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Energylike parameters

The foregoing discussion of power and energy relates in an interesting and often practical way to matters encountered in electronics technology. Consider, for instance, the ratings for fuses intended for protection of power semiconductors. Whereas industrial-type fuses are selected according to their current ratings, semiconductor-protection fuses also have I²t designations often described as energy let-through ratings. This nomenclature can be confusing, however; I²t is not dimensionally equivalent to energy. In actual practice, one consults the spec sheet for the I²t rating of the semiconductor device to be protected. The I²t rating of the fuse must then be less than that of the semiconductor device. The fuse also must be capable of carrying the normal current range drawn by the semiconductor device.

The basic idea is to ascertain that a fault condition will enable the fuse to blow before the semiconductor device can be damaged. Because of the involvement of time in this manner, it can be loosely stated the fuse prevents damage from exces­sive energy despite the fact that I²t does not strictly represent energy. I²t is a practical means of matching fuses to power devices.

Such fuses also can carry maximum voltage ratings just as industrial-type fuses do. The I²t concept is useful in that a proper fuse can be specified without resorting to much empirical (and costly) testing.

Another energy like parameter is the volts-second values that must be taken into consideration when passing a duty-cycle modulated wave through a transformer. Volt-seconds are not dimensionally equivalent to energy, but this parameter can relate closely to both power and energy considerations. This is because the secondary voltage of the transformer is governed not only by the turns ratio but is affected by the duty cycle. The explanation of this often-overlooked phenomenon will be found in this chapter.

It is natural enough to associate power ratings with power devices, but in some instances energy ratings provide more meaningful information to the designer. For example, the protective capability of the MOV (metal-oxide transistor) is determined by its energy absorption rating-usually in joules. And, secondary breakdown in power transistors is a destructive mode that is essentially energy dependent. This is why narrow-pulse operation tends to yield a more favorable SOA (safe operating area of the load line) than dc or continuous operation. (2447)

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Power and energy at the submicroscopic level

The photon is the particle like entity responsible for the energy content of electromag­netic waves. The theory of the photon holds that its energy is directly proportional to its frequency. That is why X-ray radiation is said to be more energetic than ordinary visible light. This concept becomes more meaningful when exemplified in the following way. Co insider two 50 W (watt) RF (radio frequency) power generators, one at 10 MHz, the other at 100 MHz. Ten times as many 10 MHz photons as 100 MHz photons must be generated to produce the power level of 50 W. That is, 10 MHz photons have only 1/10 the energy content of 100 MHz photons. Although practical design and operation of RF equipment is ordinarily carried out without consideration of photonic principles, those working at higher frequencies involved in opto-voltaic devices, lasers, and fiber optics often profit from consideration of the energy content of the photon.

A classic example of the need to differentiate between power and energy is Einstein's explanation of photoelectric emission of electrons. It was shown that below a certain frequency of light, no amount of power could provoke photoelectric emission. Conversely, above a certain high-frequency threshold of the illumination, the smallest measurable power level sufficed to induce a proportionately tiny photocurrent. This simply stated concept merited the Nobel Prize. If, however, sloppy-use has been mace of the terms energy and power, the idea would have been mean­ingless. Energy and power are not interchangeable terms. Blue light is higher in frequency than red light. A certain threshold energy is needed to liberate electrons from the photo emissive cathode. Blue-light photons possess this energy; red-light photons do not, and no matter how many of them are involved (high power), no photoelectrons can be produced. (1859)

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