Agilent Technologies 90B manuel d'utilisation

Application Note 90B DC POWER SUPPLY HANDBOOK[...]

3 TA BLE OF CONTENTS Introduction ................................................................................................................... ........................................ 6 Definitions .................................................................................................................... ............................[...]

4 Typical Switching Reg ulated Power Supplies ..................................................................................... .......... 27 Summ ary of Basic Switching Regulator Config urations ............................................................................ ... 30 SCR Regulation....................................................[...]

5 Constant Voltage Remo te Program ming With Voltage Control ....................................................................... 84 Program ming with Unity Voltage Gain ............................................................................................ ............. 84 Program ming with Variable Voltage Gain ...........................[...]

6 INTRODUCTION Regulated power supplies em ploy engineering techniques drawn from the latest advances in many disciplines such as: low-lev el, high-power, and wideband am plification techniques; operational amplifier and feedback principles; pulse circuit techniques; and the constantly expanding frontiers of solid state component developm ent. The [...]

7 A UTO-PA RA LLEL POWER SUPPLY SYSTEM A UTOMA TIC (A UTO) SERIES OPERA TION A master- slave series connection of the outputs of two or more power supplies used for obtaining a voltage greater than that obtainable from one supply. Auto-Series operation, which is perm issible up to 300 volts off ground, is characterized by one-knob control, equal or[...]

8 A UTOMA TIC (A UTO) TRACKING OPERA TION A m aster-slav e connection of two or more power supplies each of w hich has one of its output terminals in comm on with one of the output terminals of all of the other power supplies. Auto- T racking operation is characterized by one-k nob control, proportional output voltage from all supplies, and no inte[...]

9 CONSTA NT CURRE NT P OWER SUP PLY OUTPUT CHA RACTERISTICS CONSTA NT VOLTA GE POWER SUPPLY A regulated power supply that acts to maintain its output voltag e constant in spite of changes in load, line, temperature, etc. Thus, for a chang e in load resistance, the output voltage of this type of supply remains constant while the output current chang[...]

10 CONSTA NT VO LTA GE/CONS TANT CURRENT (CV/CC) OUTPUT CHARA CTERISTIC CONSTA NT VO LTA GE/CURRENT LIMITING (CV /CL) POWE R S UP P LY A supply sim ilar to a CV/CC supply except for less precise regulation at low values of load resistance, i.e., in the current limiting region of operation. One form of current lim iting is shown abov e. CONSTA NT VO[...]

11 CROWBA R CIRCUIT An overv oltage protection circuit that monitors the output v oltage of the supply and rapidly places a short circuit (or crowbar) across the output term inals if a preset voltage level is exceeded. CURRENT FOLDBA CK Another form of current limiting often used in fixed output v oltage supplies. For load resistance sm aller than [...]

12 LOA D EFFECT (LOA D REGULATION) Formerly known as load regulation, load effect is the chang e in the steady- state value of the dc output voltag e or current resulting from a specified change in the load current (of a constant-v oltage supply) or the load voltag e (of a constant-current supply), with all other influence quantities m aintained co[...]

13 TYPICA L OUTP UT IMPEDA NCE OF A CONSTANT VOLTA GE POWER SUPPLY PA RD (RIPPLE A ND NOISE) The term PARD is an acronym for "Periodic and Random deviation" and replaces the former term ripple and noise. PARD is the residual ac component that is superim posed on the dc output voltage or current of a power supply. I t is measured over a sp[...]

14 PROGRA MMING SPEED The maxim um tim e required for the output voltage or current to chang e from an initial v alue to within a tolerance band of the newly prog ramm ed value following the onset of a step chang e in the program ming input signal. Because the prog ramming speed depends on the loading of the supply and on whether the output is bein[...]

15 REMOTE SENSING ( REMOTE ERROR SENSING) A means whereby a constant voltage power supply m onitors and regulates its output voltag e directly at the load terminals (instead of the power supply output terminals). Two low current sensing leads are connected between the load terminals and special sensing terminals located on the power supply, permitt[...]

16 STA BILITY (SEE DRIFT) TEMPERA TURE COEFFICIENT For a power supply operated at constant load and constant ac input, the m aximum steady -state change in output voltag e (for a constant voltage supply) or output current (for a constant current supply ) for each degree change in the ambient tem perature, with all other influence quantities maintai[...]

17 PRINCIPLES OF OPERA TION Electronic power supplies are defined as circuits which transform electrical input power- -either ac or dc- -into output power-- either ac or dc. This definition thus excludes power supplies based on rotating machine principles and distinguishes power supplies from the more general categ ory of electrical power sources w[...]

18 A sim ple unregulated power supply consisting of only a rectifier and filter is not capable of providing a ripple free dc output voltag e whose value remains reasonably constant. To obtain ev en a coarse approximation of the ideal output characteristic of Figure 1, som e type of control element (reg ulator) must be included in the supply. Regula[...]

19 Ty pical Series Regulated Pow er Supply Figure 3 shows the basic feedback circuit principle used in Agilent series regulated power supplies. The ac input, after passing throug h a power transform er, is rectified and filtered. By feedback action, the series regulator alters its v oltage drop to keep the regu lated dc output voltag e constant des[...]

20 to variations of the line and load. Hence, their line and load reg ulation and transient recovery tim e* are superior to supplies using any of the other regulation techniques. These supplies also exhibit the lowest ripple and noise, are tolerant of ambient tem perature changes, and with their circuit sim plicity, have a hig h reliability. *Power[...]

21 E R -E S E S -E O I R = R R = R P (1) Then multiply ing both sides by RR RP, we obtain E R R P = E S R P + E S R R –E O R R .( 2 ) Figure 4 y i elds a second equation relating the am plifier output to its gain and voltag e input E O =E S (- A) (3) which when substituted in equation (2) and solv ed for Es yields E R R P E S = R P + R R (1+A) (4[...]

22 Figure 5. Operat i onal A mpl i fier w i t h DC I nput Signal A large electroly tic capacitor is then added across the output terminals of the operational am plifier. The impedance of this capacitor in the m iddle range of frequencies (where the overall g ain of the amplifier falls off and becomes less than unity ) is much lower than the impedan[...]

23 (2) The use of a fixed dc input voltage m eans that the output voltage can only be one polarity, the opposite of the reference polarity.** (3) The series regulator can conduct current in only one direction. This, tog ether with the fact that the rectifier has a given polarity , means that the power supply can only deliver current to the load, an[...]

24 minim izing size increases. Figure 7 shows an earlier A gilent power supply using SCR' s as the preregulating elem ents. Silicon Controlled Rectifiers, the semiconductor equiv alent of thyratons, are rectifiers which remain in a non- conductive state, even when forw ard voltage is provided from anode to cathode, until a positive trigg er pu[...]

25 half cycle of input ac and hold the v oltage drop across the series reg ulator constant in spite of changes in load current, output voltag e , or input line voltag e. Figure 8 shows how varying the conduction angle of the SCR's affects the amplitude of the output v oltage and current delivered by the SCR bridg e rectifier of Figure 7. An ea[...]

26 switching pow er transistors, fast recovery diodes, and new filter capacitors with low er series resistance and inductance, have propelled switching supplies to a position of great prominence in the power supply industry. Presently, switching supplies still have a strong g rowth potential and are constantly chang ing as better components becom e[...]

27 voltag e across it. In a switching supply , however, the input ac is rectified directly (Fig ure 9) and the filter capacitor is allowed to charge to a m uch higher voltag e (the peaks of the ac line). Since the energy stored in a capacitor = 0.5CV2, while its v olume (size) tends to be proportional to CV, storag e capability is better in a switc[...]

28 Included, but not show n, in the modulator chip are additional circuits that establish a minim um "dead tim e" (off time) for the switching transistors. T his ensures that both switching transistors cannot conduct sim ultaneously during m aximum duty cy cle conditions. Figure 10. Sw i t chi ng Regulated Constant Voltage Suppl y Ac Inru[...]

29 future switching supplies. Preregulated Sw itching Supply. Figure 11 shows another hig her power switching supply similar to the circuit of Figure 10 except for the addition of a triac prereg ulator. Operation of this preregulator is similar to the previously described circuit of Figure 7. Briefly, the dc input voltag e to the switches is held r[...]

30 catch diode) was not required in the two transistor reg ulators of Figures 10 and 11 because of their full-wav e rectifier configuration. Another item not found in the prev ious regulators is "flyback " diode CR F . This diode is connected to a third transformer winding which is bifilar wound with the primary. During the off periods of[...]

31 Figure 13. Basic Sw itching Regulator Conf i gurat ions Configuration B is a useful alternativ e to push-pull operation for lower power requirem ents It is called a forward, or feed- t hrough, conv erter because energy is transferred to the power transformer secondary imm ediately following turn- on of the switch. Although the ripple frequency i[...]

32 Figure 14 illustrates a ty pical SCR regulated supply w hose output is continuously v ariable down to near zero volts. Circuit operation is v ery similar to the SCR prereg ulators described previously , except that the SCR control circuit receives its input from the voltage com parison amplifier. The control circuit com putes the firing time for[...]

33 Figure 15. Ideal Const ant Current Pow er Supply O ut put Characteristi c Any one of the four basic constant v oltage reg ulators can also furnish a constant current output provided that its output voltag e can be varied down to zero, or at least over the output v oltage range required by the load. Besides the regulator, the reference and contro[...]

34 Figure 16. Constant Current Pow er Supply CONSTA NT V O LTAGE/CONSTA NT CURRENT (CV/CC) P O WER SUPP LY Because of its convenience, v ersatility, and inherent protection features, many Agilent supplies employ the CV/CC circuit technique shown in Figure 17. Notice that only low power lev el circuitry has been added to a constant voltag e supply t[...]

35 Figure 17. Constant Vol tage/Constant Current CV/CC Pow er Suppl y Figure 18 illustrates the output characteristic of an ideal CV/CC power supply. With no load attached (RL= ∞ ), I OUT = 0, and EOUT = E S , the front panel v oltage control setting. When a load resistance is applied to the output terminals of the power supply , the output curre[...]

36 Figure 18. Operat i ng Locus of a CV/CC Pow er Suppl y Full protection against any overload condition is inherent in the Constant Voltag e/Constant Current design principle because all load conditions cause an output that lies somew here on the operating locus of Fig ure 18. For either constant voltag e or constant current operation, the proper [...]

37 operation. Thus, the current limiting locus of Figure 19 slopes m ore than that of Figure 18, and the crossover “knee" is m ore rounded. A sharp knee indicates continuous reg ulation through the crossover region while a rounded k nee denotes loss of regulation before the crossov er value is reached. To avoid any possibility of performance[...]

38 regulating elements. Thus, current foldback is especially useful if the supply is operating in a remote location and a long term short-circuit occurs. For switching reg ulated supplies, current foldback does not significantly reduce dissipation within the supply. I t does, however, provide superior load protection as m entioned previously . All [...]

39 B. RFI Choke - Minimizes spik es at output of supply by slowing down turn- on of triac. C. Rectifier Damping N etwork - RC network protects other elements in supply against short-duration input line transients. D. Series Regulator Diode - Protects the series regulator ag ainst reverse v oltages which could be deliv ered by an active load or para[...]

40 possibility. The circuit insures that the power supply voltage across the load will nev er exceed a preset limit. This protection is valuable because of the extrem e voltage sensitivity of present-day sem iconductor devices. The basic elements used in m ost crowbars are: some method of sensing the output voltage, an SCR that will short the outpu[...]

41 2. The crowbar circuit creates an extra current path during norm al operation of the supply, thus changing the current that flows through the current m onitoring resistor. Diode CR1 keeps this extra current at a fixed level for which com pensation can then be made in the constant current com parator circuit. 3. In prereg ulated supplies the crow[...]

42 Figure 22A . Crow bar Response Figure 23 shows ty pical protection circuits that are used in Agilent switching regulated power supplies. Most of these protection circuits perform functions that are sim ilar to those of the linear supply of Figure 21. Howev er, their circuit placement, or the m anner in which they affect the operation of the supp[...]

43 Figure 23. Protect i on Circuits, Sw itchi ng Type Supply Additional Protection - Although not shown on Fig ure 23, all Agilent switching supplies contain some form of overcurrent protection, usually a current foldback circuit. Also included are remote sensing protection resistors and input protection components for the com parison amplifier. SP[...]

44 Figure 24. "Pi ggy-back" Pow er Supply As an illustrative exam ple, assume that the low v oltage rectifier supplying the series transistor of the "pi ggy- back" supply develops approxim ately 40 volts, and that the m ain voltage source is capable of providing a maxim um of 300 v olts. With 20 volts norm ally dropped across th[...]

45 dr op a t ap pr oxi mat el y 2 0 vol t s, le avi ng a pp ro xi mat el y 20 vol ts ac ro ss th e o ut pu t t e rmi nal s of th e " pi ggy-ba ck" supply. Agilent Technolog ies supplies may use any of three basic methods of controlling the high voltag e output of the Main Voltage Source: (1) the control sig nal from the Hig h Voltage Cont[...]

46 tens of kilov olts or more. Such a hig h-voltag e supply would cause noise problems, would be difficult to modulate or to prog ram rapidly, would be dang erous, very large, and would waste considerable power. Figure 25. A n Ideal Current Source Electronic current regulation is a m uch more tractable way to obtain hig h output impedance, although[...]

47 Figure 26. Im pedances Shunt i ng t he Load Degrade Current Regulati on As shown in Figure 27, the CCB design includes three key sections which determine its uni que regulating pro- perties-- the Program ming /Guard Amplifier, the Main Cu rrent Regulator, and the Voltag e Limit Circuit. The Program ming /Guard Amplifier is an independent, variab[...]

48 Its ohm ic value is large enoug h to give an adequate current m onitoring voltag e, yet small enoug h to minimize its temperature rise (and the resulting resistance change) caused by its own pow er dissipation. Figure 27. Precisi on Current Source Bl ock Diagram Returning to the g uard duties of the Programm ing/Guard Am plifier, the output of t[...]

49 limit m ode, a high- current transient can occur if the current regulator saturates while the instrum ent is still in voltag e limit. The Voltage Lim it Circuit in Constant Current Sources virtually eliminates v oltage or current overshoots and undershoots when going in and out of voltage lim it, without adding any significant leakage path acros[...]

50 output terminal and the g uard has no effect on the output impedance. The m eter still measures the output voltag e because the guard is at the same potential as the positiv e output terminal. The front-panel v oltmeter is internally connected to g uard; and if greater accuracy is needed, a voltmeter can be connected externally . Unlike other g [...]

51 Figure 28. Out put Characterist i cs of CV/ CC Supplies, Conventional vs. Extended Range Example of Extended Range Pow e r Supply Agilent Technologies uses two different desig n techniques in their extended range power supplies. In one ty pe, shown on Figure 29, extended rang e is achieved by adding a special tap switching network ahead of a sta[...]

52 The main secondary winding of the power transformer has three sections, each of w hich has a different turns ratio with respect to the prim ary winding . At the beginning of each half- cycle of the input ac, the control circuit determines w hether one, both, or none of the triacs will be fired. If neither triac is fired, the rectifier receives a[...]

53 Figure 30. Out put Pow er Plot The triac control circuit also monitors the unreg ulated dc to provide ac line compensation. Variations in the amplitude or frequency of the ac line modify the am plitude of the unregulated dc voltag e which, in turn, alter the position of the IOD1 and I OD2 decision lines. For example, both I OD lines decrease (mo[...]

54 The extended range power supply ov ercomes the latter problem through the use of series reg ulating transistors with higher v oltage ratings and with therm ally im proved heat sinks. The heat sinks allow the series transistors to be properly cooled during the worst case conditions that are encountered during rapid down- programm ing. In addition[...]

55 Figure 31. Bipol ar Pow er Supply / Amplifier Draw n as a CV/ CC Pow er Supply . The rear terminal strip on B PS/A instruments includes num erous control terminals to facilitate remo te resistance program ming of the CV or CC output in the power supply m ode or remote dc or ac program ming in the amplifier m ode. Digitally Controlled Pow er Sour[...]

56 Figure 32. Bipol ar Pow er Supply / Amplifier Draw n as an Amplifier Figure 33. Digi tal Vol t age Source Bl ock Di agram[...]

57 Additional circuits are also included to facilitate operation within the sy stems environm ent. The additional circuitry perform s interface, isolation, storage, overcurrent protection, and status feedback functions as explained in subsequent paragraphs. Interface and Isolation. Each input and output signal, to and from a DC PS, passes through i[...]

58 Status Feedback. T hree feedback lines are av ailable to furnish continuous status information to the controller. A flag line inform s the computer when new voltag e program ming data is being processed by the DVS. Current overload and latch lines are activ ated if the DVS experiences a current overload or latch condition. Digital Current S ourc[...]

59 A C A ND LOA D CONNECTIONS Modern power supplies are flexible, high- performance instrum ents designed to deliver a constant or controlled output with a m aximum of reliability and control versatility. I n many cases, howev er, the user inadvertently degrades this perform ance capability by m aking im proper wiring connections to the input or ou[...]

60 Point (GP ). 12. The CP should be connected to the GP as shown in Figures 40 through 43 (unless one load is already g rounded), mak i ng certain there is only one conductive path between these two points. 72 13. Connections between the power supply sensing and output terminals should be rem oved and using shielded two- wire cable, the power supp[...]

61 A utotransformers An autotransformer (or isolation transformer) connected between the ac power source and the power supply input terminals should be rated for at least 200% of the maximum rms current required by the power supply. Because a power supply input circuit does not draw current continuously, the input current wav e is not sinusoidal, a[...]

62 Figure 34. Im proper Load Connect i ons DC Distribution Terminals A single pair of terminals are designated as the positive and negative "DC Distribution Terminals" (DT's). These two terminals m ay be the power supply output, the B+ at the load, or a separate pair of term inals established expressly for distribution. Proper locati[...]

63 If rem ote sensing is employ ed, the DT 's should be located as close as possible to the load term inals - sensing leads should then be connected from the power supply sensing terminals to the DT' s (see Figure 36). (See Figure 47 for further details on rem ote sensing.) One pair of wires should be connected directly from the power sup[...]

64 The battery sym bol represents an ideal constant voltage source with perfect reg ulation and zero output impedance at all frequencies, but ev ery regulated power supply has some sm all output impedance at high frequencies. Thus a more exact circuit m odel for a power supply includes an equivalent source resistance and inductance as shown in Figu[...]

65 travel down the load distribution wires and falsely trigg er one of the other loads. Figure 37. Pow er Supply and Load Wiring Equivalent Ci rcui ts To be effective, the high frequency impedance of local decoupling capacitors C 0 , C1, C2, and C3 (Figure 38) must be lower than the im pedance of wires connected to the same load. Thus a decoupling [...]

66 Figure 38. Local Decoupl i ng Capacitors The ideal concept of a single "quiet" ground potential is a snare and a delusion. No two g r ound points have exactly the sam e potential. The potential differences in many cases are sm all, but even a difference of a fraction of a volt in two g round potentials will cause amperes of current to [...]

67 repeat, separating the dc distribution circuits from any conductive paths in common with ground currents will in general reduce or eliminate ground loop problems. Figure 39. Isol ating Gr ound Loop Pat hs from DC System The only way to av oid such common paths is to connect the dc distribution sy stem to ground with only one wire. Figure 39 illu[...]

68 DC Common One of the DC Distribution Terminals should be designated as the "DC Common Point” (CP). There should be only one DC Com mon Point per DC System . If the supply is to be used as a positive source, then the minus DC Distribution Terminal is the DC Common Point; if it is to be a negative source, then the plus DT is the CP. Here ar[...]

69 Figure 41. Preferred G r ound Connect i ons f or M ul tiple Loads, All Isolated Figure 42. Preferred G r ound Connect i ons f or Si ngle Grounded Loads c. Single Grounded Load -- T he load terminals of the g r ounded load must be desig nated as the DT's and the grounded term inal of the load is necessarily the CP (Figure 42). This method of[...]

70 connection to ground or chassis- -or when there are m ultiple loads and only one has an internal connection to ground or chassis (Fig ure 43). Figure 43. Gr ound Connect i ons f or M ul tiple Loads, One Grounded d. Multiple Loads, Tw o or More of Which are Individual ly Grounded -- This is an undesirable situation and must be elim inated if at a[...]

71 Figure 44. Gr ound Connect i ons f or M ul t iple Loads, Tw o or More G r ounded e. Load System Float ed as a DC Potent ial Above Ground In som e applications it is necessary to operate the power supply output at a fixed v oltage abov e (or below) ground potential. In these cases it is usually advantag eous to designate DC Com mon Point using wh[...]

72 DC Ground Point The CP should be connected to the GP as shown in Figures 40 through 43 (unless one load is already grounded), making certain there is only one conductive path between these two points. This connection should be such that the total impedance from the DC Comm on for example, be the separate ground term inal located on one of the po[...]

73 Some idea of how easily even the shortest leads can degrade the perform ance of a power supply at the load terminals can be obtained by comparing the output im pedance of a well-regulated power supply (typically of the order of 1 milliohm or less at dc and low frequencies) with the resistance of the various w ire sizes listed in the following ch[...]

74 Figure 48. Constant Vol t age Regul ator w it h Remot e Error Sensi ng Remote Sensing C onnections Connections between the power supply sensing and output terminals should be removed, and using shielded two-wire cable, the power supply sensing terminals should be connected to the DC Distribution Terminals as shown in Figure 49. Do not use the sh[...]

75 Figure 49. Remote Sensi ng Connect ions Typically, the sensing current is 10mA or less. To insure that the tem perature coefficient of the sensing leads will not sign ificantly affect the power supply temperature coefficient and stability specifications, it is necessary to keep the I R drop in the sensing conductors less than 20 times the power [...]

76 To reduce the degree of output overshoot w hich can result from accidentally opened remote sensing connections, many regulated power supplies include internally w ired resistors or small silicon diodes as show n in Figures 50 and 51. I f they are not part of the power supply, and if the power supply application inv olves long sensing leads, sens[...]

77 If the resistor config uration of Figure 50 is included by the m anufacturer or added by the user, it may be necessary to check that the power rating of this resistor is adequate, particularly for sizable sensing drops. Remem ber that the actual dissipation in the remote sensing protection resistors is ED2/R, where ED is the IR drop from either [...]

78 power supply im pedance at the load at high frequencies. However, the capacitor m ust be chosen with care if power supply oscillation is to be av oided, since any capacitor resonances or other tendency toward hig h impedance w ithin or near the bandpass of the power supply regulator will reduce loop stability . It is therefore comm on in extreme[...]

79 power supply system - this point must be designated as one of the two DT's for both power supplies. Thus there are exactly (N + 1) DT's in any sy stem, where N is the number of power supplies (excluding the possibility of parallel supplies sharing the same distribution terminals or series pow er supplies with unused intermediate term i[...]

80 REMOTE PROGRA MMING Remote prog ramming , a feature found on many Ag ilent power supplies, permits control of the reg ulated output voltag e or current by means of a rem otely varied resistance or v oltage. I t is generally accom plished by restrapping the supply 's rear terminals so that the front panel control is disabled and a rem ote co[...]

81 Figure 54. Constant Vol t age Suppl y with Resi stance Programming Program ming a power supply with a 200 ohms/v olt programm ing coefficient to an output level of 30 volts would require and R P of 6K. The power supply w ill force through this prog ramming resistor a 5mA constant current thus resulting in 30 v olts across the power supply output[...]

82 Figure 55. Remote Programmi ng Connect i ons The wire size of the program ming leads m ust be adequate to withstand any program ming surges (consider effects of any larg e storage capacitors which hav e to be charged or discharged through the prog ramming leads). The temperature coefficient of a v ery long program ming leads m ay degrade power s[...]

83 ohms. It appears at first g lance that the circuit of Figure 56B also has one drawback - - nam ely, the output voltag e must always be switched in ascending or descending sequence. As Figu re 56C shows, however, the sam e voltage divider can hav e its tap points returned to the switch contacts in any sequence, perm itting output voltag e values [...]

84 causing the output v oltage to rise to some v alue higher than the m aximum voltage rating of the supply . With some loads this could result in serious dam age. To protect loads from accidental opening of the remote programming leads, a zener diode should be placed directly across the power supply programming terminals. This zener diode is selec[...]

85 basis. Programming wi th Variable Voltage Gain Figure 58 illustrates the m ethod by which the power supply can be program med using an external v oltage with a voltag e gain dependent upon the ratio of R P to R R . Note that this method is no different from the circuit normally used for constant voltage control of the output except that an exter[...]

86 In situations w here only low program ming v oltages are being used, forward conducting silicon diodes (0.7V per junction) can be used in place of zener diodes. CONSTA NT CURRE NT RE MOTE PROGRA MMING Most of the general principles discussed under Constant Voltage Prog ramm ing are also applicable when considering rem ote programm ing for consta[...]

87 Figure 59. Ideal Remot e Programmi ng Charact eri st i cs As Figure 60 indicates, all power supplies dev i ate somewhat from the ideal. The application of a short-circuit across the program ming term inals results in an output voltage which is slig htly different from zero (ty pically between +20 m illivolts and -50 m illivolts). While the linea[...]

88 accuracy will deliv er zero volts with z ero programm ing resistance. Thus, the first step in improving the program ming accuracy of Figure 60 is to short the prog ramming terminals and note the output v oltage. Norm ally, this voltag e will be slightly negative. I f this is not the case the comparison am plifier packag es can sometim es be inte[...]

89 than the new output voltag e being program med. When this exponential rise reaches the newly prog rammed voltag e level, the constant voltage amplifier resum es its normal regu lating action and holds the output constant. Thus, the rise time can be determ ined using a universal tim e constant chart or the formula shown in Figure 61. If no load r[...]

90 Figure 62. Speed of Response - Programmi ng Dow n Since up-prog ramming speed is aided by the conduction of the series regulating transistor, while downprogram ming norm ally has no active elem ent aiding in the discharge of the output capacitor, laboratory power supplies normally program upward m ore rapidly than downward. In m any Agilent labo[...]

91 OUTPUT VOL T A GE A ND CURRENT RA TINGS DUTY CYCLE LOA DING In som e applications the load current varies periodically from a minim um to a maxim um v alue. At first it mig ht seem that a reg ulated power supply having a current rating in excess of the average load requirement (but less than the peak load value) would be adequate for such applic[...]

92 peak load condition. Figure 63. Short- t erm O verload Equivalent Ci rcui t and O utput Volt age Thus, the equations can be used to evaluate whether the v oltage sag and recov ery time resulting from a ov erload condition lie within acceptable lim its, permitting the use of a power supply having a current rating less than the[...]

93 peak load dem and. For short term overloads, a quick approximation can be made to determ ine the amount of vol ta ge s ag: (I P – I L ) ∆ T ∆ V ≈ = C O where: ∆ V = Th e vo lt a ge s ag E NORM I P = R L PE A K = Peak load current demand, I L = The current limit or constant current setting, Co = The output capacitor (in farads), and ∆[...]

94 DUA L OUTPUT USING RE SISTIVE DIV I DE R Often it is required to use both a positive and neg ative dc power source having approximately the same voltag e and current capability. I t might seem reasonable to meet such requirements using a single regulated dc supply with a resistive v oltage div i der center-tapped to g round. Figure 65 shows, how[...]

95 Figure 64B. Reverse Current Loading Solut i on. Figure 65. Center- t apped Pow er Supply O ut put[...]

96 PA RA LLEL OPERA TION The operation of two constant voltag e power supplies in parallel is normally not feasible because of the larg e circulating current which results from even the sm allest voltage difference which inev itably exists between the two low impedance sources. Howev er, if the two power supplies feature CV /CC or CV/CL automatic c[...]

97 of current monitoring resistors in the master and slave supplies, the output current contribution will alw ays be equal regardless of the output v oltage or current requirement of the load. Normally , only supplies having the sam e model num ber should be connected for Auto-P arallel operation, since the two supplies must hav e the same voltag e[...]

98 Figure 67. A uto-Series Operat i on of Tw o Suppl i es Comparing Figure 67 with previous block diagram s for the constant voltage power supply, there is no difference in the circuit location of Resistor R2 and the front panel voltag e control normally found in Ag ilent laboratory ty pe power supplies. Thus, Auto-Series operation can be achiev ed[...]

99 A UTO TRA CKING OPERATION Auto- T racking or automatic track ing operation of power supplies is sim ilar to Auto- Series operation except that the master and slav e supplies have the same output polarity with respect to a comm on bus or ground. Figure 68 shows two supplies connected in Auto-Track ing with their neg ative output terminals connect[...]

100 As Figure 69 indicates, it is only necessary to add a single external current m onitoring resistor to a rem ote program ming constant v oltage power supply in order to convert it to constant current operation. (Also any remote sensing protection resistor or diode connected inside the supply from –S to - OUT must be remov ed.) Because the prop[...]

101 PERFORMA NCE MEA SUREMENTS CONSTA NT V O LTA GE P OWER S UP P LY MEA S UREMENTS Figure 70 illustrates a setup suitable for the m easurement of the six m ost important operating specifications of a constant voltag e power supply: source effect, load effect, PARD, load effect transient recovery tim e, drift, and temperature coefficient. The autom[...]

102 Figure 70. Constant Vol t age M easurement Setup Failure to connect the m onitoring instrum ent to the proper points shown in Figure 71 will result in the measurem ent not of the power supply characteristics, but of the power supply plus the resistance of the leads between its output terminals and the point of connection. Ev en using clip leads[...]

103 A . FRONT PA NEL B. REA R PA NEL Figure 71. Proper Connecti ons f or M oni t ori ng and Load Leads Check Curr ent Limit Contr ol Setting. When measuring the constant voltage perform ance specifications, the constant current or current limit control must be set w ell above the maxim um output current that the supply w ill draw. The onset of cons[...]

104 supply, connect both leads to either the positiv e or the negative sensing term inals, whichever is g rounded to chassis. Signals on the face of the CR T as a result of either of these tests are indicative of shortcom ings in the measurem ent setup. The most likely causes of these defects and proper corrective measures are discussed further und[...]

105 The power supply w ill perform w ithin its load effect specification at any rated output v oltage combined with any rated input line v oltage. CV PA RD (Ripple and Noise) Definiti on: The term PARD replaces the former t erm ripple and noise. PARD is t he Periodic and Random Deviation of the dc output voltage from its average value, over a speci[...]

106 Figure 72. Measurement of PARD (Ripple and Noise) for a CV Suppl y Either a twisted pair or preferably a shielded two- wire cable should be used to connect the output terminals of the power supply to the v ertical input terminals of the scope. When using shielded two wire, it is essential for the shield to be connected to ground at one end only[...]

107 measurem ents where both the power supply and the oscilloscope case are connected to g r ound (e. g., if both are rack- mounted), it may be necessary to use a differential scope with floating input as show n in Figure 72C. I f desired, two single- conductor shielded cables may be substituted in place of the shielded two- wire cable. Because of [...]

108 Noise Spik e Measurements When a high frequency spike measurem ent is being made, the oscilloscope m ust have a bandwidth of 20MHz or more. Measuring noise with an instrument that has insufficient bandwidth m ay conceal high frequency spikes detrimental to the load. The test setups illustrated in Figures 72A and 72B are generally not acceptable[...]

109 CV Load Effect Transient R ecovery Time (Load Transient Recov ery) Definition: The time "X" for the output voltage to recover and to stay within "Y" millivolts of the nom inal output voltage following a "Z" amp step change in load current, where: "Y" is specified separately for each model but is generally[...]

110 transient recovery time of a power supply , the spike amplitude for load switching tim es of less than 1 microsecond cannot be accurately determined, unless a very w i deband scope is used. Of all power supply specifications, transient recovery time is subject to the widest variation in definition, and is not defined at all by som e power suppl[...]

111 CV Drift (Stability ) Definition: The change in output voltage (dc to 20Hz) for the first eight hours following a 30 minute warm-up period. During the warm-up and measurement interval all parameters, such as load resistance, ambient temperature, and input line voltage are held constant. Drift includes periodic and random dev iations over a band[...]

112 downprogram ming. This is done to present the worst possible conditions for prog ramming in each direction. A method for m easuring the program ming speed of an Agilent power supply is as follows: Figure 77. CV Programming Speed Test Setup 1. Restrap the power supply rear barrier strip for rem ote resistance programm ing, constant voltag e. The[...]

113 and the output voltag e (EOUT) in both the up and down programm ing directions. Figure 78. Ty pical Programming Speed Waveforms The constant voltage prog ramming speed of a power supply using a rem ote programming voltage is identical to the speed obtained when using a remote resistance prov ided that the remote voltag e changes rapidly enough.[...]

114 the power supply w hich will be shorted to ground. All constant current measurem ents are made in term s of the change in v oltage across this resistor; the current performance is calculated by dividing these voltag e changes by the ohmic v alue of RM. Figure 79. Constant Current M easurement Set up Many of the precautions listed for the prev i[...]

115 Figure 80. Four-Terminal Current M oni t ori ng Resi st or Keep Temperature of R M Constant Resistor R M should be protected ag ainst stray air currents (open doors or windows, air conditioning vents), since these will change the resistance v alue, degrading the stability and temperature coefficient m easurements. Check Voltage C ontrol Setting[...]

116 Figure 81. External Vol t met er M easurement Error on CC Pow er Supply CC Source Effect (Line Regulation) Definition: The change ∆ I OUT in the steady state value of dc output current due to a change in ac input voltage over the specified range from low line (e. g., 104 volts) to high line (e. q., 127 volts), or from high line to low line. M[...]

117 Most of the comm ents pertaining to the ground loop and pick up problems associated with constant v oltage ripple and noise measurem ent also apply to the measurem ent of constant current ripple and noise. Figure 82 illustrates the most im portant precautions to be observed when m easuring the ripple and noise of a constant current supply. The [...]

118 Figure 82. Measurement of PARD for a CC Power Suppl y[...]

119 CC Temperature Co efficient Definition: The change in output current per degree Celsius change in the ambient temperature following a 30 minute warm-up. D uring the measurement interval the ac line voltage, output current setting and load resistance are held constant. The constant current power supply m ust be placed in an oven and operated ove[...]

120 INDEX A AC power, input connections input wire size ................................................................................................................... 61 interchanging ac and acc leads .......................................................................................... 60 interchanging ac and g round leads ...............[...]

121 constructed from constant voltage supply ......................................................................... 99 definition ............................................................................................................................. 8 limitations .............................................................................[...]

122 E Efficiency , definition ......................................................................................................... ............ 11 of preregulated supplies .................................................................................................... 23 of SCR regulated supplies ...........................................[...]

123 of high performance constant current supply .................................................................... 49 measurement method .............................................................................................. 112, 119 I nrush current, definition ...............................................................................[...]

124 transient recovery .................................................................................................... 107, 117 Piggy -Back reg ulator, definition ............................................................................................... .... 44 Power supply /amplifier ......................................................[...]

125 S Safety ground, in power cord ................................................................................................... ..... 68 Sampling resistor (see current monitoring resistor) SCR's, definition .............................................................................................................. ............. 24 in [...]

126 A gil ent Technologi es’ Test and Measurement Support , Services, and A ssi st ance Agilent Technolog ies aims to m aximize the v alue you receive, while m inimizing y our risk and problems. We strive to ensure that y ou get the test and measurem ent capabilities you paid for and obtain the support you need. Our extensive support resources an[...]