1/9 AN1657 application note february 2003 design specification the typical electrical specification of an smps of a 17" crt monitor for pc is summarized in table 1. two goals concerning the off-mode consumption of the smps have been set: the first one is to meet the present energystar ? requirements, which envisage less than 2w absorbed from the mains; the second more ambitious goal is to comply with iea's "1w initiative" as well as to be eligible for g eea l abel. both voluntary standards require to achieve a power consumption below 1w. table 1. 90w smps for crt monitor: electrical specification qr approach and the l6565 the smps will be realized with a quasi-resonant (qr) flyback converter based on the l6565, a control ic specifically designed to handle such kind of converters. referring to [1] and [2] for a detailed description of the device and the topology, it is here worthwhile reminding that qr operation implies that the trans- input voltage range (vin) 88 to 264 vac mains frequency (f l ) 50/60 hz maximum output power (pout) 92 w outputs horizontal deflection vout = 200v 3% iout = 0.33a full load ripple = 1% video amplifier vout = 80v 5% iout = 0.13a full load ripple = 1% vertical deflection vout = 15v 10% iout = 0.33 a full load ripple = 1% heater vout = 6.5v 10% iout = 0.6a full load ripple = 2% micro vout = 5.0v 2% iout = 0.05a full load ripple < 1% minimum switching frequency in normal mode (f min , @ vin = 100 v dc , full load) 25 khz target efficiency (vin =88 to 264 vac, full load) ( h )> 85% suspend-mode input power (vin = 88 to 264 vac) <15 w off-mode input power (@pout = 125 mw on 5v output, vin = 88 to 264 vac) < 2w (energystar ) < 1w (iea, geea) by claudio adragna smps for crt monitors with the l6565 this note shows and discusses a couple of designs of a 90w wide-range-mains smps for crt monitor based on the qr controller l6565. the first design refers to a low-cost smps that meets current ener- gystar ? requirements on off-mode consumption (pin<2w). the second design is an evolution of the first one so as to be compliant with iea's "1w initiative". both have been realized and tested on the bench. the result of their evaluation is presented along with some significant waveforms.
AN1657 application note 2/9 former always works close to the boundary between continuous and discontinuous conduction mode and thereby at a switching frequency that depends on the input voltage and the output current. the ripple across the input bulk capacitor modulates the switching frequency in itself. this characteristic, besides be- ing advantageous in terms of emi emissions (it spreads the spectrum), makes it more difficult to see the noise on the screen. furthermore, with qr operation mosfet's turn-on occurs with zero or minimum drain voltage, which minimizes the switching noise generated. finally, since the converter always oper- ates in discontinuous conduction mode the reverse recovery characteristics of the secondary rectifiers are not invoked, which goes in favor of a "quiet" operation too. the above-mentioned characteristics, coupled with the high degree of safety under short circuit conditions inherent in its operation, make qr approach ideal for noise-sensitive applications as monitors are. the l6565 is an excellent low-cost solution to implement reliable and energy-efficient qr flyback convert- ers both under maximum and minimum load conditions. the internal functions of the ic (frequency fold- back and burst-mode operation at light load) as well as its inherent low consumption (less than 70 a start- up current and less than 3.5 ma quiescent current) make designer's life easier when they face the chal- lenging tasks of meeting energy-saving requirements. additionally, the l6565 offers a safety feature (device disable upon secondary rectifier short circuit) that can be fruitfully put to use in the present context to achieve an ultra-low consumption at light load. to pro- tect the converter in the event of such failure, an internal comparator senses the voltage on the current sense pin of the ic and disables the gate driver if this voltage exceeds 2v. to re-enable the driver, the supply voltage of the ic must fall below the uvlo threshold and then exceed again the start-up threshold. energystar ? compliant design the first proposed schematic is shown in figure 1. only its more significant features will be commented, please refer to [2] for the standard characteristics of an l6565-based qr flyback. figure 1. l6565-based, energystar ? compliant, 90w smps for crt monitor: electrical schematic 1 3 2 f1 250vac 5a 88 to 264 vac c1 220 m f 400v 5 1 4 8 7 3 ic1 l6565 1 8 c2 47 nf 250v c8a,b 4.7 nf y2 r2 47 k w 3w 7 4 6 c6 4.7 f 2 q1 stp6nk60zfp c5 100 pf c3 47 f 25v bd01 stbr606 d2 stth1l06 4 3 1 2 c17 22 nf c18 15 nf 250v 18 15 13 14 16 200v 0.33a 80v 0.13a 6.3v 0.6a +15v 0.33a -15v 0.33a gnd 17 c10 100 f 250v c9 220 f 100v c11 22 f 100v c12 1000 f 16v d8 uf4002 d7 stth1l06 d6 uf4006 11 12 10 c16 47 f 25v d9 uf4002 d10 uf4002 c14 470 f 25v l1 1h 5v 0.05a c13 2.2 m f 10v r1a 68 k w d1 1n4148 d3 1n4148 r3 22 w r4 1 k w c4 330 pf 100v r5 47 k w r6a 1.5 m w r6b 1.5 m w r7 15 k w c7 10 nf d4 1n4148 d5 1n4148 dz1 18v 0.5w r9 33 w r10 1 k w r11a,b 0.56 w ic2 pc817a ic3 tl431 c15 470 f 25v r13a,b 4.7 m w r16 47 w r17 2.7 k w r19 1.8 k w r20 330 k w r21 4.7 k w ic4 l78l05cz 1 2 3 tr1 100 k w r8 6.2 k w r18 10 k w r14 100 k w 1w r1b 68 k w emi filter r12 4.7 k w r15 33 k w
3/9 AN1657 application note the converter is started up by r1a, r1b and the diode d1 that draw some current from the ac side of the bridge rectifier. this inexpensive circuit wakes up the system in less then 3s @ 88 vac and contributes to light load losses with 240 mw @ 264 vac. despite this dummy consumption it is anyway possible to meet the target of less than 2w input consumption thanks to the favorable features of the l6565. supply- ing the ic from the ac side of the bridge helps reduce the power consumption on the start-up resistors and eliminates any chance of spurious restarts at converter's power down. r6a and b along with r7 correct the overcurrent setpoint so as to minimize the power capability change of the converter over the entire input voltage range. c7 filters out any noise that might be coupled to the pin. r8 and c6 provide soft-start. at start-up c6 is charged by the output of the l6565 e/a (pin 2) with a current defined by 2.5 / r8 and the e/a works temporarily closed-loop. as the e/a saturates high there is no more current through c6, the loop opens, the voltage on pin 1 (e/a input) goes to zero and pin 2 stays high at about 6v. when the l6565 turns off (because its supply voltage vcc goes below the uvlo threshold) the capacitor is discharged internally in few milliseconds - because the impedance of the pins becomes low - in this way ensuring a correct soft-start even when the l6565 is continuously restarted (e.g. in case of overload or short circuit). output voltage regulation is done with a tl431+optocoupler arrangement on the secondary side and the information is fed back to the current sense pin (#4) of the l6565. regulation is thus performed by mod- ulating the voltage offset generated by the phototransistor current on r10. c5 adds a small filtering effect to increase noise immunity. this feedback arrangement helps reduce the load of the self-supply system (winding 7-8, d3, r3, c3). in fact with the usual arrangement, where the phototransistor sinks current from pin 2 (with pin 1 grounded), the regulation current, typically 3 ma at light load, adds up to the operating current of the ic. with this circuit, to create about 1v offset, which is required at light load, the phototrans- istor needs to draw only 1 ma. this load reduction will counteract the natural decay of the self-supply volt- age when the converter is lightly loaded. please note that with this technique the zcd masking time of the l6565 (refer to [2] for details) is fixed at 3.5 s. the circuit made up of r4, c4, d4 and dz1 provides overvoltage protection in case of failure of the feedback loop. r4 and c4 smooth the waveform generated by the self-supply winding to suppress the leading edge spike that could mislead the circuit. during mosfet's off-time the winding generates a voltage proportional to the output voltage. thus, if the feedback loop opens (e.g. the optocoupler fails), which causes the output voltage to rise above the regulated value, the voltage provided by r4, c4 will increase as well. dz1 will be turned on and inject an additional offset on the current sense pin after mosfet's turn off. as the voltage on the pin reaches 2v an internal comparator will be triggered, the l6565 will shut down and the converter will be stopped until l6565's vcc voltage, after falling below the uvlo threshold, goes again above the start-up threshold. this may take some hundreds milliseconds, then the system will work in a continuous restart mode, the energy throughput will be very low and the output voltage will not reach dangerous values. table 2. l6565-based 90w smps for crt monitor: transformer specification the linear regulator that supplies the 5v line for the p takes its input from the +15v line. using the 6.5v line would improve efficiency (especially in off-mode) even further. to do so, however, an ldo (low core philips etd44, 3c85 material bobbin horizontal mounting, 18 pins air gap ? 1 mm for an inductance 1-4 of 380 h leakage inductance < 10 h windings spec & build winding wire s-f turns notes pri1 4xawg29 2-4 19 pin 4 is cut for safety sec1 (200v) awg25 17-18 48 sec2 (80v) awg25 15-16 32 sec3 (6.5v) awg25 13-14 3 evenly spaced sec4 (+15v) awg25 11-12 6 bifilar with sec5 sec5 (-15v) awg26 10-11 6 bifilar with sec4 pri2 4xawg29 1-2 19 aux (+15v) awg29 8-7 7 evenly spaced
AN1657 application note 4/9 dropout) regulator is needed (with the l7805 at least 2v dropout must be ensured), which is slightly more expensive, and the transformer must supply a sufficient voltage (5v plus the dropout and the tolerances) under all operating conditions. actually, in suspend mode with the heater still supplied the 6.5v drops at 5.5v, which forces the use of the +15v line. either with a better transformer construction - so that the 6.5v line never falls below, say, 6v or turning off the heater during suspend mode, an ldo regulator could pro- vide 5v powered by the 6.5v line, thus reducing the converter actual load by about 150 mw. there is no special circuit that handles the off-mode operation: simply, when the monitor enters off- mode the loads drop to negligible values on all of the outputs except the 5v one that must still supply the p governing monitor operation. in these conditions p's consumption is estimated at 25 ma max. as a result, the l6565 will enter its natural burst-mode operation, where a series of few switching cycles are repeated at the frequency of the internal starter or at a submultiple of its. the consumption measurements under these operating conditions are shown in table 4 in the "experimental results" section. "1w initiative" compliant design to make such design the starting point will be the energystar ? compliant circuit, which a circuitry dedicated to handle the off-mode will be added to. besides the additional circuitry, minimum adaptations in the power section and some changes in the control circuit will be required. figure 2 shows the resulting schematic. conceptually, the way off-mode is handled consists of forcing the l6565 to work in a very low frequency controlled burst-mode (continuous restart), so as to cut all frequency-related losses. to see how this is achieved practically it is worth examining the schematic of figure 2, specifically looking at the new and modified parts as compared to the schematic of figure 1. starting from the primary side, the major addition is the high-voltage active start-up circuit (r1a, r1b, r2, d1,dz1 and q1) along with the associated network (r9, r10, q3) to turn it off when the converter is run- ning. this addition has a not negligible impact on the part count and the total cost but is essential. figure 2. l6565-based, "1w initiative" compliant, 90w smps for crt monitor: electrical schematic c1 220 m f 400v 5 1 4 8 7 3 ic1 l6565 1 8 c2 47 nf 250v c8a,b 4.7 nf y2 r3 47 k w 3w 7 4 6 c6 4.7 f 2 q2 stp6nk60zfp c7 100 pf c3 47 f 25v bd01 stbr606 d2 stth1l06 1 3 c16 22 nf c17 15 nf 250v 18 15 13 14 16 200v 0.33a 80v 0.13a 6.5v 0.6a +15v 0.33a -15v 0.33a gnd 17 c10 100 f 250v c9 220 f 100v c11 22 f 100v c12 1000 f 16v d8 uf4002 d7 stth1l06 d6 uf4006 11 12 10 dz3 10v 0.5w d9 uf4002 d10 uf4002 c13 2200 f 25v l1 1h 5v 0.05a c15 2.2 m f 10v d1 1n4148 d3 1n4148 r4 22 w r5 1 k w c4 330 pf 100v r6 47 k w r7a 1.5 m w r7b 1.5 m w r8 15 k w c5 10 nf d4 1n4148 d5 1n4148 dz2 18v 0.5w r12 33 w r13 1 k w r14a,b 0.56 w ic2 pc817a ic3 tl431 c14 470 f 25v r16a,b 4.7 m w r19 680 w r20 2.7 k w r22 1.8 k w r23 330 k w r24 4.7 k w ic4 le50cz 2 1 2 3 tr1 100 k w dz4 8.2v 0.5w r25 5.6 k w r27 10 k w off c18 470 nf q4 bc547 r11 6.2 k w r26 470 w r21 10 k w r10 47 k w r9 220 k w r1a 4.7 m w r1b 4.7 m w dz1 12 v 0.5w r2 6.8 k w q1 stq1nc60r q3 bc547 f1 250vac 5a 88 to 264 vac emi filter 4 3 1 2 r17 100 k w 1w r15 4.7 k w r18 33 k w
5/9 AN1657 application note the importance of the circuit lies not only in the reduction of the associated losses from the 240 mw of the resistive start-up to less than 10 mw, but also in the fact that, arranged as a current source like in the schematic of figure 2, it provides constant wake-up and restart times for the converter, regardless of the input ac voltage. the benefits resulting from that will be clearer after discussing how the system handles off-mode operation. the current sourced by the generator and that charges the v cc capacitor c3 is: , where v z is the zener voltage of dz1, v f the forward drop across d1 and v th the threshold voltage of q1. with an appropriate selection of v z it is possible to compensate the temperature drift of both v f ( ? -2 mv/ c) and vth ( ? -7 mv/c). experience shows that zener diodes with v z =12v have a temperature coeffi- cient around +10 mv/c, thus one of them will be selected. with this circuit, neglecting the start-up current absorbed by the l6565, which is typically two orders of magnitude smaller, the wake-up time is defined by r2, c3 and the turn-on threshold (v ccon ) of the l6565: , whereas the restart time, that is the time needed for vcc to go from the uvlo threshold (v ccoff ) to v ccon during a continuous restart, depends on the v cc hysteresis (v cchys = v ccon - v ccon ) of the l6565: . the circuit that is actually responsible for handling off-mode operation is on the secondary side and is the network comprising r25 to r27, c18, dz4 and q4. to adapt the existent circuit to the new operation, r19 (r16 in the schematic of fig. 1) has been increased to 680 w and the capacitor c13 has been replaced by the zener diode dz3. the bulk capacitor c13 on the +15v line has been increased from 470 to 2200f and the l7805 replaced by an le50cz. during normal operation a logic signal (open collector) governed by the p keeps the off input low, so that q4 is always off and the off-mode circuit is disabled. when the monitor is to enter off-mode the p acts so that the loads of all the outputs are cut off, then it reduces its own consumption and opens the pull-down that was grounding the off input. q4 is immediately turned on because the voltage on the +15v is already higher than threshold (v t ) defined by dz4, r25 and r27. q4 will then draw a relatively large current from the photodiode, limited by r19+r20 (that is why c13 has been replaced by dz3: this will still give a high frequency pole to the output-to-control transfer function but will keep the current through the photodiode under control as q4 is turned on). as a result there will be quite a large current in the phototransistor too, which will bring the voltage on the current sense pin above the 2v threshold that shuts down the driver of the l6565 and stops pwm activity. the ic, however, remains active and pin 2 stays high, thus keeping the start-up generator off as long as vcc - which is decaying because of the quiescent consumption of the ic and the phototransistor current - is above v ccoff . as v cc = v ccoff the ic turns off, pin 2 goes low and the start-up generator is turned on again. this is the most critical moment: since the output voltage starts from about 15v and needs to decay below v t before q4 turns off, the phototransistor will keep on sinking quite a large current even after the start- up generator is enabled. as a result, v cc will be pulled well below v ccoff . the start-up generator will take a time longer than t restart to bring v cc back to v ccon and restart pwm activity. on the secondary side, as the l6565 stops switching the voltage on c13 starts to decay slowly and goes on like that as long as the l6565 does not switch. even during this time the voltage on c13 must not go below the minimum value that correctly supplies the linear regulator. to have maximum headroom the le50cz has been used, which features less than 0.5v dropout. besides, this part has lower quiescent current and the low dropout allows keeping a lower average input voltage, which helps reduce the actual load on the converter. as pwm restarts, the voltage on c13 quickly builds up and reaches v t in few milliseconds, which turns on q4 again restarting another cycle. c18 filters switching noise and allows a clean q4 turn-on. this time the phototransistor current will drop to zero before vcc falls below v ccoff and there will be no v cc undershoot. i ch v z v f v th C + r2 ----------------------------------- = t wake up C c3 i ch -------- - v ccon ? t restart c3 i ch -------- - v cchys ?
AN1657 application note 6/9 under steady-state operation the l6565 will be then shortly activated with a repetition rate t rep essentially given by t decay + t restart , where t decay is the time that v cc takes to span v cc hysteresis downward: , and i q is the quiescent consumption of the l6565. the input of the linear regulator is a sawtooth going from a peak ? v t to a valley that depends on t rep , c13 and the input current to the le50cz. also vcc is a sawtooth going up and down from v ccon to v ccoff . when the monitor is to resume its normal operation the off input will be grounded thus inhibiting q4 and disabling the off-mode circuit. however, the l6565 will be able to respond only when it is active. under the worst-case conditions, that is the off input is grounded just after the ic has been shut down, the l6565 will be responsive only after a time t rep . the power management software must take this into ac- count, re-enabling the loads on all of the outputs with a delay not shorter than t rep after grounding the off input. experimental results in the following tables the results of some bench evaluations are summarized. some waveforms at full load and off-mode under different line conditions are shown for user's reference, with some stress on the off-mode management of the "1w initiative" compliant design. as to the full-load performance there is no significant difference between the two designs presented: table 3 is common to both. same applies to table 4 that shows the consumption during suspend-mode opera- tion, a particular low-consumption mode envisaged by vesa standards. no special action is taken to keep the consumption below the limit (pin < 15w): the converter can comfortably fulfill this requirement. table 3. l6565-based, 90w smps for crt monitor: line regulation and full load efficiency table 4. l6565-based, 90w smps for crt monitor: suspend-mode consumption v ac [v] 88 110 132 176 220 264 pin [w] 103.9 101.0 99.6 98.4 98.4 98.6 vout [v] 199.90 199.87 199.84 199.83 199.86 199.82 79.45 79.40 79.39 79.37 79.38 79.39 14.12 14.13 14,14 14.14 14.16 14.16 -14.20 -14.20 -14.21 -14.21 -14.22 -14.22 6.58 6.60 6.62 6.63 6.64 6.65 pout [w] 90.59 90.61 90.63 90.64 90.69 90.69 h [%] 87.2 89.7 91.0 92.1 92.2 92.0 load conditions: 200v: 0.33a; 80v: 0.13a; 15v: 80 w ; 6.5v: 10 w ; 5v: 0.05a v ac [v] 88 110 132 176 220 264 pin [w] 12.6 12.9 13.3 13.3 13.4 13.6 load conditions: 200v: 0.005a, 80v: 0.065a; 15v = open; 6.5v: 10 w (heater on); 5v: 0.025a. pout ? 9.3w pin [w] 8.3 8.3 8.5 8.5 8.6 8.7 load conditions: 200v: 0.005a, 80v: 0.065a; 15v = 6.5v = open (heater off); 5v: 0.025a (ldo regulator supplied from 6.5v line). pout ? 6.3w. note: a) applicable to energystar compliant design only. b) efficiency improvement is due to onset of l6565 burst-mode operation. t decay c3 i q ------- - v cchys ?
7/9 AN1657 application note table 5. l6565-based, energystar ? compliant design: off-mode consumption table 6. l6565-based, "1w iniziative" compliant design: off-mode consumption figure 3. line filter (not tested for emi compliance) used during prototype bench evaluation figure 4. l6565-based, 90w smps for crt monitor: full load, v in =100 vdc (left), v in =380 vdc (right) figure 5. l6565-based, 90w smps for crt monitor: open feedback and ovp, main waveforms v ac [v] 88 110 132 176 220 264 pin [w] 1.4 1.4 1.4 1.4 1.5 1.6 load conditions: 200v = 80v = 15v = 6.5v = open; 5v: 0.025a. pout = 0.125w pin [w] 1.2 1.2 1.2 1.2 1.3 1.4 load conditions: 200v = 80v = 15v = 6.5v = open; 5v: 0.025a (ldo regulator supplied from 6.5v line) v ac [v] 88 110 132 176 220 264 pin [w] 0.25 0.31 0.38 0.49 0.62 0.75 load conditions: 200v = 80v = 15v = 6.5v = open; 5v: 0.025a. pout = 0.125w to the ac source b82732 47 mh, 1.3a epcos b81133 470 nf, x2 epcos to the smps b81133 680 nf, x2 epcos q1 drain q1 drain 200v output smoothed auxiliary winding pin 4 of l6565 gate drive smoothed auxiliary winding pin 4 of l6565
AN1657 application note 8/9 figure 6. l6565-based, energystar ? design: off-mode, vin = 100 vdc (left), vin = 380 vdc (right) figure 7. l6565-based, "1w initiative" compliant design: off-mode main waveforms figure 8. l6565-based, "1w initiative" compliant design: entering (left) and exiting (right) off-mode q1 drain l6565 vcc q1 drain l6565 vcc q1 drain l6565 vcc v (c13) pin2 l6565 lowest valley l6565 off hv gen. on phototr. on l6565 off hv gen. on phototr. off q1 drain l6565 vcc v (c13) pin2 l6565 vcc of l6565 v (c13) pin2 of l6565 soft-start q1 drain vcc of l6565 pin4 of l6565 collector of q4
information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences 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 stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectro nics. stmicroelectronics acknowledges the trademarks of all companies referred to in this document. the st logo is a registered trademark of stmicroelectronics ? 2003 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - brazil - canada - china - finland - france - germany - hong kong - india - israel - italy - japan -malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states. http://www.st.com 9/9 AN1657 application note conclusions two designs of a crt monitor smps based on the qr controller l6565 have been realized and the re- sults of their bench evaluation have been presented. the first one is a low-cost design able to meet current energystar ? requirements on off-mode consumption (pin < 2w). the second design, at the price of a slight part count and cost increase, meets the target pin <1w, which makes it compliant with iea's "1w initiative" as well as eligible for geea label. references and related documentation [1] "l6565 quasi-resonant smps controller" datasheet [2] "l6565 quasi-resonant controller" (an1326) [3] "25w quasi-resonant flyback converter for set-top box application using the l6565" (an1376) [4] "eval6565n, 30w ac-dc adapter with the l6565 quasi-resonant pwm controller" (an1439)
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