Introduction to Aluminum Electrolytic Capacitors
The development of the electrolytic capacitor (e-cap) has been one of the main factors in the successful miniaturization and increased performance of many modern-day electronics. The basic e-cap construction is shown in the figure below:
Figure 1 – Typical wet electrolytic capacitor (Courtesy of KEMET)
Since capacitance is a function of surface area, aluminum foils are first etched to create a rough contour with maximal contact area, resulting in high capacitance and optimal CV-value.
A second foil layer and a paper separator are added to fully complete the capacitor structure, producing an excellent terminal contact with this electrolyte. This aluminum-electrolyte-paper sandwich is then rolled or “wound” into a can and sealed with two terminals.
Aluminum Electrolytic Capacitors in Automotive Systems
DC-link capacitors are used in order to provide a stable DC-voltage, limiting voltage fluctuations even under high ripple current loads and fluctuations created by the inverter. The DC-link capacitors are acting as a local energy source, connected to the DC- board-net close to the power electronics (Þ low impedance).
Key Requirements, for Automotive DC-Link Capacitors
- Ripple current capability
- Low ESR
- High Temperature Capability
- Low Thermal Resistance (especially when mounted heat-sinked to metallic chassis)
- Operational Life
- Low Impedance
- Low Inductance
- High Reliability
An EMC filter is often used to protect the board-net from voltage spikes created by the switching power electronics.
The requirements for an EMC filter capacitor in automotive inverters is similar to that of DC-link capacitors.
Advancing from Through-Hole to SMD Designs
Increasing levels of miniaturization have forced the development of SMD alternatives to what was once only served by through-hole options.
The PES and PEV series of SMD aluminum electrolytic capacitors is based on the successful PEH and PEG series of through-hole radial crown capacitors. This introduction brings the expectations of high CV and high ripple current performance to an SMD solution.
The following considerations should be remembered when implementing a reflow process:
- Vapor heat transfer systems are not recommended
- Thermal systems such as infrared radiation and hot blast should be used
- Avoid repeated reflowing
Recommended reflow profile:
Solder Paste Alloy:
Solder passed allow should be selected to be suitable for the above recommended reflow profile. Sn/Ag or Sn/Ag/Cu allows with recommended peak solder temperatures in the range of 235C to 240C should be used.
Example of Successful Reflow
PCB Land Patterns and Footprints
The ripple current capability of the capacitor is further increased if the capacitor body is mounted to a chassis or metallic body with low thermal resistance. The information in the datasheet shows both natural convection and heat-sinking conditions.