Agilent 90B - Manual

12 LOAD EFFECT (LOAD REGULATION) Formerly known as load regulation, load effect is the change in the steady-state value of the dc output voltageor current resulting from a specified change in the load current (of a constant-voltage supply) or the loadvoltage (of a constant-current supply), with all ...

14 PROGRAMMING SPEED The maximum time required for the output voltage or current to change from an initial value to within atolerance band of the newly programmed value following the onset of a step change in the programming inputsignal. Because the programming speed depends on the loading of the su...

15 REMOTE SENSING (REMOTE ERROR SENSING) A means whereby a constant voltage power supply monitors and regulates its output voltage directly at the load terminals (instead of the power supply output terminals). Two low current sensing leads are connected betweenthe load terminals and special sensing ...

17 PRINCIPLES OF OPERATION Electronic power supplies are defined as circuits which transform electrical input power--either ac or dc--intooutput power--either ac or dc. This definition thus excludes power supplies based on rotating machineprinciples and distinguishes power supplies from the more gen...

18 A simple unregulated power supply consisting of only a rectifier and filter is not capable of providing a ripplefree dc output voltage whose value remains reasonably constant. To obtain even a coarse approximation of theideal output characteristic of Figure 1, some type of control element (regula...

19 Typical Series Regulated Power Supply Figure 3 shows the basic feedback circuit principle used in Agilent series regulated power supplies. The acinput, after passing through a power transformer, is rectified and filtered. By feedback action, the seriesregulator alters its voltage drop to keep the...

20 to variations of the line and load. Hence, their line and load regulation and transient recovery time* are superiorto supplies using any of the other regulation techniques. These supplies also exhibit the lowest ripple and noise,are tolerant of ambient temperature changes, and with their circuit ...

22 Figure 5. Operational Amplifier with DC Input Signal A large electrolytic capacitor is then added across the output terminals of the operational amplifier. Theimpedance of this capacitor in the middle range of frequencies (where the overall gain of the amplifier falls offand becomes less than uni...

23 (2) The use of a fixed dc input voltage means that the output voltage can only be one polarity, the opposite ofthe reference polarity.** (3) The series regulator can conduct current in only one direction. This, together with the fact that the rectifierhas a given polarity, means that the power su...

24 minimizing size increases. Figure 7 shows an earlier Agilent power supply using SCR's as the preregulating elements. Silicon ControlledRectifiers, the semiconductor equivalent of thyratons, are rectifiers which remain in a non-conductive state,even when forward voltage is provided from anode to c...

25 half cycle of input ac and hold the voltage drop across the series regulator constant in spite of changes in loadcurrent, output voltage, or input line voltage. Figure 8 shows how varying the conduction angle of the SCR'saffects the amplitude of the output voltage and current delivered by the SCR...

26 switching power transistors, fast recovery diodes, and new filter capacitors with lower series resistance andinductance, have propelled switching supplies to a position of great prominence in the power supply industry.Presently, switching supplies still have a strong growth potential and are cons...

27 voltage across it. In a switching supply, however, the input ac is rectified directly (Figure 9) and the filtercapacitor is allowed to charge to a much higher voltage (the peaks of the ac line). Since the energy stored in acapacitor = 0.5CV2, while its volume (size) tends to be proportional to CV...

28 Included, but not shown, in the modulator chip are additional circuits that establish a minimum "dead time" (offtime) for the switching transistors. This ensures that both switching transistors cannot conduct simultaneouslyduring maximum duty cycle conditions. Figure 10. Switching Regulat...

29 future switching supplies. Preregulated Switching Supply. Figure 11 shows another higher power switching supply similar to the circuit of Figure 10 except for the addition of a triac preregulator. Operation of this preregulator is similar to thepreviously described circuit of Figure 7. Briefly, t...

30 catch diode) was not required in the two transistor regulators of Figures 10 and 11 because of their full-waverectifier configuration. Another item not found in the previous regulators is "flyback" diode CR F . This diode is connected to a third transformer winding which is bifilar wound ...

31 Figure 13. Basic Switching Regulator Configurations Configuration B is a useful alternative to push-pull operation for lower power requirements It is called aforward, or feed-through, converter because energy is transferred to the power transformer secondaryimmediately following turn-on of the sw...

32 Figure 14 illustrates a typical SCR regulated supply whose output is continuously variable down to near zerovolts. Circuit operation is very similar to the SCR preregulators described previously, except that the SCRcontrol circuit receives its input from the voltage comparison amplifier. The cont...

34 Figure 16. Constant Current Power Supply CONSTANT VOLTAGE/CONSTANT CURRENT (CV/CC) POWER SUPPLY Because of its convenience, versatility, and inherent protection features, many Agilent supplies employ theCV/CC circuit technique shown in Figure 17. Notice that only low power level circuitry has bee...

35 Figure 17. Constant Voltage/Constant Current CV/CC Power Supply 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 voltage control setting. When a load resistance is applied to the output term...

36 Figure 18. Operating Locus of a CV/CC Power Supply Full protection against any overload condition is inherent in the Constant Voltage/Constant Current designprinciple because all load conditions cause an output that lies somewhere on the operating locus of Figure 18.For either constant voltage or...

37 operation. Thus, the current limiting locus of Figure 19 slopes more than that of Figure 18, and the crossover“knee" is more rounded. A sharp knee indicates continuous regulation through the crossover region while a rounded knee denotes loss ofregulation before the crossover value is reached....

38 regulating elements. Thus, current foldback is especially useful if the supply is operating in a remote locationand a long term short-circuit occurs. For switching regulated supplies, current foldback does not significantlyreduce dissipation within the supply. It does, however, provide superior l...

39 B. RFI Choke - Minimizes spikes at output of supply by slowing down turn-on of triac. C. Rectifier Damping Network - RC network protects other elements in supply against short-duration input line transients. D. Series Regulator Diode - Protects the series regulator against reverse voltages which ...

40 possibility. The circuit insures that the power supply voltage across the load will never exceed a preset limit. This protection is valuable because of the extreme voltage sensitivity of present-day semiconductor devices. The basic elements used in most crowbars are: some method of sensing the ou...

41 2. The crowbar circuit creates an extra current path during normal operation of the supply, thus changing thecurrent that flows through the current monitoring resistor. Diode CR1 keeps this extra current at a fixedlevel for which compensation can then be made in the constant current comparator ci...

42 Figure 22A. Crowbar Response Figure 23 shows typical protection circuits that are used in Agilent switching regulated power supplies. Most ofthese protection circuits perform functions that are similar to those of the linear supply of Figure 21. However,their circuit placement, or the manner in w...

43 Figure 23. Protection Circuits, Switching Type Supply Additional Protection - Although not shown on Figure 23, all Agilent switching supplies contain some form of overcurrent protection, usually a current foldback circuit. Also included are remote sensing protection resistorsand input protection ...

44 Figure 24. "Piggy-back" Power Supply As an illustrative example, assume that the low voltage rectifier supplying the series transistor of the "piggy-back" supply develops approximately 40 volts, and that the main voltage source is capable of providing amaximum of 300 volts. With 2...

45 drop at approximately 20 volts, leaving approximately 20 volts across the output terminals of the "piggy-back"supply. Agilent Technologies supplies may use any of three basic methods of controlling the high voltage output of theMain Voltage Source: (1) the control signal from the High Vol...

46 tens of kilovolts or more. Such a high-voltage supply would cause noise problems, would be difficult tomodulate or to program rapidly, would be dangerous, very large, and would waste considerable power. Figure 25. An Ideal Current Source Electronic current regulation is a much more tractable way ...

47 Figure 26. Impedances Shunting the Load Degrade Current Regulation As shown in Figure 27, the CCB design includes three key sections which determine its unique regulating pro-perties--the Programming/Guard Amplifier, the Main Current Regulator, and the Voltage Limit Circuit. The Programming/Guard...

48 Its ohmic value is large enough to give an adequate current monitoring voltage, yet small enough to minimizeits temperature rise (and the resulting resistance change) caused by its own power dissipation. Figure 27. Precision Current Source Block Diagram Returning to the guard duties of the Progra...

49 limit mode, a high-current transient can occur if the current regulator saturates while the instrument is still involtage limit. The Voltage Limit Circuit in Constant Current Sources virtually eliminates voltage or current overshoots andundershoots when going in and out of voltage limit, without ...

50 output terminal and the guard has no effect on the output impedance. The meter still measures the outputvoltage because the guard is at the same potential as the positive output terminal. The front-panel voltmeter isinternally connected to guard; and if greater accuracy is needed, a voltmeter can...

51 Figure 28. Output Characteristics of CV/CC Supplies, Conventional vs. Extended Range Example of Extended Range Power Supply Agilent Technologies uses two different design techniques in their extended range power supplies. In one type,shown on Figure 29, extended range is achieved by adding a spec...

52 The main secondary winding of the power transformer has three sections, each of which has a different turnsratio with respect to the primary winding. At the beginning of each half-cycle of the input ac, the control circuitdetermines whether one, both, or none of the triacs will be fired. If neith...

53 Figure 30. Output Power Plot The triac control circuit also monitors the unregulated dc to provide ac line compensation. Variations in theamplitude or frequency of the ac line modify the amplitude of the unregulated dc voltage which, in turn, alterthe position of the IOD1 and IOD2 decision lines....

54 The extended range power supply overcomes the latter problem through the use of series regulating transistorswith higher voltage ratings and with thermally improved heat sinks. The heat sinks allow the series transistorsto be properly cooled during the worst case conditions that are encountered d...

55 Figure 31. Bipolar Power Supply/Amplifier Drawn as a CV/CC Power Supply. The rear terminal strip on BPS/A instruments includes numerous control terminals to facilitate remoteresistance programming of the CV or CC output in the power supply mode or remote dc or ac programming inthe amplifier mode....

57 Additional circuits are also included to facilitate operation within the systems environment. The additionalcircuitry performs interface, isolation, storage, overcurrent protection, and status feedback functions asexplained in subsequent paragraphs. Interface and Isolation. Each input and output ...

59 AC AND LOAD CONNECTIONS Modern power supplies are flexible, high-performance instruments designed to deliver a constant or controlledoutput with a maximum of reliability and control versatility. In many cases, however, the user inadvertentlydegrades this performance capability by making improper ...

60 Point (GP). 12. The CP should be connected to the GP as shown in Figures 40 through 43 (unless one load isalready grounded), making certain there is only one conductive path between these two points. 72 13. Connections between the power supply sensing and output terminals should be removed andusi...

61 Autotransformers An autotransformer (or isolation transformer) connected between the ac power source and the power supplyinput 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 continuous...

62 Figure 34. Improper Load Connections DC Distribution Terminals A single pair of terminals are designated as the positive and negative "DC Distribution Terminals" (DT's). These two terminals may be the power supply output, the B+ at the load, or a separate pair of terminalsestablished expr...

63 If remote sensing is employed, the DT's should be located as close as possible to the load terminals - sensingleads should then be connected from the power supply sensing terminals to the DT's (see Figure 36).(See Figure 47 for further details on remote sensing.) One pair of wires should be conne...

64 The battery symbol represents an ideal constant voltage source with perfect regulation and zero outputimpedance at all frequencies, but every regulated power supply has some small output impedance at highfrequencies. Thus a more exact circuit model for a power supply includes an equivalent source...

65 travel down the load distribution wires and falsely trigger one of the other loads. Figure 37. Power Supply and Load Wiring Equivalent Circuits To be effective, the high frequency impedance of local decoupling capacitors C 0 , C1, C2, and C3 (Figure 38) must be lower than the impedance of wires c...

66 Figure 38. Local Decoupling Capacitors The ideal concept of a single "quiet" ground potential is a snare and a delusion. No two ground points have exactly the same potential. The potential differences in many cases are small, but even a difference of a fractionof a volt in two ground pote...

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. Isolating Ground Loop Paths from DC System The only way to avoid such common paths is to connect the dc distribution system...

68 DC Common One of the DC Distribution Terminals should be designated as the "DC Common Point” (CP). There should beonly one DC Common Point per DC System. If the supply is to be used as a positive source, then the minus DCDistribution Terminal is the DC Common Point; if it is to be a negative ...

69 Figure 41. Preferred Ground Connections for Multiple Loads, All Isolated Figure 42. Preferred Ground Connections for Single Grounded Loads c. Single Grounded Load --The load terminals of the grounded load must be designated as the DT's and the grounded terminal of the load is necessarily the CP (...

71 Figure 44. Ground Connections for Multiple Loads, Two or More Grounded e. Load System Floated as a DC Potential Above Ground In some applications it is necessary to operate the power supply output at a fixed voltage above (or below) ground potential. In these cases it is usuallyadvantageous to de...

72 DC Ground Point The CP should be connected to the GP as shown in Figures 40 through 43 (unless one load is alreadygrounded), making certain there is only one conductive path between these two points. This connection should be such that the total impedance from the DC Common for example, be the se...

73 Some idea of how easily even the shortest leads can degrade the performance of a power supply at the loadterminals can be obtained by comparing the output impedance of a well-regulated power supply (typically ofthe order of 1 milliohm or less at dc and low frequencies) with the resistance of the ...

74 Figure 48. Constant Voltage Regulator with Remote Error Sensing Remote Sensing Connections Connections between the power supply sensing and output terminals should be removed, and using shieldedtwo-wire cable, the power supply sensing terminals should be connected to the DC Distribution Terminals...

75 Figure 49. Remote Sensing Connections Typically, the sensing current is 10mA or less. To insure that the temperature coefficient of the sensing leadswill not significantly affect the power supply temperature coefficient and stability specifications, it is necessaryto keep the IR drop in the sensi...

77 If the resistor configuration of Figure 50 is included by the manufacturer or added by the user, it may benecessary to check that the power rating of this resistor is adequate, particularly for sizable sensing drops.Remember that the actual dissipation in the remote sensing protection resistors i...

78 power supply impedance at the load at high frequencies. However, the capacitor must be chosen with care ifpower supply oscillation is to be avoided, since any capacitor resonances or other tendency toward highimpedance within or near the bandpass of the power supply regulator will reduce loop sta...

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 system, where N is the number of power supplies (excluding the possibility of parallel supplies sharing the same distribution terminals or series power su...

80 REMOTE PROGRAMMING Remote programming, a feature found on many Agilent power supplies, permits control of the regulated outputvoltage or current by means of a remotely varied resistance or voltage. It is generally accomplished byrestrapping the supply's rear terminals so that the front panel cont...

81 Figure 54. Constant Voltage Supply with Resistance Programming Programming a power supply with a 200 ohms/volt programming coefficient to an output level of 30 voltswould require and R P of 6K. The power supply will force through this programming resistor a 5mA constant current thus resulting in ...

82 Figure 55. Remote Programming Connections The wire size of the programming leads must be adequate to withstand any programming surges (considereffects of any large storage capacitors which have to be charged or discharged through the programming leads).The temperature coefficient of a very long p...

83 ohms. It appears at first glance that the circuit of Figure 56B also has one drawback -- namely, the output voltage mustalways be switched in ascending or descending sequence. As Figure 56C shows, however, the same voltagedivider can have its tap points returned to the switch contacts in any sequ...

84 causing the output voltage to rise to some value higher than the maximum voltage rating of the supply. Withsome loads this could result in serious damage. To protect loads from accidental opening of the remote programming leads, a zener diode should be placed directly across the power supply prog...

85 basis. Programming with Variable Voltage Gain Figure 58 illustrates the method by which the power supply can be programmed using an external voltage with avoltage 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 voltag...

86 In situations where only low programming voltages are being used, forward conducting silicon diodes (0.7V perjunction) can be used in place of zener diodes. CONSTANT CURRENT REMOTE PROGRAMMING Most of the general principles discussed under Constant Voltage Programming are also applicable whencons...

88 accuracy will deliver zero volts with zero programming resistance. Thus, the first step in improving theprogramming accuracy of Figure 60 is to short the programming terminals and note the output voltage.Normally, this voltage will be slightly negative. If this is not the case the comparison ampl...

90 Figure 62. Speed of Response - Programming Down Since up-programming speed is aided by the conduction of the series regulating transistor, whiledownprogramming normally has no active element aiding in the discharge of the output capacitor, laboratorypower supplies normally program upward more rap...

91 OUTPUT VOLTAGE AND CURRENT RATINGS DUTY CYCLE LOADING In some applications the load current varies periodically from a minimum to a maximum value. At first it mightseem that a regulated power supply having a current rating in excess of the average load requirement (but less than the peak load val...

93 peak load demand. For short term overloads, a quick approximation can be made to determine the amount ofvoltage sag: (I P – I L ) ∆ T ∆ V ≈ = C O where: ∆ V = The voltage sag E NORM I P = R L PEAK = Peak load current demand, I L = The current limit or constant current setting, Co = The output cap...

94 DUAL OUTPUT USING RESISTIVE DIVIDER Often it is required to use both a positive and negative dc power source having approximately the same voltageand current capability. It might seem reasonable to meet such requirements using a single regulated dc supplywith a resistive voltage divider center-ta...

96 PARALLEL OPERATION The operation of two constant voltage power supplies in parallel is normally not feasible because of the largecirculating current which results from even the smallest voltage difference which inevitably exists between thetwo low impedance sources. However, if the two power supp...

98 Figure 67. Auto-Series Operation of Two Supplies Comparing Figure 67 with previous block diagrams for the constant voltage power supply, there is nodifference in the circuit location of Resistor R2 and the front panel voltage control normally found in Agilentlaboratory type power supplies. Thus, ...

99 AUTO TRACKING OPERATION Auto-Tracking or automatic tracking operation of power supplies is similar to Auto-Series operation except thatthe master and slave supplies have the same output polarity with respect to a common bus or ground. Figure 68shows two supplies connected in Auto-Tracking with th...

100 As Figure 69 indicates, it is only necessary to add a single external current monitoring resistor to a remoteprogramming constant voltage power supply in order to convert it to constant current operation. (Also anyremote sensing protection resistor or diode connected inside the supply from –S to...

101 PERFORMANCE MEASUREMENTS CONSTANT VOLTAGE POWER SUPPLY MEASUREMENTS Figure 70 illustrates a setup suitable for the measurement of the six most important operating specifications of aconstant voltage power supply: source effect, load effect, PARD, load effect transient recovery time, drift, andte...

102 Figure 70. Constant Voltage Measurement Setup Failure to connect the monitoring instrument to the proper points shown in Figure 71 will result in themeasurement not of the power supply characteristics, but of the power supply plus the resistance of the leadsbetween its output terminals and the p...

103 A. FRONT PANEL B. REAR PANEL Figure 71. Proper Connections for Monitoring and Load Leads Check Current Limit Control Setting. When measuring the constant voltage performance specifications, the constant current or current limit controlmust be set well above the maximum output current that the su...

104 supply, connect both leads to either the positive or the negative sensing terminals, whichever is grounded tochassis. Signals on the face of the CRT as a result of either of these tests are indicative of shortcomings in themeasurement setup. The most likely causes of these defects and proper cor...

106 Figure 72. Measurement of PARD (Ripple and Noise) for a CV Supply Either a twisted pair or preferably a shielded two-wire cable should be used to connect the output terminals ofthe power supply to the vertical input terminals of the scope. When using shielded two wire, it is essential forthe shi...

107 measurements where both the power supply and the oscilloscope case are connected to ground (e. g., if both arerack-mounted), it may be necessary to use a differential scope with floating input as shown in Figure 72C. Ifdesired, two single-conductor shielded cables may be substituted in place of ...

108 Noise Spike Measurements When a high frequency spike measurement is being made, the oscilloscope must have a bandwidth of 20MHz ormore. Measuring noise with an instrument that has insufficient bandwidth may conceal high frequency spikesdetrimental to the load. The test setups illustrated in Figu...

109 CV Load Effect Transient Recovery Time (Load Transient Recovery) Definition: The time "X" for the output voltage to recover and to stay within "Y" millivolts of the nominaloutput voltage following a "Z" amp step change in load current, where: "Y" is specified sepa...

112 downprogramming. This is done to present the worst possible conditions for programming in each direction. Amethod for measuring the programming 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 remote res...

113 and the output voltage (EOUT) in both the up and down programming directions. Figure 78. Typical Programming Speed Waveforms The constant voltage programming speed of a power supply using a remote programming voltage is identical to the speed obtained when using a remote resistance provided that...

114 the power supply which will be shorted to ground. All constant current measurements are made in terms ofthe change in voltage across this resistor; the current performance is calculated by dividing these voltagechanges by the ohmic value of RM. Figure 79. Constant Current Measurement Setup Many ...

115 Figure 80. Four-Terminal Current Monitoring Resistor Keep Temperature of R M Constant Resistor R M should be protected against stray air currents (open doors or windows, air conditioning vents), since these will change the resistance value, degrading the stability and temperature coefficient mea...

116 Figure 81. External Voltmeter Measurement Error on CC Power 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., 1...

117 Most of the comments pertaining to the ground loop and pickup problems associated with constant voltageripple and noise measurement also apply to the measurement of constant current ripple and noise. Figure 82illustrates the most important precautions to be observed when measuring the ripple and...

118 Figure 82. Measurement of PARD for a CC Power Supply

120 INDEX A AC power, input connections input wire size ................................................................................................................... 61interchanging ac and acc leads .......................................................................................... 60in...