ST AN1826 Application note

	AN1826
®	APPLICATION NOTE

TRANSIENT PROTECTION SOLUTIONS:

Transil™ diode versus Varistor

A. BREMOND / C. KAROUI

Since the seventies, electronic modules are more and more present in our life. This is the case for our entertainment, automotive, telecommunication, production tools, house access equipment, and it is not really a secret that these smart functions are very sensitive to transients coming from human body itself (ESD1), electromagnetic part switching and industrial transients (EOS2), atmospheric effects (lightning). These surges are more often lethal for the electronic functions and the constant decrease of silicon elementary cell size (transistor) makes them more and more compact but more and more fragile.

To assume a correct definition of a protection stage, we have to ask the two following questions :

-what we have to protect?

-against what we have to be protected?

The answer to the first question depends on both the technology used to realize the part to be protected and normal operating waveform of the signal on the line to be protected. Needed parameters are the maximum voltage that technology can withstand, the type of normal operating signal (unidirectional : 0-Vcc or bi-directional : ±Vcc), the maximum and minimum voltages of this signal and its frequency. The answer to the second one is closely linked to the environment where modules are operating. In each application we have to focus on one type of disturbances, i.e. in telecom we are faced to atmospheric effects, in domestic and industrial worlds, disturbances are EOS while in computer or mobile phones, the main surge cause is due to ESD.

To avoid dramatic consequences on systems submitted to over voltage transients, the best way is to use protection devices based on clamping action.

In this kind of products we have the choice between two technologies :

-silicon devices named Transil™ - so called TVS3

-and ceramic components - Varistor for instance.

Figure 1 shows the electrical characteristics of a clamping device, this curve has two areas.

Fig. 1: Electrical characteristics of bi-directional clamping devices.

The first one located between 0V axis and VRM4 is the normal operating zone; in this area the signal is not be affected by the transient suppressor (low leakage current and negligible intrinsic capacitance).

The second one is the surge suppression zone in which the surge is clamped at VCL5.

VCL = VBR + Rd × IPP

VBR6: knee voltage of the I/V curve

Rd: dynamical resistance.

IPP: current value during surge.

	I
	IPP
	IRM	V
	VRM
		VCL
	VBR

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AN1826 - APPLICATION NOTE

In this zone, the main property of the protection device is to have a clamping voltage as close as possible to the operating voltage. This property is defined by the clamping factor CF.

C F = VCL

VBR

Fig. 2: Electrical characteristics of unidirectional clamping devices.

	I
	IPP
	IRM		V
	IR	VRM
			VCL
		VBR
		VR

When the signal moving through the line to be protected is unidirectional (0 to +Vcc or -Vcc to 0) the surge suppressor has to be unidirectional, otherwise the protection function is not optimized.

Figure 2 shows the electrical characteristics of a unidirectional clamping device. In this case the positive behavior of the device is still the same while the negative one is totally different.

Negative over voltage occurring on the protected line is clamped to a typical forward drop voltage of a diode.

Figure 3 gives the electrical characteristics of both Transil™ diode and Varistor measured on curve tracer. Both tested components have been chosen in the same power and voltage (14V) ranges in order to make an accurate comparison.

Fig. 3: Electrical characteristics measured on curve tracer.

Current (µA)

200

150

100

50

0

Varistor

-50

-100

-150

-200 -20 -10 0 10 20

Voltage (V)

Current (µA)

200

150

100

50

0

Transil

-50

-100

-150

-200 -20 -10 0 10 20

Voltage (V)

The first remark we can do, regarding these curves, is that the Varistor operates uniquely in bi-directional mode while the Transil™ diode can be both uni or bi-directional (here this is a unidirectional type). The second point is the leakage current which is higher for the Varistor particularly close to the knee of the curve. The third point to focus on is the slope of these characteristics in the clamping zone (vertical part of the curve after VBR), here one can easily see that the Transil™ diode slope is tighter than the Varistor one.

And taking into account that the curve slope represents 1 , so for the same surge current the clamping

Rd

voltage will be lower for the Transil™ and then its clamping factor will be better.

To verify this point we performed measurements on both Transil™ and Varistor during EOS and ESD surge test conditions. The EOS tests are based on IEC61000-4-5 1.2/50µs (8/20µs current) composed waveform while ESD surges were performed in accordance with IEC61000-4-2 system standard.

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