full load even at turn off the transition is clean, without any abnormal behavior like restart or glitches both on the primary or secondary side. 9.0 short-circuit tests at full load in a power supply the short circuit protection is a very important function because it involves the safety of the equipment powered and it can be the source of annoying problems during the equipment qualification phase. a frequent problem with universal mains input converters is a not constant behavior of the protection overall the input voltage range. in fact, we have to protect the shorted output components and the primary side switch from overheating or melting because during short the average output current can be very high as well as the power involved at primary side. to limit the power involved in fault condition avoiding any inexpensive component over sizing a common solution adopted by the controllers is the so called hic-cup mode functionality, so, if a short happens, the reflected low impedance provides for the auxiliary voltage disappearing and consequently the controller is off. then it restarts with a normal start-up phase and its time constant, then it works until the fault is detected again and a new cycle is triggered. the on-off working ratio provides for the average power delivering figure 23: 85 vac - 50hz ch1:vdd ch2:vc12 (vaux) ch3:+3v3 vout figure 24: 265 vac - 50hz ch1:vdd ch2:vc12 (vaux) ch3:+3v3 vout
AN1487 - application note 14/32 limitation. however, sometimes, using old concept controllers may happen that at low mains a good, low frequency hic-cup mode is achieved, but it becomes inefficient at high mains. this is due to the auxiliary capacitor charging by a simple resistor, providing for the effect of a charging time proportional to the input mains voltage and the result is an hic-cup with an on-off working ratio not short enough to limit the output diode dissipation of the shorted output also at high mains. the consequence in case of long-term shorts is a catastrophic failure for the circuit due to the rectifier overheating. thanks to the viper integrated start-up current source, the circuit behavior is very similar overall the input voltages range because, like during the start-up phase, the vcc capacitor is charged at constant current. this internal functionality eliminates definitively the problem of a poor short circuit protection bringing a superior safety level of the circuit against circuit fault. moreover, no any circuit over sizing of neither the power semiconductor nor the heat sinks is required so saving additional costs. on the board the short circuit tests have been done testing the circuit at maximum and minimum and nominal input voltage, but to avoid the insertion of a high amount of similar pictures, only the most significant have been inserted. the circuit parameters checked in all conditions have been the drain voltage and the mean value of the output current. the drain voltage is an important parameter to check during shorts at maximum input voltage to insure the reliability against long term-shorts. this because, especially with rcd clamper networks on the trafo primary, the effect of an higher peak of the primary current due to the heavier output load during the short charges the leakage inductance with more energy than during the normal operation, then an higher spike can appear on the mosfet drain. if the spike exceeds the bvdss and the avalanche capability or the clamping diode vrrm, we have the circuit destruction. using the viper, thanks to the easy clamping of the e/a and the maximum power deliverable, this problem can be easily managed. moreover, in this design, a transil clamp has been used and it's the best solution to avoid over voltages on the drain because it clamps almost at a constant level differently to a rcd clamper. hence, in the left pictures and the following ones it is easy to check that during shorts the current has a value close or less to the nominal one, and the drain voltage peak is always well below the bvdss, so preventing component melting for excessive dissipation. the auto-restart of the circuit has been also checked and it is correct at short removal for all conditions. in figure 27, find the viper most significant control waveforms during the hic-cup mode operation: it is possible to check the auxiliary voltage ramp across c12 due to its charging and discharging and the comp pin voltage. this last trace gives an idea about the clamping of the comp pin by the zener diode d11: the voltage peak is 3.54v while its maximum value can rise up 4.5v, so limiting the output power deliverable to the output. the short circuit has also been checked for the other transformer output (figure 28 and figure 29) and for all the smps post regulated outputs. in order to check the transformer behavior the measurement of the transformer output voltages during a short has been done. this because the transformer coupling may figure 25: 115 vac - 50hz ch1:vdd ch2:vc12 (vaux) ch3:+3v3 vout figure 26: 220 vac - 50hz ch1:vdd ch2:vc12 (vaux) ch3:+3v3 vout
AN1487 - application note 15/32 cause an unpredictable voltage variation, that is dependent on the current flowing in each winding. as shown in figures 30, 31, 32 and 33 all the output voltages remain at low values, thanks to the good coupling between the transformer windings. figure 27: 265 vrms 9.1 post-regulated outputs like the other output, both the two controllers keep under control the circuit preventing in all conditions the circuit from catastrophic failures. the l4971 dedicate to 5v regulation during short limits the current and the overheating working in burst-mode. the working time in this case is dependent on the impedance of the short circuit, thus always limiting the components overstress. the st34063 instead limits the duty cycle at its minimum value when the output current exceeds the threshold set by the sensing resistor r8. ch2:vc12 (vaux) ch3:vpin5 - vcomp figure 28: 15v trafo output short- 85 vrms ch1:drain voltage ch2:vc12 (vaux) ch3:vc14 (+15v) ch4:ishort circuit figure 29: 15v trafo output short- 265 vrms ch1:drain voltage ch2:vc12 (vaux) ch3:vc14 (+15v) ch4:ishort circuit
AN1487 - application note 16/32 9.2 12v output short a short circuit on this output is not able to provide for the hic-cup working mode of the primary controller and the current is limited by the linear regulator l78m12. 9.3 +35v output short a short on this output provides of course the same behavior of a short of c25 as shown before. the circuit starts work in burst mode protecting the circuit component from overstress as shown before. figure 30: +3.3/+35v outputs: short at 265 vrms - short c19 (+3.3v) ch1: drain voltage ch2: c25 voltage (+35v) ch3: c14 voltage (+15v) ch4 :ishort circuit figure 31: +3.3/+35v outputs: short at 265 vrms - short c25 (+35v) ch1: drain voltage ch2: 3.3vout ch3: c14 voltage (+15v) ch4: ishort circuit figure 32: short at 220 vrms - short c20 (+5v) ch2:vd5 ch3:+3v3 vout ch4:ishort circuit figure 33: short at 220 vrms - short c15 (+17/22v) ch1:+3v3 vout ch2:+17/22 vout ch3:vd3 ch4:ishort circuit
AN1487 - application note 17/32 10.0 short circuit protection at low load after the full load tests some additional checks on the short circuit protection with reduced loads have been done. 10.1 half load at vin=115vac: shorting each output by the active load the over current protection works correctly, providing for the hic-cup working mode. the hic-cup mode intervention is obtained even shorting the unregulated voltages (+15v and +35v) at the transformer output. both the dc/dc converters protect the circuits like tested before as well as the 12v linear regulator limits the output current. at vin=220vac: the circuit behaves like at 115v. 10.2 reduced load - no lnb and tuner voltages loaded even in this condition, all the smps output voltages are protected on all the input voltage range. decreasing the load of the output delivering current the output current protection works still correctly. 11.0 short circuit protection without load this condition is the worst case for most of the power supply. in fact the coupling between the windings in this case changes the circuit behavior, while the low peak current at primary side sometimes is not able to trigger the controllers protection circuits even in this abnormal condition the circuit performs well, working in burst mode like at full load. as visible on figure 34, the most salient parameters are measured showing that the circuit components are still working inside their rating and with margin, so insuring a good reliability. in figure 35 and figure 36, the measure results for the others transformer outputs are shown . 12.0 switch on and turn off in short circuit condition - full load - short on 3v3 figure 37 and figure 38 describe the smps behavior during the start-up phase with an output voltage shorted. as clearly visible the circuit starts correctly then it works in hic-cup mode protecting itself. the start-up phase is clean in all conditions, without showing any dangerous transition for the smps circuitry. even at turn off in short circuit the smps functionalities are good, protecting properly the circuit. no any abnormal transition or level has been observed during the tests. 35v 12v 17/22v 5v 3.3v 7.5ma 0.2 a 0.175a 0.75a 2a pout tot = 12.6w 35v 12v 17/22v 5v 3.3v 0ma 0.4 a 0a 1.5a 4a pout tot = 12.6w
AN1487 - application note 18/32 figure 34: +3v3 output: short at 265 vrms 13.0 open loop protection in any smps a dangerous fault that could happen is the so called open loop operation, due to a failure of the feedback network. if unprotected, in this condition the smps can deliver output voltages much higher than the nominal values destroying the smps and the load. using the viper with a secondary feedback the solution for this problem is integrated in the viper and free of charge: in fact just using the internal error amplifier dedicated to primary feedback and designing properly the auxiliary winding of the transformer it is possible to protect the circuit without adding any additional external component to the circuit. ch1:drain voltage ch2:vc14 (+15v) ch3:vc25 (+35v) ch4:ishort circuit figure 35: +15v output: short at 265 vrms ch1:drain voltage ch2:vc14 (+15v) ch3:vc25 (+35v) ch4:ishort circuit figure 36: +35v output: short at 265 vrms ch1:drain voltage ch2:vc14 (+15v) ch3:vc25 (+35v) ch4:ishort circuit
AN1487 - application note 19/32 the measures have given the following results, overall the input voltage range: in both condition the measured voltages are not critical for the circuitry. obviously, only the transformer voltages have been measured because the post-regulators keep their output voltages at the nominal value even in this condition. 14.0 emi conducted noise measurements (pre-compliance test) the following pictures (figure 39 and figure 40) are the quasi-peak conducted noise measurements at full load and nominal mains voltages. the limits shown on the diagrams are the en55022 class b, which is the most widely rule for domestic equipments like a stb. as visible on the diagrams, there is a good margin of the measures with respect to the limits. please note that the limit to be considered on the diagrams is the higher line. nominal voltages 35v 15v 3v3 v c12 open loop at full load 38.2v 16.4v 3.6v 13.4v open loop at no load 47v 20.5v 4.8v 13.4v limits: en55022 - class b quasi-peak measure start freq.: 150 khz stop freq.: 30mhz figure 37: start at 220 vrms - 50hz ch1:vdd ch2:vc11 (vaux) ch3:vpin5 - vcomp figure 38: turn-off at 220 vrms - 50hz ch1:vdd ch2:vc11 (vaux) ch3:vpin5 - vcomp
AN1487 - application note 20/32 15.0 conclusion a complete smps dedicated to digital set-top boxes has been designed, assembled and completely tested. the results show that it is reliable and suitable to meet the current international rules for similar equipments, meeting also a low-cost, complexity and volume and good performances as requested by the market. figure 35: vin = 115 vrms 50 hz - full load ch1:drain voltage ch2:vc14 (+15v) ch3:vc25 (+35v) ch4:ishort circuit figure 36: vin = 220 vrms 50 hz - full load ch1:drain voltage ch2:vc14 (+15v) ch3:vc25 (+35v) ch4:ishort circuit
AN1487 - application note 21/32 annex 1: component list ref . part type description supplier c10 100uf- 400v elcap elna c12 47uf-25v yxf elcap rubycon c13 47uf-25v yxf elcap rubycon c14 470uf-35v yxf elcap rubycon c15 220uf-50v yxf elcap rubycon c16 2200uf-6.3v yxf elcap rubycon c18 2200uf-6.3v yxf elcap rubycon c19 470uf-6.3v yxf elcap rubycon c2 12n cercap avx c20 1000uf-6.3v yxf elcap rubycon c21 n56 cercap avx c22 2n2-4kv (y2) 44ld22 cercap-safety cera-mite c23 2n2 cercap avx c24 220n cercap avx c25 220uf-63v yxf elcap rubycon c26 100n-275vac - b81133 x cap epcos c27 100n-275vac - b81133 x cap epcos c28 1n0-1kv 30lvd10 cercap hv cera-mite c29 1n0-1kv 30lvd10 cercap hv cera-mite c3 22n cercap avx c33 1n0 cercap avx c34 1n0 cercap avx c35 100pf-1kv hrr cercap hv murata c36 100n cercap avx c37 100n cercap avx c39 100n cercap avx c4 22n cercap avx c40 100n cercap avx c5 100n cercap avx c6 22n cercap avx c7 22n cercap avx c8 100n cercap avx c9 150n cercap avx
AN1487 - application note 22/32 d1 stta106 ultra fast rec. rectifier stmicroelectronics d11 bzx79-c3v9 zener diode philips d12 1n4148 gen. purpose diode wishay d13 1n4148 gen. purpose diode wishay d14 1n4148 gen. purpose diode wishay d15/ 1.5ke200a transil stmicroelectronics d2 stta106 ultra fast rec. rectifier stmicroelectronics d3 byv10-60 schottky rectifier stmicroelectronics d4 byw98-200 fast rec. rectifier stmicroelectronics d5 1n5822 schottky rectifier stmicroelectronics d6 byw100-200 fast rec. rectifier stmicroelectronics d7 stps10l60cf power schottky rectifier stmicroelectronics d8 df06m bridge rectifier gen. semicond. f1 fuse 2a - 5*20 f2 not used - shorted ic1 viper100afp integrated controller stmicroelectronics ic2 tcdt1102g optocoupler vishay telefunken ic3 tl431acz shunt regulator stmicroelectronics ic4 st34063a dc-dc converter stmicroelectronics ic5 l7812acv linear regulator stmicroelectronics ic6 l4971_dip8 step-down switching reg. stmicroelectronics jp1 39-26-3030 2 poles connector molex jp2 mks1862-6-0-1212 12 poles connector stocko l1 2.7uh elc08d inductor panasonic l2 1mh elc08d inductor panasonic l3 we-fi 744 70 21 100uh-2a inductor wurth electronik l4 b82732-r2901-b30 2*27mh filter coil epcos q1 bc548a small signal bjt zetex q2 bc556 small signal bjt zetex q3 bc548a small signal bjt zetex q4 bc548a small signal bjt zetex r1 39r - 1/4w - 5% sfr25 beyschlag r10 1k2 - 1/4w - 5% sfr25 beyschlag r11 4k7 - 1/4w - 5% sfr25 beyschlag r12 390 - 1/4w - 5% sfr25 beyschlag annex 1: component list (continued)
AN1487 - application note 23/32 r13 1k5 - 1/4w - 5% sfr25 beyschlag r14 4k7 - 1/4w - 1% mba0204 beyschlag r15 24k - 1/4w - 5% sfr25 beyschlag r16 10k - 1/4w - 5% sfr25 beyschlag r17 220r - 1/4w - 1% mba0204 beyschlag r18 2k7 - 1/4w - 1% mba0204 beyschlag r19 82r - 1/4w - 1% mba0204 beyschlag r2 2k2 - 1/4w - 5% sfr25 beyschlag r20 ntc_16r s236 ntc thermistor epcos r21 1k0 - 1/4w - 5% sfr25 beyschlag r22 1k0 - 1/4w - 5% sfr25 beyschlag r23 12k - 1/4w - 5% sfr25 beyschlag r24 270r - 1/4w - 5% sfr25 beyschlag r25 4k7 - 1/4w - 5% sfr25 beyschlag r26 10m - 1w - 5% vr37 beyschlag r27 2k2 - 1/4w - 5% sfr25 beyschlag r28 680r - 1/4w - 5% sfr25 beyschlag r29 33r - 1/4w - 5% sfr25 beyschlag r4 2k7 - 1/4w - 5% sfr25 beyschlag r5 10k - 1/4w - 5% sfr25 beyschlag r8 0r68-1w - 5% pr01 beyschlag r9 20k - 1/4w - 5% sfr25 beyschlag t1 g6988-01 power transformer orega hs1 6099b heat sink for ic1 thermalloy hs2 6099b heat sink for d7 thermalloy hs3 6043pb heat sink for ic5 thermalloy annex 1: component list (continued)
AN1487 - application note 24/32 annex2: transformer specification
AN1487 - application note 25/32 annex2: transformer specification (continued)
AN1487 - application note 26/32 annex2: transformer specification (continued)
AN1487 - application note 27/32 annex2: transformer specification (continued)
AN1487 - application note 28/32 annex2: transformer specification (continued)
AN1487 - application note 29/32 annex2: transformer specification (continued)
AN1487 - application note 30/32 annex2: transformer specification (continued)
AN1487 - application note 31/32 annex2: transformer specification (continued)
AN1487 - application note 32/32 information furnished is believed to be accurate and reliable. however, stmicroelectronics 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 results from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectronics. the st logo is a registered trademark of stmicroelectronics ? 2001 stmicroelectronics - printed in italy- all rights reserved. stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - u.s.a. http://www.st.com

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