Electronics 101 |
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1. Introduction |
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2. Passive Components and Stray Parameters |
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The reader is certainly familiar with “lumped” passive components: resistors, capacitors and inductors. These are well-characterized elements that can be purchased from distributors. | |||
Most readers are less familiar with “distributed” passive elements that are equally important in power electronics. They are an undesired part of any power circuit and have to be dealt with at the design stage. All common topologies, like a bridge, are capable of handling a load with significant amount of inductance, but they are not designed to handle the “strays”. Here are some examples:
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Figure 1. Stray inductances in a half-bridge. Some are in the DC side of the loop (red), some in the AC side (blue). The inductances in the DC side are responsible for the voltage transients on the bus. These transients are proportional to Lstray di/dt and depend, in part, on the control scheme. The inductances in the DC loop can be compensated with a decoupling capacitor.
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The parasitics embedded in a high frequency transformer are a good example that deserves careful analysis. The magnetics designer has the difficult task of balancing the parasitics to optimize overall performance, as any improvement in any one of these stray parameters degrades the other two. | |||
Leakage inductance(s). An ideal transformer would only have a “magnetizing inductance”. In a real transformer there is a leakage inductance in series with the primary and with the secondary windings, due to the fact that the two windings are not perfectly coupled (Figure 2). This is not a problem for the power circuit, that is already capable of handling the magnetizing inductance, but degrades the performance of the transformer in bandwidth and in its power handling capability. These inductances are charged and discharged every cycle by the power circuit and part of the energy stored in the leakage inductance is dissipated in the semiconductors, increasing their operating temperature. | |||
Figure 2. Stray parameters in transformers. The leakage inductances are due to the imperfect coupling of the windings. The parasitic capacitances are most pronounced in compact, high-frequency designs, where they are least desired and they complicate the design of the EMI filter. Both reduce the bandwidth of the transformer.
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Interwinding capacitance(s). An ideal transformer has no capacitances. A real transformer has winding and inter-winding capacitances that have a significant impact on the entire power system:
Worse yet, the charge and discharge occur during the switching transients. Thus they increase the switching losses and limit the upper frequency capability of the system. Like the leakage inductance, they degrade the bandwidth and the response time of the entire system.
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Winding resistance. This increases losses and temperature in the transformer itself. It complicates the thermal design of the overall system and reduces its power-handling capability. | |||
The interwinding capacitances in motors operated at medium-high voltage are also a source of problems. They have been known to cause insulation breakdown and corrosion in the ball bearings. | |||
Stray capacitances are also responsible for electro-static discharge (ESD). ESD is an unpleasant experience for human beings as well as for semiconductors. Human beings normally survive it; semiconductors can be permanently damaged by it. Hence, the ESD precautions implemented in all manufacturing plants and laboratories. | |||
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