All drones are susceptible to many of the same fault and failure conditions. These conditions can cause problems that range from the merely annoying to the catastrophic.
The number of drones on the market has risen quickly in recent years. The Federal Aviation Administration is predicting continuing strong growth in products of interest to both hobbyists and professional/commercial drone users. Sold at a wide array of price points, they range from relatively inexpensive consumer models to far more capable drones intended for aerial imaging and data collection, including real estate photography, industrial and utility inspection, and agricultural applications, such as crop inspection. State and local governments are also turning to drones to gather information for emergency services, including search and rescue operations.
No matter how a particular drone is used or how much it costs, all drones are susceptible to many of the same fault and failure conditions. These conditions can cause problems that range from the merely annoying (a drone that won’t start or take flight) to the catastrophic (a crash that causes major property damage or personal injury). A battery that catches fire during charging or a mid-flight failure due to any of a number of electrical issues are common examples that highlight why robust electrical protection is essential. Fortunately, a growing array of tools and techniques are available to implement passive battery safety systems, electrostatic discharge (ESD) protection, and stalled motor protection.
Figure 1 illustrates a generic drone design that highlights some of the areas that drone makers must take into account when designing circuit protection for their products’ various electrical subsystems and some of the most common circuit protection components for each application.
Protecting the battery and charging circuits
Obviously, drones require on-board batteries to power their operation. Lithium polymer (LiPo) batteries are among the most common battery types used for drones because they offer the advantage of high energy density in relation to their size and weight, with a higher voltage per cell, so they can power the drone’s on-board systems with fewer cells than other rechargeables. They also discharge more slowly than other types, so they’ll hold a charge longer when not in use. However, if not charged or used properly, they can’t provide peak performance for long and can even begin to smoke and catch fire.
Over-discharge and over-charge are two externally created events that can cause problems in Lithium-Ion batteries. During over-discharge, if the cell voltage drops lower than approximately 1.5V, gas will be produced at the anode. When voltage drops to less than 1V, copper from the current collector dissolves, causing internal shorting of the cell. Therefore, under-voltage protection is required and is provided by the battery protection IC. Over-charging creates gassing and heat buildup at the cathode when cell voltage reaches approximately 4.6V. Although cylindrical cells have internal protection from pressure, activated CIDs (current interrupt devices) and internal PTCs (positive temperature coefficient discs that increase in resistance when heated), LiPo cells do not have internal CIDs and PTCs. External overvoltage, over-gas, and over-temperature protection is especially critical for Li-polymer cells.
A variety of circuit protection options are available to help guard drone batteries against over-current and over-temperature conditions, including metal hybrid PPTC with thermal activation (MHP-TA) devices, PolySwitch PPTC devices, low resistance SMD PPTC devices, and surface-mount fuses. MHP-TA devices combine the advantages of low thermal cut-off temperatures, high hold-current ratings and compact size, which are invaluable for protecting LiPo batteries. The latest MHP-TA devices offer a 9VDC rating and a higher current rating than typical battery thermal cutoff (TCO) devices. They are capable of handling voltages and battery charge rates common in high-capacity LiPo cells. Many provide resettable and accurate over-temperature protection and their compact footprint and thin form factor simplifies circuit protection in ultra-thin battery pack designs.
For other battery chemistries, such as Lithium-Ion (Li-Ion), nickel metal hydride (NiMH), or nickel cadmium (NiCd), PolySwitch PPTC resettable devices may offer a better solution. Not only are they compatible with high-volume electronics assembly, but their UL, CSA, and TÜV agency recognitions make it easier for designers to meet regulatory requirements. Their low resistance helps increase battery operating time and they enhance over-temperature protection from thermal events.
Small-footprint, low-height-profile POLYFUSE LoRho Surface Mount Resettable PPTCs are well-suited for protection circuit modules for Li-ion and LiPo battery packs, providing fast over-current and over-temperature protection with ultra-low internal resistance, voltage drop, and power dissipation. By resetting automatically, they provide a low maintenance alternative to one-time fuses for over-current protection. Because they’re packaged for surface-mounting on a printed circuit board, they can be mounted within an electronic protection module on the board, simplifying the assembly process.
Although fuses and PTCs are both over-current protection devices, PTCs are automatically resettable; traditional fuses need to be replaced after they are tripped. A fuse will completely stop the flow of current (which may be desirable in critical applications), but after most similar over-current events, PTCs continue to enable the equipment to function, except in extreme cases.
[Continue reading on EDN US: Surface-mount fuses]
Todd Phillips is the Global Market Manager for the Electronics Business Unit at Littelfuse.
Henry Yu is a Senior Technical Marketing Engineer in the Electronic Business Unit at Littelfuse.
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