When a fault occurs in a linear transformer or power supply, some of the components may begin to overheat. In the case of the switch mode charger, some of the components that can fail are the magnetics or the switching FETs. In a linear charger, where no FETs or electronics perform the power conversion, the transformer windings are typically the components at risk.
There are several circuit protection schemes that can be used in power supplies to help guard against 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. This method can protect against both overtemperature and overcurrent conditions. However, some disadvantages of this approach are 1) it requires two components and therefore can increase cost, and 2) 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, but these 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 240 VAC. The PPTC device can help provide both overcurrent and overtemperature protection in one convenient package and offers designers the option of placing resettable protection on the primary side of the transformer, potentially replacing two components with a single device. Furthermore, because PPTC devices do not 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.
Performance Comparison: Thermal fuse vs. PPTC device
Tyco Electronics recently conducted comparison tests of their PolySwitch LVR series of PPTC devices as primary protection elements on a variety of transformers. The performance characteristics of the PPTC devices were compared to those of thermal fuses and ceramic PTC devices.
Many linear power supply designs utilize a one-shot thermal fuse as a primary protection solution. Figure 1 shows an effect of overheating on such a transformer. In this test, 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.
Figure 1. Effect of secondary short on 240 VAC transformer utilizing a thermal fuse as the primary protection element
Figure 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.
Figure 2. Effect of secondary short on 240VAC transformer utilizing a PPTC device as the primary protection element
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 to provide some improved protection for the transformer windings, as well as the secondary circuitry.
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.
||Max Coil Temp (°C)
||Max Current (mA)
| Thermal Fuse
|| >100 min
| Thermal Fuse
|| 51 min
| Thermal Fuse
|| 66 min
| PPTC Device
|| 11 min
| PPTC Device
| PPTC Device
|| 11 min
Performance Comparison: CPTC vs. 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. To obtain 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 typically reaches a surface temperature of 180 to 220°C in its high resistance state. This renders it an unsuitable solution 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 can make 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 to 120°C) in the tripped state. The PPTC device can also 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 Figure 4. 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.
Figure 4. Time-to-trip comparison of CPTC device versus PPTC device in secondary short on 120VAC transformer
A disadvantage of the CPTC device in this application may be its high surface temperature. Thermal images in Figure 5 illustrate 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.
Figure 5. Thermal images compare surface temperatures of CPTC and PPTC devices in their tripped state
The PPTC device's demonstrated ability to help protect low voltage designs - typically below 72V - has made it the preferred technology for resettable overcurrent and overtemperature protection in portable electronics, computers, and telecommunications equipment. The latest generation of PPTC devices is capable of operating at line voltages of 120VAC and 240VAC, and can be used as either as a secondary or primary side protection element on transformers and power supplies.
In addition, due to the dependence of device resistance on temperature, PPTC devices can offer very effective overtemperature protection. Their resettable functionality, small size, low resistance and fast time-to-trip can provide a practical and cost-effective alternative to thermal fuses and CPTC devices.