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Sponsored by 43 obviates the need for using a significant amount of heat sinking. To fully leverage the PPTC device, you can thermally bond it to a metal-core PCB or LED heat sink. To help prevent damage from an ESD event, you can place small-form-factor, low-ca- pacitance—typically, 0.25-pF—ESD-pro- tection devices in parallel with the LEDs. Designers typically use MOVs for transient-overvoltage suppression in ac line-voltage applications. New thermally enhanced MOVs help protect a variety of low-power systems against damage from overcurrent, overtemperature, and over- voltage faults, including lightning strikes, ESD surges, loss of neutral, incorrect input voltage, and power induction (Figure 3). Under normal operating conditions the ac line voltage you apply to an MOV should not exceed the device's maximum ac root-mean-square voltage rating, and, if the transient energy does not exceed the MOV's maximum rating, short events clamp to a suitable voltage level. How- ever, a sustained abnormal overvoltage or limited-current condition, such as a loss of neutral, may cause the MOV to go into thermal runaway. Designers frequently pro- tect the MOV from thermal overheating by placing a thermal-cutoff device in series with the MOV. A typical line-voltage tran- sient-protection scheme may also incor- porate an overcurrent-protection element, such as a fuse, to protect the system from damage from an overcurrent overload that exceeds a predetermined level. Standard unprotected MOVs are typically 275V-ac rms for a universal input-voltage range. In a loss-of-neutral condition, they can overheat—with negative conse- quences—even if you use a fuse or power resistor upstream. The MOV in Figure 3, an ac 2Pro, includes a PPTC element to help prevent thermal runaway, maintaining varistor surface temperature at less than 150°C. In the event of an overvoltage tran- sient, such as a loss-of-neutral event, the PPTC element heats up, trips, and goes into a high-resistance state, helping to re- duce the risk of MOV-device failure. p

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