Reduced size of low-inductance Aluminum Electrolytic capacitors

Driven by emerging industry trends such as Industry 4.0 and the Industrial Internet of Things (IoT), automation of manufacturing and assembly processes continues to gain widespread adoption, while low-inductance electrolytic capacitors help reduce costs in robotics and other industrial equipment, Improve performance.

Both polypropylene film and electrolytic capacitors are suitable for tasks such as bulk smoothing and decoupling in high-power industrial applications. These include the output of switch-mode power supplies and stabilizing the DC link of variable-frequency motor drives and fixed-frequency generators. Compared to other capacitor technologies, electrolytic capacitors provide high capacitance in a small size at low cost and are generally preferred for applications up to about 600V.

Every real capacitor will have a corresponding inductance that creates voltage spikes when high-frequency ripple current passes through the device. Capacitors designed for low parasitic inductance can reduce the magnitude of these voltage peaks, allowing designers to use lower voltage classes <of power semiconductor devices. In addition, the use of low-inductance devices reduces the number of capacitors required per bank, helping to reduce overall cost and size and weight.

The parasitic inductance of the capacitor

Unlike an ideal capacitor capable of instantaneously transferring all stored energy to the load, a real capacitor has unwanted parasitic elements which can be considered as equivalent inductance and resistance (ESL and ESR) in series with the capacitor . Unwanted inductance can cause effects including induced voltage spikes that can damage sensitive components connected to the circuit. In addition, the interaction between stray inductance and device capacitance can also cause noise, which affects circuit stability and power quality.

In general, inductance tends to resist changes in current, and the magnitude of the effect depends on frequency. Capacitive reactance decreases with frequency, while inductive reactance tends to increase with frequency. These two reactance’s become equal in magnitude but opposite in phase at the self-resonant frequency of the capacitor, producing a cancelling effect so that the total reactance is zero, and the impedance of the capacitor is entirely due to ESR:

Because |Z|√(|Xc|-|XL|)2+|ESR|2

Additional XL=2πfL and Xc=1/2πfC??

At the self-resonant frequency, XL=XC, the self-resonant frequency is determined by the following formula: f=1/2π√LC

Below the self-resonant frequency, the element behaves as a capacitor, and the impedance tends to decrease with increasing frequency. As the frequency increases, the impedance characteristic starts to deviate and reaches a minimum at the self-resonant frequency. Above this frequency, the inductive nature dominates and the impedance increases. Lowering the ESL of the capacitor increases the self-resonant frequency.

Low Inductance Capacitor Requirements

One application that can benefit from low inductance electrolytic capacitors is bulk capacitance, which typically suffers from high frequency switching. Additionally, DC-link applications such as industrial inverter drives require low-ESL capacitors to minimize self-heating while enhancing protection of power devices. The ESL of standard electrolytic DC link capacitors along with associated connections, cables and other components together create voltage spikes that require a snubber on each inverter phase leg. Reducing the ESL of the capacitor itself can reduce the overall inductance to the point where the snubber circuit for each inverter phase leg can be completely eliminated.

Internal capacitor design

The main internal components that affect the ESL of larger screw terminal electrolytic capacitors include the deck terminal, internal connection tabs, and windings, as shown in Figure 1. By optimizing the internal layout, ESL can be effectively reduced and all magnetic fields generated by the capacitor current can be eliminated, which can be achieved by techniques such as reducing the distance between winding components and terminals, and reducing the distance between lugs. Figure 1 compares the internal layout of a standard capacitor to a low-inductance version, illustrating how redesigning these features can reduce inductance by as much as 40%.?

This article was originally published on the EDN China website, compiled by Franklin 2022-11-18