Wanting to be a good parent, one of the first things that I wanted my child to recognize and understand was what to avoid. This included the same list of potential harms that had been passed down to me from my parents; including such classics as “Don’t touch the stove when it’s hot!” and “Stop running in the house!” However, I took great care in making sure that my daughter was aware of the danger of playing around electrical outlets. My intent was to elevate her fear of electricity so high that she would not even think of pulling those plastic covers off, even after she was strong enough to do so. And it worked!
Having ample apprehension about electricity is one of those fears that will actually keep you healthy. And designers, builders, and suppliers of electrical and electronic products bear the responsibility of minimizing the probability of these fears manifesting and posing a threat to anyone’s safety. For circuit boards, that means following good design tips for high current and when incorporating high voltage PCB design considerations that will aid your CM in building safe, reliable boards and avoiding potential dangers. Before laying out the essential considerations to include, let’s define a high voltage PCB and the potential threats lying in wait that you want to avoid.
What is a High Voltage PCB?
Circuit boards are most often classified according to how many surfaces have components mounted, single or double-sided, or according to the PCB stackup, single layer or multilayer. Although, correct these characterizations fall woefully short in giving insight as to what specific design considerations need to be taken into account to ensure performance, reliability and manufacturability. More helpful is the list below, which categorizes PCBs according to signal characteristics.
PCB Signal Attributes
- High speed
- High power
- High current
- High voltage
It should be noted that a board may have signals that exhibit one or more of the attributes listed. In fact, it is common for a high power PCB to contain signals that are either high current, high voltage, or both. As all of these characterizations are relative, a more definitive description for a high voltage board is desirable. Unfortunately, this is not available precisely, yet an appropriate value can be estimated if the intended application is also added to the board classification. For example, the National Electric Code (NEC) defines high voltage as above 100V for AC and DC. For automotive systems, high voltage is 30 – 1000V for AC and 60 – 1500V for DC. However, since a voltage of 50V or more can be quite dangerous, even fatal, for most people it is safe to adopt this level to define high voltage PCBs, generically.
Where is the Danger for High Voltage PCBs?
As shown in the figure above, PCBs can indeed catch on fire! The probability of this highly dangerous occurrence is significantly greater for high voltage boards and there are several situations that can be the root cause. One threat that can lead to this type of inferno is high voltage arcing, which is possible when unexpected high voltages or spikes are incident on the board. Voltage spikes are most often introduced by external power supply connections due to inline power fluctuations and/or inadequate protection circuitry and can cause surface tracking, where an alternative current path is created from the source to another conductive element.
High voltage can mean high power and high current. For designs where the current levels are high, it is imperative that copper weights are large enough to allow for current variations. Additionally, skin effect, which is the tendency for high frequency current density to be higher at the surface of a conductor than in the center, can pose a threat for creepage and clearance between adjacent (or nearby) traces. Therefore, high voltages can be the source of damage to components, traces and/or the board itself. Even worse, nearby circuitry or personnel can also be threatened. Fortunately, these types of outcomes can usually be avoided by consideration of employing high voltage PCB design considerations early in the board development process.
High Voltage PCB Design Considerations to Avoid Danger
Having your boards fail in the field due to high voltage contingencies is not only dangerous, but can also be costly due to replacements and recalls. In the latter case, redesigns and additional manufacturing can significantly reduce profit margin and negatively impact the customer relationship, not to mention the impact on your company’s reputation in the industry. These potential dangers and unnecessary expenditures can be avoided by following guidelines; such as those below, that include high voltage PCB design considerations meant to aid your CM in building boards that are unlikely to fail due to high voltage contingencies.
Essential High Voltage PCB Design Considerations
- Know your board’s application environment
- Employ good heat dissipation and thermal relief techniques
- Select high-quality arc resistant materials
- Use heavy copper traces, if needed
- Maximize spacing and clearance distances
- Utilize shielding, if necessary
- Apply conformal coating
The above guidelines, if followed, will assist your CM in building high-quality, reliable boards that are less susceptible to high voltage threats.