Appliance Design - March 2008 - (Page 22)

POWER Over-current and thermal runaway 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. 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 – ROV10431K). 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. Fig. 1. Effect of loss of neutral condition on various devices. Fig. 2. Typical 2Pro LVM2P-015R10431 device response under IEC61000-4-5 surge immunity test. How it works Although the AC line voltage applied to an MOV is not generally expected to exceed the device’s maximum continuous operat22 applianceDESIGN March 2008 Fig. 3. Typical 2Pro LVM2P-015R10431 device response to loss of neutral event. www.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - March 2008

Appliance Design - March 2008 Contents Editorial Shipments/Forecasts News Watch A New Appliance is Set to Break into Homes Soon - the Micro CHP Unit, which Generate both Heat and Power. Thermally Enhanced Varistors Help Protect Low-Power Systems Against Damage Caused by Over-Current, Over-Temperature and Over-Voltage Faults. Hybrid Controller Reduces Standby Power Consumption and Improves Active-Mode Efficiency. Battery-Management ICs Solve Design Challenges for Cordless Appliances Using High-Voltage, Lithium-Ion Battery Technology. A Semiconductor Solution Protects the Relay in a Temperature Controller for a Cooking Appliance. New Polyurethane Foam Insulation System Optimizes both Insulation Performance and Productivity. Innovations in Decorative, Pre-Finished Metals Expand Range of Design Options for Appliance Designers. Design Marts Association Report: AHAM Advertiser's Index

Appliance Design - March 2008

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