application note AN253/1088 power supply design basics by p. antoniazzi in mains-supplied electronic systems the ac input voltage must be converted into a dc voltagewith the right value and degree of stabilization. figures 1 and 2 show the simplest rectifier circuits. in these basic configurations the peak voltage across the load is equal to the peak value of the ac voltage supplied by the transformer's secondary winding. for most applications the output ripple pro- duced by these circuits is too high. however, for some applications - driving small motors or lamps, for example - they are satisfactory. if a filter capacitor is added after the rectifier diodes the output voltage waveform is improved consider- ably. figures 3 and 4 show two classic circuits com- monly used to obtain continuous voltages starting froman alternatingvoltage. the figure 3 circuit uses a center-tappedtransformer with tworectifier diodes while the figure 4 circuit uses a simple transformer and four rectifier diodes. figure 1 : basic half wave rectifier circuit. figure 2 : full wave rectifier wich uses a cen- ter-tapped transformer. figure 3 : full wave rectified output from the transformer/rectifier combination is fil- tered by c1. figure 4 : this circuit performs identically to that shown in figure 3. figure 5 shows the continuous voltage curve obtai- ned by adding a filter capacitor to the figure 1 circuit. the section b-c is a straight line. during this time it is the filter capacitor that supplies the load current. the slope of this line increases as the current increa- ses, bringing point c lower. consequentlythe diode conduction time (c-d) increases, increasing ripple. with zero load current the dc outputvoltage is equal to the peak value of the rectified ac voltage. figure 6 shows how to obtain positive and negative outputsreferred to a common ground. useful design data for this circuit is given in figures 7, 8 and 9. in particular, the curves shown in figure 7 are helpful in determiningthe voltage ripple for a given load cur- rent and filter capacitor value. the value of the volt- age ripple obtained is directly proportional to the load current and inversely proportional to the filter capacitor value. aimed at system designers whose interest focusses on other fields, this note reviews the basic power supply design knowhow assumed in the rest of the book. 1/6
figure 5 : output waveforms from the half-wave rectifier filter. figure 8 : dc to peak ratio for half wave rectifi- ers. figure 6 : full-wave split supply rectifier. figure 9 : dc to peak ratio for full-wave rectifi- ers. figure 7 : ripple voltage vs. filter capacitor value (full-wave rectifier). figure 10 : dc characteristics of a 50 va non- regulated supply. application note 2/6
the performance of asupply commonlyused in con- sumer applications - in audio amplifiers, for example - is described in figure 10 and table 1. when a low ripple voltageis required an lc filter net- work may be used. the effect on the output voltage of this addition is shown in figure 11. as figure 11 shows, the residual ripple can be reduced by 40 db. but often the inductor is costly and bulky. often the degree of stability provided by the circuits described above is insufficient and a stabilizer circuit is needed. figure 12 shows the simplest solution and is satisfactory for loads of up to about 50ma. this circuit is often used as a reference voltage to apply to the base of a transistor of to the input of an op amp to obtain higher output current. the simplest example of a series regulator is shown in figure 13. in this circuit thetransistor is connected as a voltage follower and the output voltage is about 600 - 700mv lower than the zener voltage. the re- sistor r must be dimensioned so that the zener is correctly biased and that sufficient base current is supplied to the base of q1. for high load currents the base current of q1 is no longer negligible. to avoid that the current in the ze- ner drops to the point where effective regulation is not possible a darlington may be used in place of the transistor. when better performance is required the op amp cir- cuit shown in figure 14 is recommended.in this circuit theoutputvoltage is equalto the reference voltage ap- plied to the input of the op amp. with a suitable output buffer higher currents can be obtained. the outputvoltage of the figure 14 circuit can be va- ried by adding a variable divider in parallel with the zener diode and with its wiper connected to the op amp's input. the design of stabilized supplies has been simpli- fied dramatically by the introduction of voltage regu- lator ics such as the l78xx and l79xx - three-terminal series regulators which provide a very stable output and include current limiter and thermal protection functions. figures 16, 17 and 18 show how these circuits are used. refer to the da- tasheets for more information. figure 11 : ripple reduction produced by a sin- gle section inductance-capacitance filter. figure 12 : basic zener regulator circuit. figure 13 : the series pass zener-based regula- tor circuit can supply load currents up to about 100ma. table1 . mains (220v) secondary voltage dc output voltage (v o ) i o =0 i o = 0.1a i o =1a +20% +15% +10% 10% 15% 20% 28.8v 27.6v 26.4v 24v 21.6v 20.4v 19.2v 43.2v 41.4v 39.6v 36.2v 32.4v 30.6v 28.8v 42v 40.3v 38.5v 35v 31.5v 29.8v 28v 37.5v 35.8v 34.2v 31v 27.8v 26v 24.3v application note 3/6
figure14: the op-amp-based regulator can supply 100ma with excellent regulation. figure15: zener regulator circuit modified for low-noise output. figure16: a three terminal 1a positive regulator circuit is very simple and performs very well. application note 4/6
figure17: a three terminal 1a negative voltage regulator. figure18: complete 12v 1a split supply regulator circuit. 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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