By Mel Clark and Kent Walters
This note highlights the differences between thyristor surge protective device (TSPD) crowbars and conventional silicon transient voltage suppressors (TVS) clamp components used for protecting vulnerable silicon microchips from voltage spikes. Each type has its unique features which complement each other.
The advantage of the TSPD is its low on-state voltage after triggering which provides much higher surge current capability for a relatively small chip size compared to a clamp device. A 1.5KE200C (bidirectional TVS clamp) will handle 5A for a 10/1000us surge while a TSPD with the same chip size can safely conduct 200A for the same surge duration.
The reason for this major difference is illustrated by comparing characteristic curves of the two devices. Figure 1 depicts the familiar V-1 curve for a bidirectional TVS. Peak pulse power rating (PPP) is the product of clamping voltage (VC) times peak pulse current rating (IPP), and is limited by the maximum allowable junction temperature.
The TSPD supplied by Microsemi is a multi-layer device which breaks down with a positive resistance, then quickly switches back to a low on-state voltage as shown in Figure 2. TVS and TSPD devices are available as unidirectional or bidirectional.
[Figure 1. TVS V-I Curve (Bidirectional shown)]
[Figure 2. TSPD V-I Curve (Bidirectional shown)]
The TSPD on-state condition allows greater surge current conduction (for equivalent power) compared to a clamp device, since the TSPD on-state voltage is vastly lower than the TVS clamping voltage. For specific parameter details, refer to the Microsemi Data Book.
For restoration to nonconduction, TSPD current must be interrupted or reduced below the holding current (IH). This limits the use of TSPDs to applications associated with low driving currents such as signal and communication lines. In contrast, silicon TVSs restore to nonconduction when the driving voltage drops below breakdown voltage (V(BR)) so they are not subject to this limitation.
Present surge current capability of TSPDs includes 50A, 100A and 200A for a 10/1000us surge. Operating voltages typically cover a broad range, from 12V up through several hundred volts.
Typical uses for TSPDs include protection across signal and control lines exposed to harsh lightning conditions. These include central office telephone systems, PABXs, consumer subscriber stations, faxes and modems. Other applications include protection of instrumentation and SCR switches for remote controls.
Silicon avalanche TVSs find use across signal lines in less harsh environments for protection from secondary lightning effects and electrostatic discharge (ESD). TVSs are also used across power supplies where the device must restore to nonconduction without interrupting or severely limiting voltage or current flow to the load.
The capacitance of the TSPD is considerably lower than its TVS counterpart, particularly at the lower end of the voltage spectrum. This lower capacitance minimizes signal attenuation for high data rate signals.
TVS devices are available over a continuous voltage spectrum from five volts up through several hundred volts, providing more voltage selections compared to the TSPD.
The TSPD crowbar is most useful for protection across communication and control lines exposed to harsh lightning environments. The TVS is applicable for protection from secondary transient effects, ESD and across power buses. Each of these two device types is suited for specific uses and complement each other in providing optimal protection.
Copyright Microsemi Corporation 1996
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