How to select and apply the right components to protect medical devices, users, and patients
driving the unit’s paddles (Figure 11). Temperature indicators are specialized versions of temperature sensors such as thermistors. Although it may seem obvious that potentially hot areas such as power supplies or higher voltage sources need to be monitored for excess heating, even an I/O port such as USB-Type C can be handling significant current and thus overheat. This may be due to an internal failure or even a faulty load or shorted cable being plugged into it. To manage this potential problem, a device such as the SETP0805- 100-SE setP positive temperature coefficient (PTC) temperature indicator helps protect USB Type-C plugs from overheating. It has been designed to accommodate the unique specifications of this USB standard and is capable of helping to protect even the highest levels of USB Type-C power delivery. Available in an 0805 (2.0 x 1.2mm) package, it protects systems consuming 100 watts or higher, providing sensitive and reliable temperature indication as its resistance increases from a nominal 12 ohms (Ω) at 25⁰C to 35 kilohms (kΩ) at 100⁰C (typical values). GDTs may conjure up images in engineers’ minds of large, bulky tubes with visible sparks, but they are in reality very different. These tubes are placed between a line or
Figure 6: The V07E250PL2T MOV is a through-hole leaded, 7mm disk rated for operation to 390 volts and can handle transients up to 1,750 A. Image source: Littelfuse, Inc. sub-circuits or allow the high- side drivers of a half or H-bridge MOSFET configuration to “float” off ground. Another objective SSRs, also called optoisolators, allow one voltage to switch and control an independent, unrelated voltage with near-perfect galvanic isolation (no ohmic path) between input and output. They serve multiple broad objectives. One is functional: they can eliminate ground loops between separated +/18 kilovolts (kV) to +/-30 kV. Applications include protection of USB 2.0, USB 3.0, HDMI, eSATA, and display port interfaces, to cite a few possibilities. Note that the similarly named TVS diode array provides the same basic functionality but has higher capacitance and thus is better suited for lower speed interfaces. The SP3019-04HTG is an example of a such a diode array (Figure 10). It integrates four channels of ultra-low-capacitance (0.3 pF) asymmetrical ESD protection in a six-lead SOT23 package, and also features an extremely low typical leakage current of 10 nanoamperes (nA) at 5 volts. As with the TVS diode, typical applications are for protection of USB ports as well as the LCD/LED user interface display (again, areas 2 and 3 in Figure 9).
Figure 7: MLVs such as the V12MLA0805LNH can withstand repeated transient pulses without performance deterioration. Image source: Littelfuse, Inc.
Figure 5: The abrupt switch of the MOV from high impedance to low impedance when a transient voltage occurs clamps that voltage to an acceptable level. Image source: Littelfuse, Inc.
USB port (areas 1 and 5 in Figure 3). TVS diodes also protect sensitive electronics from
high-voltage transients and can respond to overvoltage events faster than most other types of circuit protection devices. They clamp and thus limit voltage to a certain level using a p-n junction that has a larger cross-sectional area than that of a normal diode, allowing the TVS diode to conduct large currents to ground without sustaining damage. TVS diodes are generally used to protect against electrical overstress such as those induced by lightning strikes, inductive load switching, and electrostatic discharge (ESD) associated with transmission or data lines and electronic circuits. Their response time is on the order of nanoseconds, which is advantageous for protecting relatively sensitive I/O interfaces in medical products, telecommunication and industrial equipment, computers, and consumer electronics. They have a defined clamping relationship between the transient voltage versus voltage across, and current
through the TVS, with specifics defined by the TVS model under consideration (Figure 8). The SMCJ33A is a unidirectional TVS diode with a 53 volt clamping voltage and 28 A peak current rating in a 5.6 x 6.6mm SMT package; a bidirectional version (B suffix) is also available for use when both positive and negative- going transients are anticipated. In a representative application such as a portable ultrasound scanner with a high voltage pulse generator to drive the piezoelectric transducers, TVS diodes could be used to protect the USB ports as well as the LCD/LED user interface display (areas 2 and 3 in Figure 9). Diode arrays use steering diodes centered around a large TVS diode (such as a Zener diode) to help
they serve is safety related and especially important for medical devices where their isolation provides an impassable barrier. This containment is needed where there are high internal voltages along with user or patient contact with instrumentation leads, knobs, probes, and enclosures. The CPC1017NTR is representative of a basic single-pole, normally open (1-Form-A) SSR. It is packaged in a diminutive 4mm2, four-lead housing while providing 1,500 volts RMS (VRMS) isolation
between input and output. It’s extremely efficient, requiring just 1 mA of LED current to operate, can switch 100 mA/60 volts, and provides arc-free switching without the need for external snubbing circuits. Further, it does not generate EMI/RFI and is immune to external radiated electromagnetic
fields – characteristics that are required in some medical
instrumentation and systems. In an application such as a defibrillator, designers can use it to electrically separate the low-voltage circuitry from the high voltages of the bridge
Figure 8: Shown is the general relationship for a TVS between voltage transients, voltage across the TVS, and current through the TVS,
with specific values determined by the
reduce the capacitance seen by I/O lines. These devices have a low off-state capacitance of 0.3 to 5 picofarads (pF) and are suitable for ESD levels from
selected TVS diode model. Image
source: Littelfuse, Inc.
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