By Mel Clark and Kent Walters
This note is intended to clarify the confusion that exists in silicon transient voltage suppressor (TVS) nomenclature and to provide guidance for cross referencing. Examples will illustrate the selection process for choosing equivalent or near equivalent parts.
Some series have their part numbers based on the nominal breakdown voltage (V(BR)) of the device while others are based on the rated working voltage (VWM). Examples of both numbering systems include the P6SMB6.8 Motorola series based on a nominal V(BR) and the Microsemi SMBJ5.0 series which is based on VWM. Both of these devices are surface mount types rated for 600 W of peak pulse power (PPP). Cross referencing is normally applicable only for devices of the same PPP rating for equivalent TVS performance levels.
Initially, TVS device offerings were identified by part numbers based on breakdown voltage, V(BR). Subsequent product introductions were then labeled with the more practical working/operating voltage, VWM, base. With the VWM value integrated into the part number, a design engineer could more easily identify and select the best part suited for his/her application. A major advantage to using the VWM based nomenclature is that the minimum V(BR) and VWM are identical for +/-5% and +/-10% tolerance parts. Hence, the +/-10% part becomes a candidate for substitution as a +/-5% device if required and may even provide a more permanent cost effective solution.
The first parameter to match in selecting an equivalent is the minimum breakdown voltage, V(BR), since most other ratings correlate well with this characteristic. Then check the fit of VWM, peak pulse current (IPP) and maximum clamping voltage (VC). For example, selecting a replacement for the Motorola P6SMB10A, 10V nominal V(BR), 600 W surface mount package part with a Microsemi equivalent is shown below:
Note that the electrical parameters of the Microsemi part match very closely and well within the needs for circuit performance and protection requirements. Both parts share essentially the same electrical characteristics and DO-214AA package outline although marked with different labels. This exercise in equivalent selection provides guidance to the user in choosing alternative sources and also allowing for broader supplier base.
Cross referencing is also required in converting from thru-hole to surface mount technology since most axial leaded part numbers are based on V(BR), while most surface mounts are based on VWM. Illustrated below is an example cross referencing an axial lead 1.5 kW device for operating in a 36 V circuit to an electrically equivalent surface mount chosen from the Microsemi Product Data Book.
Here we show that the SMCJ36A surface mount is an acceptable electrical replacement for the axial leaded 1.5KE43A. In this example, the minimum V(BR) of the SMCJ36A compared favorably with that of the 1.5KE43A for matching. Subsequently, the remaining parameters were also a close match for the same 1.5 kW device rating.
Some parameters match more closely than others. In most cases, especially for ESD protection, the VWM can be as high as 15 V for a 5 V operating circuit and still provide good performance. This has been proven in volume production.
In the event that a perfect match cannot be made, integrated circuits are normally quite forgiving for very short duration pulses, even though a substitute TVS may clamp a few volts higher. If you need additional guidance in cross referencing devices or in selecting a TVS for your application, please call Microsemi Corp. - Scottsdale for assistance.
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