When trying to select components for high-voltage applications, designers can face an uphill battle in trying to find components that will meet safety requirements while also yielding as compact a design as possible.
Modern designers are very familiar with the challenge to squeeze as much as possible into the space available yet when the operating voltages run into hundreds (or even thousands) of volts, the challenge becomes somewhat more difficult – not least because component manufacturers and safety bodies define minimum spacing distances to ensure safety and correct operation.
In these applications, dielectric materials can change and behave differently to how they behave in low voltage applications. Electrical charge may flow through the insulator and arcing between conductors is another possibility. Even a single arcing event can damage electronic equipment or the components they are made from. Should the dielectric break down then users may be exposed to a shock hazard.
Selecting materials and components
When selecting components and materials, safety considerations have to be paramount. The voltage rating must be suitable for the application, which usually means double the maximum applied voltage. Additionally, the creepage and clearance distances (defined below) must meet any safety requirements.
• Creepage: Electric fields (measured in kilovolts per millimeter – kV/mm) tend to spread over the surface of a dielectric between electrodes with different voltages applied to them. Greater creepage distances are required for stronger fields.
• Clearance: This is the distance required between to electrodes that are separated by air to prevent arcing.
In these definitions, ‘electrode’ is any sort of conductor and can refer to component pins, tracks on a PCB, pins on a connector – or any other electrical conductor that is placed close to another and sitting at a different potential. Dielectric can be an insulator inside a component, insulating surface on a PCB or, in some cases, the outer packaging of a component.
As a practical example, consider a simple multi-layer ceramic capacitor (MLCC). Even a working voltage of 300V can create a strong electric field such that the applied energy will arc across the surface of an incorrectly chosen component, instead of following the desired path through the MLCC. This can cause dielectric breakdown, creating a short circuit and, ultimately, failure of the MLCC.
While applied voltage is key to defining appropriate creepage and clearance distances, in the real world a number of factors must be taken into consideration, analyzing each application individually. These include environmental conditions, risks to operators or equipment, or specific industry- or market-specific safety standards that are required to be met. High humidity or dielectric surface contamination can be a catalyst for ionization, making arcing more likely. To protect against contamination, components such as resistors or capacitors may be coated with a protective, glass-based coating. To prevent contamination of PCBs, a conformal coating may be added.
One example of a high-voltage passive component with extended creepage and clearance distances is AVX’s SXP molded, radial, multilayer capacitors. There are multiple families within the extensive SXP range. The SXP4 family has capacitance values from 100pF to 2700pF, in a 22.4mm x 16.3mm x 5.84mm case size with lead spacing of 19.8 mm.
Advanced technology can allow components to remain physically small while still exhibiting larger creepage and clearance distances. For example, the C1210W683KDRACTU X7R MLCC from KEMET employs their ArcShield technology whereby a shield electrode is added internally. This effectively reduces the electric field strength over the surface of the component and so, arcing is prevented. Using this innovative approach, operating voltages as high as 1kV can be achieved in components with a package size as small as 0603.
Connector manufacturers define the voltage rating and publish this in their technical data sheets to allow designers to select the correct product for any situation or application. The system designer is responsible for achieving the correct creepage and clearance distances within the system, including any connectors used. As such, the designer must review specifications carefully and only select connectors that meet (or exceed) the requirements of the system, and any relevant standards.
Often, high-voltage connectors are designed to be application-specific so that they meet the typical industry requirements. By selecting a product described as being for a particular application (e.g. “rated to 2000V DC for medical applications, per standard IEC 60601”) and confirming the specification in the data sheet, designers can be confident that the connector is suitable for the application.
Sometimes, it is obvious just by looking at a connector that it is intended for high-voltage applications. An example of an extreme high-voltage application is connecting the overhead cables to the rolling stock on electrically powered trains. The HVTT and HVTE cable assemblies from TE Connectivity are rated to 15/25 kV and feature AC-withstand voltage of 50/90kV and impulse-withstand voltage to 125/175kV. Looking at the assemblies (see Figure 2) the smooth surfaces and textures indicate that the HVTT and HVTE are designed to not only avoid the accumulation of surface contaminants but also to avoid high concentrations of electric field strength and to maximize discharge. The physical size (diameters of 90 to 135mm) and application-sensitive design deliver creepage distances of 650mm to 1000mm.