Figure 1. Effect of a secondary short on a 240-V-ac transformer utilizing a thermal fuse as the primary protection element.
The electric motors, transformer, and controllers used in home and professional-grade appliances are often subjected to mechanical overloads, overheating, stalls, lost neutral, severe overvoltages, humidity, and other damaging factors. New polymeric positive temperature coefficient (PPTC) devices are designed for operation at line voltages of 120 V ac and 240 V ac and can help appliance engineers prevent safety and fire hazards, as well as reduce warranty return and replacement costs resulting from motor failure.
Protecting increasingly sophisticated and complex control boards from misconnection, power surges, or short circuit damage is of particular concern to the equipment manufacturer. Although appliance transformers, their enclosures, and connections are capable of withstanding higher voltage transients, the use of sensitive solid-state devices on the board necessitates improved overcurrent, overtemperature, and overvoltage control.
Coordinating overcurrent and overvoltage protection can also help designers comply with safety agency requirements, minimize component count, and improve equipment reliability. A metal oxide varistor (MOV) overvoltage protection device used in a coordinated circuit-protection strategy with a line-voltage-rated PPTC overcurrent device helps manufacturers meet IEC 6100-4-5, the global standard for voltage and current test conditions for equipment connected to ac mains.
Figure 2. Effect of a secondary short on a 240-V-ac transformer utilizing a PPTC device as the primary protection element.
When a fault occurs in a transformer or power supply, some of the components may begin to overheat. Several circuit-protection schemes can be used to help guard against damage caused by these fault conditions and the resultant overtemperature damage, including thermal fuses, current fuses, and circuit breakers. A common solution is to use a thermal fuse on the primary side and an overcurrent fuse on the secondary side.
However, in many appliance applications, resettable devices such as PPTC devices, ceramic positive temperature coefficient (CPTC) devices, and bimetal breakers are the preferred solution. These devices do not generally require replacement after a fault event, and they allow the circuit to return to the normal operating condition after the power has been removed and/or the overcurrent condition is eliminated.
Although the fuse is perhaps one of the simplest and lowest-cost solutions to transformer protection, most equipment manufacturers find it easy to justify the cost of resettable protection if it helps protect against overcurrent damage caused by electrical short, overloaded circuit, or customer misuse. It can also help reduce warranty returns.
To analyze the performance of PPTC devices, a comparison test was recently conducted using Raychem Circuit Protection PolySwitch LVR devices as primary protection elements on a variety of transformers. The performance characteristics of the PPTC devices were compared to those of thermal fuses.
Some power-supply designs utilize a single-use thermal fuse as a primary protection solution. Figure 1 illustrates the effect of overheating on such a transformer. In this test, a short on the secondary side resulted in coil temperatures exceeding 200ºC. The thermal fuse—rated at 115ºC and mounted near the center of the core—failed to open, and the insulation on the windings melted,
destroying the transformer.
Figure 2 shows the results of a test in which a similar transformer was tested with a PPTC device installed as a primary protection element. A primary input voltage of 253 V ac was applied, and a secondary short was simulated. Surface temperatures of the primary (Tprim) and secondary (Tsec) windings as well as those of the PPTC device (T-PPTC) were measured. The PPTC device started to trip when its external temperature reached approximately 95ºC, at which time the primary coil temperature was about 95ºC. Once it had tripped and limited the current, the coils began to cool.
The performance characteristics of the PPTC devices versus thermal fuses studied in similar tests on a 120-V-ac transformer with a short on the secondary side are shown in Table 1. These data demonstrate the advantages of the PPTC device’s faster time-to-trip and its ability to limit the maximum coil temperature, thereby helping protect the transformer windings, as well as the secondary circuitry.
Table 1. Comparison of performance characteristics of thermal fuses and PPTC devices used as primary protection elements on a 120-V-ac transformer with a short on the secondary.
CPTC devices help provide resettable protection; however, their application may be limited due to their relatively high operating temperature, high resistance, and large size. The composition of the CPTC device tends to be brittle, which makes it vulnerable to damage from shock, vibration, and the thermal stress of heating and cooling found in many appliance applications.
Traditional bimetal circuit breakers, although widely used to help protect the electric motors found in appliances, do not latch and require additional action to interrupt their on-off cycle. The bimetal strip is constructed of two different metals bonded together. When the bimetal circuit breaker’s current rating is exceeded, heat generated by the excessive current causes the bimetal strip to bend and open a set of contacts to stop current flow. With no current flowing to the circuitry, the device returns to its normal shape, closing the contacts so current flow may resume. In the case of a stall, the bimetal circuit breaker continues to cycle until power is removed.
The cycling nature of the bimetal circuit breaker has disadvantages. Among those are material fatigue and a tendency for contacts to spark or weld shut. If the device “fails closed,” it can cause overcurrent damage to the motor as well as to sensitive follow-on electronics. Potential noise or “chatter” and electromagnetic interference (EMI) can make bimetal circuit breakers incompatible with advanced electronic control systems.
Protection for AC Mains Applications
Figure 3. Coordinated overvoltage and overcurrent protection on ac mains circuit.
From small countertop appliances to professional-grade ovens, increasing complexity and functionality are driving the industry toward circuit integration and board size reduction. Protecting sensitive electronic components from voltage transients, short circuits, and customer misuse is of primary concern to manufacturers.
In the past, control board designs often used no overcurrent protection on the primary or secondary side, relying on the transformer to sink sufficient heat to prevent control board damage in the event of a fault condition. However, the increased use of sensitive solid-state devices on the board now requires that voltage levels be limited.
Electrical equipment can be exposed to potential damage from large voltage or power transients on the ac mains inputs caused by lightning strikes or power station load-switching transients. IEC 61000-4-5 is the global standard for voltage and current test conditions for equipment connected to ac mains.
Coordinating overcurrent and overvoltage protection at the ac mains input can help designers comply with safety agency requirements and minimize component count and cost. Figure 3 shows how an MOV is used in combination with a PPTC device to help improve equipment reliability in the harsh ac environment and help fulfill the IEC-61000 test requirements.
The MOV device’s high current-handling and energy-absorption capability, fast response, and low cost make it suitable for overvoltage protection in power supplies, control board transformers, and
electric motors. The PPTC overcurrent protection device is also rated at 240 V ac, permitting maximum intermittent voltages of up to 265 V ac, and it can be installed with the MOV device in the ac mains input lines.
Unlike a single-use current fuse, the resettable PPTC device helps protect against damage from conditions where faults may cause a rise in temperature with only a slight increase in current draw. When installed on the primary side of the circuit, in proximity
to potential heat-generating components such as magnetics, field-effect transistors (FET), or power resistors, the PPTC device helps provide both overcurrent and overtemperature protection with a single installed component.
Certain mains-overload conditions may cause the MOV device to remain in a clamped state, where it will continue to conduct current. This may eventually result in an overtemperature failure of the device. While not directly applicable to passing IEC 61000-4-5 tests, placing the PPTC device in close thermal proximity to the MOV device can help protect the MOV device in extended overload conditions by transferring heat to the PPTC device. This causes the PPTC device to trip faster, limiting the current through the MOV device.
This application of the PPTC device allows designers to leverage the temperature response of the PPTC device and replace other thermal protection devices in the circuit. Not only does the PPTC device perform dual functions in this case, it also provides a fully resettable solution. Because the device resets after the fault is cleared and power to the circuit is removed, maintenance or replacement is not normally required. This helps reduce warranty and service costs and improves customer satisfaction.
The PPTC and MOV devices chosen for a particular application depend on the IEC 61000-4-5 class rating for the equipment, as well as the operating conditions of the equipment itself. When selecting a PPTC device, the primary consideration is to match the hold-current rating of the device to the primary current drawn by the electrical equipment under normal operating conditions.
Coordinating overcurrent, overtemperature, and overvoltage protection can help designers minimize component count and reduce warranty returns resulting from failed motors and control board transformers. The low resistance, fast time-to-trip, low profile, and resettable functionality of the PPTC device help circuit designers provide a safe and dependable product and comply with regulatory agency requirements.
About the Author
Faraz Hasan is global industrial and appliance marketing manager for Raychem Circuit Protection products at Tyco Electronics Corp. He earned a BME with honors from AMU, Aligarh, India. He also holds a postgraduate diploma in marketing and sales management from Bharatiya Vidya Bhavan, New Delhi, India. To contact Hasan, e-mail firstname.lastname@example.org.
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