The Basics of Power Conversion and the New Film Capacitors

The Basics of Power Conversion and the New Film Capacitors

As electronic circuits evolve and advance, the demand for stable components capable of handling more power in extreme environments increases. Innovations in power supply designs, such as the use of wide bandgap semiconductors, require capacitors that operate at higher frequencies, at higher temperatures, and in smaller spaces. All the while, expectations of high reliability, safety, and long life also increase.

Film capacitors have been around for a long time, but modern technologies and processes have radically expanded the capabilities of these tried and true devices. Now, higher capacitance densities, frequencies, environmental ratings, low losses, and life expectancies are all being realized. Today’s power film capacitors are the ideal solution for power conversion in sustainable energy, energy storage, industrial, or automotive applications.

Understanding Power Conversion

Figure 1: Energy Conversion Model

Power conversion circuitry takes energy from a power source and converts it into an output format usable by end devices. Energy sources could be the traditional power grid, renewable energy generators like solar or wind, or stored energy in batteries or capacitor banks. Those sources provide energy that is not conditioned for end devices, but rather it is conditioned for transmission or the raw output from the source.

Power conversion systems have an input stage, where the power is converted from AC or DC to the desired DC level, and then an output stage where the DC voltage is converted to the AC or DC level required by the end devices. These systems also include an intermediate stage where the DC-link capacitor, or capacitor bank, reside. The DC-link capacitor is responsible for filtering the voltage, and providing energy storage for a clean, consistent, and fast energy source to the output stage.

In each conversion stage, input and output, snubber capacitors (1, in figure above), are used to suppress undesirable voltage and current pulses created by the switching stages of the semiconductor devices. In either the input or output stage, if AC voltage is coming in or going out, AC filter capacitors can be found (2, in figure above). DC filter or DC-link capacitors (3, in figure above) are found in all power conversion circuits, in the input or intermediate stages. DC filter capacitors can also be found in the output stage, if it is a DC/DC converter stage.

Snubbers and Resonant Tank Capacitors

Capacitors placed across switching devices and used to suppress the inherent voltage transient noise generated by switching are called snubbers. A snubber circuit is made up of a resistor and a capacitor, and in many cases, the resistance of the traces or wiring and the equivalent series resistance (ESR) of the capacitor can serve as the R part of the RC snubber circuit.

A resonant circuit, tank circuit, or resonant tank circuit, is a resonator made up of an inductor and a capacitor and tuned to resonate at specific frequencies. This resonant circuit can be used to significantly increase the efficiency of certain types of power converters.

Figure 2: R75H Capacitor in snubber, resonant tank, and DC-Link applications

Snubbers and resonant tank circuits present special challenges for capacitors. They must withstand high DC voltage pulses, operate at high frequencies, and endure high temperatures. For these types of circuits, the higher the dV/dt, the more efficient. And as circuits get smaller and more space-constrained, capacitance density and power density are becoming increasingly important.

The new R75H series of pulse capacitors are ideal for a variety of applications. With a high capacitance density and high current capability, they can operate at the high frequencies needed for high-efficiency converters. The R75H series has one of the highest dV/dt characteristics in the market, with its single metalized polypropylene construction. These capacitors have a high-temperature rating, up to 125 °C, and are self-healing and extremely reliable.

AC Filter Capacitors

Capacitors placed on AC voltage lines to filter them are called AC filter capacitors. On three-phase AC power lines, these capacitors can be placed in either a delta or wye configuration. In a delta configuration, the capacitors are connected between the different phases, but in a wye configuration, the capacitors are connected between each phase and a central point. This neutral point is sometimes connected to the ground or sometimes left as a floating neutral, depending on the system design.

These capacitors provide filtering for the AC voltage lines (input or output). Because these are large devices that can be found on high power lines with filter inductors (for instance LCL filters), and connected in banks with several capacitors in series/parallel, one of the most important requirements for these types of devices is safety. They need to be highly reliable and completely safe.

Figure 3: C44P-R capacitors in an AC filter (wye config) application

The new C44P-R series AC filter capacitors represent a step forward in technology. They have a high current capability and a long life expectancy. They are metalized polypropylene film capacitors with self-healing capability- in the case of a dielectric breakdown, the arc energy is enough to close the channel, resulting in a self-healed device at the cost of just a small capacitance drop. These capacitors are also constructed with an overpressure safety mechanism. When the internal soft resin in the capacitor expands due to high internal self-heating, the can elongates and disconnects the terminals internally.  Once the capacitor is disconnected, the potential for catastrophic failure is avoided, but the capacitor is no longer operational and can be safely replaced.

DC-Link Capacitors

Capacitors in the DC circuits at the output of either the input or output stages of a converter are called DC-link capacitors. These serve as filters on the DC voltage, as well as energy storage capacitance to provide instantaneous current to all downstream circuits. They can also be used in certain applications to store energy for failsafe power loss operations.

DC-link capacitors must be able to withstand high power, high ripple currents, and many charge/discharge cycles.

They need to do this reliably and safely in extreme conditions, as many of these power converters are found in windmills, solar farms, and other renewable energy source circuits.

Figure 4: C44U-M capacitors in a DC-link application

The new C44U-M series DC link capacitors are large can capacitors up to 116 mm in diameter, allowing for high capacitance density, high DC voltage load capability, and high ripple current., they have such high-power density and high voltage capability that they actually can be considered as an excellent substitute of screw terminal electrolytic capacitors, reducing the size of the final solutions in which they are needed with better performance in ripple current and lower losses due to their low ESR, and extended operational life. They also have similar self-healing capabilities of the AC filter capacitors, increasing the long-term reliability and lifetime even further.

Applications and Industries

Together, these innovative and improved devices serve the power converter needs of all kinds of industrial and commercial applications. They are particularly ideal for advanced technologies, in that they support high-frequency converters and harsh conditions, allowing them to work with new silicon and wide bandgap (WBG) semiconductors. These higher efficiency power converters support the latest in energy storage, renewable generators such as wind and solar, and electric vehicles.

Power Conversion Capacitor Solutions

The new film capacitors for power conversion meet the needs of today’s most demanding applications. The entire package of R75H pulse snubbers, the C44P-R AC filters, and the C44U-M DC link capacitors provide the full capacitance solution for power converters in any extreme environment.

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