Capacitor innovations helping engineers manage ripple current effects
KEY TAKEAWAYS:
- Ripple current will accelerate capacitor aging if left unchecked
- New materials may help, but design advice is essential
- WBG transistors make ripple current ratings harder to overlook
Choosing capacitors with a higher ripple current rating doesn’t address the bigger challenge of developing power stages with higher voltages operating in more extreme temperatures.
OEMs have seen improvements recently in the way ripple current impacts capacitor behavior. Capacitor manufacturers have achieved this by addressing the main causes: equivalent series resistance (ESR), thermal paths and the materials used.
Anyone developing power distribution networks, working in EVs or developing with wide bandgap (WBG) devices may have seen the improvements. These have been mostly evolutionary, but significant.
Any current across a resistive element generates heat, so when ripple current–the unwanted element in power supplies–combines with ESR in a capacitor, the result is self-heating, and that heat accelerates the aging effect of a capacitor.
WBG devices, specifically silicon carbide (SiC) and gallium nitride (GaN), are being designed into new applications, including EV inverters and older applications with new demands, such as power supplies in AI-driven data centers. The reason for WBG adoption is their higher switching frequencies, which enable higher voltages and higher efficiency.
Capacitors are key components in power supplies. As power relates to area under the curve, OEMs now need capacitors that have higher ripple current ratings. The approaches most suppliers are taking include finding ways to lower ESR, improving the way heat is dissipated or spread to remove it from the capacitor, and increasing the AC current capacity per unit of volume.
Material developments that impact ripple current in capacitors
Hybrid polymer-electrolytic aluminum capacitors have emerged as one solution, particularly for automotive and industrial applications. These devices often combine polymer (which has low ESR) with a liquid electrolyte (for higher voltage levels and self-healing capabilities).
Some hybrid aluminum capacitors now available boast significantly higher ripple current ratings than their predecessors, and with that comes increased capacitance and lower ESR. These combine to deliver substantially lower self-heating characteristics than conventional electrolytics.
But even for conventional aluminum electrolytic capacitors, manufacturers are finding ways of lowering ESR and improving current paths to raise ripple current ratings. One technique involves adding electrode tabs, which can also improve vibration and shock resistance. This can be attractive for applications that encounter higher levels of mechanical stress.
Developments like these have been rolling out quietly for a while. The focus has been on switched-mode power supplies and, specifically, DC-links. Rather than searching for higher ripple current, engineers could ask for capacitors that feature lower ESR or longer lifetimes. The practical way to verify that is to look at ripple current ratings, which may be documented in the data sheet, but not always. Consider talking to an Avnet field application engineer if you want to find the shortest route to the right capacitor.
Capacitor advancements that tackle ripple current effects
For multi-layer ceramic capacitors (MLCCs), the discussion is more about dielectric developments and loss optimization than about ripple current numbers, but the effect is the same.
Lower losses under higher AC voltages translate into higher ripple current handling and lower self-heating. The enabling technology is the dielectric. In some cases, MLCCs can be used in place of film capacitors in some high-frequency power applications. The benefits are smaller sizes as well as lower losses.
MLCCs can deliver miniaturization and reduced losses, provided that the design requirements respect the capacitor’s voltage, DC bias and thermal limits. Those gains implicitly convey improved ripple current ratings at the kinds of switching frequencies that would have previously dictated using film capacitors.
Silicon capacitors are fabricated using a semiconductor process with a metal-insulator-metal configuration. This results in high-density capacitors that can behave more predictably and remain more stable when under DC bias conditions or higher operating temperatures.
Design advice for power engineers
There is now more design advice available to power engineers about how to manage ripple current. This includes thinking about film, aluminum and polymer parts in terms of ripple current, which will be different for each type.
This advice may now include how to add design margins or create thermal paths rather than simply putting a figure on a data sheet. They may also stress that the ripple current stated is not a constant, but is influenced by the application, frequency and temperature. Look for maximum ripple currents across different temperatures and frequencies.
The way ripple current is reported can make it difficult to compare and contrast between suppliers offering similar parts. This is where Avnet can help. With field application engineers close to the source, OEMs can get first-line advice on which capacitor best fits their application.
The effects of ripple current are unavoidable, and in the absence of the perfect capacitor, there will always be some unwanted resistance in the path. The result is dissipated power. Even with an ESR of just 1 mΩ, a ripple current of around 32 ARMS would generate 1 W of heat.
Understanding where the advances are being made and what solutions best fit your application can help engineers prevent ripple from becoming a source of heat, aging and failure.
