[FAQ] What is Creepage and Clearance in Isolated Switches and Switch Drivers?

Creepage and Clearance in Isolated Switches and Switch Drivers

Creepage and clearance distances are industry standard requirements that dictate the minimum spacing between the any two exposed metal contacts to prevent arcing through air or along surfaces. They’re important design factors in systems that include an isolation barrier between domains of high and low voltages. For an integrated circuit, the creepage and clearance distances are measured between the two closest contacts that have a significant voltage between them. In automotive systems, awareness of creepage and clearance requirements is growing as electric vehicles move to higher battery voltages. This FAQ will define creepage and clearance, review design methods to minimize them, and close with what determines them in an example using the TPSI2140-Q1 isolated switch.

Clearance

Clearance is the shortest distance between two metal contacts measured through the air, often making it a shorter distance than the creepage. This measurement is primarily a function of the minimum required electrical voltage that causes an arc between the contacts. It is shown in red in the figure below.

Figure 1. Clearance of an integrated circuit

Creepage

Creepage is the shortest distance between two metal contacts on either side of an isolation barrier measured along a surface. This measurement can be made along the path that travels across the package of an integrated circuit or along the PCB. The creepage of the device will often be longer than the clearance due to the surface-bound nature of the path. The benefit of this comes after an arc has occurred. The heat produced from an arc will burn the surface of a PCB, creating a scorched, carbonized patch on the PCB material. Since carbon has higher conductivity, this effectively reduces the creepage distance since the material is more conductive. Creepage is shown in blue in the figure below.

Figure 2. Creepage of an integrated circuit 

Ways to Increase Effective Creepage Distances

There are several ways a design can minimize creepage without actually enlarging the physical distance between two conductive contacts. These modifications include physical changes made to the board and the application of conformal coating.

Conformal Coating – This is a thin, polymeric film that is added to the board to protect the surfaces from pollution. Pollution reduces the creepage distance between two points as it is by definition particles, liquids, even gases that are conductive or can become conductive. Commonly, the pollutant is water in the form of humidity, which can also cause corrosion or mold growth. Thus, conformal coating is a requirement in automotive, marine, aerospace, and military applications.

Physical PCB Modifications ­– There are several board modifications that can be implemented to increase the creepage distances between two high voltage contacts:

• Notches and Grooves – Notches, a rectangular trench, and grooves, a V-shaped or rounded trench, can be added between two contacts to increase the surface distance. The benefit of these is in fitting several of them in parallel between contacts to significantly increase creepage.
• Ribs – A rib is a feature where the PCB material has been raised, creating a protrusion above the PCB surface. Similar to notches and grooves, a benefit of ribs is in placing several in parallel. The drawback with ribs is that they may not fit under integrated circuits with low PCB clearance. An additional benefit, however, is that the protrusion creates a longer clearance distance.
• Slots – Slots are long, narrow holes created in the PCB to force the creepage path to lengthen laterally. The slot method is usually the most cost-effective and easiest way to increase creepage.

Figure 3. Types of physical modifications from edge and top view

There are downsides to these creepage solutions. First, although these methods for lengthening creepage seem straightforward, they can take up a significant amount of space. Increasingly, designs are cramped for space and each of these modifications need some length to be effective. This can encroach on other circuit real estate. Second, there is the cost. The slot method may be the cheapest but it will still add non-negligible costs to the fabrication of the board. Third, implementing a cutout of any kind will weaken the mechanical integrity of the board. This means it is more liable to break should it experience strong enough vibrations or stress.

Creepage Walkthrough

There are several important factors that determine creepage and clearance. This section will review the most important of the factors by designing the TPSI2140-Q1 into a 1000V system as an example. Let’s start by looking at the creepage distances we care about.

Figure 4. TPSI2140-Q1 footprint creepage

In figure 4, the distances that are important for the creepage calculation are shown with the TPSI2140-Q1 footprint. There is a measurement of 8.1mm creepage from the primary side to the secondary side, as well as a 6.04mm measurement between the high voltage pins on the secondary side. The reason this measurement is only 6.04 and not more is because it is the distance between conductive surfaces. There are two distances along non-conductive surfaces from S1 to SM and SM to S2 that together equal the effective non-conductive distance. This is shown below in figure 5.

Figure 5. Effective creepage distance between pins

Working voltage - The first factor to consider is the working voltage. As stated previously, this system is working at 1000V. This means that under ­normal operation, the maximum voltage differential between any two conductive surfaces is 1000V. For the TPSI2140-Q1 requirement must be met from secondary to primary and S1 to S2.

Pollution degree – Pollution is considered a foreign substance of any kind that reduces the electrical resistance of the insulation. Pollution degrees range from 1 to 4, wherein pollution degree 1 there is no pollution or slight, non-conductive contamination and pollution degree 4 connotates conductive dust and wetness. The TPSI2140-Q1 is rated for an environment of pollution degree 2, meaning it can withstand pollutants that are normally non-conductive but become conductive when introduced to condensation.

Material Type – The next factor to consider is the material type. Over time, high voltages can create leakage paths through a material. As the pathways are created, the material is carbonized, making the pathway more conductive. The comparative tracking index measures the breakdown of materials and assigns classes based on the voltage range it can withstand without breakdown. The TPSI2140-Q1 package falls in material group 1, so the main concerns when designing with this device will be creepages along the surface of the PCB.

Table 1. Comparative tracking indices

At this point, we have established the working voltage to be 1000V. We have determined the TPSI2140-Q1 can withstand environments of pollution degree 1 and 2. Lastly, we know the material classification for the TPSI2140-Q1 package, with a degree of freedom in the PCB material type. The next step is to refer to the IEC standard 60950 to find what the creepage requirements are for this system. Below is an excerpt from the standard, covering the creepages that concern us:

Table 2. 1000V creepage requirements according to IEC 60950-1 Table 2N

The shorter of the two creepage distances on the TPSI2140-Q1 footprint is 6.04 mm. Without modifications, this design will meet the requirements of a pollution degree 1 environment and a pollution degree 2 environment if the PCB is fabricated with a group I material. In order to meet the requirements with a PCB material belonging to group 2 or 3, the board must be modified physically. One option would be to drill slots into the board as shown below:

Figure 6. TPSI2140-Q1 footprint modified with slots

TPSI305x-Q1 Creepage and Clearance Synopsis

The TPSI305x-Q1 has a creepage and clearance of 8.5 mm. This differs from the TPSI2410-Q1 in that the measurement is only taken between the primary and secondary sides. This is because the highest working voltage differential between any two pins on the secondary side will not exceed 15V, or the device’s gate drive specification. Similar to TPSI2140-Q1, the package is material group I and rated for environments of pollution degree 2, making the PCB material the decisive factor in creepage requirement for a given working voltage. Placing TPSI305x-Q1 in the 1000V setting from the TPSI2140-Q1 example, a PCB of material groups 1 and 2 will meet the creepage requirement but will need modification for group 3.

Conclusion

Creepage and clearance are the distances across a surface and through air, respectively, between two metal contacts. There are requirements for each in high voltage designs to lower the risk of arcing and carbonization of electronic materials. These distances depend on the working voltage, the degree of pollution and the material group classification. While the industry requirements cannot be avoided, there are ways to lower creepage through physical board modification, such as notches and slots, and reduce pollution degree in the form of conformal coating. It is important to refer to the IEC standards while designing in high voltage applications, especially as these designs come closer to human contact in EVs and EV charging.

For More Information

There are additional resources available on TI.com about creepage and clearance:

Application Brief - Solutions to meet the higher automotive isolation creepage & clearance needs

TI Precision Labs Video - What are Creepage and Clearance?