Capacitors used in medical power systems must have low loss and high reliability
by Ashley Stanziola, Technical Applications Engineer at Kyocera AVX
Medical devices are advancing in ways that allow them to have multiple capabilities and better monitoring. These developments are causing systems to become more complex and use higher power. Power design for medical devices is extremely important because having an unreliable, faulty power supply can be catastrophic for patients. Therefore, it is crucial to choose high-quality parts with low-loss characteristics that are suitable for conversion efficiency. When choosing capacitors, the characteristics that are most important for power quality are Equivalent Series Resistance (ESR), Leakage, Dissipation Factor (DF), and the components’ temperature vs. time stability, as well as voltage over lifetime stability.
These characteristics are important for medical power design because lower ESR and DF values reduce energy losses and heat generation, which is essential for thermal management, device integrity, and efficiency. Minimizing leakage current prevents battery drain and ensures long-term reliability. Temperature and voltage stability ensure consistent performance across varying temperatures and voltages. Stability ensures that the capacitor’s capacitance does not drift over time, which is critical for the precise operation of medical equipment.
KYOCERA AVX classifies its medical products into two groups, critical and non-critical. Critical medical products meet the requirements for devices with high risks for the patient, such as implantable and life-sustaining applications. These devices include pacemakers, cochlear implants, cardioverter-defibrillators, and neural stimulators, e.g., brain and spinal stimulators. Non-critical products meet the requirements for applications with low and medium risk to the patient. These devices may include some external electronics, as well as oximeters, glucose meters, external insulin pumps, and blood pressure, heart rate, and patient monitors.
KYOCERA AVX Components has three series of tantalum capacitors with long-life operations for critical applications. These series are HRC5000 TAZ, HRC5000 TBC, and HRC6000 TBC. The HRC5000 TAZ series is a molded tantalum with extremely low ESR, low DC leakage (as low as 0.1µA), capacitance ranges from 0.1µF to 330µF, and a voltage range from 4V to 50V. Their characteristics not only make these parts optimal for power quality but have also allowed them to win Medical Design Briefs Magazine’s Product of the Year Award in 2013. They are offered in case sizes from R (2.05mm x 1.30mm) to H (7.24mm x 3.81mm).
Both the HRC5000 TBC and the HRC6000 TBC series are KYOCERA AVX’s TACmicrochip construction. This construction has a tantalum wafer substrate with tantalum powder pressed and sintered onto the surface. Individual anodes are defined on the surface using a sawing operation, and the wafers are processed through dielectric formation and cathode deposition. Next, a lid is placed over the structure, and epoxy is flowed into the channels between the anodes. This is followed by a dicing operation that separates the capacitors, which then proceed through burn-in, test, and packaging processes. The HRC5000 and HRC6000 TBC both have low ESR and extremely low DC leakage. The TBC HRC5000 has capacitance ranges from 0.47µF to 47µF, a voltage range from 4V to 40V, and case sizes from L (1.60mm x 0.84mm) to B (3.60mm x 2.90mm). The TBC HRC6000 has capacitance ranges from 0.33µF to 68µF, a voltage range from 4V to 16V, and also has case sizes from L (1.60mm x 0.84mm) to B (3.60mm x 2.90mm).
Both the HRC5000 TAZ and TBC establish their reliability by using design/process change controls, statistical screening, Weibull burn-in, maverick piece/lot screening, DPA, and lot conformance testing. Meanwhile, the HRC6000 uses KYOCERA AVX’s Q Process to effectively remove components that may experience parametric shifts through customer processing or display instability through life testing. The figure compares the ESR for all three series and leakage current characteristics for both HRC5000 TAZ and HRC5000 TBC Series. The data is for a 47µF, 6V capacitor in HRC5000 TAZ (TAZG476M006C*LB5, Size G: 6.73mm x 2.79mm), HRC5000 TBC (TBCS476M006C*LB5, Size S: 3.20mm x 1.80mm) and HRC6000 TBC (TBCS476M006C*LQ6, Size S: 3.20mm x 1.80mm). For power quality, it is important to look at ESR because the loss can reduce efficiency and cause heat in the medical device. Also, it is important to look at leakage because it can significantly reduce part reliability. As shown in the image, the HRC5000 TBC and the HRC6000 TBC have a lower ESR compared to the HRC5000 TAZ. Also, the HRC5000 TBC has a lower DC Leakage than the HRC5000 TAZ.
KYOCERA AVX offers two options to establish part reliability. The first is the industry standard, Weibull grading. Even though Weibull Grading has been used for decades to characterize the reliability of Tantalum Capacitors, there is evidence to suggest that there are inefficiencies with this method. Firstly, the burn-in process requires an extreme amount of voltage acceleration. Subjecting capacitors to these voltages can cause the part’s DC Leakage performance to degrade. Another issue is that healed parts that are mechanically weak can pass the Weibull assessment. These unreliable parts can result in an early failure that can be catastrophic within the system. The last issue is that the part’s early life failure is not included in the Weibull reliability calculation. Due to these issues, KYOCERA AVX offers Q Process as an alternative option to establish reliability.
The Q Process improves part performance with an updated burn-in process and procedure that incorporates an additional reflow and statistical 3-sigma screening at 125°C for pre- and post-burn-in. The result of the Q Process is the ability to detect and remove parts that display elevated leakage characteristics. Removing the parts prior to the burn-in process corrects the mechanically weak parts issue seen in Weibull Grading. This is due to the process preventing part defects from healing and shifting back into the normalized population. Furthermore, the Q Process also removes the DC Leakage degradation, due to acceleration, by determining the optimum burn-in time and voltage. The new burn-in has been revised to occur at 125°C, and the voltage is based on the design of the components under test.
When designing power for medical devices, conversion efficiency is key. Capacitors used in medical power systems must have low loss and high reliability. Additionally, their long-term leakage must be considered to prevent damage to the part as well as the device. KYOCERA AVX has perfected its tantalum capacitors that are used for critical applications. HRC5000 TAZ, TCP SERIES – DLA 09009, and HRC5000 TBC are offered with Weibull Grade reliability, and HRC6000 TBC with Q Process. These series have electrical characteristics that are highly desirable for medical power design.