High-voltage Multi-Layer Ceramic Capacitors (MLCCs) are great for roles such as coupling, DC blocking, snubbing, or power-supply resonators. Their high volumetric efficiency delivers high capacitance values and high breakdown voltage within compact dimensions. A 1206 (imperial: 0.125” x 0.063”) chip-size MLCC can provide several thousand picofarads and a voltage rating of 2-3000V.
There are barriers to miniaturization, however. Although the MLCC can be chosen with a voltage rating well above the normal system operating voltage, high voltages applied during hi-pot testing or exceptional operating conditions can damage the capacitor or nearby circuitry if the device is not carefully selected and properly designed-in.
An AC hi-pot test voltage is usually 1000V plus twice the normal operating voltage. For a 240V line-powered appliance this is about 1500V, which is much greater than the voltage needed for corona discharge to occur that can lead to arcing across the surface of the capacitor or circuit board. Arcing can degrade the component and the circuit board, potentially causing test failure or reduction of performance and lifetime in the field.
The culprit is creepage: the natural tendency for an electric field to spread out over a dielectric surface. As the applied voltage rises, the surrounding air can become sufficiently ionized for corona discharge to occur. An applied voltage of about 300V is enough for inception. Then, depending on the characteristics of the dielectric material such as CTI (comparative tracking index), the effects of contaminants on the dielectric surface, and environmental factors such as humidity, an arc can be set up between the capacitor terminations or other exposed conductors on the board that are sitting at a lower potential.
Creepage can be increased to avoid these effects. One way is to use larger-size components. This may be an option if space constraints are not super tight, or if achieving the smallest possible circuit size is not a critical goal. However, MLCCs in larger packages can be vulnerable to cracking caused by flexing of the circuit board. This can happen during use in the field, or during stressful manufacturing processes such as reflow soldering.
A lot has been written about flex cracking of MLCCs, and component makers have overcome the problem by increasing compliance in the device terminals. Several manufacturers offer soft-termination systems for MLCCs, such as the AVX FLEXITERM high-voltage series, Johanson Dielectrics Polyterm MLCCs , or KEMET FT-CAP devices. These can allow for up to several millimeters of board flex.
Another approach is to adapt the internal MLCC structure to reduce the voltage gradient between the capacitor termination and the component surface. This has the potential to allow high-voltage capacitors in small package sizes – miniaturization with safety and robustness. KEMET, with its ArcShield MLCCs, and Johanson Dielectrics, with its high-voltage chip capacitor series, have both taken this approach. These devices enable MLCCs in case sizes such as 1206, 0805 and even 0603 to be used safely in high-voltage applications.
However, we should still approach high-voltage design with caution. While these internal-electrode schemes certainly give engineers a lot more freedom to miniaturize, good PCB design practice remains essential to ensure the end product will pass hi-pot tests and survive in the field.
The effective creepage distance can be reduced when the component is mounted close to the surface of the PCB. Keeping the board surface free of contaminants such as solder particles is essential to prevent high-voltage discharges.
Various design techniques can also be applied, such as milling or routing a slot to create an air gap between pads of chip components such as MLCCs that are subjected to high applied voltages. Alternatively, resistance to discharge and arcing can be improved without adding a slot by eliminating solder mask between the pads. Adding a radius at the corners of the pads can also help to avoid concentrations of electric-field strength.
Another potential problem to avoid, by design, is the possibility for inner circuit layers to have an inadvertent role in promoting arcing. In his analysis of board design for high-voltage circuitry, John Maxwell of Johanson Dielectrics recommended ensuring that no terminations on inner circuit layers should exist in between the terminations of the devices exposed to high voltages capable of causing arcing. If this basic aspect of board-design practice is not observed, the capacitor can interact with the trace on the inner PCB layer to create conditions for arcing.
The lesson is that, although new component choices can increase engineers’ freedom to deliver more and more fantastic products within smaller and smaller form factors, ensuring safety and reliability will always demand diligence in design.