Analog Angle Blog

Circuit protection: When one plus one is more than four

Adding components to provide circuit protection against internal and external mishaps is one of those thankless design jobs, similar to buying insurance. When it’s not needed, it seems like an added burden; when you do need it, it’s hard to know if it will be enough. Although following regulatory mandates and best practices is a good place to start. Among the most common class of faults for which protection is needed involves overvoltage events, resulting from internal or external short circuits, surges, and component failures.

Three component-based strategies are available to provide overvoltage protection: 1) diverting the associated excess current to ground via a switch which goes to a very low impedance once a threshold voltage is exceeded; 2) dissipating the excess energy via a voltage clamp across the protected line; and 3) disconnecting the line of concern in a fuse-like action when a threshold is crossed.

There are many components available to implement these strategies. Some of them act as crowbars and temporarily short-out line when the fault occurs (Figure 1) while others function as clamps which limit the transient voltage to a preset limit until the fault extinguishes (Figure 2). Note that the term “crowbar” dates back to the very early days of electric power systems when workers would literally drop a metal crowbar across an out-of-control power bus to short it out.

Figure 1 When the crowbar protection function triggers, it becomes a low-impedance path between the line it is protecting and ground, thus diverting the overvoltage surge to ground. Source: Bourns

Figure 2 In contrast to the crowbar, the clamp limits the overvoltage surge to a predefined value. Source: Bourns

Among the many options for protection are gas discharge tubes (GDTs), thyristors, metal oxide varistor (MOV) and multi-layer varistor (MLV) devices, transient voltage suppressors (TVS), and even Zener diodes, to cite a few. It’s common to see several of these devices used in combination to provide overall protection and to work around the inherent shortcomings of each in a mutually beneficial relationship. Obviously, there’s much more to the faults and protection components and their actions.

For example, in order to provide an overvoltage protection solution, which has virtually no leakage current and thus longer operational life, designers often resort to a dual-component arrangement. This hybrid approach combines two discrete components: a series-connected GDT and MOV (Figure 3) with a combined voltage-versus-time curve (Figure 4). Obviously, this dual-component approach requires more circuit-board “real estate” and adds another component to the bill of materials (BOM).

Figure 3 The hybrid approach of connecting a GDT and MOV in series provides a more-effective overvoltage protection solution. Source: Bourns

Figure 4 The response versus time of the hybrid GDT + MOV arrangement shows how it combines the basic response attributes of each device. Source: Bourns

But there’s a larger concern and complication: the circuit-board layout in the region of the MOV and GDT is often subject to regulatory mandates defining minimum creepage and clearance distances. Clearance is the shortest distance in air between two conductive parts; creepage distance means the shortest distance along the surface of a solid insulating material between two conductive parts.

These distances increase with voltage. Thus, the actual placement of the MOV and GDT components adds another concern and constraint to board-layout considerations.

Recently, I saw a relatively new protection device which is a combination of two existing devices, but more than just a simple, obvious co-packaging of two separate components. Devices in the Bourns’ IsoMOV Series of hybrid protection components combine both a MOV and a GDT in a single package, offering the equivalent functionality of a discrete MOV and GDT in series (Figure 5).

Figure 5 Two schematic symbols for the IsoMOV (right) shows it as a merger of their individual standard symbols. Source: Bourns

A look at the construction of the IsoMOV shows that it’s not just an obvious, simplistic co-packaging of a MOV and a GDT in a single shared enclosure. Instead, the integrated device merges the two to create the functional equivalent of discrete MOV and GDT in series (Figure 6).

Figure 6 The physical construction of the IsoMOV is a completely different realization of the hybrid function. Source: Bourns

After assembly of the core, the leads are attached and the unit is epoxy coated. The result is a familiar radial-disc MOV package that is only slightly thicker and with smaller diameter than the similarly rated conventional devices (Figure 7). Further, due to the patent-pending design of the metal oxide technology, the IsoMOV component also has a higher current rating for the same size. Both the footprint penalty and creepage/clearance issues are eliminated.

Figure 7 The radial-lead disk package of the IsoMOV looks like a standard MOV, except it’s smaller and yet has a higher current rating than an equivalent MOV alone. Source: Bourns

The circuit protection device is more than just “the best of both worlds” as there are other advantages to the design. MOV failures (yes, they have well-known failure modes) are generally characterized by a so-called “surge hole” at the edge of the metallized area, which is typically caused by an elevated temperature inside the MOV at that edge during a surge. Bourns says this technology is designed to substantially reduce or eliminate this failure mode.

It’s always interesting when a combination product is more than just the sum of its constituent parts. Here, the combination provides both performance and regulatory-compliance benefits in addition to obvious space savings. By stepping back and thinking “out of the box” (actually, it’s “in the box” here) and looking at the internal construction details, the protection device offers genuine benefits.

We often see higher levels of functional integration via co-packaging that yields a smaller enclosure or IC, which is generally a good thing, but sometimes there is a downside in performance compromise. However, that doesn’t appear to be the case here. In fact, it’s the second time I have seen this occurrence for small non-IC components in recent years. Several vendors have devised combinations of rechargeable batteries plus supercapacitors in a single enclosure, which offer much more than just a smaller co-package. Instead, it demonstrates a fundamental rethinking of the device construction and physics (see first item in Related Content). The result is far superior to the sum of the two individual energy-storage components.

Have you used other “combination” components which offered benefits beyond the obvious space savings, or did they instead force a compromise in key specifications? On a larger scale, have you used combination test instruments which combine nominally unrelated functions in a single package, such as a spectrum analyzer and an oscilloscope (no, a basic multimeter does not count here)? Was there performance synergy or did you experience frustrating limitations?

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