How to select and apply the right components to protect medical devices, users, and patients
many designs, it is a final line of protection, as it acts decisively and irrevocably. Fuses are available for current values from under one ampere to hundreds of amperes or higher and can be designed to withstand hundreds or thousands of volts between their two terminals during fault-induced open-circuit conditions. A typical fuse is the Littelfuse 0215.250TXP , a 250 milliampere (mA), 250 volt AC (VAC) fuse in a 5 x 20 millimeter (mm) ceramic enclosure (Figure 1). Like most fuses, it is a cylindrical or cartridge-shaped housing that is not soldered into the circuit but instead goes into a fuse holder for ease of replacement. Fuses are also available in rectangular and “blade” housings as well as those that can be soldered; note that the soldering profile must be carefully observed to avoid damaging the fuse element. Despite their apparent simplicity, fuses have many variations, subtleties, and other factors that must be taken into account when selecting the appropriate one
back-to-back Zener diodes. Their symmetrical and sharp breakdown characteristics enable them to provide excellent transient suppression performance. When a high-voltage transient occurs, the varistor impedance decreases by many orders of magnitude from a near open circuit to a highly conductive level, clamping the transient voltage to a safe level in a few milliseconds (Figure 4). As a result of this clamping action, the potentially destructive energy of the transient pulse is absorbed by the varistor (Figure 5). MOVs are offered in a variety of packages such as the 390 volt, 1.75 kiloampere (kA) V07E250PL2T , which is a small disk with through-
Figure 2: The 2016L100/33DR 33 volt, 1.1 A PPTC device can be used in low voltage applications where resettable protection is needed; it reacts in under
Figure 4: The voltage- current (V-I) curve of the MOV shows its normal high resistance region as well as its very low impedance region, which occurs when the voltage increases beyond a design threshold.
0.5 s at an overcurrent of 8 A. Image source: Littelfuse, Inc.
for a circuit (References 2 and 3). Fuses are commonly used on input AC lines, output leads where a total short-circuit may occur, or internally where any overcurrent is a serious concern such that the current flow must be fully stopped, and the problem’s source determined and fixed before operation can resume. PPTC devices serve two main types of applications: safety regulation such as for a USB port, power supply, battery, or motor control; and risk prevention for an I/O port. During abnormal conditions such as overcurrent, overload, or overtemperature, the PPTC resistance will increase dramatically, which limits the power supply current in order to protect circuit components. Once a PPTC device trips into a high resistance state, a small amount of current continues to flow through the device. PPTC devices require a low joule heating “leakage” current or external heat source in order to maintain their tripped condition. After the fault condition is removed and the power is cycled, this heat source is
resistance status and the circuit is restored to a normal operating condition. Although PPTC devices are sometimes described as “resettable fuses” they are, in fact, not fuses but nonlinear thermistors that limit current. Because all PPTC devices go into a high resistance state under a fault condition, normal operation can still result in hazardous voltage being present in parts of the circuit. A good example of a PPTC is the Littelfuse 2016L100/33DR , a surface mount, 33 volt, 1.1 A PPTC device for low voltage (≤60 volts) applications where resettable protection is needed (Figure 2). It has a footprint of 4 x 5mm and will trip in under 0.5 seconds (s) at an overcurrent of 8 A. In a typical ventilator, the 2016L100/33DR might be used to protect the battery management system’s MOSFET from high currents due to external short circuits or provide overcurrent protection for USB chipsets (Figure 3). MOVs are voltage dependent, nonlinear devices that have an electrical behavior similar to
Image source: Littelfuse, Inc.
hole leads that measures just 7mm in diameter (Figure 6). They are often used on an input AC line to prevent damage due to AC line voltage transients (area 1 in Figure 3). Note that MOVs can be connected in parallel for improved peak current and energy handling capabilities, as well as in series to provide voltage
ratings higher than those normally available, or ratings between the standard offerings. MLVs are similar to MOVs and provide the same basic function but have different internal construction and thus somewhat different characteristics. MLVs are fabricated by wet stack printing layers of zinc oxide (ZnO) and metal inner electrodes, sintering, terminating, glassing, and finally plating. In general, for the same MOV voltage rating, smaller MLV parts have a higher clamp voltage at higher currents, while larger parts have a higher energy capability. The V12MLA0805LNH MLV, for example, was tested with multiple pulses at its peak current rating (3 A, 8/20 microseconds (µs)). At the end of the test – 10,000 pulses later – the device voltage characteristics are still well within specification (Figure 7). This device should be considered for transient protection in the ventilator power supply and
Figure 1: The Littelfuse 0215.250TXP is a 250 mA, 250 VAC fuse in a ceramic body with a 5mm diameter and a length of 20mm. Image source: Littelfuse, Inc.
Figure 3: In this ventilator block diagram, PPTC devices could be used in the battery management system as well as the USB port sections (areas 2 and 5). Image source: Littelfuse, Inc.
eliminated. The device can then return to a low
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