Vol.6, No.1

Programmable power supplies begin to employ switch-mode topologies!

Some "test" power supplies now take advantage of the small size and greater density available with switch-mode techniques over linear methods.

BY PAUL BIRMAN and SARKIS NERCESSIAN
Kepco, Inc.
Flushing, NY

The world of power supplies has long been divided into two distinct parts. Power modules for end products have almost universally adopted high-frequency switch-mode topologies--flyback, forward converter, and resonant--to reduce cost and weight while maintaining good performance. At the same time, supplies used for testing applications mostly rely on linear regulator techniques to obtain the dynamic range necessary to impose an accurate "zero-up" control.

Now, however, the paths taken by the two types of supplies are starting to converge. Switch-mode techniques that have long been part of end-use supplies are finding their way into the design of test power supplies.

Programmable supplies that employ switch-mode topologies are slowly emerging. One example is the MST series from Kepco (see photo), which uses a combination of switch-mode and linear technology. The MST supplies are 200-W designs in a plug-in 1.8-in.-wide module. The narrow width allows up to nine instruments to share a standard 19-in. rack. Hewlett-Packard Co. (Santa Clara, CA) and other companies are also now offering programmable supplies that use a switch-mode topology.

The principal advantages of a switch-mode topology are small size and greater volumetric efficiency. A combined switch-linear approach like that of the MST achieves nearly 1-W/in.3 density. This is well shy of the 10 to 30-W/in.3 density possible in board-mounted dc/dc converters, but far better than the 0.2 to 0.3-W/in.3 densities characteristic of straight linear designs, which rely on mains-frequency magnetics.

Dynamic range limited

Two main obstacles hinder designing a programmable switch-mode supply. The first is the inherently limited dynamic range of switch-mode topologies. The second is filtering out noise.

To put the first problem into perspective, consider that a programmable power supply with a GPIB (IEEE-488.2) interface will normally have at least 12-bit resolution, a dynamic range of 1/4095, or a resolution of 0.0244%. However, for a typical FET-based 100-kHz switch-mode supply, the contraints of minimum to maximum pulse width impose a dynamic range limit of 1/20 or 5%. That's far short of the capabilities of a linear stabilizer and well shy of the requirements of a 12-bit GPIB controller.

Linear supplies, on the other hand, which rely on a basic series-pass topology, can easily achieve a dynamic range of 1/10,000, and an advanced "operational power supply" can exceed 1/100,000 and approach 1/1,000,000. (This is the ratio of the cutoff leakage to the saturation current of the devices used, usually bipolar transistors.) Higher resolution in a linear supply is limited by factors like noise and jitter in the reference voltage and the temperature sensitivity of comparison amplifiers.

One way to get around the limited dynamic range of a switch-mode supply is to introduce a small dissipating circuit either in series or in parallel with the output to guarantee a minimum duty cycle. If, in addition, a current-mode regulating scheme is used that makes the control loop roll off with a single pole, sufficiently high loop gain can be attained to provide the needed 12-bit resolution for bench supplies.

Another solution is to combine high-frequency switch-mode technology with a linear post regulator to provide control in the last 5% of the range. Since the linear part need handle only a small part of the power, the loss of efficiency is modest and is comparable to the efficiency loss that results from adding a power-factor-correcting front end to an in-product power supply.

Filtering noise

The second obstacle to designing a programmable switch-mode supply with enough dynamic range is noise. The noise fed through to the output must be kept below the 1/4095 level or less than 0.0244%. In a 0 to 100-V supply, this means keeping the output noise below 24.4 mV.

Power supplies for test applications require higher noise suppression than in-product power supplies. Because test power supplies often derive measurements about a circuit or component being examined, any noise from the supply's transistors, diodes, or other components might obscure the desired test result or make it inaccurate.

Noise suppression can be achieved by placing snubbers at the points of noise, thus minimizing the rate of change (dV/dt and di/dt slopes) by controlling the rise and fall time of the switch. Beyond that, the power supply designer must prevent the remaining noise from propagating outside through either the output terminals or the mains connection. Filters are needed to impede both common-mode and normal-mode noise. In addition, special attention to the physical layout of foil patterns on pc boards is also needed to minimize coupling, reduce parasitics, and aid shielding.

Other considerations

One drawback of the smaller test power supplies made possible by the use of switch-mode techniques is the limited space available for the traditional front-panel controls and display. However, if such power supplies are thought of as mainly intended for automatic test applications, then the control function may be relegated to the digital communications bus.

The concept of the "virtual panel" has become popular for the "instruments-on-a-card" that constitute a VXI-based test system. For ultra-small test power supplies, this allows users to access all of the usual controls and readouts, provided they have a computer on which to display them. It is possible to create such virtual panels using National Instruments' LabVIEW or LabWindows.

It is possible to use a serial bus to expand the reach of the conventional GPIB+. A a single virtual panel on a single computer screen, driving a single IEEE-488 port, provides control to and read back from up to 27 separate models that can be spaced as far as 300 m from the controller.

At present, the complexities involved in designing switch-mode power supplies for test-power applications has limited the approach to multi-power supply systems requiring small size and high efficiency. These advantages are less important for typical benchtop power supplies, but high-frequency conversion techniques are finding their way into those supplies as well.