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90B
User Manual
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Agilent Technologies 90B User Manual

DC POWER SUPPLY HANDBOOK
Application Note 90B
TABLE OF CONTENTS
Introduction...........................................................................................................................................................6
Definitions ..............................................................................................................................................................6
Ambient Temperature..........................................................................................................................................6
Automatic (Auto) Parallel Operation ..................................................................................................................6
Automatic (Auto) Series Operation.....................................................................................................................7
Automatic (Auto) Tracking Operation................................................................................................................8
Carryover Time....................................................................................................................................................8
Complementary Tracking....................................................................................................................................8
Compliance Voltage ............................................................................................................................................8
Constant Current Power Supply ..........................................................................................................................8
Constant Voltage Power Supply..........................................................................................................................9
Constant Voltage/Constant Current (CV/CC) Power Supply..............................................................................9
Constant Voltage/Current Limiting (CV/CL) Power Supply............................................................................10
Crowbar Circuit.................................................................................................................................................11
Current Foldback...............................................................................................................................................11
Drift....................................................................................................................................................................11
Efficiency...........................................................................................................................................................11
Electromagnetic Interference (EMI)..................................................................................................................11
Inrush Current....................................................................................................................................................11
Load Effect (Load Regulation)..........................................................................................................................12
Load Effect Transient Recovery Time ..............................................................................................................12
Off-Line Power Supply......................................................................................................................................12
Output Impedance of a Power Supply...............................................................................................................12
PARD (Ripple and Noise).................................................................................................................................13
Programming Speed...........................................................................................................................................14
Remote Programming (Remote Control)...........................................................................................................14
Remote Sensing (Remote Error Sensing)..........................................................................................................15
Resolution..........................................................................................................................................................15
Source Effect (Line Regulation)........................................................................................................................15
Stability (see Drift)............................................................................................................................................16
Temperature Coefficient....................................................................................................................................16
Warm Up Time..................................................................................................................................................16
Principles of Operation.......................................................................................................................................17
Constant Voltage Power Supply........................................................................................................................17
3
Constant Current Power Supply ........................................................................................................................32
Constant Voltage/Constant Current (CV/CC) Power Supply............................................................................34
Constant Voltage/Current Limiting Supplies....................................................................................................36
Protection Circuits.............................................................................................................................................38
Special Purpose Power Supplies........................................................................................................................43
AC and Load Connections..................................................................................................................................59
Checklist For AC and Load Connections..........................................................................................................59
AC Power Input Connections............................................................................................................................60
LOAD CONNECTIONS FOR ONE POWER SUPPLY...................................................................................61
Load Connections For Two or More Power Supplies.......................................................................................78
Remote Programming.........................................................................................................................................80
Constant Voltage Remote Programming With Resistance Control...................................................................80
4
Constant Voltage Remote Programming With Voltage Control.......................................................................84
Constant Current Remote Programming............................................................................................................86
Remote Programming Accuracy........................................................................................................................86
Remote Programming Speed .............................................................................................................................88
Output Voltage and Current Ratings................................................................................................................91
Duty Cycle Loading...........................................................................................................................................91
Reverse Current Loading...................................................................................................................................93
Dual Output Using Resistive Divider................................................................................................................94
Parallel Operation..............................................................................................................................................96
Auto Parallel Operation.....................................................................................................................................96
Series Operation ................................................................................................................................................97
Auto-Series Operation.......................................................................................................................................97
Auto Tracking Operation...................................................................................................................................99
Converting a Constant Voltage Power Supply To Constant Current Output....................................................99
PERFORMANCE MEASUREMENTS ..........................................................................................................101
Constant Voltage Power Supply Measurements..............................................................................................101
CV Load Effect Transient Recovery Time (Load Transient Recovery)......................................................109
Constant Current Power Supply Measurements..............................................................................................113
Index...................................................................................................................................................................120
5

INTRODUCTION

Regulated power supplies employ engineering techniques drawn from the latest advances in many disciplines such as: low-level, high-power, and wideband amplification techniques; operational amplifier and feedback principles; pulse circuit techniques; and the constantly expanding frontiers of solid state component development.
The full benefits of the engineering that has gone into the modern regulated power supply cannot be realized unless the user first recognizes the inherent versatility and high performance capabilities, and second, understands how to apply these features. This handbook is designed to aid that understanding by providing complete information on the operation, performance, and connection of regulated power supplies.
The handbook is divided into six main sections: Definitions, Principles of Operation, AC and Load Connections, Remote Programming, Output Voltage and Current Ratings, and Performance Measurements. Each section contains answers to many of the questions commonly asked by users, like:
What is meant by auto-parallel operation? What are the advantages and disadvantages of switching regulated supplies? When should remote sensing at the load be used? How can ground loops in multiple loads be avoided? What factors affect programming speed? What are the techniques for measuring power supply performance?
In summary, this is a book written not for the theorist, but for the user attempting to solve both traditional and unusual application problems with regulated power supplies.

DEFINITIONS

AMBIENT TEMPERATURE

The temperature of the air immediately surrounding the power supply.

AUTOMATIC (AUTO) PARALLEL OPERATION

A master-slave parallel connection of the outputs of two or more supplies used for obtaining a current output greater than that obtainable from one supply. Auto-Parallel operation is characterized by one-knob control, equal current sharing, and no internal wiring changes. Normally only supplies having the same model number may be connected in Auto-Parallel, since the units must have the same IR drop across the current monitoring resistors at full current rating.
6
AUTO-PARALLEL POWER SUPPLY SYSTEM

AUTOMATIC (AUTO) SERIES OPERATION

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 permissible up to 300 volts off ground, is characterized by one-knob control, equal or proportional voltage sharing, and no internal wiring changes. Different model numbers may be connected in Auto-Series without restriction, provided that each slave is capable of Auto-Series operation.
AUTO-SERIES POWER SUPPLY SYSTEM
7

AUTOMATIC (AUTO) TRACKING OPERATION

A master-slave connection of two or more power supplies each of which has one of its output terminals in common with one of the output terminals of all of the other power supplies. Auto-Tracking operation is characterized by one-knob control, proportional output voltage from all supplies, and no internal wiring changes. Useful where simultaneous turn-up, turn-down or proportional control of all power supplies in a system is required.
AUTO-TRACKING POWER SUPPLY SYSTEM

CARRYOVER TIME

The period of time that a power supply's output will remain within specifications after loss of ac input power. It is sometimes called holding time.

COMPLEMENTARY TRACKING

A master-slave interconnection similar to Auto-Tracking except that only two supplies are used and the output voltage of the slave is always of opposite polarity with respect to the master. The amplitude of the slaves' output voltage is equal to, or proportional to, that of the master. A pair of complementary tracking supplies is often housed in a single unit.

COMPLIANCE VOLTAGE

The output voltage rating of a power supply operating in the constant current mode (analogous to the output current rating of a supply operating in the constant voltage mode).

CONSTANT CURRENT POWER SUPPLY

A regulated power supply that acts to maintain its output current constant in spite of changes in load, line, temperature, etc. Thus, for a change in load resistance, the output current remains constant while the output voltage changes by whatever amount necessary to accomplish this.
8
CONSTANT CURRENT POWER SUPPLY
OUTPUT CHARACTERISTICS

CONSTANT VOLTAGE POWER SUPPLY

A regulated power supply that acts to maintain its output voltage constant in spite of changes in load, line, temperature, etc. Thus, for a change in load resistance, the output voltage of this type of supply remains constant while the output current changes by whatever amount necessary to accomplish this.
CONSTANT VOLTAGE POWER SUPPLY
OUTPUT CHARACTERISTIC

CONSTANT VOLTAGE/CONSTANT CURRENT (CV/CC) POWER SUPPLY

A power supply that acts as a constant voltage source for comparatively large values of load resistance and as a constant current source for comparatively small values of load resistance. The automatic crossover or transition between these two modes of operation occurs at a "critical" or "crossover" value of load resistance Rc = Es/Is, where Es is the front panel voltage control setting and Is is the front panel current control setting.
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CONSTANT VOLTAGE/CONSTANT CURRENT
(CV/CC) OUTPUT CHARACTERISTIC

CONSTANT VOLTAGE/CURRENT LIMITING (CV/CL) POWER SUPPLY

A supply similar 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 limiting is shown above.
CONSTANT VOLTAGE/ CURRENT LIMITING
(CV/CL) OUTPUT CHARACTERISTIC
10

CROWBAR CIRCUIT

An overvoltage protection circuit that monitors the output voltage of the supply and rapidly places a short circuit (or crowbar) across the output terminals if a preset voltage level is exceeded.

CURRENT FOLDBACK

Another form of current limiting often used in fixed output voltage supplies. For load resistance smaller than the crossover value, the current, as well as the voltage, decreases along a foldback locus.

DRIFT

The maximum change in power supply output during a stated period of time (usually 8 hours) following a warm-up period, with all influence and control quantities (such as; load, ac line, and ambient temperature) maintained constant. Drift includes periodic and random deviations (PARD) over a bandwidth from dc to 20Hz. (At frequencies above 20Hz, PARD is specified separately.)

EFFICIENCY

Expressed in percent, efficiency is the total output power of the supply divided by the active input power. Unless otherwise specified, Agilent measures efficiency at maximum rated output power and at worst case conditions of the ac line voltage.

ELECTROMAGNETIC INTERFERENCE (EMI)

Any type of electromagnetic energy that could degrade the performance of electrical or electronic equipment. The EMI generated by a power supply can be propagated either by conduction (via the input and output leads) or by radiation from the units' case. The terms "noise" and "radio-frequency interference" (RFI) are sometimes used in the same context.

INRUSH CURRENT

The maximum instantaneous value of the input current to a power supply when ac power is first applied.
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LOAD EFFECT (LOAD REGULATION)

Formerly known as load regulation, load effect is the change in the steady-state value of the dc output voltage or current resulting from a specified change in the load current (of a constant-voltage supply) or the load voltage (of a constant-current supply), with all other influence quantities maintained constant.

LOAD EFFECT TRANSIENT RECOVERY TIME

Sometimes referred to as transient recovery time or transient response time, it is, loosely speaking, the time required for the output voltage of a power supply to return to within a level approximating the normal dc output following a sudden change in load current. More exactly, Load Transient Recovery Time for a CV supply is the time "X" required for the output voltage to recover to, and stay within "Y" millivolts of the nominal output voltage following a "Z" amp step change in load current --where:
TRANSIENT REC OVERY TIME
(1) "Y" is specified separately for each model but is generally of the same order as the load regulation
specification.
(2) The nominal output voltage is defined as the dc level halfway between the steady state output voltage
before and after the imposed load change.
(3) "Z" is the specified load current change, typically equal to the full load current rating of the

OFF-LINE POWER SUPPLY

A power supply whose input rectifier circuits operate directly from the ac power line, without transformer isolation.

OUTPUT IMPEDANCE OF A POWER SUPPLY

At any frequency of load change, ∆EOUT/∆IOUT. Strictly speaking, the definition applies only for a sinusoidal load disturbance, unless the measurement is made at zero frequency (dc). The output impedance of an ideal constant voltage power supply would be zero at all frequencies, while the output impedance for an ideal constant current power supply would be infinite at all frequencies.
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TYPICAL OUTPUT IMPEDANCE OF A CONSTANT
VOLTAGE POWER SUPPLY

PARD (RIPPLE AND 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 superimposed on the dc output voltage or current of a power supply. It is measured over a specified bandwidth, with all influence and control quantities maintained constant. PARD is specified in rms and/or peak-to-peak values over a bandwidth of 20Hz to 20MHz. Fluctuations below 20Hz are treated as drift. Attempting to measure PARD with an instrument that has insufficient bandwidth may conceal high frequency spikes that could be detrimental to a load.
DC OUTPUT OF POWER SUPPLY AND
SUPERIMPOSED PARD COMPONENT
13

PROGRAMMING SPEED

The maximum time required for the output voltage or current to change from an initial value to within a tolerance band of the newly programmed value following the onset of a step change in the programming input signal. Because the programming speed depends on the loading of the supply and on whether the output is being programmed to a higher or lower value, programming speed is usually specified at no load and full load and in both the up and down directions.
PROGRAMMING SPEED WAVEFORMS

REMOTE PROGRAMMING (REMOTE CONTROL)

Control of the regulated output voltage or current of a power supply by means of a remotely varied resistance or voltage. The illustrations below show examples of constant voltage remote programming. CC applications are similar.
REMOTE PROGRAMMING USING RESISTANCE CONTROL
REMOTE PROGRAMMING USING VOLTAGE CONTROL
14

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 between the load terminals and special sensing terminals located on the power supply, permitting the power supply output voltage to compensate for IR drops in the load leads (up to a specified limit).
POWER SUPPLY AND LOAD CONNECTED NORMALLY
POWER SUPPLY AND LOAD CONNECTED FOR REMOTE SENSING

RESOLUTION

The smallest change in output voltage or current that can be obtained using the front panel controls.

SOURCE EFFECT (LINE REGULATION)

Formerly known as line regulation, source effect is the change in the steady-state value of the dc output voltage (of a CV supply) or current (of a CC supply) due to a specified change in the source (ac line) voltage, with all other influence quantities maintained constant. Source effect is usually measured after a "complete" change in the ac line voltage; from low line to high line or vice-versa.
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STABILITY (SEE DRIFT)

TEMPERATURE COEFFICIENT

For a power supply operated at constant load and constant ac input, the maximum steady-state change in output voltage (for a constant voltage supply) or output current (for a constant current supply) for each degree change in the ambient temperature, with all other influence quantities maintained constant.

WARM UP TIME

The time interval required by a power supply to meet all performance specifications after it is first turned on.
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PRINCIPLES OF OPERATION

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 category of electrical power sources which derive electrical power from other energy forms (e. g., batteries, solar cells, fuel cells).
Electronic power supplies can be divided into four broad classifications:
(1) ac in, ac out-line regulators and frequency changers (2) dc in, dc out-converters and dc regulators (3) dc in, ac out-inverters (4) ac in, dc out
This last category is by far the most common of the four and is generally the one referred to when speaking of a "power supply". Most of this Handbook is devoted to ac in/dc out power supplies, although a brief description of a dc-to-dc converter is presented later in this section.
Four basic outputs or modes or operation can be provided by dc output power supplies:
• Constant Voltage: The output voltage is maintained constant in spite of changes in load, line, or
temperature.
• Constant Current: The output current is maintained constant in spite of changes in load, line, or
temperature.
• Voltage Limit: Same as Constant Voltage except for less precise regulation characteristics.
• Current Limit: Similar to Constant Current except for less precise regulation.
As explained in this section, power supplies are designed to offer these outputs in various combinations for different applications.

CONSTANT VOLTAGE POWER SUPPLY

An ideal constant voltage power supply would have zero output impedance at all frequencies. Thus, as shown in Figure 1, the voltage would remain perfectly constant in spite of any changes in output current demanded by the load.
Figure 1. Ideal Constant Voltage Power Supply Output Characteristic
17
A simple unregulated power supply consisting of only a rectifier and filter is not capable of providing a ripple free dc output voltage whose value remains reasonably constant. To obtain even a coarse approximation of the ideal output characteristic of Figure 1, some type of control element (regulator) must be included in the supply.

Regulating Techniques

Most of today's constant voltage power supplies employ one of these four regulating techniques: a. Series (Linear) b. Preregulator/Series Regulator c. Switching d. SCR

Series Regulation

Series regulated power supplies were introduced many years ago and are still used extensively today. They have survived the transition from vacuum tubes to transistors, and modern supplies often utilize IC's, the latest in power semiconductors, and some sophisticated control and protection circuitry.
The basic design technique, which has not changed over the years, consists of placing a control element in series with the rectifier and load device. Figure 2 shows a simplified schematic of a series regulated supply with the series element depicted as a variable resistor. Feedback control circuits continuously monitor the output and adjust the series "resistance" to maintain a constant output voltage. Because the variable resistance of Figure 2 is actually one or more power transistors operating in the linear (class A) mode, supplies with this type of regulator are often called linear power supplies.
Figure 2. Basic Series Regulated Supply
Notice that the variable resistance element can also be connected in parallel with the load to form a shunt regulator. However, this type of regulator is seldom used because it must withstand full output voltage under normal operating conditions, making it less efficient for most applications.
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Typical Series Regulated Power Supply

Figure 3 shows the basic feedback circuit principle used in Agilent series regulated power supplies. The ac input, after passing through a power transformer, is rectified and filtered. By feedback action, the series regulator alters its voltage drop to keep the regulated dc output voltage constant despite variations in the ac line, the load, or the ambient temperature.
The comparison amplifier continuously monitors the difference between the voltage across the voltage control resistor, RP, and the output voltage. If these voltages are not equal, the comparison amplifier produces an amplified difference (error) signal. This signal is of the magnitude and polarity necessary to change the conduction of the series regulator which, in turn, changes the current through the load resistor until the output voltage equals the voltage (EP) across the voltage control.
Figure 3. Series Regulated Constant Voltage Power Supply
Since the net difference between the two voltage inputs to the comparison amplifier is kept at zero by feedback action, the voltage across resistor R current I
flowing through RR is constant and equal to ER/RR. The input impedance of the comparison amplifier
P
is very high, so essentially all of the current I programming current I
is constant, EP (and hence the output voltage) is variable and directly proportional to RP.
P
Thus, the output voltage becomes zero if R
is also held equal to the reference voltage ER. Thus the programming
R
flowing through RR also flows through RP. Because this
P
is reduced to zero ohms.
P
Of course, not all series regulated supplies are continuously adjustable down to zero volts. Those of the OEM modular type are used in system applications and, thus, their output voltage adjustment is restricted to a narrow range or slot. However, operation of these supplies is virtually identical to that just described.

Pros and Cons of Series Regulated Supplies

Series regulated supplies have many advantages and usually provide the simplest most effective means of satisfying high performance and low power requirements.
In terms of performance, series regulated supplies have very precise regulation properties and respond quickly
19
to variations of the line and load. Hence, their line and load regulation and transient recovery time* are superior to 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 simplicity, have a high reliability.
*Power supply performance specifications are described in the Definitions section of this handbook.
The major drawback of the series regulated supply is its relatively low efficiency. This is caused mainly by the series transistor which, operating in the linear mode, continuously dissipates power in carrying out its regulation function. Efficiency (defined as the percentage of a supply's input power that it can deliver as useful output: P
OUT/PIN
), ranges typically between 30 and 45% for series regulated supplies. With the present need for
energy conservation, this inefficiency is being scrutinized very closely.
For many of today's applications, the size and weight of linear supplies constitute another disadvantage. The power transformer, inductors, and filter capacitors necessary for operation at the 60Hz line frequency tend to be large and heavy, and the heat sink required for the inherently dissipative series regulator increases the overall size.

The Series Regulated Supply - An Operational Amplifier

The following analogy views the series regulated power supply as an operational amplifier. Considering the power supply as an operational amplifier can often give the user a quick insight into power supply behavior and help in evaluating a power supply for a specific application.
An operational amplifier (Figure 4) is a high gain dc amplifier that employs negative feedback. The power supply, like an operational amplifier, is also a high gain dc amplifier in which degenerative feedback is arranged so the operational gain is the ratio of two resistors.
Figure 4. Operational Amplifier
As shown in Figure 4, the input voltage E voltage is fed back to this same summing point through resistor R input current to the amplifier can be considered negligibly small, and all of the input current I both resistors R
and RP. As a result:
R
is connected to the summing point via resistor RR, and the output
R
. Since the input impedance is very high, the
P
flows through
R
20
IR =
ER-E
R
S
R
=
ES-E
R
O
P
Then multiplying both sides by RRRP, we obtain
E
= ESRP + ESRR –EORR.(2)
RRP
Figure 4 yields a second equation relating the amplifier output to its gain and voltage input
E
=ES (-A) (3)
O
which when substituted in equation (2) and solved for Es yields
E
R RP
ES =
+ RR (1+A)
R
P
Normally, the operational amplifier gain is very high, commonly 10,000 or more. In equation (4)
If we let A → ∞ Then Es→ O
(1)
(4)
(5)
This important result enables us to say that the two input voltages of the comparison amplifier of Figure 4 (and Figure 3) are held equal by feedback action.
In modern well-regulated power supplies, the summing point voltage Es is at most a few millivolts. Substituting Es = 0 into equation (1) yields the standard gain expression for the operational amplifier
R
P
EO = -E
R
R
R
(6)
Notice that from equation 6 and Figure 4, doubling the value of RP doubles the output voltage.
To convert the operational amplifier of Figure 4 into a power supply we must first apply as its input a fixed dc input reference voltage E
(see Figure 5).
R
21
Figure 5. Operational Amplifier w i t h DC I nput Signal
A large electrolytic capacitor is then added across the output terminals of the operational amplifier. The impedance of this capacitor in the middle range of frequencies (where the overall gain of the amplifier falls off and becomes less than unity) is much lower than the impedance of any load that might normally be connected to the amplifier output. Thus, the phase shift through the output terminals is independent of the phase angle of the load applied and depends only on the impedance of the output capacitor at medium and high frequencies. Hence, amplifier feedback stability is assured and no oscillation will occur regardless of the type of load imposed.
In addition, the output stage inside the amplifier block in Figure 4 is removed and shown separately. After these changes have been carried out, the modified operational amplifier of Figure 5 results.
Replacing the batteries of Figure 5 with rectifiers and a reference zener diode results in the circuit of Figure 6. A point by point comparison of Figures 3 and 6 reveals that they have identical topology--all connections are the same, only the position of the components on the diagram differs!
Thus, a series regulated power supply is an operational amplifier. The input signal to this operational amplifier is the reference voltage. The output signal is regulated dc. The following chart summarizes the corresponding terms used for an operational amplifier and a power supply.
Operational Amplifier Constant Voltage Power Supply
Input Signal Reference Voltage Output Signal Regulated DC Amplifier Regulator Output Stage Series Regulating Transistor Bias Power Supply Rectifier/Filter Gain Control Output Voltage Control
As a result of the specific method used in transforming an operational amplifier into a power supply, some restrictions are placed on the general behavior of the power supply. The most important of these are:
(1) The large output capacitor Co limits the bandwidth. *
22
(2) The use of a fixed dc input voltage means 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, together with the fact that the rectifier
has a given polarity, means that the power supply can only deliver current to the load, and cannot absorb current from the load.
* Special design steps have been added to the design of most Agilent low voltage supplies to permit a significant reduction in the size of the output capacitor merely by changing straps on the rear terminal strip (see page 89).
** In Agilent’s line of Bipolar Power Supply/Amplifiers this output capacitor is virtually eliminated by using a special feedback design. In addition, BPS/A instruments are capable of ac output, conduct current in either direction, and their outputs are continuously variable through zero (see page 54).
Figure 6. Operational Amplifier Representation of Adjustable CV Power Suppl y

Series Regulator with Preregulator

Adding a preregulator ahead of the series regulator allows the circuit techniques already developed for low power supplies to be extended readily to medium and higher power designs. The preregulator minimizes the power dissipated in the series regulating elements by maintaining the voltage drop across the regulator at a low and constant level. This improves efficiency by 10 to 20% while still retaining the excellent regulation and low ripple and noise of a series regulated supply. In addition, fewer series regulating transistors are required, thus
23
minimizing size increases.
Figure 7 shows an earlier Agilent power supply using SCR's as the preregulating elements. Silicon Controlled Rectifiers, the semiconductor equivalent of thyratons, are rectifiers which remain in a non-conductive state, even when forward voltage is provided from anode to cathode, until a positive trigger pulse is applied to a third terminal (the gate). Then the SCR "fires", conducting current with a very low effective resistance; it remains conducting after the trigger pulse has been removed until the forward anode voltage is removed or reversed. On more recent preregulator designs, the SCR's are replaced by a single triac, which is a bidirectional device. Whenever a gating pulse is received, the triac conducts current in a direction that is dependent on the polarity of the voltage across it. Triacs are usually connected in series with one side of the input transformer primary, while SCR's are included in two arms of the bridge rectifier as shown in Figure 7. No matter which type of element is used, the basic operating principle of the preregulator circuit is the same. During each half cycle of the input, the firing duration of the SCR's is controlled so that the bridge rectifier output varies in accordance with the demands imposed by the dc output voltage and current of the supply.
Figure 7. Constant Voltage Power Supply with SCR Preregulator
The function of the preregulator control circuit is to compute the firing time of the SCR trigger pulse for each
24
half cycle of input ac and hold the voltage drop across the series regulator constant in spite of changes in load current, output voltage, or input line voltage. Figure 8 shows how varying the conduction angle of the SCR's affects the amplitude of the output voltage and current delivered by the SCR bridge rectifier of Figure 7. An earlier firing point results in a greater fraction of halfcycle power from the bridge and a higher dc level across the input filter capacitance. For later firing times, the dc average is decreased.
Figure 8. SCR Conduction Angle Control of Preregulator Output
The reaction time of an Agilent preregulator control circuit is much faster than earlier SCR or magamp circuits. Sudden changes in line voltage or load current result in a correction in the timing of the next SCR trigger pulse, which can be no farther away than one half cycle (approximately 8 milliseconds for a 60Hz input). The large filter capacitance across the rectifier output allows only a small voltage change to occur during any 8 millisecond interval, avoiding the risk of transient drop-out and loss of regulation due to sudden changes in load or line. The final burden of providing precise and rapid output voltage regulation rests with the series regulator, while the preregulator handles the coarser and slower regulation demands.
The preregulator SCRs, together with the leakage inductance of the power transformer, limit high inrush currents during turn-on. A slow-start circuit allows gradual turn-on of the SCRs while the leakage inductance acts as a small filter choke in series with the SCRs. Thus, both the supply's input components and other ac connected instruments are protected from surge currents.

Switching Regulation

The rising cost of electricity and continuous reductions in the size of many electronic devices have stimulated recent developments in switching regulated power supplies. These supplies are smaller, lighter, and dissipate less power than equivalent series regulated or series/preregulated supplies.
Although basic switching regulator technology and its advantages have been known for years, a lack of the necessary switching transistors, rectifier diodes, and filter capacitors caused certain performance problems that were costly to minimize. As a result, these supplies were used only in airborne, space, or other applications where cost was a secondary consideration to weight and size. However, the advent of high-voltage, fast
25
switching power transistors, fast recovery diodes, and new filter capacitors with lower 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 growth potential and are constantly changing as better components become available and new design techniques emerge. Concurrently, performance is improving, costs are dropping, and the power level at which switching supplies are competitive with linear supplies continues to decrease. Before continuing with this discussion, a look at a basic switching supply circuit will help to explain some of the reasons for its popularity.

Basic Switching Supply

In a switching supply, the regulating elements consist of series connected transistors that act as rapidly opened and closed switches (Figure 9). The input ac is first converted to unregulated dc which, in turn, is "chopped" by the switching element operating at a rapid rate(typically 20KHz). The resultant 20KHz pulse train is transformer-coupled to an output network which provides final rectification and smoothing of the dc output. Regulation is accomplished by control circuits that vary the on-off periods (duty cycle) of the switching elements if the output voltage attempts to chance.
Figure 9. Basic Swit chi ng Supply
Operating Advantages and Disadvantages. Because switching regulators are basically on/off devices, they avoid the higher power dissipation associated with the rheostat-like action of a series regulator. The switching transistors dissipate very little power when either saturated (on) or nonconducting (off) and most of the power losses occur elsewhere in the supply. Efficiencies ranging from 65 to 85% are typical for switching supplies, as compared to 30 to 45% efficiencies for linear types. With less wasted power, switching supplies run at cooler temperatures, cost less to operate, and have smaller regulator heat sinks.
Significant size and weight reductions for switching supplies are achieved because of their high switching rate. The power transformer, inductors, and filter capacitors for 20KHz operation are much smaller and lighter than those required for operation at power line frequencies. Typically, a switching supply is less than one-third the size and weight of a comparable series regulated supply.
Besides high efficiency and reduced size and weight, switching supplies have still another benefit that suits the needs of the modern environment. That is their ability to operate under low ac input voltage (brownout) con­ditions and a relatively long carryover (or holdup) of their output if input power is lost momentarily. The switching supply is superior to the linear supply in this regard because more energy can be stored in its input filter capacitance. To provide the low voltage, high current output required in many of today's applications, the series regulated supply first steps down the input ac and energy storage must be in a filter capacitor with a low
26
voltage across it. In a switching supply, however, the input ac is rectified directly (Figure 9) and the filter capacitor is allowed to charge to a much higher voltage (the peaks of the ac line). Since the energy stored in a capacitor = 0.5CV2, while its volume (size) tends to be proportional to CV, storage capability is better in a switching supply.
Although its advantages are impressive, a switching supply does have some inherent operating characteristics that could limit its effectiveness in certain applications. One of these is that its transient recovery time (dynamic load regulation) is slower than that of a series regulated supply. In a linear supply, recovery time is limited only by the speeds of the semiconductors used in the series regulator and control circuitry. In a switching supply, however, recovery is limited mainly by the inductance in the output filter. This may or may not be of significance to the user, depending upon the specific application.
Also, electro-magntic interference (EMI) is a natural byproduct of the on-off switching within these supplies. This interference can be conducted to the load (resulting in higher output ripple and noise), it can be conducted back into the ac line, and it can be radiated into the surrounding atmosphere. For this reason, all Agilent Technologies switching supplies have built-in shields and filter networks that substantially reduce EMI and control output ripple and noise.
Reliability has been another area of concern with switching supplies. Higher circuit complexity and the relative newness of switching regulator technology have in the past contributed to a diminished confidence in switching supply reliability. Since their entry into this market, Agilent has placed a strong emphasis on the reliability of their switching supplies. Field failures have been minimized by such factors as careful component evaluation, MTBF life tests, factory "burn-in" procedures, and sound design practices.

Typical Switching Regulated Power Supplies

Currently, switching supplies are widely used by Original Equipment Manufacturers (OEMs). This class of switching supply provides a high degree of efficiency and compactness, moderate-to-good regulation and ripple characteristics, and a semi-fixed constant voltage output.
Figure 10 shows one of Agilent’s higher power, yet less complex, OEM switching supplies. Regulation is accomplished by a pair of push-pull switching transistors operating under control of a feedback network consisting of a pulse width modulator and a voltage comparison amplifier. The feedback elements control the ON periods of the switching transistors to adjust the duty cycle of the bipolar waveform (E) delivered to the output rectifier/filter. Here the waveform is rectified and averaged to provide a dc output level that is proportional to the duty cycle of the waveform. Hence, increasing the ON times of the switches increases the output voltage and vice-versa.
The waveforms of Figure 10 provide a more detailed picture of circuit operation. The voltage comparison amplifier continuously compares a fraction of the output voltage with a stable reference (EREF) to produce the VCONTROL level for the turn-on comparator. This device compares the VCONTROL input with a triangular ramp waveform (A) occurring at a fixed 40KHz rate. When the ramp voltage is more positive than the control level, a turn-on signal (B) is generated. Notice that an increase or decrease in the VCONTROL voltage varies the width of the output pulses at B and thus the ON time of the switches.
Steering logic within the modulator chip causes switching transistors Q1 and Q2 to turn on alternately, so that each switch operates at one-half the ramp frequency (20KHz).
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Included, but not shown, in the modulator chip are additional circuits that establish a minimum "dead time" (off time) for the switching transistors. This ensures that both switching transistors cannot conduct simultaneously during maximum duty cycle conditions.
Figure 10. Switchi ng Regul ated Constant Voltage Supply
Ac Inrush Current Protection. Because the input filter capacitors are connected directly across the rectified line, some form of surge protection must be provided to limit line inrush currents at turn-on. If not controlled, large inrush surges could trip circuit breakers, weld switch contacts, or affect the operation of other equipment connected to the same ac line. Protection is provided by a pair of thermistors in the input rectifier circuit. With their high negative temperature coefficient of resistance, the thermistors present a relatively high resistance when cold (during the turn-on period), and a very low resistance after they heat up.
A shorting strap (J1) permits the configuration of the input rectifier-filter to be altered for different ac inputs. For a 174-250Vac input, the strap is removed and the circuit functions as a conventional full-wave bridge. For 87-127Vac inputs, the strap is installed and the input circuit becomes a voltage doubler.
Switching Frequencies, Present and Future. Presently, 20KHz is a popular repetition rate for switching regulators because it is an effective compromise with respect to size, cost, dissipation, and other factors. Decreasing the switching frequency would bring about the return of the acoustical noise problems that plagued earlier switching supplies and would increase the size and cost of the output inductors and filter capacitors.
Increasing the switching frequency, however, would result in certain benefits; including further size reductions in the output magnetics and capacitors. Furthermore, transient recovery time could be decreased because a higher operating frequency would allow a proportional decrease in the output inductance, which is the main constraint in recovery performance.
Unfortunately, higher frequency operation has certain drawbacks. One is that filter capacitors have an Equivalent Series Resistance and Inductance (ESR and ESL) that limits their effectiveness at high frequencies. Another disadvantage is that power losses in the switching transistors, inductors, and rectifier diodes increase with frequency. To counteract these effects, critical components such as filter capacitors with low ESRs, fast recovery diodes, and high-speed switching transistors are required. Some of these components are already available, others are not. Switching transistors are improving, but remain one of the major problems at high frequencies. However, further improvements in high-speed switching devices, such as the new power Field Effect Transistors (FETs) would make high frequency operation and its associated benefits, a certainty for
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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 the previously described circuit of Figure 7. Briefly, the dc input voltage to the switches is held relatively constant by a control circuit which issues a phase adjusted firing pulse to the triac once during each half-cycle of the input ac. The control circuit compares a ramp function to a rectified ac sinewave to compute the proper firing time for the triac.
Although the addition of preregulator circuitry increases complexity, it provides three important benefits. First, by keeping the input to the switches constant, it permits the use of lower voltage, more readily available switching transistors. The coarse preregulation it provides also allows the main regulator to achieve a finer regulation. Finally, through the use of slow-start circuits, the initial conduction of the triac is controlled; providing an effective means of limiting inrush current.
Note that the preregulator triac is essentially a switching device and, like the main regulator switches, does not absorb a large amount of power. Hence, the addition of the preregulator does not significantly reduce the overall efficiency of this supply.
Figure 11. Switching Supply with Preregulator.
Single Transistor Switching Regulator. At lower output power levels, a one transistor switch becomes practical. The single transistor regulator of Figure 12 can receive a dc input from either one of two sources without a change in its basic configuration. For ac-to-dc requirements, the regulator is connected to a line rectifier and SCR preregulator and for dc-to-dc converter applications it is connected directly to an external dc source.
Like the previous switching supplies, the output voltage is controlled by varying the ON times of the regulator switch. The switch is turned on by the leading edge of each 20KHz clock pulse and turned off by the pulse width modulator at a time determined by output load conditions.
While the regulating transistor is conducting, the half-wave rectifier diode is forward biased and power is transferred to the output filter and the load. When the regulator is turned off, the "flywheel" diode conducts, sustaining current flow to the load during the off period. A flywheel diode (sometimes called a freewheeling or
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catch diode) was not required in the two transistor regulators of Figures 10 and 11 because of their full-wave rectifier configuration.
Another item not found in the previous regulators is "flyback" diode CR transformer winding which is bifilar wound with the primary. During the off periods of the switch, CR
. This diode is connected to a third
F
is
F
forward biased, allowing the return of surplus magnetizing current to the input filter, and thus preventing saturation of the transformer core. This is an important function because core saturation often leads to the destruction of switching transistors. In the previously described two transistor push-pull circuits, core saturation is easier to avoid because magnetizing current is applied to the core in both directions. Nevertheless, matched switching transistors and balancing capacitors must still be used in these configurations to ensure that core saturation does not occur.
Figure 12. Single Transistor Sw i t ching Regulator

Summary of Basic Switching Regulator Configurations

Figure 13 shows three basic switching regulator configurations that are often used in today’s power supplies. Configuration A is of the push-pull class and this version was used in the switching supplies shown in Figures 10 and 11. Other variations of this circuit are used also, including two-transistor balanced push-pull and four transistor bridge circuits.
As a group, push-pull configurations are the most effective for low-voltage, high-power and high performance applications. Push-pull circuits have the advantage of a ripple frequency that is double that of the other two basic configurations and, of course, output ripple is inherently lower.
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Figure 13. Basic Swit chi ng Regulator Configurations
Configuration B is a useful alternative to push-pull operation for lower power requirements It is called a forward, or feed-through, converter because energy is transferred to the power transformer secondary immediately following turn-on of the switch. Although the ripple frequency is inherently lower, output ripple amplitude can be effectively controlled by the choke in the output filter. Two-transistor forward converters also exist wherein both transistors are switched simultaneously. They provide the same output power as the single transistor versions, but the transistors need handle only half the peak voltage.
Configuration C is known as a flyback, or ringing choke, converter because energy is transferred from primary to secondary when the switches are off (during flyback). In the example, two transistors are used and both are switched simultaneously. While the switches are on, the output rectifier is reverse biased and current in the primary inductance rises in a linear manner. When the switches are turned off, the collapsing magnetic field reverses the voltage across the primary, and the previously stored energy is transferred to the output filter and load. The two diodes in the primary protect the transistors from inductive surges that occur at turn-off.
Flyback techniques have long been used as a means of generating high voltages (e. g., the high voltage power supply in television receivers) and, as you might expect, this configuration is capable of providing higher output voltages than the other two methods. Also, the flyback regulator provides a greater variation of output voltage with respect to changes in duty cycle. Hence, the flyback configuration is the most obvious choice for high, and variable, output voltages while the push-pull and forward configurations are more suitable for providing low, and fixed, output voltages.

SCR Regulation

SCR regulation techniques permit the design of low cost, compact power supplies with efficiencies of approximately 70%. Their main disadvantages are a higher ripple and noise, a less precise regulation, and a slower transient recovery time relative to the other three regulation methods. However, these supplies are widely used in high power applications where a lower degree of performance can be tolerated.
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