Analysis of AC Power LED Lighting Protection Technology

Metal oxide varistors (MOVs) are commonly used for transient overvoltage suppression in AC transmission line voltage applications. Lightning, switching of inductive loads, or switching of capacitor banks can all cause transient overvoltage conditions. In these applications, there is also the potential risk of sustained abnormal overvoltage/limited current conditions, which may cause the MOV to enter a thermal runaway state and further cause overheating, gassing, and possible fire.

New thermally enhanced MOVs can help protect many low-power systems from damage caused by overcurrent, overtemperature, and overvoltage faults, including lightning strikes, electrostatic discharge (ESD) surges, loss of neutral, incorrect input voltage, and power induction. These devices help protect AC-powered LED lighting systems, as shown in Figure 1.

Integrated devices help reduce component count

Under normal operating conditions, the AC line voltage applied to the MOV will not exceed the maximum AC root mean square voltage (VAC RMS) rating of the device, and short-duration transient events can be clamped to an appropriate voltage level, assuming that the transient energy does not exceed the highest rating of the MOV. However, sustained abnormal overvoltage/limited current conditions (such as loss of neutral) may cause the MOV to enter thermal runaway.

The MOV is usually protected from overheating by placing a thermal cutoff (TCO) device in series with the MOV. Typical line voltage transient protection schemes may also integrate an overcurrent protection element (such as a fuse) to protect the system from damage caused by overcurrent overloads exceeding a predetermined level.

Overcurrent Conditions and Thermal Runaway

Tyco Electronics' AC2Pro. device integrates a polymer positive temperature coefficient (PPTC) component and an MOV component into a thermal protection device to provide a resettable function in the event of overcurrent or overvoltage. This integrated device approach helps manufacturers meet industry requirements such as IEC61000-4-5 and IEC60950.

[page]Unprotected standard MOV components are typically limited to 275VAC RMS for a universal input voltage range. Under loss of neutral conditions, they may produce negative results by overheating, even if a fuse or source resistor is used upstream in the circuit.

The PPTC element in the AC 2Pro device helps prevent thermal runaway, keeping the varistor surface temperature below 150°C and preventing the device from reaching unsafe temperatures caused by overvoltage transients.

How it works

Although the AC line voltage applied to an MOV will not typically exceed the maximum continuous working voltage rating of the device, it is possible for overvoltage transients to occur that exceed these limits. The 2Pro device integrates PPTC technology with the MOV to provide improved overcurrent and thermal protection when the MOV is exposed to a continuous overvoltage condition for a long period of time.

In the event of an overvoltage transient, such as the loss-of-neutral event shown in Figure 2, the PPTC element in the 2Pro device heats up, trips, and enters a high-impedance state, helping to reduce the risk of MOV device failure.