International Appliance Manufacturing 2008 - (Page 37) Tyco Electronics recently conducted comparison tests of their Raychem Circuit Protection PolySwitch™ LVR devices as primary protection elements on a variety of transformers. The performance characteristics of the PolySwitch LVR PPTC devices were compared to those of thermal fuses. Figure 1. Effect of a secondary short on a 240 VAC transformer utilizing a thermal fuse as the primary protection element. 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 120VAC transformer with a short on the secondary side are shown in the following table (Figure 3). 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. the global standard for voltage and current test conditions for equipment connected to AC Mains. Overcurrent Circuit Protection Technology Comparison When a fault occurs in a transformer or power supply, some of the components may begin to overheat. There are several circuit protection schemes that 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 are the preferred solution. These devices do not generally require replacement after a fault event, and allow the circuit to return to the normal operating condition after the power has been cycled and/or the overcurrent condition is eliminated. 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 253VAC was applied and a secondary short was simulated. Surface temperatures of the primary (Tprim) and secondary (Tsec) windings as well as that of the PPTC device (T-PPTC) were measured. The PPTC device Coordinating Protection for AC Mains Applications 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. Electrical equipment can be exposed to potential damage from large Figure 2. Effect of a secondary short on a 240VAC transformer utilizing a PPTC device as the primary protection element. International Appliance Manufacturing 2008 37
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