1. Прочитайте первую часть данного ниже текста и найдите ответы на следующие вопросы:

1. When does so-called counter EMF appear?

2. What purpose are snubbers, or clamp circuits are used for?

3. What circumstances can be the reason of catastrophic destruction?

4. What happens when a reverse base-emitter voltage is actually applied during turn off?

5. When does the secondary breakdown usually take place?

Switching transients-a hidden gremlin in power control

I.

Making and breaking the flow of current in an electric circuit is the utmost in simplistic procedures or is it? Suppose the current has been flowing for some time and the knife switch is suddenly opened. Because of the energy' stored in the magnetic field of the inductor, you know that the abrupt cessation of current in the circuit does not come about in step with the physical breakage of contact between the switch elements. Several things happen before you can truly identify an open circuit. This so-called counter EMF accompanied by an arc dissipating energy in the form of heat, light, and sound. Additionally, considerable RF energy can manifest itself as interference in communications and other sensitive equipment. Once the arc has depleted the energy stored in the magnetic field of the inductor, it will extinguish itself, allowing the circuit to finally become open. A price might be paid for this process in the form of burned or fused switch blades.

Unfortunately, the same process tends to occur in a solid-state device used in switching applications. The semiconductor material inhibits the formation of the destructive arc up to a point. This feature enables the device to serve as a switch within the boundaries of its safe operating area. On the other hand, such devices are very unforgiving when excessive current is switched or considerable inductance is present. Various circuit techniques can be used to prevent destruction of the power switching-device by absorption of the excess energy. Circuits used for this purpose are generally referred to as numbers, or clamp circuits. An important aspect of the use of such protective circuits is that the stray inductance in a switching suffice to produce destructive counter EMFs in nanoseconds, even though there is no inductance in the load proper. This happens when high currents are being switched at high repetition rates, and with short turn-off times.

The practical aspect is that the snubbed circuit cannot be optimally effective unless it is connected to the power-switching device via very short leads. Otherwise the protected power switch can suffer catastrophic destruction; it is common that elegantly calculated snubbed and clamp networks fail to absorb nanosecond energy excesses at the actual terminals of the switching device. This condition might not impart sudden death to the switching device, but it is often the cause of a mysteriously short life span.

When a bipolar power transistor is switched to its off conductive state, it is said to be reverse biased, although this might correspond to zero base-emitter voltage. Often, however, a reverse base-emitter voltage is actually applied during turn off. This shortens the storage time of minority carriers and reduces the fall time of turn off. Reduced fall time decreases switching losses because less time is spent during simultaneous collector voltage and collector current. You must be careful, however, for even though switching losses are lessened by reverse bias, the vulnerability to destruction from secondary breakdown generally increases. For practical purposes, keep in mind that secondary breakdown usually takes place at lower energy levels under reverse-bias operation (when the transistor is switched off) than when the transistor is forward biased (during the on state of the transistor). This situation is not desired because the switch-off interval is just when the inductive kick back of circuit inductance produces the dreaded switching transient. (3508)