AC power LED lighting protection technology - Power Circuit - Circuit Diagram
Metal oxide varistors (MOVs) are commonly used for transient overvoltage suppression in AC line voltage applications. Switching of lightning, inductive loads, or switching of capacitor banks can cause transient overvoltage conditions. In these applications, there are potential risks such as persistent abnormal overvoltage/limited current conditions, which may cause the MOV to enter a thermal runaway state and further cause overheating, outgassing and possible ignition.
The new Thermally Enhanced MOV helps protect many low-power systems from damage caused by overcurrent, overtemperature, and overvoltage faults, including lightning strikes, electrostatic discharge (ESD) surges, neutral losses, erroneous input voltages, and power sensing . These devices help protect the AC power LED lighting system, 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 RMS rating of the device, and it is assumed that as long as the transient energy does not exceed the maximum rating of the MOV, a short time instant The event can be clamped to a suitable voltage level. However, persistent abnormal overvoltage/limited current conditions (such as neutral loss) may cause the MOV to enter a thermal runaway state.
The MOV is typically protected from overheating by placing a thermal cut-off (TCO) device in series with the MOV. A typical line voltage transient protection scheme can also integrate an overcurrent protection component (such as a fuse) to protect the system from damage caused by overcurrent overloads above a predetermined level.
Overcurrent conditions and thermal runaway
Tyco Electronics' 2Pro. device integrates a polymer positive temperature coefficient (PPTC) component and a MOV component in a thermal protection device to provide a resettable function in the event of an overcurrent or overvoltage condition. This integrated device approach helps manufacturers meet industry requirements such as IEC61000-4-5 and IEC60950.
Unprotected standard MOV components are typically limited to 275VAC RMS for a common input voltage range. Under neutral conditions, they may have negative consequences due to overheating, even if a fuse or power supply resistor is used upstream of the circuit.
The PPTC component in the AC 2Pro device helps prevent thermal runaway, keeps the varistor surface temperature below 150 °C and prevents the device from reaching unsafe temperatures caused by overvoltage transients.
working principle
Although the AC line voltage applied to an MOV typically does not exceed the maximum continuous operating voltage rating of the device, overvoltage transients that exceed these limits may occur. The 2Pro device integrates PPTC technology with the MOV to improve overcurrent and thermal protection when the MOV is under constant overvoltage conditions for a long time.
In the event of an overvoltage transient, such as the neutral loss event shown in Figure 2, the PPTC component in the 2Pro device will heat up, trip, and enter a high impedance state, helping to reduce the risk of MOV device failure.
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