Capacitor Selection Methods in Medical Power Supplies

　　Switching power supplies are used to control these power supplies. Due to their significant advantages, switching power supplies have become the standard power supply for most electronic products. Capacitors can be used to reduce ripple and absorb the noise generated by the switching regulator. They can also be used for post-regulation to improve the stability and transient response of the equipment. There should be no ripple noise or residual jitter in the power supply output. Tantalum capacitors are often used in these circuits to reduce ripple, but tantalum capacitors may be affected by the noise of the switching regulator and produce unsafe transients.

　　To ensure reliable operation, the rated voltage of tantalum capacitors must be reduced. For example, the operating voltage of a D-type tantalum capacitor rated at 10uF/35V should be reduced to 17V. If used to filter ripple at the power input, a 35V rated tantalum capacitor can work reliably on a voltage rail as high as 17V.

　　High voltage power bus systems generally have difficulty achieving a 50% reduction in rated voltage. This situation limits the use of tantalum capacitors for applications with voltage rails greater than 28V. Currently, due to the need to derate tantalum capacitors, the only viable solution for high voltage filtering applications is to use larger leaded electrolytic capacitors instead of tantalum capacitors.

　　New Tantalum Capacitors

　　To address the issue of reduced rated voltage, Vishay R&D has developed a new series of SMD solid tantalum capacitors with higher rated voltage levels, rated up to 75WVDC. The use of 50V rated voltage capacitors in 28V and higher voltage rails has caused concerns among designers, but with Vishay's new 63V and 75V tantalum capacitors, the industry-recognized safety indicator of a 50% reduction in rated voltage can be achieved. Thinner and more consistent dielectric formation enables the rated voltage of SMD solid tantalum capacitors to reach 75V, thus achieving a technological breakthrough in increasing the rated voltage. Multiple processes have been improved in the forming process: reducing the concentration of mechanical stress generated during the forming process, reducing local overheating of the electrolyte during the capacitor forming process, and improving the consistency of electrolyte concentration and purity during the dielectric forming process. The new capacitors T97 series have a rated voltage of 75V, and the 83 series has a rated voltage of 63V.

　　Wireless inductive coupling charging

　　A large number of inductive chargers use flyback converters. Inductive charging provides charging power for medical device batteries, and inductive chargers are also used in a large number of portable devices (such as toothbrushes).

　　Reducing the size of rechargeable batteries can help reduce the size of implantable medical devices that use wireless inductive charging circuits. Wireless inductive chargers can safely charge tiny thin-film rechargeable energy storage devices (such as the Cymbet EnerChip) installed on the device. Inductive chargers use the operating principle of parallel LC (inductor, capacitor) resonant energy storage circuits. Figure 1 shows the CBC-EVAL-11 RF inductive charger evaluation kit from Cymbet.

　　Vishay 595D series 1000uF tantalum capacitor is used as C5 capacitor of Cymbet receiver circuit board to provide pulse current for loads such as radio transmitter. This inductive charger has good isolation between input and output, which is an important requirement for medical equipment.

　　In some high voltage inductive charger applications, a high voltage stable capacitor is required as a resonant capacitor. Since the primary coil of the inductive charger needs to be driven by an AC voltage, the capacitor must be adjusted accordingly. The inductive charger needs to have a high breakdown voltage (VBD) performance, and in some applications, protection against high voltage arc discharge is also required. To avoid arc discharge, the circuit board is generally coated with a protective coating, or the high voltage side is isolated from the rest of the circuit board by properly arranging the component layout, etc. However, this method often requires a lot of circuit board space because high voltage circuits usually use large leaded through-hole plug-in capacitors.

　　High voltage arc protection capacitor solution

　　To solve this problem, Vishay has introduced a series of HVArc (high voltage arc) protection MLCC (multilayer ceramic capacitors) that prevent arc discharge while saving space. These new devices have maximum capacity within higher voltage ratings and improved voltage breakdown tolerance. High voltage arc discharge will cause short circuits and may damage other components. Standard high voltage SMD capacitors will eventually fail and short circuit, depending on the number of arc discharges and the part with problems. Vishay HVArc protection capacitors can absorb all energy, so this capacitor can work normally under high voltage, at least until the high voltage breakdown limit is reached, without destructive arc discharge.

　　The VBD distribution of HVArc protection capacitors is controlled by the unique design of the device, and the VBD can reach 3kV or more. This product uses NPO and X7R dielectrics.

　　New non-magnetic capacitor for MRI

　　Capacitors and other electronic components used in the circuits inside or around magnetic resonance imaging (MRI) equipment need to be shielded or encapsulated outside the MRI room. The dielectric, electrode or termination materials of the capacitors may contain ferrous or magnetic materials. The magnetic field level of MRI systems is constantly increasing to improve image resolution, and the capacitors used in the MRI room will cause magnetic field distortion. Therefore, it is necessary to reduce or completely eliminate the magnetic materials in most capacitors.

　　The latest series of MLCCs use non-ferrous materials in the electrode and termination structures to meet the requirements of eliminating magnetization. X7R and NPO dielectrics can be used for non-magnetic structures. The outer dimensions are 0402 to 1812, which meet EIA specifications. Vishay also uses dedicated capacitor sorting equipment during final testing to ensure that all non-magnetic capacitors meet technical requirements.