CONTENT INTRODUCTION THE BASICS OVERCURRENT PROTECTION PRINCIPLE OF OPERATION CONSTRUCTION AND OPERATION OPERATING PARAMETERS DESIGN CONSIDERATION FOR PPTC DESIGN CRIETERIA DIFFERENT TYPES OF POLYFUSE ADVANTAGES OVER CONVENTIONAL FUSES APPLICATION CONCLUSION REFERENCE
INTRODUCTION Polyfuse :A New Standard For Circuit Protection Fuses work between life and death of a circuitry.Choosing the right fuse is important for prolonged protection of the circuitry from over-current and voltage spikes. Conventional fuse is either ‘ok’ or ‘blown,’ but PolyFuse is a resettable fuse by itself. Polyfuse is a resettable fuse that doesn’t need to be replaced like the conventional fuse. Many manufacturers also call it PolySwitch or MultiFuse. Polyfuse are designed and made of PPTC material in thin chip form. It is placed in series to protect a circuit. Polyfuse provide over-current protection and automatic restoration. center
Radial-leaded metal-oxide varistor devices from Raychem can be combined with PolySwitch devices to help provide protection for electric motors, telecom equipment and various other systems
PPTC polymeric positive temperature coefficient device (PPTC, commonly known as a resettable fuse) is a passive electronic component used to protect against overcurrent faults in electronic circuits. They are actually non-linear thermistors, however, and cycle back to a conductive state after the current is removed, acting more like circuit breakers, allowing the circuit to function again without opening the chassis or replacing anything.passive electronic componentcurrentelectronic circuitsnon-linearthermistorscircuit breakers chassis
THE BASICS Technically Polyfuses are not fuses but Polymeric Positive Temperature Coefficient Thermistors. For thermistors characterized as positive temperature coefficient, the device resistance increases with temperature. The PPTC circuit protection devices are formed from thin sheets of conductive semi-crystalline plastic polymers with electrodes attached to either side. The conductive plastic is basically a non-conductive crystalline polymer loaded with a highly conductive carbon to make it conductive. The electrodes ensure the distribution of power through the circuit. Polyfuses are usually packaged in radial, axial, surface mount, chip or washer form. These are available in voltage ratings of 30 to 250 volts and current ratings of 20 mA to 100A.
Polyfuse is a series element in a circuit. The PPTC device protects the circuit by going from a low-resistance to a high- resistance state in response to an overcurrent condition, as shown in Figure1. This is referred to as "tripping" the device. OVERCURRENT PROTECTION
PRINCIPLE OF OPERATION Polyfuse device operation is based on an overall energy balance. Under normal operating conditions, the heat generated by the device and the heat lost by the device to the environment are in balance at a relatively low temperature, as shown in Point 1of Figure-2. If the current through the device is increased while the ambient temperature is kept constant, the temperature of the device increases. Further increases in either current, ambient temperature or both will cause the device to reach a temperature where the resistance rapidly increases, as shown in Point 3 of Figure-2.
Any further increase in current or ambient temperature will cause the device to generate heat at a rate greater than the rate at which heat can be dissipated, thus causing the device to heat up rapidly. At this stage, a very large increase in resistance occurs for a very small change in temperature, between points 3 and 4 of Figure-2. This is the normal operating region for a device in the tripped state. This large change in resistance causes a corresponding decrease in the current flowing in the circuit. This relation holds until the device resistance reaches the upper knee of the curve (Point 4 of Figure-2). As long as the applied voltage remains at this level, the device will remain in the tripped state (that is, the device will remain latched in its protective state). Once the voltage is decreased and the power is removed the device will reset.
CONSRUCTION AND OPERATION PPTC fuses are constructed with a non-conductive polymer plastic film that exhibits two phases. 1-semicrystalline phase 2-amorphous phase SEMICRYSTALLINE PHASE
OPERATING PARAMETERS · Initial Resistance · Operating Voltage · Holding Current · Trip Current · Time to Trip · Tripped State · Leakage Current · Trip Cycle · Trip Endurance · Power Dissipation · Hysteresis
DESIGN CONSIDERATIONS FOR PPTC DEVICES. The Fig. below illustrates the hold and trip current behavior of the PPTC devices as a function of temperature. Hold and Trip Current Some of the critical parameters to consider when designing PPTC devices into a circuit include device hold current and trip current, the effect of ambient conditions on device performance; device reset time, leakage current in the tripped state and the automatic or manual reset conditions.
Region A shows the combination of current and temperature at which the PPTC device will trip and protect the circuit. Region B shows the combination of current and temperature at which the device will allow normal operation of the circuit. In Region C it is possible for the device to either trip or o remain in low resistance state depending on the individual device resistance and its environment.. It can be affected by the design choices such as: 1. Placing the device in proximity to a heat generating source such as a power field effect transistor (FET), a resistor or a transformer resulting in reduced hold current, power dissipation and time to trip. 2. Increasing the size of the traces or leads that are in electrical contact with the device resulting in increased heat transfer and greater hold current, slower time to trip and greater power dissipation
EFFECT OF AMBIENT CONDITIONS ON DEVICE PERFORMANCE The heat transfer environment of the device can significantly affect the device performance. In general, by increasing the heat transfer of the device, there is a corresponding increase in power dissipation, time to trip and hold current. The opposite occurs if the heat transfer from the device is decreased. Furthermore, changing the thermal mass around the device changes the time to trip of the device. If the heat generated is greater than the heat lost to the environment, the device will increase in temperature resulting in a trip event. The rate of temperature rise and the total energy required to make a device trip depends on the fault current and heat transfer environment. Under normal operating conditions the heat generated by the device and the heat lost to the environment are in balance. TIME TO TRIP The time to trip of a PPTC device is defined as the time needed from the onset of a fault current to trip the device. Time to trip depends upon the size of the fault current and the ambient temperature.
DESIGN CRITERIA To select the best device for a specific application, circuit designers should consider the following design criteria: 1. Choose the appropriate form factor 2. Choose a voltage rating 3. Choose a hold current rating 4. Check trip time 5. Check maximum interrupt current
DIFFERENT TYPES OF POLYFUSES Surface Mount Resettable Fuses This surface mount polyfuse family of polymer of polymer based resettable fuses provides reliable over current protection for a wide range of products such as computer motherboards, USB hubs and ports, CD/DVD drives, digital cameras and battery packs. Each of these polyfuse series features low voltage drops and fast trip times while offering full resettability. This makes each an ideal choice for protection in datacom and battery powered applications where momentary surges may occur during interchange of batteries or plug and play operations
Radial-Leaded Resettable Fuses Due to the automatic resetting of the polyfuse, these components are ideal for applications, where temporary fault conditions (eg: during hot plugging) can occur. The radial-leaded RLD-USB-series 709 is specifically designed for universal serial bus (USB) applications with lower resistance, faster trip times and lower voltage drops.
Battery Strap Resettable Fuses This type profile strap type polyfuse family of resettable fuses provides thermal and over charge protection for rechargeable battery packs commonly used in portable electronics such as mobile phones, notebook computers and camcorders.
Advantages over conventional fuses 1. Over current protection 2. Low base resistance 3. Latching operation 4. Automatic resettability 5. Short time to trip 6. No arching during faulty situations 7. Small dimensions and compact designs 8. Internationally standardized and approved 9. No accidental hot plugging 10. Withstand mechanical shocks and vibrations 11. Life time- up to 10 times longer
APPLICATIONS PolyFuses are used in automobiles, batteries, computers and peripherals, industrial controls, consumer electronics, medical electronics, lighting, security and fire alarm systems, telecommunication equipment and a host of other applications where circuit protection is required. Some of its applications in protecting various equipments are discussed below: TRANSFORMERS PROTECTION: The equipment powered by a transformer get overheated due to excessive current or short-circuit. A PolyFuse on the secondary side of the transformer will protect the equipment against overload. SPEAKER PROTECTION: Nowadays speakers are designed and sold independently of amplifiers. Therefore, there are possibilities of damage due to mismatches. The protection choices for loudspeaker systems are limited. Fuses protect the speaker, but a blown fuse is always a source of frustration. Using a Polyfuse in series with the speaker as shown in Figure-10 will protect it from over-current/over- heating damage. Choosing a correct trip-current rated Polyfuse is important to match the power level of the speaker.
Batteries are constantly charged and discharged over their life-cycle. Over-charge results in an increase in the temperature of the electrolyte. This could cause either a fire or an explosion. Polyfuse play a vital role in the charging and discharging cycles of batteries. The Polyfuse low resistance overcomes the additional series resistance introduced by the MOSFETs and the low trip temperature can provide protection against thermal runaway in the case of an abusive overcharge. BATTERY PROTECTION
PPTC resettable fuses are designed for today’s demanding electronic and electrical industries. The concept of a self-resetting fuse of course predates this technology. Bimetal fuses, for example are widely used in appliances such as hairdryers, but these are generally large current devices. PPTC resettable fuses compete with another common overcurrent protection device, namely positive temperature coefficient (PTC) ceramic thermistors. However, PPTC fuses offer several advantages. The vast majority PPTC fuses on the market have trip times in the range 1–10 seconds, but there are PPTC fuses with trip times of a few milliseconds. Generally speaking, however, these devices are considered slow-trip fuses. The blow time depends on the overcurrent, so that a fuse that may open within a few milliseconds with a severe overload, may take tens of seconds for a light overload. They are ideal for all low voltage DC and AC application. CONCLUSION