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october 2007 rev 2 1/14 AN1344 application note vipower: 108 w power supply using viper100a-e introduction the viper100a-e is designed to deliver 100 w for the upper voltage range or 50 w for universal input. this application note describes a power supply that delivers over 100 w for both voltage ranges using a voltage doubler in the front end. the viper100a-e combines a state-of-the-art pwm circuit along with an optimized 700 v avalanche rugged vertical power mosfet. it is part of stmicroelectronics? propri etary vipower, (vertical intelligent power). it uses a fabrication process, which allows the integration of analog control circuits with vertical power device on the same chip. this document covers the implementation and results for achieving 18 v at 6 a power supply that runs from both european and domestic mains. (90-132 v ac and 180- 264 v ac , 47-63 hz). www.st.com
contents AN1344 2/14 contents 1 key features of the vi per100a-e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 general circuit descripti on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 transformer construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4 burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5 thermal consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6 overcurrent limiting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 7 transient response 50% step chang e . . . . . . . . . . . . . . . . . . . . . . . . . . 12 8 output ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9 emi consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 10 performance and cost consideration . . . . . . . . . . . . . . . . . . . . . . . . . . 12 11 conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 12 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
AN1344 key features of the viper100a-e 3/14 1 key features of the viper100a-e adjustable switching frequency up to 20 khz current mode control burst mode operation in standby mode, meets "blue angel" undervoltage lock-out with hysteresis integrated start-up supply avalanche rugged overtemperature protection primary or secondary regulation figure 1. board layout the power supply has low ripple voltage, good transient response, and be able to current limit by power limiting and cycling on and off during a hard short. one use of this application is to replace a bulky 60 hz transformer wit h a lighter, better regulated, more efficient alternative for an audio or entertainment system.
general circuit description AN1344 4/14 2 general circuit description the power supply has been designed for the upper voltage range. the lower voltage range utilizes a voltage doubler to raise the bulk vo ltage to 2 times the peak of the input line voltage. in the doubling mode, the current charges one capacitor for each phase of the line, therefore doubling the voltage. when sw1 is open, both capacitors are charged in series resulting in a bulk voltage equal to the peak of the line input. a wire jumper can be installed at production for units destined for countries using the lower range. the switching frequency operates at 100 khz. the output can deliver 18 v from no load to 6 a continuous. the mode of operation ranges from discontinuous at high line minimum load to continuous at low line max load. this mode of operation was chosen to minimize the high peak currents of the discontinuous mode of operation. the viper100a-e can be regulated in secondary mode with an optocoupler giving excellent regulation or in the primary mode. primary regulation works by regulating the v dd pin at the output of the auxiliary windin g. depending on the coupling of the transformer, a 15% regulation can be achieved. in this application, by taking advantage of the dual regulation, a current limit scheme is obtained. this viper100a-e advantage, along with the transformer design, constitutes the overcurrent circuit. the transformer is designed for a turn ratio of operation for a universal input and an inductance to run in continuous conduction mode at one-half the output load. t he coupling between the secondar y to auxiliary winding along with the viper100a-e dual regulation plays an important part in the current limit. under typical operation, the output is tightly regulated through u2 and u3, the optocoupler and tl431 respectively. as the output current increases, it causes the voltage at the auxiliary output to increase. r4 is selected to trim the voltage at v dd to reach 13 v when the output current exceeds the maximum limit. at this point, primary regulation takes over and the output starts to fold-back. the output uses an stmicroelectronics 100 v schottky diode for better efficiency. c9 and c10 are low esr capacitors which manage the ripple current. u3 provides the reference and the feedback to tightly regulate the output. c7, c8, and r6 form the feed back loop compensation to optimize st ability during transients. table 1. electrical specification parameter results input voltage 90-132 v ac with jumper in, 180 - 264 v ac no jumper output voltage j2 load regulation (0.6 to 6 a) from set point +/- 0.6% line regulation (at max load) +/- 0.05% efficiency 86% @120 v dc and 87% @ 375 v dc output ripple voltage 15 mv max input power at no load 1.5 w typical transient response, 50% load step +/- 350 mv, +/- 1.9%, 200 s settling time emi en55022 and fcc class b
AN1344 general circuit description 5/14 figure 2. electrical schematic table 2. transformer specification parameter value primary inductance 525 h primary leakage inductance 7.9 h core etd34 inductance rating (al factor) 329 nh/t note split primary - gapped core d3 stps20h100c t c14 2.2nf y1 cap r1 33 thermistor r14 470k j2 con2 1 2 d5 bzw50 - 180 c1 .1uf x cap c4 100pf 1kv close for 120vac c2 330uf 200v c20 330uf 200v r9 1k c6 4.7nf 50v r8 10 r12 470k c15 .1uf 50v f2 fuse 2.5a 5x20mm j1 con 1 2 3 1 2 3 c10 1800uf 25v r5 4.22k 1% r3 200 2w br1 600v 2a bridge 4 3 2 1 r11 20k 1% c16 .001uf 1kv r13 22 .5w d1 stta106 st 600v 1 2 u2 h11a817a 1 2 4 3 d2 1n4148 1 2 sw1 sw spst 1 2 18 v @ 6a c7 22nf 50v 2 x 6mh l1 2 1 4 3 r4 8.2 l4 10uh c11 470uf 25v r7 220 c12 .1uf 50v c17 .1uf x cap d4 nu c8 1uf 50v e34351e tx1 cramer coil 2 3 9 4 7 5 . . . . . . c5 180uf 16v u1 2 4 1 3 5 6 vd d source osc drain comp hs u3 tl431 2 3 1 r10 3.16k 1% r6 6.2k c9 1800uf 25v r0 0 r00 0 ohms viper100a -e
general circuit description AN1344 6/14 2.1 transformer construction figure 3. cross section of the transformer the transformer is wound with a split primary to reduce leakage inductance and minimize the snubbing needed. the auxiliary winding is placed on th e outside to achieve the coupling needed for the current limiting function. figure 4. pc board top legend and bottom foil (112 mm x 83 mm single sided)
AN1344 general circuit description 7/14 figure 5. voltage and current waveforms figure 5 illustrates the voltage drain to source, and the current thro ugh the viper100a-e. vr3 is the v rms across r3 to snub the diode. the maximum voltage drain to source measured 609 v out of the 700 v, specified maximum. the current shows the power supply being in continuous conduction mode with a peak of 2 a. the snubber r3-c4 reduces ringing thus lowering the maximum peak voltage on the power mosfet and reducing the emi. in these waveforms the transil, d3, was replaced by an rc clamp, (r2=39 k, 2 w and c3=4700 pf). the clamp circuit worked the same under normal operation, but during start up or during short circuit operation, the voltage on the drain of viper100a-e reached as high as 750 v. the device is avalanche rugged and was able to withstand the momentary energy. using the transil at this power level is preferred in order to reduce the stresses. table 3. component list quantity reference value part number 1 br1 600 v, 1.5 a bridge 2kbp06m 2 c1, c17 0.1 f, x cap p4610 2 c2, c20 330 f, 200 v p6116 1 c4 100 pf, 1 kv p4116 1 c5 180 f, 16 v p10245 1 c6 4.7 nf, 50 v p4793 1 c7 22 nf, 50 v p4517 1 c8 1 f, 50 v p10312 1 c9 1800 f, 25 v panasonic fc 1 c10 1800 f, 25 v p10283 1 c11 470 f, 25 v p6242c 2 c12, c15 0.1 f, 50 v p4923 1 c14 2.2 nf, y1 cap p10463
general circuit description AN1344 8/14 1 c16 0.001 f, 1 kv p4128 1 d1 600 v stmicroelectronics stta106 1 d2 1n4148 1 d3 2x10 a, 100 v stmicroelectronics stps20h100ct 1 d4 3.3nz nu 1 d5 stmicroelectronics bzw50-180 1 f2 2.5 a, 5x20 mm fuse 1 j1 con 1 j2 con2 1 l1 2x6 mh plk1084 1 l4 10 h m6007 1 r0 0 wire 1 r00 0 ? wire 1 r1 33 ? thermistor nw 96f3302 1 r3 200 ? , 2 w 1 r4 8.2 ? 1 r5 4.22 k ? , 1% 1 r6 6.2 k ? 1 r7 220 ? 1 r8 10 ? 1 r9 1 k ? 1 r10 3.16 k ? , 1% 1 r11 20 k ? , 1% 2 r12, r14 470 k ? , 1/4 w 1 r13 22 ? , 1/2 w 1 sw1 sw spst 1 tx1 cramer coil e34351e 1 u1 viper100a-e stmicr oelectronics viper100a-e 1 u2 h11a817a 1 u3 tl431 stmicroelectronics tl431z table 3. component list (continued) quantity reference value part number
AN1344 layout considerations 9/14 3 layout considerations some simple rules to improve the performance and minimize noise should be followed: 1. minimize power loops. switched power current paths inner loop area must be as small as possible. this can be achieved by carefu l layout of the printed circuit board. this avoids radiated and conducted emi noise, and improves efficiency by eliminating parasitic inductance, thus reducing or eliminating the need for snubbers and emi filtering. 2. use separate tracks for low level signal and power traces carrying fast switching pulses. this can be seen on the viper100a-e pin 4. ground is split between power and signal traces on the printed circuit lay out. when signal paths share the same trace as a power path, instabilities may result. the compensation co mponents, c7, r6, and c9 are on a separate trace connected directly to the source of the device. 4 burst mode when the output current is too low, the minimum on time, fixed by the internal blanking time, is too high to control the output voltage. in this case the burst mode operation takes over automatically. the viper100a-e switch stays off when the voltage on the compensation pin goes below 0.5 v. this results in missing cycles as shown in figure 6 . v in is 115 v ac , minimum output current is at 40 ma. figure 6. good burst mode as can be seen, there is a burst of pulses followed by a pause of 600 ms. this repetitive burst reduces power consumption while maintaining a negligible ripple on the output. the v dd voltage is stable, just above the low threshold of 8 v of the internal under voltage lock out. the under voltage lock out can be reached by further reducing the output current. as the current decreases, the v dd voltage on the primary side also decreases. when v dd falls below the under voltage lock out of 8 v, another type of burst mode appears which is controlled by the v dd voltage. this is calle d ?bad? burst mode (see figure 7 ) because it has drawbacks, but the output vo ltage is still under control.
burst mode AN1344 10/14 figure 7. bad burst mode at lighter load, the v dd voltage drops below the under voltage threshold, the start up circuit is reset, and the v dd capacitor charges back up to the high threshold of 11 v through the start up current source. as shown in figure 7 the reoccurrence of this cycle is about 300ms. the worst output voltage swing is 2.4 v, which occurs at 20ma. at no-load condition, the output voltage swing becomes negligible (45 mv). this mode of operation leads to the following drawbacks: 1. because the start up current source is turned on to supply the capacitor from a high voltage rail, efficiency is dramatically reduced. 2. the recurring period leads to as much as 13% variation in the output voltage. for this audio application it does not matter, but th e designer should review all aspects of operation. 3. below the minimum current of 40 ma, the dynamic behavior is very poor which is typical of all power supplies. if the demand of current occurs during the recharging phase, the output capacitor is discharged and normal operation returns only at the next starting phase. in conclusion for this design a 40 ma minimum load is needed, 0.6% of maximum load, to keep the unit in optimal performance. however, below this range, the output voltage is still under control and no stresses are applied to the unit. the transformer was optimized for the current scheme and not for blue angel. table 4. stand-by input power input voltage input wattage at no-load input wattage at 40 ma 90 v ac 0.85 w 1.77 w 115 v ac 1.1 w 1.8 w 132 v ac 1.3 w 1.86 w
AN1344 thermal consideration 11/14 5 thermal consideration temperature measurement was taken at room ambient of 24 c, convection air-cooled resulted in the viper100a-e tab temperature of 91.1 c at 115 v ac input with a 6 a output. results may vary depending on final application. 6 overcurrent limiting this power supply was designed for an audio application where music peaks can exceed the maximum current of the power supply. in a sound entertainment system it is imperative for the power supply to not shut down during such peaks. it is acceptable for the voltage to decrease as the current increases. this maintains constant power for the unit. under a short circuit condition, this unit cycles on and off or "hiccup mode". in figure 8 the output voltage versus the output current is shown. maximum output power reached is 163 w. the viper100a-e also has thermal shutdown with hysteresis that is located close to the power mosfet portion of the die, which protects it from exceeding the temperature limit of the i.c. figure 8. output voltage versus the output current figure 9. transient response 50% step change figure 10. output ripple figure 11. emi measurement
transient response 50% step change AN1344 12/14 7 transient response 50% step change the output current is modulated from 3 a to 6 a, 50% duty cycle at a line input of 115 v ac . the result is 322 mv or 1.8% dynamic regulation with a settling time of 500 microseconds. 8 output ripple the ripple was measured using an hp probe socket attached after the output connector. this minimizes stray noise being picked up by the scope probe ground lead, which shows up as high frequency noise. the top trace shows the reduction in cost from eliminating l4 and c11. this gives a ripple, at 6 a load, of 125 mv peak to peak. with the low pass filter the ripple is reduced to about 13 mv excluding voltage spikes. 9 emi consideration when dealing with emi, it is always best to reduce noise at its source. figure 11 shows fcc class b plots comparing emi at 6 amps load with snubber r3 and c4 in and out. the blue trace, or lower trace, has the rc snubber across the diode. the emi is reduced by 4 to 8 db. adding a 2w resistor and a capacitor here is much less expensive than adding across the line capacitors and inductors in the emi filter. this unit passed both en55022 class b and fcc class b. 10 performance and cost consideration this design has been optimized for performance. cost can be reduced by substituting a 17v zener for the tl431. the output regulation falls to the +/- 5% voltage set point, plus a +0.084/c temperature drift of the zener. the cost of the tl431 and 3 other passive components can then be eliminated. if more output ripple voltage can be tolerated, than l4 and c11 can be eliminated. 11 conclusion this design delivers over 100 w for both volt age ranges by utilizing the viper100a-e with a voltage doubler in the front end. the power supply has excellent regulation, current limiting, short circuit protection, meets both en55022 and fcc class b and best of all is from stmicroelectronics.
AN1344 revision history 13/14 12 revision history table 5. document revision history date revision changes 04-jan-2005 1 minor text changes 18-oct-2007 2 ? document reformatted no content change ? viper100a replaced by viper100a-e
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