ST AN320 APPLICATION NOTE

AN320

Application note

Operation of a Trisil™ crowbar type protection diode

Introduction

In the field of parallel protection, the devices used have two main functions in transient operation.

■ Limit the voltage.

■ Divert the surge current.

If the first function is carried out perfectly by an avalanche junction, confirmed by the success of the Transil™ series, the second is limited by voltage permanently present across the diode terminals.

Use of increasingly sophisticated but fragile electronic components and publication of new standards do not allow the use of Transil diodes in certain applications. This problem is solved by the use of a semiconductor device with two conducting states such as the thyristor (or Triac in the bidirectional version).

STMicroelectronics has developed this type of component under the trade name of Trisil. This Application note describes the operation of the Trisil.

Figure 1. I / V characteristic of a Trisil

TM: Trisil is a trademark of STMicroelectronics TM: Transil is a trademark of STMicroelectronics

Transient operation

Standby operation

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July 2010 Doc ID 5649 Rev 3 1/13

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Trisil characteristics AN320

1 Trisil characteristics

1.1 Electrical characteristic

The electrical characteristic of the Trisil is similar to that of a Triac (see Figure 1) except that the component has only two terminals.

Triggering in this case is not done via a gate but by an internal mechanism dependent on the current flowing through it.

1.2 Operation seen from the outside

In normal operation, the Trisil is biased at a voltage lower than or equal to the standby voltage (V presence of the Trisil connected across the equipment to be protected does not disturb its operation (see Figure 2).

The characteristic data at this point includes:

● Leakage current

● Electrical capacity

● Reliability of the component in blocking mode

). At that point of the characteristic, the leakage current is about 10 nA and the

Figure 2. Stand by characteristics

As the voltage increases beyond V

, the Trisil impedance drops from practically infinite to a

few ohms. The Trisil remains biased at its avalanche voltage and its operation is then identical to that of a Transil diode (see Figure 3).

The characteristic parameters at this level are the limiting voltage (breakover voltage of the component, V

) and the time for switching between the blocked and conducting states.

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AN320 Trisil characteristics

Figure 3. Avalanche characteristic of the Trisil

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For current values higher than I

, the voltage across the Trisil drops to a few volts and the

high currents permitted without damage are possible due to the low value of this voltage, since the physical limit is dependent on the dissipated power (see Figure 4).

Figure 4. Triggering, and on-state characteristics

The characteristic parameter is then the possibility of withstanding surge currents (peakpoint current, I

Return to standby operation by turning off the Trisil takes place when the current flowing through it drops below I

. This is the characteristic parameter for switching from the

conducting to the blocked state (see Figure 5).

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Trisil characteristics AN320

Figure 5. Return to standby operation

The surge current associated with the disturbance is diverted through the Trisil as soon as it begins to operate in the avalanche mode (see Figure 3) and the voltage limitation results from the electrical characteristic at this point. The behavior of the Trisil is here identical to that of the Transil. The difference depends on the level of the breakover current, I

, where

the triggering of the thyristor structures take place.

This phenomenon results in absolute limitation independently of the current level, and a capacity to divert currents much higher than those possible for an avalanche diode (Transil).

Furthermore, this limitation is independent of the avalanche voltage of the device.

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