Electronics 101

 

2. Passive Components and Stray Parameters

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:

  • Stray inductances are everywhere. They can make a current-sensing resistor virtually useless. Stray inductances in the main current flow cause destructive overvoltages during di/dt transients. Stray inductances in capacitors reduce their effectiveness. Figure 1 shows the stray inductances that are frequently found in a half-bridge.

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.

 

  • Stray capacitances cause current spikes in the power semiconductors and increase losses in the system. One typical example is the stray capacitance of the cables between the motor drive and the motor: they appear to the control as short-circuits in the load.  Stray capacitances have a major impact on the performance of those applications that operate a high frequency, like the voltage regulators for microprocessors. For more details see http://www.irf.com/product-info/igbt/pcb-layoutguidelines.pdf
 
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.

 

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:

  • Like the leakage inductance, they have to be charged and discharged at every cycle. Unlike the leakage inductance, the entire energy stored in these capacitances is lost, dissipated in the power semiconductors and in the winding resistance, thereby increasing the temperature of the transformer and of the semiconductors.

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.

  • The current that flows in the interwinding capacitances has to be collected by the harmonic filter at the input that is required by applicable regulations. This current increases its size and cost and significantly degrades the EMI performance of the entire system.
 
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.