Passive Electronic Components
Lecture 11
Page 13 of14
01-Jun-2018
Circuit protection components
Lecture Plan
1. Fuses
2. Thermistors
3. Varistors
Circuit protection components may be divided into:
· overcurrent protection components,
· overvoltage protection components.
Overcurrent protection components protect a circuit from high current that may result from circuit malfunction or wrong handling. There are three basic categories of current limiters:
· circuit breakers,
· fuses,
· positive temperature coefficient (PTC) devices.
We shall consider the two last types of devices. Circuit breakers, which include magnetic and/or thermal units, have size, cost, and mode of operation that rule them out for many PCB applications.
Overvoltage protection components are placed on PCB to protect expensive semiconductor devices (ICs) from the effects of transients. It may be high voltage induced by lightning or generated by rubbing of dielectric materials. Overvoltage protective components are:
· gas discharge devices,
· semiconductor devices (Zener diodes, thyristors),
· metal-oxide varistors.
We shall consider the last type of devices. Gas discharge devices are relevant only to special high voltage applications. Semiconductor devices are not classified as passive components.
1. Fuses.
Fuse is a type of sacrificial overcurrent protection device. Its essential component is a metal element that melts when too much current flows across it and interrupts the current. Fuse was patented by Thomas Edison in 1890.
Construction and principle of operation.
Typical fuse comprises metallic part that is called fusing element. It may be strip, wire, film deposited on ceramics (Fig.1, 2, 3). Fusing element is supposed to be destructed by abnormally high current. In fuses that interrupt high electrical power fusing element is commonly surrounded by sand filler and encapsulated in fire-proof fuse body (Fig.1). In the smallest chip fuses the element is deposited on a chip substrate surface (Fig.3). The element is connected to the fuse terminals. The element configuration, mass, and material are selected to achieve the desired electrical and thermal characteristics. The current passing through the fuse element generates a heat. The heat is dissipated to the ambiance. The higher is thermal resistance between the element and ambiance the less power is needed for fuse activation, the less electrical resistance of fuse is required, the less is voltage drop on the fuse. Notches in the fusing element concentrate heat dissipation in small areas increasing thermal resistance of the fuse.
When the current reaches some critical value the element temperature reaches its melting point, melts and fuse opens. The higher is the current the shorter time is needed for element to melt. Such inverse time/current characteristic (Fig.4) is desirable because it meets the property of electronic circuits to carry low level overloads safely for relatively long time and to fail quickly under significant overload. A properly selected fuse can provide effective protection over a broad range of the current. Typically fusing element is made of some elemental metal (copper, tin, etc.). Elemental metals have very high TCR (more than 4×103 ppm/K) that results in quick rise of fuse resistance and acceleration of fuse clearing.
Blade fuse
Fig.1. Blade fuse construction
Miniature fuses
Fig.2. Miniature fuse construction
Subminiature fuses
Chip fuses
Fig.3. Chip fuse construction
The main parameters of a fuse.
Current rating , A – nominal value of current. It is maximal safe current that fuse can pass through for unlimited time at particular ambient temperature. Commonly a fuse is derated 25% regarding normal load. For example 10 A fuse is typically used for load of 7.5 A. Fuse actuation is a thermal process therefore maximal safe current depends on ambient temperature: the lower is a temperature the higher is a maximal safe current. According to recommendations of manufacturer the fuse has to be in addition derated or uprated depending on the circuit temperature.
Voltage rating , V – maximal voltage that can be applied to the terminals of actuated (burnt out) fuse.
Breaking capacity , A – maximum current that a fuse can safely interrupt at rated voltage (and any voltage below ).
Time/current characteristics – graphically specified functional relationship between current value and clearing time (interval of time between the instant of current starting and the instant of current interruption). According to relationship between clearing time and current a fuse may be fast acting or slow blow (time delay).
Particular fuse standard specifies some points in this curve. For example IEC 60127-4 standard specifies time/current characteristics as the following:
Current / Time1.25 / ³ 1 hour
2 / < 2 min
10 / t <0.001 s (FF)
0.001£ t £ 0.01 (F)
0.01< t £ 0.1 (T)
0.1< t £ 1.0 (TT)
Designations: FF - very quick acting, F - quick acting, T - time-lag, TT - long time-lag.
Fig.4. Typical time/current characteristics of fuse.
2. Thermistors.
A thermistor is a thermally sensitive resistor. There are two basic types of thermistors:
· negative temperature coefficient (NTC) thermistors,
· positive temperature coefficient (PTC) thermistors.
PTC thermistors.
PTC thermistors are resistors with a high positive temperature coefficient of resistance. PTC thermistors may be linear or non-linear (switching). Only switching PTC thermistors may be directly used as circuit protection devices. Nevertheless, we shall consider both types of PTC thermistors.
Linear PTC Thermistors are linear resistors with a high positive temperature coefficient of resistance.
· Thermally sensitive silicon thermistors, sometimes referred to as “silistors”. These devices exhibit a fairly uniform positive temperature coefficient (about +0.77% /K) through most of their operational range, but can also exhibit a negative temperature coefficient region at temperatures in excess of 150°C. These devices are most often used for temperature compensation of silicon semiconducting devices in the range of -60°C…+150°C.
· Pure metal (Platinum, Nickel) thermistors. They may be manufactured using thin-film or wirewound technology. Nickel thin-film thermistors have TCR about +0.41% /K at +25°C. Platinum thin-film thermistors have TCR about +0.38% /K in 0…+100°C temperature range.
· Thick-film thermistors commonly have TCR range +0.10… +0.36% /K depending on resistance values.
Switching PTC Thermistors are non-linear resistors with a high positive temperature coefficient of resistance. Commercial switching PTC thermistors are ceramic or polymer. At some critical temperature coefficient of resistance of these devices increases sharply resulting in significant resistance increase. The resistance change can be as much as several orders of magnitude within a temperature span of a few degrees.
Resistance – Temperature Characteristics of PTC Thermistors:
Silistor and Switching Type
Ceramic switching type PTC thermistors. Ceramic PTC devices have ability:
· to operate in high-voltage circuits,
· to return after cooling to normal operating resistance with great accuracy.
They exhibit a very small negative temperature coefficient of resistance until the device reaches a critical temperature that is referred to as its Curie (switch, transition) temperature. When critical temperature is exceeded resistance of the thermistor rises several orders of magnitude.
The raw material constitutes doped polycrystalline ceramic based on barium titanate. Generally, such ceramic is known as a good insulating material. A low resistivity is achieved by doping the ceramic with materials of a higher valency than that of the atoms in crystal lattice. The material structure is composed of many individual monocrystallites (as shown in the picture).
At the temperatures above Curie temperature the potential barriers are formed in the boundaries of these monocrystallites. They prevent free electrons from diffusing into adjacent areas. It results in high resistance of the grain boundaries. This effect is absent at the temperatures below Curie temperature. That is why in a certain range of temperatures above Curie temperature resistance of the PTC thermistor rises sharply.
Mixtures of barium carbonate, titanium oxide and other materials whose composition produces the desired electrical and thermal characteristics are ground, mixed and compressed into disks, washers, rods, slabs or tubular shapes depending on the application. These blank parts are then sintered, preferably at temperatures below 1400 °C. Afterwards, they are provided with terminals and finally coated or encased.
Ceramic PTC devices’ size, which is inherently larger than that of equivalent miniature fuses, can be a problem in products with high component density and limited space. Ceramic PTC materials also have a high thermal mass, which means that their reaction time to a moderate overcurrent may be longer than sensitive components’ time to damage. Moreover, ceramic PTC materials have relatively high resistance in “ON” mode. This can preclude their use in low-voltage circuits in which voltage drop across thermistor can interfere with the load’s operation.
Polymer switching type PTC thermistors. A more recent development in PTC technology overcomes ceramic devices’ size and reaction-time shortcomings.
Polymer PTCs are made of a slice of plastic with carbon grains embedded in it. When the device is cool, the carbon grains are in close contact with each other and form a conductive path through the device. As the device heats up, the plastic expands and the grains start to disconnect raising the total resistance of the device.
In general, polymer devices suit to the circuits operating at less than 60V with normal currents of less than 15A.
The relationship between electrical and thermal parameters of a thermistor at steady state condition follows from equality of electrical power dissipated in thermistor and thermal power escaped from thermistor to ambiance.
,
U – voltage applied to thermistor,
T – thermistor temperature,
Ta – ambient temperature,
Rth – thermal resistance between thermistor and ambiance,
R(T) – electrical resistance of thermistor at temperature T.
Using of PTC thermistor for protection from overcurrent resulted from load resistance decrease.
The above figure illustrates operating states of a PTC fuse. V is a voltage drop across PTC fuse, I is its current.
· Function is represented by brown curvein above picture.
· Three load lines correspond to three different resistance values of the load L (RL2RL1RL).
Solution (single or multiple) of the following equation system
corresponds to intersection(s) of respective load line and function graph in the above graph. Red load line corresponds to overload state with the lowest load resistance RL. In this state current is limited by high resistance of thermistor. Rated operation corresponding to blue line is non-resetting automatically after overload ending. Rated operation corresponding to green line automatically resets after overload ending.
Using of PTC thermistor for protection from overcurrent resulted from voltage rise
At that RL2=RL1 . When voltage V0 rises from value V01 to value V02 the circuit current drops down and corresponds to point E in the below graph.
Using of PTC thermistor for temperature protection
Position of current-voltage curve of the thermistor after temperature rise is indicated by red line in the below graph.
Common applications of PTC thermistors are:
§ Motor start.§ Fluorescent lamp start.
§ Degaussing.
§ Power semiconductors protection / § Self-regulated heaters (in thermostats).
§ Resettable fuses.
§ Liquid level sensors.
Compact fluorescent lamp circuit Transformer protection
Liquid level sensors Power semiconductors protection
NTC thermistors are resistors with a high negative temperature coefficient. They are much more commonly used than PTC thermistors. The first NTC thermistor was discovered in 1833 by Michael Faraday, who reported on the semiconducting behavior of silver sulfide. Commercial production of thermistors began only in the 1930s.
NTC thermistors are the most sensitive of all the temperature sensing elements and have a rapid response time.
Interchangeability with tolerances 0.1…0.2 °C is another important feature.
NTC thermistors are hard and rugged sensors. They are able to handle mechanical and thermal shocks better than any other temperature measuring device.
The listed properties make the thermistors ideal for precise temperature sensing and compensation. The resistance of a thermistor as a function of temperature is approximated by expression
,
where
B - material constant (B value), K;
T - thermistor temperature, K;
Ro - resistance of thermistor at temperature To.
Usually To = 25°C = 298K. Ro ranges from some Ohms to several megaohms. The B value is a function of the material composition. Its range is commonly 2500K … 5000K. Thermistors with B values 3000K … 4000K are often used for measurement purposes.
Manufacturing of NTC thermistors. The raw materials are different oxides of metals such as manganese, iron, cobalt, nickel, copper and zinc. The oxides are milled to a powdery mass, mixed with a plastic binder and then compressed into the desired shape. Standard shapes are disk and wafer. Wafer is diced to the blanks. The blanks or disks are sintered at high temperatures (1000 … 1400 °C) to produce polycrystalline thermistor body. Disks are provided with terminals by baking a silver paste onto the flat surfaces and attaching the leads. Depending on the application, thermistors are coated or additionally incorporated in different kinds of housing. Finally thermistors are subjected to a special ageing process to ensure high stability of their electrical values. Otherwise their resistance may change even at room temperature due to solid-state reactions in the polycrystalline ceramic material.
SMD NTC thermistors (chip thermistors) may be produced using thick-film technology. They may single layer (like chip resistors) or multilayer (similar to MLCC structure). Multilayer structure allows lower resistance at high B constant when compared to single layer structure.
Parameters of NTC thermistors.
Resistance Temperature Coefficient
The resistance temperature coefficient (a) of a thermistor is the rate of change of the resistance per 1°C temperature change. It is expressed in percent change per 1°C (or 1K) and is given by:
.
Typical values of resistance temperature coefficient are in -2…-5 %/°C range.
Dissipation Constant
Dissipation constant (or dissipation factor) of a thermistor is the amount of power needed to raise the temperature of the thermistor due to self-heating by 1K when thermistor is mounted on standard 1.2mm thick glass-epoxy PCB. Dissipation constant is measured in [W/K].
,