Column: EMC
sometimes two capacitors are connected in series. If the two capacitors are electrolytics, they are subjected to higher leakage current, compared with film capacitors. Also, due to their looser capacitance tolerance values, it is important to balance the two capacitors using resistors. Tese ensure that the voltage sharing between the two capacitors is equal, and provide the ‘bleed’ function when they discharge; they are also essential for preventing electrolytic capacitors from ageing prematurely.
R-C filters R-C filters are oſten used for applications where a signal’s phase shiſt must be minimised. For example, in motor drive applications, current phase oſten needs to be sampled for controlling the motor. Tis is usually done by having a shunt resistor with the bridge side switches. Since the motor rotates fast, delay of the sampling signal must be minimised, so in such an application, an R-C filter rather than an L-C filter is oſten
preferred aſter the shunt resistor. R-C filters are also very common in the
peripherals of a microcontroller chip such as an ADC. Resistors are also used as current-limiting
components to prevent in-rush current, and as ‘bleed’ resistors for capacitors. In-rush current, oſten caused by low line impedance during power-up, can damage the board. Figure 3 shows a PFC+LLC charger that suffered from an in-rush current incident; the weak points in the system are the sensing resistors, which were completely destroyed. In some applications, resistive components
are used in the front end of a power system to improve the power and crest factors and mains harmonics. Compared to inductors, resistors have no phase shiſt, ideal for power- factor correction. Tey also allow the circuit to withstand supply voltage surges without needing a varistor. However, the drawback here is that they absorb power and thus lower overall system efficiency.
Figure 3: This PFC+LLC charger was the victim of an in-rush current incident
Essential components Despite the drawbacks of using resistors, they are essential for many applications involving filters. From managing signal integrity and resonance in circuit boards, to storing higher impulse loads and improving the power factor, resistors are widely used in many designs.
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