Metal Oxide Varistors (MOVs) are typically used for transient over-voltage suppression in AC line voltage applications. Lightning, inductive load switching, or capacitor bank switching may cause transient over-voltage conditions. In these applications, there also exists the potential for a sustained abnormal over-voltage/limited-current condition that may cause the MOV to go into thermal runaway, resulting in overheating, out-gassing and possibly fire.

New thermally enhanced MOVs help protect a wide variety of low-power systems against damage caused by over-current, over-temperature and over-voltage faults, including lightning strikes, electrostatic discharge (ESD) surges, loss of neutral, incorrect input voltage and power induction.

These devices help provide protection in a wide range of AC line applications, including AC mains LED lighting systems, PLC network adapters, cell-phone chargers, AC/DC power supplies (up to 30 VA as input power for 230 VAC input voltage), modem power supplies, AC panel protection modules, AC power meters, and home appliances.

Integrated device

Enlarge this picture Fig. 1. Effect of loss of neutral condition on various devices.

Under normal operating conditions the AC line voltage applied to an MOV is not expected to exceed the device’s maximum AC root mean voltage (VACRMS ) rating and, provided that the transient energy does not exceed the MOV’s maximum rating, short-duration transient events are clamped to a suitable voltage level. However, a sustained abnormal over-voltage/limited-current condition, such as a loss of neutral, may cause the MOV to go into thermal runaway.

Protecting the MOV from thermal over-dissipation is frequently accomplished with a thermal cut-off (TCO) device placed in series with the MOV. A typical line-voltage, transient-protection scheme may also incorporate a thermal-protection element, such as a fuse, to protect the system from damage caused by an over-current condition that exceeds a predetermined level.

Over-current and thermal runaway

Enlarge this picture Fig. 2. Typical 2Pro LVM2P-015R10431 device response under IEC61000-4-5 surge immunity test.

Tyco Electronics’ 2Pro™ device combines a polymeric positive-temperature coefficient (PPTC) over-current component with an MOV component into one thermally protected device to help provide resettable current limiting for over-current conditions and voltage clamping during over-voltage events. This integrated-device approach was developed to help manufacturers meet industry test requirements, reduce component count, and improve equipment reliability.

Standard unprotected MOVs are typically rated to 275 VACRMS for a universal input voltage range. In a loss of neutral condition they may overheat with negative consequences, even if a fuse or power resistor is used upstream. In a worst case scenario, as shown in Fig. 1, a voltage of 400 VAC instead of 230 VAC, derived from a loss of neutral, is applied. In such an unlimited current condition, the unprotected MOV will first fall to low impedance of a few Ohms, but due to the high amount of energy present, it is likely to rupture. If there are devices placed on the AC line return path to limit current flow, these may also overheat due to the failure of the MOV.

Enlarge this picture Fig. 3. Typical 2Pro LVM2P-015R10431 device response to loss of neutral event.

Fig. 1 illustrates the effects of these abnormal over-voltage conditions on three devices or a combination of devices:

• Raychem LVM2P-015R10431 (2Pro device).
• Single MOV (10mm, 275 VRMS – ROV10-431K).
• MOV/4 W power resistor (10 Ohms).

The 2Pro device’s PPTC element helps prevent thermal runaway, maintaining varistor surface temperature at less than 150 DegC, and preventing the device from reaching unsafe  temperatures caused by over-voltage transients.

This approach helps manufacturers comply with IEC 60950 and IEC 60335 and helps equipment remain operational after specified lightning tests according to IEC 61000-4-5 (Fig. 2). Also, because the PPTC element is in series with the MOV, no additional over-current protection is required, which reduces component count and optimizes board space.

How it works

Enlarge this picture Fig. 4. Typical circuit diagram utilizing the 2Pro circuit protection device.

Although the AC line voltage applied to an MOV is not generally expected to exceed the device’s maximum continuous operating voltage rating, over-voltage transients may occur that exceed these limits. By integrating PPTC technology with the MOV, the 2Pro device improves over-current and thermal protection under conditions where the MOV is exposed to prolonged continuous over-voltage events.

In the event of an over-voltage transient, such as a loss of neutral event shown in Fig. 3, the PPTC element of the 2Pro device heats up, trips and goes into a high-resistance state, helping to reduce the risk of MOV device failure.

The LVM2P-015R10431 devices used in the test scenarios described here are 10 mm radial-leaded devices that utilize three leads. In the electrical schematic, shown in Fig. 5, the first lead is connected to the PPTC element, the second lead is connected to the MOV/PPTC series connection (providing a path to ground), and the third lead is connected to the MOV.

The single, small form-factor, circuit-protection device helps reduce component count and footprint size, and provides a safe failure mode in case of varistor overstress. It also helps equipment comply with IEC 60950/IEC 60335 and IEC 61000-4-5 testing requirements.

Coordinated protection

Enlarge this picture Fig. 5. 2Pro in ungrounded system.

The thermally enhanced MOV is suitable for over-current and over-voltage protection in a wide range of telephony and VoIP equipment applications. These include cordless phones, VoIP gateways, data modems, set-top boxes, security systems, MDF (Main Distribution Frame) modules, analog linecards and ISDN (Integrated Services Digital Network) linecards.

The widespread adaptation of VoIP gateways in homes and enterprise environments has resulted in increasingly stringent safety requirements for Customer Premise Equipment. The 2Pro device helps equipment manufacturers comply with UL 60950 and remain operational after specified lightning tests. It also helps equipment comply with surge tests per TIA-968-A, IEC 60950, and ITU-T K.20/K.21. The UL 497A listed protector also helps provide ESD protection.

Fig. 5 and Fig. 6 illustrate how the device is used in both an ungrounded and grounded circuit protection scheme.

Summary

Enlarge this picture Fig. 6. 2Pro in grounded system.

Coordinated circuit protection helps reduce board space requirements and component count, and improves equipment reliability. The resettable functionality of the thermally protected MOV can also help manufacturers reduce warranty returns and customer complaints related to transient over-current and over-voltage incidents.

For more information, enter: applianceprotection@tycoelectronics.com

• Raychem, 2Pro, TE Logo and Tyco Electronics are trademarks.