Using Thermally Protected MOVs in TVs or Power Supply Applications

ABSTRACT

Metal Oxide Varistors (MOVs) are commonly used for transient over-voltage suppression in many  applications  such  as:  Transient  Voltage  Surge  Suppressors  (TVSS),  Uninterruptable  Power Supplies  (UPS),  AC  Power  Taps,  AC  Power  Meters  or  other  products.  Events  such  as  lightning, inductive load switching, or capacitor bank switching, are often the sources of these transients. Additionally, in these applications, the possibility for a sustained abnormal over-voltage, with a limited current condition may exist which necessitates the need to protect the MOVs from an over-dissipation thermal condition.

The UL1449 standard clearly defines abnormal over-voltage, limited current test conditions.  The intent of this paper is to outline these conditions, explore the use of MOVs in combination with a TCO (Thermal Cut-Off or Thermal Cut-Out) device, and compare performance with a thermally self-protected MOV technology.  Graphs depicting the Epoxy temperature of MOVs without thermal protection will be shown along with a MOV/TCO combination and the internally protected MOV.

1.   Introduction

 Under normal operating conditions, the AC line voltage applied to an MOV is not expected to exceed the MOV’s Maximum ACRMS  Voltage Rating. Occasionally, over-voltage transients may occur that exceed these limits.  These transients are clamped to a suitable voltage level by the MOV provided the transient energy does not exceed the MOV’s maximum rating.  If, unlike a short duration transient, an MOV is subjected to a sustained abnormal over-voltage, limited current condition (as is required in UL1449), the MOV may go into thermal runaway resulting in overheating, smoke, and potentially fire. For end products to comply with UL1449, some level of protection must be afforded to the MOV to prevent this failure mode.   That protection has traditionally been a thermal fuse or Thermal Cut-Off (TCO) device.

 2.   The UL1449 Abnormal Over-voltage Standard

 In AC line applications, Neutral and Ground are typically near or at the same potential. In the event of a loss of a Neutral-Ground connection, there exists a risk that a sustained over-voltage may be applied to an MOV that is rated for a much lower continuous voltage.  In an unlimited current condition, the MOV will first fail short, but due to the high amount of energy available, it most often ruptures instantaneously. If, however, there are loads tied to the AC line that limit current flow, the MOV can overheat  and  potentially  cause  the  TVSS  device  to  overheat  resulting  in  smoke,  out-gassing  and eventually fire. This potential condition is specifically identified and addressed in the UL1449 TVSS Standard. See Table 1. In many cases, it requires that end-product manufacturers include a thermal protection element for an MOV.

Table 1: Test Voltage Selection Table

Table 1. defines the test voltage that should be applied to various TVSS devices depending on the designer’s desired device1  rating.  Each test voltage is applied across each conductor pair with a short circuit current of 5A, 2.5A, 0.5A and 0.125A respectively across each of four TVSS devices.  Since this test is destructive, four devices are needed to test for each of the four short circuit currents. The four devices must be energized for 7 hours, or until current or temperatures within the TVSS device attain equilibrium, or until the TVSS becomes disconnected from the AC Line.

For example, in a standard 120V AC Line application, the requirement is for a 240VACRMS  test voltage to be applied across all conductor pairs.   There are three pairs; Line–Neutral (L–N), Line–Ground (L–G), and Neutral–Ground (N–G).  This test voltage is chosen because very commonly in the U.S., 120V AC power is commonly fed from a center–tapped 240V transformer. If a break occurs at X—X (see Figure 1.), then the load in the bottom phase acts as a current limiter and the line fuse may not clear. Thermally unprotected MOVs are typically rated from 130Vacrms to 150Vacrms and will heat up, out- gas and may catch fire in such circumstances. 

If you are interested in receiving a PDF with the full article. Feel free to email me directly susanb@solidstateinc.com.  Look forward to your questions and providing you with the complete information.