By David DeLeonardo, Analog Specialist AE, Future Electronics
The components used for the protection of electronic circuits are perhaps the least understood components in common use today. In order to clear up some of the confusion surrounding this class of components, this TWO PART (October Issue) article will provide an overview of (1) the types of events that can damage circuits, (2) the most commonly used components to address these events and, finally, (3) tips for identifying what solution is best for your application.
Here in PART ONE, we will limit our discussion to the most common damaging events that threaten electronic circuits and 4 of the 9 most common components used to address them. In PART TWO, we will finish up with an overview of the other 5 types of protection components; provide a set of application tips and a Device Comparison and Characterization Chart.
Most Common Damaging Events
1. ESD: This refers to Electro Static Discharge. ESD results when a built up charge of static electricity on one object is suddenly released to another object whether due to instantaneous contact or the spontaneous breakdown of the dielectric separating the two charges. We are all familiar with the experience of walking across a carpeted floor in winter and getting a shock when we touch a door knob or other uncharged object. In this case, our body had accumulated sufficient electric charge to raise our skin to between 5K and perhaps as much as 15K volts. When subjecting a device or system to ESD testing, a capacitor of particular value is raised to a given test voltage and then connected to the device or system under test via a resistor of particular value. The values of these circuit parameters are determined by the different models being used for the test as well as the ESD Class the device is being tested to. Below, the three most common models are detailed below:
|Component||Human Body Model (MIL-STD 883, METHOD 3015.7)||Machine Model|
|R2 (Ω)||1500 ± 1%||0Ω||330|
|C1 (pF)||100 ± 10%||200 ± 5%||150|
The main differences between the various models used to reproduce ESD events are the value of the associated passive elements. That is, the value of the charging resistance, the discharging resistance and the capacitive storage element. These, in turn, determine the frequency and severity of the ESD event.
As can be seen in Figure 1, the Machine Model is much more severe than either the human body model or the IEC 1000-4-2 model. This is in keeping with the fact that machines usually have more capacitance to store charge and less impedance in the way of its release than is the case with a human body.
Plots for the waveforms resulting in the application of each of the ESD test models detailed in Figure 1 are shown in Figure 2:
Figure 2 illustrates the effect of the different values of R1, R2 and C1 in the models described in Figure #1 above. As can be seen, with R2 set to zero as in the Machine model, there is a ringing that is absent in the Human Body model where R2 = 1.5KΩ. The IEC model peaks higher than the other models due to its higher C and low R2 value.
2. Overload: Overload is when the output of a power delivery device is required to provide a current to the load that is in excess of its designed capabilities. This can be either a transient condition that does not cause any immediately apparent damage (but may in fact shorten the life of the PD device or it could be a sustained condition resulting in spontaneous disassembly.)
3. Short Circuit: A short circuit is when unintentionally low impedance is introduced into a circuit resulting in excessive current flow and subsequent damage. This can be the result of a component failure, introduction of a foreign object or missed operation within the circuit.
4. Input Voltage Transient: This is a very rapid spike in the input voltage to a device or PCB. Rise times can be on the order of pico seconds and amplitude peaks can be many times the rated input voltage. These transients can either be conducted from other equipment on the line or induced INTO the line via
induction from sources outside the system. For example, lightning strikes many miles away can induce substantial transient voltages into especially long lengths of conductors.
5. Input Surge: This is a sustained input voltage well in excess of the nominal designed value. Since these can be of a duration measured in seconds rather than μSec, the energy contained in them makes them very challenging to protect against. Fortunately, these are relatively VERY rare events as compared to Input Transients.
6. Misuse/Vandalism: This is when the device is either accidentally or intentionally subjected to some input that is both sustained and substantially beyond the designed limits. For example, the input power may be connected so that the polarity is reversed.
Most Common Protection Components
In this section, we will take a brief look at the most commonly used protection components.
1. ESD Diodes: These are diodes either integrated into an IC or added externally as either discretes or part of a diode array. They are connected between a given (small) signal line and VCC and GND in such a manner so as to clamp that signal line (by forwarding conducting) to remain a diode drop within VCC or GND depending the polarity of the ESD event. See Figure 3:
A distinction should be noted between ESD diodes and TVS diodes. ESD diodes only function in forward conduction and are applied only to address lower energy ESD events. This is in contrast to TVS diodes which are able to function in both conduction directions and are built for much higher energy events.
2. TVS Diodes: These devices are Zener diodes (specially constructed for fast energy absorption) that conduct in the reverse direction when a transient on the line being protected exceeds their breakdown voltage. Devices rated below 5.6V primarily rely on the “Zener effect” while devices above 5.6V primarily rely on avalanche breakdown.
3. MOV – Metal Oxide Varistors: MOVs are passive devices comprised of ceramic bulk poly crystalline zinc oxide that has been mixed with relatively small amounts of other metal oxides such as bismuth, cobalt, manganese, etc. This mixture is sintered to form a variably sized granular structure. The boundaries between the grains behave like P-N semiconductor junctions. Like diodes, they do not conduct until a given threshold voltage is reached, after which, their conduction rises exponentially with increased voltage. Thus, this granular structure can be thought of as a matrix of series connected diode strings in parallel with other similar strings. Control of the geometry, composition and grain size allows production of parts of varying voltage behavior and energy handling ability. See Figure 4:
MOVs can range in size from small 0603 SMT devices to dinner plate sized discs that are then assembled into series connected stacks to arrive at the desired voltage.
In order to avoid damaging leakage currents, the MOV must be rated for a clamp voltage further from the normal operating voltage than is the case for other technologies such as TVS diodes.
The MOV is degraded each time it clamps and conducts substantial current. This is in contrast to TVS diodes which can withstand an indefinite number of clamping events provided they do not exceed their energy rating.
MOVs dissipate the energy in the transient by dropping voltage across their intrinsic P-N junctions that are distributed across their entire bulk mass. Thus, they can dissipate much more energy than a TVS of the same mass since they only dissipate energy in their one (or two) P-N junctions.
Above are some typical MOV devices that range in size from small 0603 SMT devices to “boat anchors.”
4. GDT – Gas Discharge Tubes: In the context of protection devices, a GDT is a device that employs two or three electrodes sealed in an inert gas environment and is designed to allow creation of an arc between the terminals when an over-voltage event is present across two of the terminals. Thus, they are sometimes referred to as Plasma Surge Arresters. When these devices are activated by an over-voltage event, they change from a very high impedance/open circuit to a near-short circuit. After the voltage across the two activated terminals falls below the “DC Hold Over Voltage” of the device, the arc will clear and the device will return to its prior high impedance state. Since GDTs are only conducting at a relatively very low impedance value (<20Ω) as compared to TVS and MOVs, they can conduct event currents MUCH higher than either of those devices.
Because of their complementary natures, GDTs and MOVs are often used together in AC line applications as shown in Figure 6.
Here, the RFI Filter functions as the required impedance between the MOVs on the right (closest to the LOAD) the GDTs across the Lines and to GND. This is necessary so as to allow the clamp current into the MOVs to cause a voltage rise across the dividing impedance which, in turn, results in the rise in voltage at the GDT terminals required to trigger them into their low impedance state.
Figure 7 shows the schematic for two and three terminal GDTs and a picture of some typical devices:
In our October issue, Part II of this article will provide (1) an overview of the remaining commonly used protective devices, (2) some tips on how to identify which protective device is best for your application and (3) a chart summarizing the characteristics of each type of device and their relative pros and cons.