CTVS Multilayer Varistors for Automotive – Functionality
Multilayer varistors or so-called Ceramic Transient Voltage Suppressors (CTVS) are voltage-dependent resistors with a symmetrical V/I characteristic curve whose resistance decreases with increasing voltage.
Because of their application as overvoltage protection devices, they are also often referred to as TVS (Transient Voltage Suppressors) on silicon basis. Connected in parallel with the electronic device or circuit that is to be guarded, CTVS form a low resistance shunt when voltage increases above a CTVS type-specific threshold value and thus prevent any further rise in the transient overvoltage.
The chip of a multilayer CTVS consists of a stack of alternating ceramic/electrode layers. The thickness of the ceramic layers affects the protection level.
In the active volume of the ceramic chip there are a number of internal electrode layers with an overlapping area affecting the pulse absorption capability. The internal electrodes are connected to the terminals or external electrodes.
The nickel barrier termination is a suitable for lead-free reflow soldering.
V/I characteristic curve
CTVS are operated in one of two modes. These modes concern two different segments of the CTVS V/I curve (1) which lies within a well defined tolerance band (2):
“High-resistance” mode (< 1 mA) If the circuit is operated at normal operating voltage the CTVS has to be highly resistive. Here, the circuit designers may generally want to know about the largest possible leakage current at the given operating voltage. Therefore, the lower limit of the tolerance band is shown.
“Low-resistance” mode (> 1 mA) In an overvoltage event the CTVS has to be highly conductive. Here, the circuit designer’s primary concern is about the worst-case voltage drop across the CTVS. The upper limit of the tolerance band is shown. The clamping voltage Vclamp is the voltage across the CTVS in an overvoltage event.
The CTVS V/I characteristic curve (1) shows the mean value of the tolerance band between the limits indicated by dashed lines (2). The mean at 1 mA represents the varistor voltage (VV = VBR), in this case 22 V. The tolerance K ±10% refers to this value, so at this point the tolerance band ranges from 19.8 to 24.2 V (region (2) ). For the CT0805K14G MLV, a maximum permissible operating voltage of 18 VDC is specified (3). Depending on the actual value of the MLV in the tolerance band, you can derive a leakage current between 6 x 10–6 A and 2 x 10–4 A at room temperature (region (3) ) as well as an increase of the voltage across the MLV to between 65 V and 80 V (region (4) ) assuming a surge current of 100 A.