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SOME STRAIGHT TALK ABOUT MOSORBS TRANSIENT VOLTAGE SUPPRESSORS
INTRODUCTION
Distinction is sometimes made between devices trademarked Mosorb (by Motorola Inc.), and standard zener/avalanche diodes used for reference, low-level regulation and low-level protection purposes. It must be emphasized from the beginning that Mosorb devices are, in fact, zener diodes. The basic semiconductor technology and processing are identical. The primary difference is in the applications for which they are designed. Mosorb devices are intended specifically for transient protection purposes and are designed, therefore, with a large effective junction area that provides high pulse power capability while minimizing the total silicon use. Thus, Mosorb pulse power ratings begin at 500 watts -- well in excess of low power conventional zener diodes which in many cases do not even include pulse power ratings among their specifications. MOVs, like Mosorbs, do have the pulse power capabilities for transient suppression. They are metal oxide varistors (not semiconductors) that exhibit bidirectional avalanche characteristics, similar to those of back-to-back connected zeners. The main attributes of such devices are low manufacturing cost, the ability to absorb high energy surges (up to 600 joules) and symmetrical bidirectional "breakdown" characteristics. Major disadvantages are: high clamping factor, an internal wear-out mechanism and an absence of low-end voltage capability. These limitations restrict the use of MOVs primarily to the protection of insensitive electronic components against high energy transients in application above 20 volts, whereas, Mosorbs are best suited for precise protection of sensitive equipment even in the low voltage range -- the same range covered by conventional zener diodes. The relative features of the two device types are covered in Table 1.
IMPORTANT SPECIFICATIONS FOR MOSORB PROTECTIVE DEVICES
Typically, a Mosorb suppressor is used in parallel with the components or circuits being protected (Figure 1), in order to shunt the destructive energy spike, or surge, around the more sensitive components. It does this by avalanching at its "breakdown" level, ideally representing an infinite impedance at voltages below its rated breakdown voltage, and essentially zero impedance at voltages above this level. In the more practical case, there are three voltage specifications of significance, as shown in Figure 1a. a) VRWM is the maximum reverse stand-off voltage at which the Mosorb is cut off and its impedance is at its highest value -- that is, the current through the device is essentially the leakage current of a back-biased diode. b) V(BR) is the breakdown voltage -- a voltage at which the device is entering the avalanche region, as indicated by a slight (specified) rise in current beyond the leakage current. c) VRSM is the maximum reverse voltage (clamping voltage) which is defined and specified in conjunction with the maximum reverse surge current so as not to exceed the maximum peak power rating at a pulse width (tp) of 1 ms (industry std time for measuring surge capability).
RELATIVE FEATURES OF MOVs and MOSORBS
Table 1.
MOV * High clamping factor. * Symmetrically bidirectional. * Energy capability per dollar usually higher than a silicon device. However, if good clamping is required the energy capability would have to be grossly overspecified resulting in higher cost. * Inherent wear out mechanism clamp voltage degrades after every pulse, even when pulsed below rated value. * Ideally suited for crude ac line protection. * High single-pulse current capability. * Degrades with overstress. * Good high voltage capability. * Limited low voltage capability. Mosorb/Zener Transient Suppressor * Very good clamping close to the operating voltage. * Standard parts perform like standard zeners. Symmetrical bidirectional devices available for many voltages. * Good clamping characteristic could reduce overall system cost. * No inherent wear out mechanism. * * * * * Ideally suited for precise DC protection. Medium multiple-pulse current capability. Fails short with overstress. Limited high voltage capability unless series devices are used. Good low voltage capability.
Motorola TVS/Zener Device Data
Articles 11-101
CHAPTER 12
In practice, the Mosorb is selected so that its VRWM is equal to or somewhat higher than the highest operating voltage required by the load (the circuits or components to be protected). Under normal conditions, the Mosorb is inoperative and dissipates very little power. When a transient occurs, the Mosorb converts to a very low dynamic impedance and the voltage across the Mosorb becomes the clamping voltage at some level above V(BR). The actual clamping level will depend on the surge current through the Mosorb. The maximum reverse surge current (IRSM) is specified on the Mosorb data sheets at 1 ms and for a logrithmically decaying pulse waveform. The data sheet also contains curves to determine the maximum surge current rating at other time intervals. Typically, Mosorb devices have a built-in safety margin at the maximum rated surge current because the clamp voltage, VRSM, is itself, guardbanded. Thus, the parts will be operating below their maximum pulse-power (Ppk) rating even when operated at maximum reverse surge current). If the transients are random in nature (and in many cases they are), determining the surge-current level can be a problem. The circuit designer must make a reasonable estimate of the proper device to be used, based on his knowledge of the system and the possible transients to be encountered. (e.g., transient voltage, source impedance and time, or transient energy and time are some characteristics that must be estimated). Because of the very low dynamic impedance of Mosorb devices in the region between V(BR) and VRSM, the maximum surge current is dependent on, and limited by the external circuitry. In cases where this surge current is relatively low, a conventional zener diode could be used in place of a Mosorb or other dedicated protective device with some possible savings in cost. The surge capabilities most of Motorola's zener diode lines are discussed in Motorola's Application Note AN784. In the data sheets of some protective devices, the parameter for response time is emphasized. Response time on these data sheets is defined as the time required for the voltage across the protective device to rise from 0 to V(BR), and relates primarily to the effective series impedance associated with the device. This effective impedance is somewhat complex and changes drastically from the blocking mode to the avalanche mode. In most applications (where the protective device shunts the load) this response time parameter becomes virtually meaningless as indicated by the waveforms in Figures 1b and 1c. If the response time as defined is very long, it still would not affect the performance of the surge suppressor. However, if the series inductance becomes appreciable, it could result in "overshoot" as shown in Figure 1d that would be detrimental to circuit protection. In Mosorb devices, series inductance is negligible compared to the inductive effects of the external circuitry (primarily lead lengths). Hence, Mosorbs contribute little or nothing to overshoot and, in essence, the parameter of response time has very little significance. However, care must be exercised in the design of the external circuitry to minimize overshoot.
SUMMARY
In selecting a protective device, it is important to know as much as possible about the transient conditions to be encountered. The most important device parameters are reverse working voltage (VRWM), surge current (IRSM), and clamp voltage (VRSM). the product of VRSM and IRSM yields the peak power dissipation, which is one of the prime categories for device selection. The selector guide, in this book, gives a broad overview of the Mosorb transient suppressors now available from Motorola. For more detailed information, please contact your Motorola sales representative or distributor.
Articles 11-102
Motorola TVS/Zener Device Data
Some Straight Talk About Mosorbs Transient Voltage Suppressors
Z
Vin MOSORB
Vout
PROTECTED LOAD
Figure 1.
VOLTAGE
VOLTAGE
Vin Vout VRSM V(BR) VRWM tp tp = PULSE WIDTH OF INCOMING TRANSIENT TIME tclamping, VERY SHORT VRSM V(BR) VRWM
Vin Vout
TIME
Figure 1a.
Figure 1b.
VOLTAGE
VOLTAGE Vin Vin OVERSHOOT
Vout VRSM V(BR) VRWM TIME tclamping, VERY LONG VRSM V(BR) VRWM
Vout
TIME tclamping, WITH OVERSHOOT
Figure 1c.
Figure 1d.
Motorola TVS/Zener Device Data
Articles 11-103
CHAPTER 12
TYPICAL MOSORB APPLICATIONS
DC Power Supplies
+ DC POWER SUPPLY +
Input/Output Regulator Protection
IC VOLTAGE REGULATOR MOSORB - +
AC INPUT
MOSORBS
Vout MOSORB -
MOSORB -
Op Amp Protection
+B MOSORB
DC Motors -- Reduces EMI
+ DC MOTOR -
Memory Protection
+5 V MOSORB MOS MEMORY -8 V MOSORB -10 V
MOSORB
IC OP AMP
MOSORB
MOSORB -B MOSORB
Inductive Switching Transistor Protection
VDD
ADDRESS BUS
VGG
RAM
ROM
DATA BUS I/O CONTROL BUS
CPU
MOSORB MOSORB
CLOCK
Gnd MOSORBS
Microprocessor Protection
KEYBOARD TERMINAL PRINTER ETC.
A I/O B FUNCTIONAL C DECODER D
Gnd MOSORBS
Computer Interface Protection Articles 11-104 Motorola TVS/Zener Device Data

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