Power Electronics

PPTC Offers New Option for Circuit Protection

Polymeric positive temperature coefficient (PPTC) devices that are rated for line voltages of 120Vac and 240Vac provide cost-effective alternatives for circuit protection.

You can use several circuit protection schemes in power supplies to help guard against fault conditions and the resultant overtemperature damage, including thermal fuses, current fuses, and circuit breakers. One solution is to use a thermal fuse on the primary side and an overcurrent fuse on the secondary side. This method can protect against overtemperature and overcurrent conditions. However, this approach requires two components, which can increase cost. Also, since fuses are single-shot devices, a fault may permanently disable the charger.

In transformer and power supply applications, circuit breakers are sometimes used for resettable protection. This can add significant cost to the design. A good solution available to date has been to use a resettable ceramic positive temperature coefficient (CPTC) device. However, this technology has not been widely applied to primary side protection because of its high operating temperature, high resistance, large size, and poor shock resistance.

A preferred solution to this problem may be to use a polymeric positive temperature coefficient (PPTC) device with an operating voltage rating of 240Vac. Unlike others, the PPTC device can help provide both overcurrent and overtemperature protection in one convenient package, while offering designers the option of placing resettable protection on the primary side of the transformer — replacing two components with a single device. Furthermore, because PPTC devices don't typically require replacement after a fault event, they can help manufacturers reduce warranty, service, and repair costs.

The overtemperature component of a circuit protection device can be critical in applications where a fault may cause a rise in the winding temperature without a substantial increase in current draw. Low-wattage power supply transformers are examples of applications where the winding resistance limits the current to low levels — even in the event of a short on the secondary.

Thermal Fuse Versus PPTC device

Recently, comparison tests of PolySwitch LVR series of PPTC devices as primary protection elements on a variety of transformers were conducted in-house. The performance characteristics of the PPTC devices were compared to those of thermal fuses and ceramic PTC devices.

Many linear power supply designs use a one-shot thermal fuse as a primary protection solution. Fig. 1 shows an effect of overheating on such a transformer. Here, a short on the secondary side resulted in coil temperatures exceeding 200°C. The thermal fuse, rated at 115°C and mounted near the center of the core, failed to open, and the insulation on the windings melted, destroying the transformer.

Fig. 2 illustrates the results of a test in which a similar transformer was tested with the 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 and secondary windings as well as that of the PPTC device were measured. The PPTC device started to trip when its external temperature reached approximately 95°C, at which time the primary coil temperature was about 95°C. Once the PPTC device 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 table. This data demonstrates the advantages of the PPTC device's faster time-to-trip and its ability to limit the maximum coil temperature, thereby helping to provide some improved protection for the transformer windings, as well as the secondary circuitry.

CPTC Versus PPTC

While CPTC devices offer the voltage ratings required for use on the primary side of transformers, their size and operating temperature characteristics have limited their use. For optimum protection against overtemperature, the circuit protection device must be placed in close contact with the transformer windings. Depending on the voltage applied, a CPTC device reaches a surface temperature of 180°C to 220°C in its high resistance state, rendering it unsuitable for overtemperature protection when the insulation ratings of the windings are lower than the CPTC surface temperature. The composition of the CPTC also tends to be brittle, which makes it vulnerable to damage from shock, vibration, or the thermal stress of heating and cooling associated with transformer applications.

In comparison to the CPTC device, the PPTC device limits the maximum temperature of the windings to a lower level and offers a lower surface temperature (100°C to 120°C) in the tripped state. The PPTC device can have lower resistance in the circuit, its impedance is less frequency dependent, and it is available in a smaller size. These characteristics can make a PPTC device a practical solution to primary side protection of linear transformers.

In tests comparing PPTC devices to CPTC devices as primary protection elements, the PPTC device reacted faster and at lower temperatures, as shown in Fig. 3. In this test on two identical transformers, the CPTC device selected had a Curie temperature of 80°C and a hold current of 80mA. The hold current of the PPTC device was 80mA. A fault was created with a secondary short, and current, coil temperature, and time-to-trip measurements were taken.

A disadvantage of the CPTC device in this application may be its high surface temperature. Thermal images shown in Fig. 4 illustrates the difference in surface temperatures of the two devices. In this comparison of a 220Vac trip, the CPTC device reached a maximum temperature of 184.5°C, and the PPTC device reached a maximum temperature of 118.9°C.

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