New IC controller tech eases power supply design

Electronic News, August 12, 1996 by Mehmet Nalbant

If you have ever been involved in the design of multiple-output power supplies, you know that the problems and complexity that you can encounter when embarking upon such a project can be considerable.

Or, perhaps the task was so intimidating, you chose to simply buy an off-the-shelf solution rather than designing it yourself.

New IC controller technology is now available which significantly reduces the complexity of designing multiple output power supplies, while at the same time delivering greatly enhanced performance using far fewer components than the most commonly used solution, i.e. a magnetic amplifier, or mag-amp. This new phase-modulated technology is available in a single, 8-pin package, which can readily be integrated into multiple output power designs through the use of a fully-proven evaluation platform. Because of this, the typically complex task facing system designers is now much simpler and easier to implement.

Generally speaking, the market demand for multiple-output power supplies has been on the increase due to the ever-expanding number of supply voltages seen in microprocessor-based systems. This increase is attributable to a number of trends, but the main contributing factor has been the evolution of silicon processes. Microprocessor speeds have set the trend for ever-increasing clock speeds. These higher clock speeds have been attainable through finer device geometries. Finer-line geometries typically mean thinner oxides, yielding overall lower breakdown voltages and hence, the need for a new, lower-supply voltage rail.

Following closely behind the need for these products are the various assembly support products such as chipsets, memories and graphics controllers. In order to keep pace with and fully utilize microprocessor speeds, these support products must also run faster and end up utilizing faster, lower-voltage rails.

The interesting fact of the matter is that the older standard interfaces to the outside world do not go away or change, i.e. RS-232, parallel port, etc. In fact, many of these standards still require existing supply voltages such as 5V, minus 5V, 12V and minus 12V. For this reason, the need for multiple output power supplies continues to increase and is being driven by the quest for ever-increasing speeds. Of course, the computer industry is the first and largest market for these products, but as other markets such as the industrial control, motion control and telecommunications markets begin to adopt similar products, the need for multiple-output power supplies will continue to rise.

Recognizing this market requirement and the needs for easier design, high performance using fewer components, in addition to the more stringent demands placed on the power supply in terms of transient response, absolute accuracy and protection features, new IC power supply controller technology has been developed to address and solve these problems.

Before phase-modulated control technology...

Several techniques have historically been used to achieve secondary-side regulation. First, in applications where less than 2 or 3 amps are required, a linear regulator is used, providing a simple, easy-to-design, relatively low-cost solution. A primary disadvantage of this approach, is that efficiency is significantly sacrificed as the difference in input and output voltage increases. Second, for higher currents, a switching regulator can be used, which offers higher efficiency than the linear approach, but which adds design complexity and nearly three times the number of components.

The third, and perhaps most common method used for currents exceeding 3 amps, is the magnetic amplifier, or mag-amp approach. Mag-amps have been around for several years. They are used in switching power supplies before the output rectifier for output voltage control. The mag-amp method is more efficient than either the linear or switching regulator approaches discussed above, but requires expensive magnetic cores and a high degree of design expertise. Inherent in the mag-amp design, is that the frequency of operation is limited by the magnetic core's poor switching characteristics, necessitating overly generous design margins. This, in turn, requires the use of heavy, bulky, expensive mag-amp cores.

As stated above, mag-amps are used in switching power supplies before the output rectifier for output voltage control. They achieve this control by relying on a saturable core that exhibits square B-H characteristics, which means that the core material has a very high permeability. When the core is not saturated, permeability remains high, but it "switches" to a very low permeability when saturated. Thus, by controlling the amount of the magnetic DC flux inside the core, it is possible to control the amount of time (or volts-second) it takes for the core to saturate under a given input voltage pulse. It is this control mechanism that is useful when mag-amps are used as secondary post regulators in switching power supplies (Figure 1).