Meeting Sustained Overvoltage Protection with Hybrid Drop-In Replacement
While circuit protection is known to be a basic requirement for AC and DC power line devices, there isn’t a common protection solution for all applications. That is because devices can have differing levels of exposure to lightning and switching transients, and in some developing and rural areas around the world, line voltage regulation can be extremely poor or inconsistent.
Solving the problem, in part, relies on component manufacturers to ensure the reliability and robustness of the protection solution. It is also incumbent upon them to meet higher density, higher performance, design flexibility, and varying requirements, and at the same time, keep manufacturing changes to a minimum. Needless to say, it can be challenging to address all circuit protection concerns. Unfortunately, even with the surge protective devices (SPD) standard, UL 1449, there is no single universal solution.
Metal oxide varistors (MOVs) are traditionally used as protection devices in AC and DC power line applications. While a new MOV is an outstanding transient overvoltage solution on its own, it will degrade over time from exposure to line voltage transients.
This article will present MOV device performance and its limitations. Addressing the issues that can occur with MOVs, the article will introduce a new innovative component hybrid overvoltage protection solution from Bourns. It will highlight how this new UL 1449-listed solution enhances performance, and also gives designers a MOV footprint-compatible device that offers additional flexibility for front-end circuit protection.
Using Conventional MOVs for Overvoltage Protection
As a radial leaded varistor device, MOVs provide bidirectional protection against overvoltage transients such as lightning, power contact, and power induction. These devices offer high current handling, high current absorption and fast reaction times capable of protecting against transient faults up to their rated limits. This is why a diverse list of power and telecom system applications employ MOVs for protection.
Because harmful MOV degradation and failure modes have been well-documented, UL (Underwriter Laboratories) requires extensive testing for a device to be UL listed. Fundamentally, as an MOV ages, its maximum continuous operating voltage decreases. The failure mode for a MOV that has experienced long-term overvoltage exposure is excessive leakage. This leakage typically leads to power dissipation and eventually can cause an overtemperature condition. A MOV’s surge history, operational time and temperature all contribute to increased leakage current, and can also produce watt loss heating that can trigger application damage. Consequently, applications that are subject to voltage transients probably need more protection than a MOV alone can provide.
Some developers will specify a Thermally Protected MOV to protect against overvoltage threats in AC power line designs. A downside to employing this approach is that Thermally Protected MOVs do not prevent the leakage current that causes the power dissipation. Another concern is that many Thermally Protected MOV fuses are only tested to 10 A in the UL 1449 limited current abnormal overvoltage test. This means they may not open an actual AC line capable of delivering over 100 amps.
Plus, a Thermally Protected MOV that successfully disconnects from the power line is no longer protecting the equipment. Along with these weaknesses, adding the associated enunciating circuitry increases the bottom-line cost of the protection scheme without increasing the level of protection.
As an alternative to lower the chances of a “thermal event”, some designers may choose an MOV with a voltage rating far above the application’s normal operating voltage. While this does reduce the stress on the MOV and slows the aging process, it also means the clamping voltage will be higher. Going this route forces the designer to spec higher voltage-rated and more expensive downstream components as they will need to be rated to survive the higher voltages let through during a surge event.
A Hybrid MOV/GDT is the Solution
Bourns® GMOV™ device is designed to overcome the shortcomings of traditional MOVs, Thermally Protected MOVs, and higher voltage-rated MOVs. This hybrid solution integrates a Bourns’ FLAT® gas discharge tube (GDT) with a high-quality conventional MOV as shown in the circuit diagram in Figure 1. This innovative design is in keeping with the company’s continued commitment to helping developers meet their space-constrained and reduced part count requirements.
Figure 1: A GMOV™ component combines a discrete MOV and GDT into a single UL 1449 listed product.
The GMOV device is packaged in the well-known standard MOV radial package making it a drop-in replacement to a similarly-rated MOV. By combining a MOV and GDT, the GMOV overvoltage protection solution provides lower leakage during the life of the MOV and a graceful failure mode that avoids high temperatures.
How it works is that during normal operating conditions the line voltage appears largely across the extremely low leakage GDT, which basically disconnects the MOV from the AC line. Working in conjunction with the GDT, the MOV is protected from small transients that present no threat to the equipment, but tend to cause premature aging of the MOV. However, if a surge event occurs, the GDT in less than a microsecond switches on and connects the MOV across the line to clamp the surge voltage to acceptable levels. Once the surge event has passed, the line voltage falls across the MOV, switching off the GDT. When the GDT is off, it in-turn disconnects the MOV from the line.
The GMOV™ component is designed to be an enhanced protection solution that minimizes degradation and catastrophic failure issues that can occur with discrete MOV’s.
Another design benefit of combining a GDT with a MOV is that the GMOV device does not impact signal or system operation. The low capacitance of the GDT assures that it will not interfere with AC or DC power line high-speed data rates. Power supplies, power systems, line voltage, telecom systems, power line communications, and white goods and appliances are among the applications that can benefit from the longer life and reliability of a Bourns GMOV device.
Circuit Protection Design Advantages
Exposure over time to line irregularities and from multiple surges can reduce circuit protection effectiveness. Leakage current associated with some protection schemes cannot be tolerated by certain applications. The robustness, lower leakage current and reduced capacitance featured by the Bourns GMOV device offers advantages over other types of protection solutions.
Energy Star applications also find reduced leakage current an advantage, especially in the presence of voltage stress. The reduced leakage current and constant capacitance of the GDT make the GMOV device well-suited for power line communications helping them to achieve acceptably high data rates.
Plus, integrating a UL 1449 listed GMOV provides the ability to save time and cost for both the design and certification phases of product development. It helps eliminate the guesswork in selecting the right combination of discrete parts. With the GMOV device, there is no need for indicating circuitry and its associated costs because the hybrid MOV/GDT design does not have a thermal fuse or any need to disconnect from the line.
GMOV Device Specifications
Helping to streamline the design process, Bourns has made specifying a GMOV device virtually identical to the selection process for a traditional MOV. The part numbering system will be very familiar to designers where the MCOV (maximum continuous operating voltage) rating is coded directly into the number. Likewise, the surge handling capacity, which is dictated by the MOV diameter, is prominently featured similar to MOV part numbers.
Physically, the GMOV is 2 mm thicker than a conventional MOV. In most through-hole PCB applications, a GMOV device will be a direct drop-in replacement. And, because it is UL Listed as a Type 5 SPD, which is identical to a MOV, it offers a direct substitute that requires minimal requalification.
A Superior Overvoltage Protection Solution
Instead of using a discrete combination of MOV and GDT, the Bourns® GMOV™ device offers an effective single-component overvoltage solution. Bourns’ breakthrough space-saving FLAT GDT technology together with a MOV in the same package meets size restrictions. This drop-in replacement for a MOV also delivers the increased reliability needed in clamping voltage transients with the added advantages of low leakage, energy savings, low capacitance, and longer operational life.
It is now possible to develop special designs for immunity from the high temporary overvoltage experienced during a voltage transient without sacrificing clamping voltage performance. Plus, UL listed combinations help to lower engineering and qualification time, and costs.