Capacitors are used in analog and digital circuits for a broad array of applications including blocking, coupling, bypassing, filtering, and energy storage. Whereas general purpose capacitors can be used in a wide range of applications, some electronic systems demand specialized components. In this article, we will explore in detail capacitors for use in pulse applications. Pulse capacitors, as these components are commonly known as, are a special class of capacitive components that are optimized to withstand rapid and intermittent voltage changes.
In an environment with high dV/dt, the peak current of a waveform can exceed the current rating of a component. When a capacitor is subjected to such an environment, its internal connections can be overloaded resulting in failure. In the case of metallized film capacitors, subjecting a component to peak currents that exceed its current rating can cause excessive heat generation in the boundaries between the film metallization and sprayed metal. Such failures can be prevented by limiting the current that a component is exposed to.
Pulses in electronic circuits
Fast rising and/or falling pulses are a major cause of component failures in electronic circuits. The high peak currents resulting from these waveforms can considerably stress the contact areas and internal connections of a capacitor. Such peak currents produce localized heating in the areas under stress. Damages of this kind can be prevented by limiting the peak currents of the waveforms.
Some of the circuits that are known to have such pulses include protection subsystems of SMPSs, pulsed laser circuits, electronic ballasts and deflection subsystems of television sets. Pulses with high values are also common in many high frequency electronic circuits. Conventional capacitors have low tolerances to fast rising/fast falling pulses, and it is necessary to use components that are optimized to withstand pulses. Using pulse grade capacitors helps to enhance the overall reliability of an electronic circuit.
For a given film capacitor, the peak current is mainly determined by the shape and amplitude of the waveform, voltage rating, capacitance and the shape of a component. The key factors that determine the maximum pulse current that a capacitor can withstand include its capacitance, peak current rating, and its maximum rate of change of voltage with time (dV/dt). These parameters are usually provided in datasheets.
Capacitors for pulse applications
Capacitors are used in electronic circuits for a broad range of applications including bypassing, coupling, filtering, and snubbering. In environments with rapid or intermittent voltage changes, pulse grade capacitors are used because they are optimized to withstand high dV/dt. These components are usually suitable for both low and high voltage values. In addition, most of them are designed to withstand high frequency environments. Pulse grade capacitors are suitable for a wide array of equipment including DC filters, pulsed lasers, flash lamps, strobe lights, particle accelerators, high energy dynodes, utility UPS systems, and so on.
Film capacitors are arguably the most popular type of capacitors for pulse applications. Metallized film capacitors have self-healing properties, and this makes them an unrivalled choice for pulse applications. Generally speaking, these capacitors have low series resistances and low inductances. The specific characteristics of a metallized film capacitor are dependent on the internal structure of a component. There are several electrode-film configurations and each yields slightly different characteristics.
Some of the key factors to consider when selecting a film capacitor for a specific pulse application include capacitance, voltage rating, pulse current capability, dV/dt rating, and power dissipation characteristics. In addition, it is important to consider the duty ratio and repetition rate of the pulses when choosing a component for your application.
For a specific capacitor, the current limit is mainly determined by the characteristics of a pulse (amplitude and form), capacitance, internal configuration and voltage rating a component. The pulse current limits of a component are used to define its dV/dt rating. This parameter and the pulse characteristic constant define the pulse handling capability of a component.
i. Pulse slope (dV/dt)
The voltage pulse slope is one of the key parameters that determine the capability of a capacitor to withstand pulses. This rating is usually given in volts per microsecond (V/µs), and it is usually provided by component manufacturers in product datasheets.
ii. Pulse characteristic constant (k)
The heat generated by a capacitor when it is subjected to rapid or intermittent voltage changes is dependent on the characteristics of the waveform. This parameter is a factor of pulse width and slope. The pulse characteristic is usually provided by component manufacturers in datasheets, and its units are V2/µs. For applications with lower duty cycles, this parameter should be derated to prevent excessive overheating that can result in component failure.
iii. Internal structure of a capacitor
The capability of a component to withstand pulses is greatly determined by its internal structure. Generally speaking, stacked film capacitors have better pulse-handling characteristics as compared to wound film components. A typical stacked film capacitor consists of individual components in parallel, and this means that failure in one component does not affect neighboring elements. This is the reason why stacked film capacitors have higher tolerances to pulses.
Although film capacitors are the most popular type of capacitors for pulse applications, some aluminum electrolytic capacitors can also be used for these applications. Aluminum electrolytic capacitors for pulse applications come in compact designs, and they can be used in a wide range of equipment including laser machines, welding machines, professional flashlights, mobile x-ray generators, and other AC and DC pulse applications.
Some applications expose electronic components to rapid or intermittent charge and discharge cycles. The peak currents produced by these waveforms can damage conventional capacitors. For such applications, capacitors that are optimized to withstand rapid and intermittent pulses are required. Although most pulse grade capacitors are based on metallized film capacitor technology, some are based on aluminum electrolytic capacitor technology. Some of the key factors to consider when selecting a film capacitor for a pulse application include pulse slope (dV/dt), pulse characteristic, and internal configuration of a component.