International Appliance Manufacturing 2008 - (Page 38) voltage or power transients on the AC Mains inputs caused by lightning strikes or power station loadswitching transients. 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 4 shows how an MOV is used in combination with a PPTC device to help improve equipment reliability in the harsh AC environment, and helps fulfill the IEC-61000 test requirements. The MOV device’s high currenthandling 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 VAC, permitting maximum intermittent voltages of up to 265 VAC and 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 heatgenerating components such as Device Thermal Fuse Thermal Fuse Thermal Fuse PPTC Device PPTC Device PPTC Device Time- to-Trip/Open >100 min 51 min 66 min 11 min 13 min 11 min Figure 4. Coordinated overvoltage and overcurrent protection on AC Mains circuit. 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 component’s temperature response and replace other thermal protection devices in the circuit. Not only Max Coil Temp (°C) 147 157 147 107 112 103 Max Current (mA) 90 89 90 87 86 88 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 cycled, maintenance or replacement are not normally required. 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. Summary 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 helps circuit designers provide a safe and dependable product and comply with regulatory agency requirements. Figure 3. Comparison of performance characteristics of thermal fuses and PPTC devices used as primary protection elements on 120VAC transformer with a short on the secondary. 38 International Appliance Manufacturing 2008
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