Current Situation

• Proliferation of Computerized Equipment
• Increased Speed and Density of Logic Circuits
• Greater Susceptibility and Sensitivity
• Change in Power Supply Technology

Prior to about 1980, the predominant power supply technology was linear. This type of technology is distinctly different from the technology available today.

Typical electronic loads today utilize non-linear switching technology. This technology is commonly known as Switch Mode Power Supplies (SMPS). These devices provide a much greater tolerance to RMS voltage fluctuations, fundamental frequency variations, and have more internal capacitance for energy storage. This energy storage provides some "ride through" during short duration blackouts (in the millisecond range). Other important benefits are: reduced size, greater efficiency and lower manufacturing costs - which are reflected to the end user. However, SMPS technology does have its own limitations and, in some circumstances, creates new problems, such as:

1. SMPS have a much greater susceptibility to transient surge voltage. This is due to the lack of transformer mass and greater internal coupling capacitance. The fast rise time, high frequency transients enter the SMPS through the low impedance paths and are dissipated within the SMPS or with more catastrophic results within the logic circuits of the hardware.
2. SMPS are very effective transient generators. They produce 180 Hz-100 kHz impulses through the rectification process which are reflected into the AC circuit to contaminate other loads.
3. SMPS produce harmonic distortion in electrical circuits. This is a product of the loss of total utilization of effective current due to the high frequency rectification process. The loss of conduction time in the SMPS must be compensated in higher peak currents. If the electrical circuits are not properly sized to handle the high peak current and harmonic current draw of the SMPS, problems, such as excessive heating of upstream transformers and motors, are imminent. The odd order harmonics 3, 5, 7, etc., are additive in the neutral in three-phase systems. Even parity sized neutral conductors will suffer excessive heating. Balanced three-phase loads are no guarantee for eliminating harmonics where SMPS are concerned.

Based on changes in power supply technology, the Allen and Segal study must be carefully applied to today's ever-changing environment. We should consider where SMPS are concerned, less RMS voltage regulation is needed and far more transient suppression and harmonic filtering are required. It should be noted it is still possible to encounter linear and SMPS technology combined in large computer systems, in which case the system's tolerance will be governed by the lowest acceptable tolerance level of any of it's components.