? february 2000 1/6 1 introduction most power supplies and associated logic circuitry need a means of powering up the switching regulator, mosfets. this application note describes a bias power supply to meet these needs and also provide standby power when the main unit is off. the viper20a, a part of stmicroelectronics proprietary vipower (vertical intelligent power), is a current mode pwm with a 700 volt avalanche rugged mosfet. it uses a fabrication process which allows the integration of analog control circuits with a vertical power device on the same chip. it can provide an output with 10w power capability for wide range input, or 20w for single input voltage range. key features of the viper20a n adjustable switching frequency up to 200khz n current mode control n burst mode operation in stand-by mode, meets blue angel standards n undervoltage lock-out with hysteresis n integrated start-up supply n avalanche rugged n overtemperature protection n primary or secondary regulation general circuit description this demoboard (see schematic in fig. 1) is a 110 to 375 vdc input discontinuous flyback, working at 100 khz. the output can deliver 12 volts at 0.5 amps continuous. this circuit can be powered from the: main bulk capacitors of an off-line power supply, power factor correction output stage, or the ac line with the addition of a low current bridge rectifier. it benefits from the emi filter present in a typical power supply. the output uses a schottky diode for better efficiency. c7 is a low esr capacitor to manage the ripple current. ic2 provides the reference and the feedback to tightly regulate the output. cs5, cs6, and rs3 form the feedback loop compensation to optimize stability during transients. the demoboard can be tested with convection air. depending on the final application, air flow might be needed to keep the viper20a under the thermal shutdown limit. AN1246 - application note vipower: auxiliary bias power supply using viper20a j.s. lo giudice
2/6 AN1246 - application note figure 1: schematic electrical specification from measured results parameter results input voltage range j1 110 to 375 vdc output j2 12v from 0 to 0.5a load regulation (0 to 0.5a) from setpoint +/- 25mv or +/- 0.2% line regulation (at max load) +/- 1mv efficiency 84% at 120vdc and 79% at 375 vdc output ripple voltage 35 mv max input power at no load 0.65w max transient response, 50% load step +/-60 mv typical c12 220pf 1kv cs6 .47u rs5 1.37k r6 150 r7 5.23k rs10 nu f1 0.5 a pico fuse 16t c3 120uf 16v c9 2.2n y cap 128t c7 680uf 16v j1 con 1 2 cs4 3.9nf ds2 stps1h100a rs4 1.6 d4 nu 14t r11 2.7k j2 con 1 2 r9 3.9 rs14 82k cs11 47n r8 1.2k cs5 33n c10 220pf 1kv ic2 tl431 a c r ds3 ll4148 rs2 4.99k c2 4.7uf 400v l2 10uh c8 330uf 16v ic1 viper20adip 2 3 8 7 1 6 4 5 vdd source drain drain osc drain comp drain d1 byt11-600 rs3 5.1k cramer coil csm 1608-027 t1 3 2 4 1 6 7 ic3 h11a817a 1 2 4 3
3/6 AN1246 - application note pc board top legend bottom foil and surface mount components * parts available from stmicroelectronics. s in legend denotes a surface mount part. quantity reference description value 1c2 f4.7 m f 400v 1c3 120 m f 16v (low esr) 1 cs4 3.9nf 50v 1 cs5 33nf 50v 1cs6 47 m f 25v 1c7 680 m f 16v (low esr) 1c8 330 m f 16v (low esr) 1 c9 y1 rated safety cap 2.2nf 1 c10 220pf 1kv 1cs11 47 m f 50v 1 c12 220pf 1kv 1 d1 *byt11-600 600v 1a ultrafast 1 ds2 *stps1h100a 100v 1a schottky 1 ds3 ll4148 1 f1 fuse 0.5 a pico 1 ic1 *viper20adip 1 ic2 *tl431 1 ic3 h11a817a (optocoupler) 1 j1 connector terminal 1 j2 connector terminal 1 l2 coilcraft pch-27-103 10 m h 1rs2 4.99k w 1% 1/8w 1rs3 5.1k w 1/8w 1rs4 1.6 w 1/8w 1 rs5 1.37 w 1% 1/8w 1r6 150 w 1/4w 1r7 5.23k w 1% 1/4w 1r8 1.2k w 1/4w 1 r9 3.9 w 1/4w 1 r11 2.7k w 1/4w 1rs14 82k w 1/4w 1 t1 cramer coil csm 1608-027 component list
4/6 AN1246 - application note transformer specification connecting the demoboard this demo board has two connectors. connect a dc voltage source capable of delivering up to 400 volts dc to connector j1. be sure to connect the positive source to the + terminal of j1 and the negative to the - terminal before turning on the input power . the minimum voltage to operate this demo board is 110 volt dc. connect an electronic load or resistive load to j2. layout considerations some simple rules to improve performance and minimize noise should be followed: 1. minimize power loops. the switched power current paths inner loop area must be as small as possible. this can be accomplished by careful layout of the printed circuit board and the use of surface mount components. this avoids radiated and conducted emi noise, and improves the 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 viper20a pin 3 on the printed circuit lay out. when signal paths are sharing the same trace as a power path, instabilities may result. the compensation components, cs6, rs3, and cs4 are on a separate trace connected directly to the source of the device. burst mode when the output current is too low, the burst mode operation takes over automatically. this results in missing cycles as shown on the following scope wave form (figure 2). vin is 375 vdc iout is at no load. as can be seen, there is one pulse out of ten pulses to reduce power consumption. the output ripple is negligible. performance and cost consideration this demo board has been optimized for performance. cost trade off can be accomplished by configuring the viper for primary regulation. the output regulation will degrade to +/- 5% but the cost of the optocoupler, the tl431 and 5 other passive components can eliminated. if more output ripple voltage can be tolerated, than l2 and c8 can be eliminated. primary inductance 1.65mh core c40 ee16
5/6 AN1246 - application note figure 2: burst mode waveform 1
6/6 AN1246 - application note 1 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 results from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this p ublication are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectron ics products are not authorized for use as critical components in life support devices or systems without express written approval of stmicr oelectronics. the st logo is a registered trademark of stmicroelectronics ? 2000 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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