As companies strive to reduce and or eliminate overhead expenses of product design, engineers will need to develop aggressive design timelines. A constant need to reinvent the internal design process to take advantage of better tools and technology is necessary. However, there is an inverse relationship between design time and cost or quality of the design.
Businesses designing high volume products can afford to invest more expenses into the design process as the overall percent of the design and engineering cost remains low compared to the over all manufacturing volume. This is not the case with low to medium volume products where the design cost can result in a large portion of the overall cost of the project. For low to medium volume products, electronic designers and engineers look to find proven or integrated solutions to speed up design turnaround while having to sacrifice per unit pricing. The purpose of this article is to highlight areas to reduce design time while mitigating risk as it relates to safety. Three significant areas that electronic engineers and PCB designers need to pay particular attention are:
- High Voltage
- Limiting Energy
- Battery usage in electronic design
We will discuss tips, tricks and shortcuts to improve the safety of your design with proven methods.
Isolation of High Voltage Circuits
One of the most dangerous parts of the PCB is the power supply. This is where voltage (often high voltage), is converted to a voltage that is suitable for the electronic design. Most of the time, this involves a buck power supply converting a high AC voltage to a low DC voltage. Designing a safe power supply takes considerable time, failure mode analysis, and expenses related to agency approval. In addition to the actual safety consideration of the power supply conversion, a switch mode supply requires designers to put more time and consideration into the PCB layout to mitigate switching noise in the layout. However, there are millions of class II power supplies sold globally each year for use in laptop computers, cell phones and a variety of other devices. Why not take advantage of the volume and existing designs available and simply use an off board, commercially available, class II power supply and simplify the PCB design? In this manner, we avoid many of the safety considerations, not to mention noise considerations, of an “on board” high voltage switch mode power supply. In many cases you will find purchasing a high volume commercially available power adapter will prove cost-effective.
High Voltage Switching
Even if we opt to purchase on off board power supply, we may still need to address switching high voltage on the PCB. Switching high voltage on and off is simpler than converting to it, however. When PCB traces of differing voltages are routed too close together a phenomenon known as arc tracking can occur. There are numerous guidelines for creepage distances on PCBs depending on voltage. The higher the voltage, the higher the required creepage distance.
Arc tracking, simply put, is when contaminants on the PCB cause a short circuit path between traces. In one of the more serious consequences, these contaminants create a resistive short. This resistance can become very hot and become a potential source of ignition. While every design should strive to maintain minimum creepage distances there may be cases where maintaining creepage distances is difficult, time-consuming, or just not practical in the space available.
When switching high voltage, be sure to use appropriately rated optical couplers or relays. These devices are constructed in a manner that respects minimum creepage distances for their rated voltage. It is best practice for AC voltage to just switch the line or “hot” not bring the neutral onto the PCB. However, it is necessary to bring both the positive and negative side of a high voltage circuit on the PCB. Always keep the high voltage side of the PCB separated from the low voltage side of the PCB. Many designs will even have a silkscreen line between the two sides for clarity. If possible, route the positive and negative side of the high voltage on opposite sides of the PCB. This is a cost-effective manner to maintain creepage distances. Another cost-effective method is to cut a slot between the low voltage side of the PCB and the high voltage side. Slots can also be cut between the high voltage traces (of differing polarities). The creepage distances for voltages in the air are much less than creepage distances on a surface.
Additional arc tracking protection can be achieved by applying a conformal coating to the PCB. The conformal coating provides an additional layer of insulation and reduces the occurrence of contaminants from meeting conductive surfaces. Creepage distances can be reduced when an appropriate coating is applied. Finally, for severe environments where excessive dust, water, humidity or other particles are present, consider potting the entire PCB assembly. While potting the PCB is the most expensive of the options, it is also the most reliable method of sealing the PCB and preventing many hazardous conditions.
Energy Considerations and Fuses
While low voltage is considered safer than high voltage, there continues to be a concern over the energy available. While a class II power supply is an energy limited device, there are many applications where the PCB might run directly from a battery. Anyone who has made the mistake of removing the positive cable of a car battery, before removing the negative cable, and accidentally touched the tool to the surrounding metal can appreciate why it is desirable to energy limit the power to our PCB. For this reason, it is recommended that the PCB be designed with an intentional “weak” spot in the incoming power circuit. This can be most easily accomplished by placing a fuse in line with a power line. PCB mounted fuses can be purchased and incorporated in many different layout options (SMD or through hole). The advantage of the fuse is predictable behavior. There are also resettable PTC fuses and active component devices that perform current limiting.
A final consideration for safety is if your product incorporates batteries and recharging circuits into the design. Lithium and lithium polymer battery technology available has given engineers the ability to have a very high watt density in much smaller packages. However, these batteries and charging circuits when designed or used improperly can lead to safety concerns. If using a battery (particularly lithium) it is recommended to use a battery that has internal protection against short circuits, overvoltage, overcharging and overheating. These batteries will cost more, however, they mitigate much of the design risk when timelines (and design volumes) are a concern.
In addition to using batteries with internal protection, there are a variety of charge management ICs available with excellent application notes, schematics, layouts and even test PCBs that can be incorporated into your design. It is well worth the time to use these resources.
Hopefully, the advice in this article will assist you during your next “budget constrained” or “aggressive timeline” project. They should allow you to reduce your design time while being confident that you have not compromised the safety of your design.