“As Electronic devices proliferate and user safety regulations evolve, designers are always looking for ways to enhance device protection while minimizing cost and board space. The problem is, circuit protection is a lot like insurance: it may seem like an unnecessary expense until it’s needed. But this protection is really needed to protect against a variety of internal and external disturbances and faults, including internal and external short circuits, overcurrent and voltage surge conditions. These conditions may temporarily or permanently disable the system; damage the system, its internal components, or loads; or even result in injury to the user.
By Bill Schweber
As electronic devices proliferate and user safety regulations evolve, designers are always looking for ways to enhance device protection while minimizing cost and board space. The problem is, circuit protection is a lot like insurance: it may seem like an unnecessary expense until it’s needed. But this protection is really needed to protect against a variety of internal and external disturbances and faults, including internal and external short circuits, overcurrent and voltage surge conditions. These conditions may temporarily or permanently disable the system; damage the system, its internal components, or loads; or even result in injury to the user.
There is no single protection scheme suitable for all faults and situations. For example, when implementing overvoltage protection (OVP), a crowbar device such as a gas discharge tube (GDT) is generally better for long-term faults, while a clamping device such as a metal oxide varistor (MOV) is better for transient event. However, GDTs maintain continuous breakdown due to “holding current”, while MOVs can fail permanently and can reach dangerously high temperatures due to thermal breakdown. Connecting the two components in series in a hybrid fashion can compensate for any potential problems, but this approach complicates board layout and increases cost. So we need to make progress in design to eliminate this hazard.
This article describes the importance of OVP protection and the various ways to implement it. Then, IsoMOV technology is introduced, which combines the advantages of GDT and MOV in a single device to achieve longer life and no holding current. Finally, several example devices from Bourns, Inc. are presented, their salient features are described, and how to select and use them for effective, low-cost protection.
Protection has multiple angles
There is no “one size fits all” when it comes to circuit and system protection. There are two reasons for this: first, there are many fault types and situations that require protection; and second, the magnitude and duration of the fault conditions determine the type and robustness of protection required.
Among many of these general failure scenarios are the following:
Overcurrent, i.e. excessive load current due to external faults, short circuits or internal component failures (including insulation faults).
Overvoltage, where a part of the system is exposed to excessive voltage due to a faulty connection.
Overheating, which causes components to overheat due to poor design, inadequate thermal management, or excessive ambient heat.
Component failure, where one internal component fails, resulting in an overcurrent/overvoltage condition that can damage other components or loads.
The consequences of failures are often more than just affecting or breaking a system, as they can cause electrical shock to users.
Crowbar and Clamping Devices for Surge Protection
In both AC and DC circuits, the most challenging fault conditions are overvoltage surges known as transient overvoltage (TOV) events. Such short pulses or spikes are often due to nearby lightning strikes or electrical switches, injecting harmful transients into electrical equipment and its sensitive electronics.
There are two broad categories of surge protection devices (SPDs) used to handle overvoltage and TOV events: crowbar devices and clamp devices. (Note that in informal discussions these terms are sometimes used interchangeably, but they are not the same).
Simply put, a crowbar device is a short circuit of the protected line, thereby diverting the surge and its current to ground, preventing it from reaching the circuit (Figure 1). When an overvoltage condition occurs, the crowbar mechanism is triggered into this low impedance mode.
Interesting digression: The term “crowbar” is said to have originated from the operation of industrial workers in the early days of electricity, who would throw a real metal crowbar over the power and ground busses when an overvoltage condition occurred.
Figure 1: When the crowbar protection is triggered, it acts as a low impedance path between the line it protects and ground, diverting overvoltage surges to ground. (Image credit: Bourns Inc.)
The crowbar remains in low-impedance mode until the current drops below the “holding current”, at which point it returns to normal high-impedance operation. It must be able to handle the current flowing through it for the time the power supply is in an overvoltage state.
In contrast, a clamp prevents the voltage from exceeding a preset level (Figure 2). When the transient voltage reaches the limit level rated by the clamping device, it clamps the voltage until the fault is extinguished, at which point the line returns to normal operating mode. It is important that the rated clamping voltage is higher than the normal operating voltage.
Figure 2: As opposed to a crowbar, a clamping device limits overvoltage surges to a predetermined value. (Image credit: Bourns Inc.)
When the transient voltage is higher than the conduction voltage of the clamping device, the current conducted by the clamping device is just enough to maintain the voltage across it at a safe, ideal value. This current, although small, can cause some safety-related issues that must be addressed, and may require additional protection, discussed further below. It must be rated for the power it must dissipate in a given time, which is usually a relatively short transient event.
Implement OVP function
Since crowbars and clamps are important protective devices, they must be simple, reliable, easy to understand, and have consistent performance attributes. In this case, they are like thermally activated fuses, a classic overcurrent protection element that is often used as an extra layer of protection.
Crowbar Assembly: The most common crowbar assembly is the GDT, which has a carefully designed and constructed spark gap in a sealed enclosure filled with an inert gas. In normal operation, before the TOV event, it looks like a near-infinite resistance (Figure 3). However, when an overvoltage surge occurs and exceeds the GDT’s design voltage, the gas ionizes and the tube “flashes” like a spark gap and switches from high impedance to very low impedance. This change will temporarily short the line until the fault goes out.
Figure 3: A GDT is a complex spark-gap device that only conducts when the voltage across it exceeds its design value; until then, it looks like an almost perfect broken circuit. (Image credit: Bourns Inc.)
GDTs are commonly used in DC circuits, telecommunications circuits, and signal circuits, all of which typically draw fairly low currents, 1 amp or less. Note that in contrast to the huge GDT seen in the movie, a low-level surge GDT is a small, packaged, PCB mountable component, with no visible flickering sparks. Smaller GDTs are rated from 75 to 600 volts; larger GDTs are rated up to several thousand volts. A problem with GDTs is that they have follow-up current (also called holding current), that is, current continues to flow even after the fault is removed.
Clamping Devices: The two most widely used options for clamping devices are Power Transient Voltage Suppressor (PTVS) diodes and Metal Oxide Varistors (MOVs), both commonly used in AC and DC circuits, motors, communication lines and high current protection of the sensing circuit (Figure 4). MOVs are rated for tens of volts to over a thousand volts.
Figure 4: Metal oxide varistors (and power transient voltage suppressors) provide a clamping voltage that covers a wide range of designs. (Image credit: Bourns Inc.)
MOVs typically conduct a small amount of leakage current even when the applied voltage is well below their nominal threshold voltage. If the MOV is subjected to a voltage surge that exceeds its rating, permanent damage can occur, resulting in increased leakage current. Although this current is usually only a few milliamps, it can also present a shock hazard in some cases.
Also, if the leakage current is large enough, the MOV will self-heat inside. When the MOV is continuously connected to the AC power supply, this self-heating will generate positive feedback, that is, the larger the leakage current, the more self-heating, and the more self-heating, the larger the leakage current will be. Subsequent surges further accelerate the cycle.
At some point, the MOV will go into thermal runaway mode, generating considerable heat and destroying the MOV. In some cases, the heat generated by the MOV can become a potential ignition source (PIS), causing nearby materials to catch fire. This impact must be considered and addressed for basic safety and safety-related standard requirements.
A better OVP solution
To provide an OVP solution with virtually no leakage current, thereby extending operating life, designers often use a two-component configuration. This hybrid approach combines two discrete devices: a GDT is connected in series with an MOV (Figure 5), and the combined voltage versus time curve is shown in Figure 6.
Figure 5: The hybrid approach of series GDT and MOV provides a more efficient OVP solution. (Image credit: Bourns Inc.)
Figure 6: Time response for a hybrid arrangement of GDT + MOV shows how the basic response properties of each device fit together. (Image credit: Bourns Inc.)
This is an efficient way for each device to compensate for possible shortcomings of the other. However, this approach also has costs.
it requires more board space
Another component is added to the Bill of Materials (BOM)
Another challenge is that the board layout in the MOV and GDT areas is complicated by regulatory requirements that define minimum creepage and clearance distances.
Gap is the shortest distance in air between two conductive parts
Creepage distance is the shortest distance along the surface of a solid insulating material between two conductive parts
The problem is that clearance and creepage distances increase with voltage. Therefore, another imposition and constraint is added when placing MOV and GDT components, which needs to be taken into account with the board layout.
To help designers address these cost, space, and regulatory concerns, Bourns, Inc. has developed the IsoMOV family of hybrid protection components. This family offers an alternative solution that combines MOVs and GDTs in a single package, providing functionality comparable to series discrete MOVs and GDTs (Figure 7).
Figure 7: The IsoMOV (right) schematic symbol shows that it is a combination of GDT (center, left) and MOV (top and bottom, left) single standard symbols. (Image credit: Bourns Inc.)
As we look at the structure of IsoMOV, we can see that it is more than a simple co-packaging of MOV and GDT in a common shell (Figure 8).
Figure 8: The physical structure of IsoMOV is an implementation of a completely different hybrid function rather than a co-package of two separate existing devices. (Image credit: Bourns Inc.)
After the core is assembled, the leads are connected and the unit is epoxy coated. The result is a familiar radial disk MOV package that is only slightly thicker and has a smaller diameter than similarly rated conventional devices (Figure 9). In addition, the IsoMOV assembly has a higher current rating in the same size due to its patented metal oxide technology design. Board space hassles and creepage/spacing issues are eliminated.
Figure 9: The radial leaded disc package IsoMOV looks like a standard MOV, but with a smaller diameter and higher current rating than a stand-alone equivalent MOV. (Image credit: Bourns Inc.)
Not only is IsoMOV “the best of both worlds”, but there are other design advantages as well. A characteristic of MOV failures is the appearance of so-called “surge holes” at the edges of the metallized area, usually due to the increased temperature inside the MOV during the surge. Bourns designed the unique EdgMOV technology specifically to substantially reduce or eliminate this failure mode.
Let’s look at an actual IsoMOV model for a more detailed look. ISOM3-275-B-L2 has a maximum rated continuous operating voltage (MCOV) of 275 volts root mean square (rms) / 350 VDC; rated current is 3 kiloamps (kA) / 15 operations, 6 kA / 1 operation operation (max). Also of particular interest is its low capacitance of 30 picofarads (pF) at 20 kilohertz (kHz), making it ideal for high-speed data lines, and its low leakage current of less than 10 microamps ( μA).
The role of standards
Design engineers must implement various forms of surge or other protection for many reasons, ranging from prudent design practice requirements to the mandate of various regulatory standards. Some of these standards are general and apply to any equipment subject to general operating conditions, such as AC line operation; others are specific to a certain type of application, such as medical equipment. Among the standards-setting organizations, there are UL, IEEE, and IEC; many of their standards are “harmonized” and therefore the same, or nearly the same.
All of these standards are complex and have many provisions; they also include exceptions, listing steps or features that can be eliminated in some cases, and additional requirements that must be added in other cases. For example, IEC 60950-1 “Information Technology Equipment C Safety” and UL/IEC 62368-1 and “Audio/Video, Information and Communication Technology Equipment Standard C Part 1: Safety Requirements” (which supersedes IEC 60950-1 in 2020) both The rated voltage of the MOV is required to be at least 125% of the rated voltage of the device. Therefore, for a main circuit of 240 volts rms, the MOV must be rated for at least 300 volts rms.
Consider the case of the common AC line plug, which is available in two and three hole versions. In theory, the three-wire version provides a safety ground, but in practice this ground is often left unconnected or unavailable. The lack of a true earth ground safety ground connection can lead to a potentially dangerous situation when there are only live and neutral wires. In this case, it is necessary to incorporate protective components into the design to prevent possible electric shock when a user touches a conductive part that is supposed to be grounded but is not. In this case, however, a small amount of MOV leakage current may still present a shock hazard.
The most common solution to prevent MOV leakage current from becoming this dangerous is to put at least one GDT in series with the MOV (Figure 10). By using IsoMOV devices, one package implements both MOV and GDT functions, saving board space. Therefore, IsoMOV is also a problem-solving component that simplifies the safety measures required to meet UL/IEC 62368-1.
Figure 10: To eliminate the risk of electric shock to users due to unavoidable leakage currents in ungrounded applications, two devices (an MOV and a GDT) can be placed in series between the live and neutral conductors of the AC line. (Image credit: Bourns Inc.)
Figure 11: An alternative to using separate MOVs and GDTs is to use a single IsoMOV device, resulting in the same or better performance and a smaller overall solution size. (Image credit: Bourns Inc.)
People often ask engineers which solution is “best”. In most cases, there are only compromises and no single, simple answer. In general, when implementing overvoltage protection, a crowbar device is better for long-term faults, while a clamp device is better for transient events. But using both devices increases board space and complicates board layout.
Now, with the advent of new technologies, there is no need to compromise. Bourns’ IsoMOVs achieve longer operating life than MOVs alone, but without the follow-on current issues of GDTs. These devices provide both surge and overvoltage protection and meet all relevant standards with a small board space footprint. In addition, their low leakage current minimizes subsequent problems, and their extremely low capacitance also makes them suitable for protecting low-voltage, high-speed circuits.
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