Examining Power Supply Architectures and Capabilities

Many of today’s power supplies or sources are much more than mere static power providers. They also are programmable, include extensive measurement facilities, and often act as a test source to determine the power-related operating limits of UUTs or even other power supplies.

Programmable AC sources, in particular, produce varying voltages, frequencies and controlled perturbations to test power supplies for compliance with worldwide AC power quality specifications and standards. “For instance, our customers use programmable functions to generate time- or cycle-based AC power transients and to create, as well as analyze, harmonic content,” said Russell C. Engle Jr., Vice President Sales and Marketing at Pacific Power Source Corp. Programmable DC supplies are often used to verify proper performance of DC-to-DC converters.

Most DC supplies, not only those designed for lab applications but also the ones providing system power, are now programmable, and often controlled by a PC. “Computer control makes today’s supplies more useful and versatile for many test and industrial applications,” commented Vince Mutascio, Vice President, Marketing at Power Ten.

“Electroplating is one example of a process control application that benefits from using computer-controlled power supplies,” Mr. Mutascio continued. “Current flowing through a plating tank can be precisely programmed for varying amplitudes and dwell times, closely controlling the thickness and uniformity of the deposition material.”

All supplies perform internal power-conversion processes which may be accomplished by a rectifier plus a series regulator (often referred to as linear) or a switch-mode implementation. Linear supplies smooth the rectified line voltage and control the output voltage/current through a regulator circuit which is placed in series with the load.

Most switch-mode power supplies (SMPS) convert the line frequency to a much higher frequency and control the output through a variety of conduction-time varying circuitry. The use of higher frequencies reduces the size and the weight, and modulating conduction time eliminates the need for internal excess-power absorption, which increases efficiency.

Each architecture has distinct advantages and disadvantages. As a result, many manufacturers use linear as well as switching technology in their AC or DC power sources. “The decision on which is best for a given application requires a working knowledge of both technologies,” emphasized Don Mulvey, Engineer at Lambda. To help you in the selection process, we will explore the pros and cons as well as the performance implications of both techniques and review programming aspects.

Programming Choices

Regardless of architecture, power supplies may be programmed locally via key pads or potentiometers, or remotely using analog or digital interfaces. Analog methods include application of an external voltage, current or variable resistance. Digital control is usually accomplished via an IEEE 488 or RS-232 interface, with the program residing in an external controller or PC. Some supplies use a microprocessor-based control architecture, making them internally or externally programmable.

Analog interfaces are often more than sufficient for simple applications such as programming the output voltage of a supply. A typical case in point is a production test of consumer appliances which may include varying the voltage between the upper and lower limits that the appliance would normally encounter.

For extended test sequences, however, digital control—most often provided by a PC—is generally mandatory. “Many users choose the PC as the primary intelligence for controlling their power supplies,” said Larry Soldaty, Marketing Manager at Elgar. “They apply it to program high- and low-voltage margin tests, power interruption for transients tests, or test sequences needed to characterize the UUT.”

Many tests require tight integration between the controlled power supply and the programmed measurement functions, especially for applications involving AC sources. In the past, this required writing several separate but interrelated test programs.

But many modern AC sources incorporate all required measurement functions to provide an integrated instrument. “This obviates the need for multiple custom programs required when controlling a group of instruments,” said Herman vanEijkelenburg, Product Marketing Manager at California Instruments. “Instead, a ready-to-run AC source application program can offer all the control, acquisition and data-reporting functions normally created by multiple custom programs.”

SMPS Architectures and Implementations

“SMPS have come a long way from their introduction as narrow output span modular power supplies,” said John O’Connor, Engineer at Hewlett-Packard. “Early supplies suffered from high in-rush currents at turn-on, wideband output noise, high peak-to-peak ripple, significant common-mode currents and large voltage overshoots when the load was abruptly reduced. But today, many power supply manufacturers address most, if not all, of these shortcomings and successfully apply switch-mode technology to create more traditional, wide-range power supplies.”

Major advantages of SMPS include high power-conversion efficiency ranging from 75% to more than 90%, and reduced size and weight. The switching elements may be metal-oxide field-effect transistors (MOSFETs), insulated gate bipolar transistors (IGBTs) or silicon controlled rectifiers (SCRs).

The MOSFET is the device of choice for high-frequency switching implementations, but IGBTs and SCRs are also often used, the latter mainly for industrial applications. “IGBTs are easily obtainable, relatively low cost and very hardy devices,” explained Ron Storm, Vice President Sales at Behlman. “Their power-handling characteristics are also more favorable than those of MOSFETs. They are driven like a field-effect transistor (FET) but, because they are bipolar, they dissipate power as a function of current, not like a FET which dissipates power as the square of the current.”

The relative merits of implementations based on these three device types are shown in Table 1. The prime differentiator between them is the achievable switching speed. But while size and weight of the SMPS decrease with higher switching frequencies, the noise increases. To contain the noise and prevent generation of excessive RFI, careful layout, shielding and filtering are often used.

“For instance, when we power high-resolution, high-speed A/D converters with DC/DC switchers, we use an LC filter on the supply line to reduce switching noise to levels that are below the noise floor of these 14- or 16-bit converters,” commented Bob Leonard of Datel. “For switchers operating at 200 kHz, a 1-m H inductor and a 33-m F tantalum capacitor with short leads work.”

Sometimes more elaborate precautions must be taken. “In recent designs, we incorporated a multiple-stage architecture,” said Tom Hayden of Keithley Instruments. “A nonresonant MOSFET switcher converts line input to DC, followed by an isolated modified-sine-wave-driven DC-DC converter, followed by an analog linear output amplifier. This combination provides the precision low-noise outputs required in high-accuracy applications and still is efficient and small.”

When to Use Linear Supplies

“While we find that more than 90% of today’s DC supply requirements easily fall within the noise limits of an SMPS, there are still applications where a low noise source is a must,” commented Mr. Soldaty. The ripple and noise of carefully designed linear supplies can be in the microvolts region, as opposed to millivolts for SMPS.

Linear power supplies provide this performance since they merely regulate and smooth voltage/current fluctuation. Unlike SMPS, they neither convert line-frequency power to high-frequency power nor employ abrupt switching and do not create undue disturbances.

The DC output of linear supplies is controlled by dropping unwanted voltage perturbations—caused by line or load changes—across a dynamic pass element which dissipates the unwanted energy as heat. “This technique allows for a very stable and quiet power source which is critical for some analog, audio, video, RF and interface circuits applications,” said Mr. Mulvey.

Lower ripple and noise output is only one advantage of the linear supply design; faster response to load changes is the other. “For relatively constant load applications, the SMPS works well and, even when the load becomes more dynamic, it typically recovers to its programmed output voltage in less than 1 ms,” said Mr. Soldaty. “But when faster recovery times are required, the linear supply again outshines an SMPS in this area.”

“Key reasons for choosing the linear implementation are that it can respond to fast load demands or produce fast changing outputs as needed to reproduce power transients containing waveforms with high-order harmonics,” concurred Mr. Engle. “If these are not required and noise margins are adequate, the switcher should be given serious consideration. Both linear and switching technology can be used to produce reliable, useful power sources.”

Trends

Further advances in the SMPS technology will still take place. As the smaller, lighter and less costly SMPS approach the performance of linear supplies, they will increasingly displace linear supplies.

“One promising technique that overcomes many problems uses a soft-switching or zero-voltage switching topology,” said Suki Randhawa of Xantrex. “This allows the use of slower switching devices and produces very low switching power losses, resulting in very high efficiency. Because a fixed switching frequency is used, the electrical noise can also be filtered more effectively.”

The inclusion of measurement facilities in power sources will provide more cost as well as time savings. “Rather than using the AC source as a programmable power source only, more ATE systems will use the AC source’s built-in measurement capabilities,” said Mr. vanEijkelenburg. “ATE designers who have already done so have found that these facilities can replace two to three other pieces of equipment such as multimeters, DSOs and power analyzers.”

Programming facilities will also expand and users will find new ways to enable integrated PC-controlled power, stimulus and measurement systems to provide added efficiencies.

Power Supplies/Sources

Programmable AC Source Has

Power-Factor Correction

The AC Source Models 6120 (2 kVA), 6110 (1 kVA) and 6106 (600 VA) deliver outputs up to 150 VAC/300 VAC at any frequency from 45 Hz to 500 Hz. They feature built-in waveform distortion simulation and control of phase angle, cycle dropout, voltage and frequency ramp-up/down. User-programmable waveform sequences are stored in EEPROM. The units can supply the repetitive high-peak current required by many electronic products. Operation is controlled from a front-panel keypad or via IEEE 488. 6120: $9,250; 6110: $6,850; 6106: $5,400. Chroma ATE, (408) 437-0797.

High-Voltage Supplies

Charge Capacitors, PFNs

The Series 500A, 102A and 152A high-voltage power supplies provide average charging rates of 500 S/s, 1,000 S/s and 1,500 S/s respectively. The three series are offered at ratings extending from 1 kV to 40 kV with continuously variable output from 0 to 100%. The units operate as constant-current supplies for charging HV capacitors or pulse-forming networks (PFN) or as constant-voltage continuous output sources. Features include remote control of high-voltage enable/reset, voltage programming and monitoring. They meet applicable UL, CSA, IEC and EN specifications and standards. From $1,750. Electronic Measurements, (908) 922- 9300.

Supplies Suited for

Lab and ATE Applications

The LLS Series Power Supplies consists of 20 models with voltage ranges up to 300 VDC and power ratings to 800 W. They operate in constant voltage or constant current modes and have adjustable overvoltage protection. Designed for laboratory and ATE applications, the LLS supplies feature a microprocessor-controlled input keypad, digital front-panel meters with simultaneous display of voltage and current, and remote analog programming. Units are available in benchtop or 19″ rack-mounting configurations. From $785. Lambda Electronics, (800) LAMBDA-4.

Precision Power Supplies

Employ Zero-Volt Switching

The XFR Series power supplies employ zero-volt switching which provides increased efficiency and low noise levels as well as better line and load regulation. The units are offered in 1,200-W and 2,800-W versions and operate in constant-voltage or constant-current output modes. Programming may be performed from the front panel or remotely via analog interconnection lines. RS-232, GPIB and fiber-optic interfaces are also available. XFR 1200-W: $1,360; XFR 2800-W: $2,495. Xantrex, (800) 667-8422.

Supplies Suit ATE, Burn-In,

Process-Control Applications

The rack-mountable P62 Series DC Supplies offer active power-factor correction and analog and IEEE 488.2 programming capabilities. All units in the series contain user-selectable constant current/constant voltage capability with auto crossover. Features include programming and readback of voltage and current with 12-bit resolution and facilities for querying settings, faults and errors. The P62 3-kW family provides outputs from 0 to 5 VDC at 0 to 450 A to 0 to 300 VDC at 0 to 10 A. From $1,750. Power Ten, (408) 738-5959.

AC Power Sources

Support IEC Testing

The i Series Programmable AC Power Sources meet the AC source requirements imposed by International Electrotechnical Commission (IEC) standards. They may be used as the AC source for performing IEC 1000-3-2 (harmonics), IEC 1000-3-3 (flicker) and IEC 1000-4-11 (voltage dips and interruption) tests. The 5001i provides 5,000-VA single-phase power at frequencies from 16 Hz to 500 Hz. For higher power demands, units can be combined. Software runs tests automatically and processes measurement data for display, analysis and reporting. From $10,975. California Instruments, (619) 279-8620.

Enhancements Added to

Programmable AC Sources

Enhancements have been incorporated in the HP 6814B (3,000 VA, single- phase) and the HP 6834B (4,500 VA, 3-phase) AC Sources. The frequency range has been extended to 5 kHz and increased peak-current capability accommodates crest factors up to 4.0. The 3-phase 4,500-VA model also has a programmable mode switch so that it may be used in single-phase applications. HP 6814B: $11,500; HP 6834B: $13,900. Hewlett-Packard, (800) 452-4844.

AC Sources Have

Waveform Editing

The ASX-Series AC Power Sources, available as single-phase and 3-phase models from 1,500 VA to 12 kVA, provide full output power from 15 Hz to 1,200 Hz. They generate time- and cycle-based transients and perform waveform synthesis and analysis under manual or program control. Other features include selectable 1-, 2- or 3-phase operation; programmable output impedance; RS-232 or GPIB interfaces, and LabVIEW for Windows and LabWindows drivers. Up to 99 static/dynamic test programs may be stored. From $2,975. Pacific Power Source, (800) 854-2433.

AC Power Sources

Available With CE Marking

The ACP and ACM Series of AC Power Sources are now available with the CE marking. Equipped with Option 100, they meet the generator requirements for the new IEC specifications, such as variable output impedance, programmed waveforms and crest factor. Tests facilitated include those enumerated by IEC 1000-3-2 (harmonics), IEC 1000-3-3 (flicker) and IEC 1000-4-11 (voltage dips and interruption). Units are available at ratings from 1,000 VA to 12.5 kVA. From $9,355. Behlman, (516) 435-0410.

High-Voltage DC Supply

Features Low Ripple

The PS300 Series of high-voltage DC power supplies includes models with 1.25 kV, 2.5 kV and 5 kV, all providing 25-W output power. Output ripple is <0.0015% of the full-scale setting, accuracy is 0.05% and regulation is 0.001%. Voltage may be set with 1-V resolution. Two 4-digit displays provide continuous readout of current and voltage and a third shows the parameter being entered. Analog voltage programming is included; IEEE 488 is optional. $1,150. Stanford Research Systems, (408) 744-9040.

AC Source Provides

High Power Density

The SmartWave™ SW 5250 (5,250 VA) AC Power Source contains three separate arbitrary waveform generators to provide the capability to produce independent complex waveforms on all three phases simultaneously. It can create fractional or multi-cycle dropouts, spikes, surges, sags and distorted waveforms. A library of 50 waveshapes is included and 50 additional user-created waveshapes can be stored. Programming may be performed from the front panel or with optional PC software. An IEEE 488.2 interface is standard. $16,355. Elgar, (800) 733-5427.

Precision Voltage/Current

Source Is Programmable

The VI-700 is a precision manually controlled or remotely programmable voltage and current source. Three ranges provide up to 20-V full scale and better than 100-m V resolution, or 20-mA full scale with 0.1-m A resolution. Both voltage and current outputs are available simultaneously. The unit features low output impedance (typical 5 mW ), low noise and ripple. VI-700: $495; BCD option for remote programming: $195; IEEE option: $495. IET Labs, (516) 334-5959.

PC Plug-In Power Source

Provides Four DC Outputs

The PC-462, an IBM-PC/AT, PS-30, EISAbus-compatible plug-in board, is a high-precision power supply and sensing unit. Four output channels are individually programmable for ranges of 0 to +6.1425 V and 0 to -6.1425 V as well as 0 to +20.475 V and 0 to -20.475 V at 250 mA. All channels are fully isolated from the PC/AT bus. Features include real-time monitoring, remote sensing, current limiting and two isolated relay driver outputs. $1,295. DATEL, (508) 339-3000.

Programmable Supplies Provide

Integrated System Solutions

The PM 2800 family of programmable power supplies is designed for lab and system environments and includes single-, dual- and triple-output configurations. They feature autoranging with full power capabilities over all ranges and extensive protection of UUTs. The supplies operate in a constant-current or constant-voltage mode, and employ linear technology to provide fast response time and low noise levels. An IEEE 488 interface is included and the Standard Commands for Programmable Instruments programming protocol is supported. From $1,895. Fluke, (800) 44-FLUKE.

Economic Benchtop Power

Supplies Use Linear Technology

The L Series benchtop power supplies are designed for applications where programmability is not required. The family consists of 14 models encompassing outputs from 0 to 300 V at power levels from 90 W to 375 W. Four output configurations are available: LS (small output), LM (medium output), LH (high output) and LT (triple output). The supplies may be used as voltage or current sources. Crossover is automatic, depending on load demands. From $335. Sorensen, Division of Elgar, (800) 525-2024.

System Power Supply Suited

For Automated Test Environment

The MPS Series Power Supplies, available in sizes ranging from 300 W to 1 kW, combine a microprocessor, firmware, a power stage and IEEE 488.1 (IEEE 488.2/SCPI optional) controllability to provide an array of operating capabilities. Parallel interfaces allow dual-channel or controller/slave operations and linkage of up to seven MPS supplies. When equipped with the IEEE 488.2 /SCPI option, LabVIEW, LabWindows and HP VEE drivers are included and a 99-point memory is provided. From $1,995. AMREL/American Reliance, (800) 654-9838.

Precision High-Voltage Supply

Simplifies Testing

The Model 248 High-Voltage Power Supply is a precision 5-kV source suited for laboratory and production testing. It features a voltage set accuracy of ± (0.01% of setting + 0.05% of range) and regulation holds the set value within 0.001% for line voltage changes of 10% and 0.005% for 100% load changes. Display accuracy is ± 1 V and the maximum output current from 0 to 5 kV is 5 mA. Programming may be performed via an IEEE interface or with front-panel keys. $2,395. Keithley Instruments, (800) 552-1115.

Triple-Output Supply

Includes Two Interfaces

The HP E3631A is an 80-W triple-output programmable power supply. It allows users to read and program current and voltage remotely via two interfaces, HP-IB and RS-232, using SCPI commands. The three outputs are: 0 to -25 V @ 1 A, 0 to +25 V @ 1 A, and 0 to 6 V @ 5 A. Separate digital meters allow simultaneous monitoring of voltage and current. Software calibration eliminates the need to open the supply. $995. Hewlett-Packard, (800) 452-4844.

Bench Power Supply Uses

Switch-Mode Technology

The ABC Models are a line of 100-W programmable bench power supplies using switch-mode technology. They come in five ranges from 0 to 10 V @ 10 A to 0 to 125 V @ 1 A. The ABC models are microprocessor-controlled zero-up switchers with menu-driven keyboard control and a 2-line ´ 16-character backlit LCD. A 12-bit GPIB interface is built in for SCPI language programming. $995. Kepco, (718) 461-7000.

Table 1

Parameters	MOSFET	IGBT	SCR
Switching Frequency	Above 50 kHz	20 to 50 kHz	60 Hz
RFI Conducted	Needs Filter	Needs Filter	Less filtering than others
Susceptibility	Good	Good	Good
Radiation	Some	Less	Very Little
Ease of Repair	More Complex	Complex	Least Complex
Size	Smallest	Small	Largest
Ripple	Function of Design	Function of Design	Function of Design